Fang Wang

Zhong-Kuan Luo

Yan Pang

Chun-Sheng Liang

Jing Chen

Dong Liu

Xiang-Hua Zhang

Yang-Hai Xu

Prof Zhong-Kuan

Luo

A dual pore carbon aerogel based air cathode for a highly rechargeable lithium-air battery

Keywords: lithium-air battery, carbon aerogel, dual pore structure, cathode passivation, sulfolane lithium-air battery, carbon aerogel, dual pore structure, cathode passivation, sulfolane

The table of contents entry should be 50-60 words long, and the first phrase should be bold. A highly conductive dual pore carbon aerogel air cathode is successfully synthesized, favorable for the fast mass transport and superior discharged products accommodation/decomposition, thus realizing a relatively high discharge rate cycling performance and impaired cathode passivation. This work targets for exploiting a high power and reversible lithium-air battery by coordinating the necessary ingredients: carbon loading and current density et al..

Introduction

Lithium-air battery, with a high energy density of 1000 Wh kg -1 which is 2-4 times higher than that of current state-of-art lithium-ion battery, has attracted unprecedentedly broad attention and is destined to be further developed to address series of problems, such as the finite oil resources, deteriorated climate and high fuel prices, and to meet the demand of long distance transport's energy storage in electric vehicle industry [1][2][3]. A typical non-aqueous rechargeable lithium-air battery involves reversible formation/decomposition of Li 2 O 2 via multiple discharge-charge cycles. However, despite such unmatched potential superiority, pore blockage, cathode passivation and degradation all result in low discharge rates and poor cycling stability (about 100 cycles), restricting its practical utilization [2][3][4][5][6][7][8][9][10][11][12][13][14][15].

To build a reversible and long-life span lithium-air battery, cathode design is an indispensable task as well as challenge. Since cathode has function of accommodating discharge products and providing free path for oxygen, e -and Li + transport, the porous structure is a prerequisite and highlighted. What's more, requirements like high conductivity, low cost and ease of fabrication all render carbon one of the most attractive materials for constructing a porous (three-dimensional) electrode. In order to achieve an optimum performance, various carbon sources (with surface defects and functional groups, hydrophobicity/hydrophilicity and oxygen adsorption of the carbon surface et al.), specific surface area, pore volume, pore size distribution and mass density must be all considered and synergistic, at last to get a compromise. Though lots of efforts have been spared in optimizing the cathode structure, pore blockage, cathode passivation and degradation are still encountered [16][17][18][19][20][21][22][23][24][25][26].

Inspired by Zhang et al. [23,[27][28], a novel highly conductive nano-porous carbon aerogel comprising of fine dual pore structure was successfully fabricated here to improve the mass transport and to reduce pore blockage and cathode passivation.

For such type battery, we postulate that the large pore is apt to facilitate oxygen penetration, and simultaneously yield and store discharged products at its abundant

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4 catalytically active carbon surface; while the small pore with little or non-catalytic active centre will be filled with highly conductive Li + electrolyte due to the coalition of hydrophilic carbon surface and capillary pressure, conducive to Li + transfer. By this new approach of cathodes designing, high oxygen-transport properties and degenerative pore blockage will be realized, favorable for the high discharge rate performance and cyclability. Free charges (e -) will mainly transfer to tri-phase boundary reaction zones where accumulates higher oxygen concentration, through carbon aerogel's highly conductive interconnected 3D network. Meanwhile, electro-active material's high specific surface area can provide more reaction sites for electrochemical reaction and its large pore volume renders large space for discharged products accommodation. Li-oxide precipitation will be eventually filled in the pores which house a catalytic active junction (with an enormous amount of both O 2 and Li + , complementary subsystems) [24,25].

1 M LiTFSI/sulfolane was chosen as electrolyte for its relative stability which was previously testified by Zhang and our group et al. [29][30][31][32][33][34][35][36]. For this newly built battery system, when controlling the specific capacity of 1000 mAh g -1 , 525 cycles and 112 cycles were obtained at a current density of 0.05 mA cm -2 and 0.10 mA cm -2 , respectively, which far exceeds that of the previous researches that identically adopted carbon aerogel as air electrode active materials [37][38][39][40][41][42][43]. Such impressive cycling behavior can be attributed to the distinctive battery structure which includes both the well modified cathode structure and relatively stable electrolyte, certainly in addition to an oxygen filled environment and the effective control of discharge rates and depth.

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As compared to common used carbonate, ester, dimethylformamide and ether-based electrolyte [44][45][46][47][48][49], sulfolane-based electrolyte could endure high overpotential during charge process and thus shows superior antioxidant ability which is bound to obtain a relatively stable Li-O 2 battery. Following the carefully controlling of the discharge rates and depth, granular to sheet-shaped or layered lithium peroxides (unlike bulk precipitates) which could undermine the cathode passivation formed during discharge process, and had already been proved to be metallic in certain lattice surface [50][51][52][53][54][55].

Experimental

Preparation of carbon aerogel and cathode, and cell assembling /disassembling

Carbon aerogel was taken as the cathode active materials here due to its light density, high specific surface area, good conductivity as well as its wide scale of pore size distribution. It was synthesized according to a resorcinol-formaldehyde (R-F) polycondensation and carbonization method previously reported by Richard W.

Pekala [37]. After successful preparation, it was mixed with pVdF powder in a molar ratio of 9:1 and stirred in a NMP containing bottle to make approriate viscous slurry, which was soon deposited on carbon paper to make various air cathodes, where the aerogel coating was applied as a layer on one side of the carbon paper. Here, cathodes with different carbon loading ranging from 0.1 mg to 0.6 mg were obtained. The aerogel loading also can be shown in the unit of mg cm -2 , which was in the range of 0.057 mg cm -2 to 0.340 mg cm -2 . Hereafter, with a whatman GF/D glassy fiber

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separator and 1 M LiTFSI/sulfolane electrolyte, the lithium-air batteries were assembled in a glove box with moisture and oxygen concentration lower than 0.5 ppm. The schematic diagram of the lithium-air battery model (CR2032) can be seen in Fig. S1. Then the cells were aged for 12 h in an inert atmosphere and exposed to oxygen for more than 30 min prior to electrochemical test. The boundaries were chosen according to both the literature and our previous experimental (see details in the Supporting Information). Intermittently, the batteries of different discharge or charge state were disassembled in the glove box. After DME solution soaking and vacuum drying for another 12 h, the cathode was sealed quickly in a gas-tight device for the subsequent XRD and SEM test.

Materials characterization and electro-chemical measurements

SEM and TEM tests were exploited to analyze the morphology of the synthesized RF aerogel and carbon aerogel, as well as the discharged/charged products. The TG-DTA test was used to evaluate the RF aerogel's thermal stability and ensure the degassing temperature for the following N 2 absorption/desorption test which aimed at studying the specific surface area, the BJH pore size distribution and pore volume. Here, XRD test was adopted to inquire the discharged and charged products composition and investigate the mechanism of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in lithium-air batteries. The standard electrical four-probe method was utilized to check the cathode and carbon aerogel's electrical conductivity.

With regard to electrochemical measurement, galvanostatic charge/discharge test,

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CV test and electrochemical impedance analysis were all performed to examine the cycling stability and cycling life, the dynamic process of the inner electrochemical reaction and mechanism, respectively. All the cycling numbers were recorded under the cut off voltage of 5.0 V for charge or above 2.0 V for discharge with fixed specific capacities or capacities. The discharge time is controlled to be 2.3 h simultaneously when the current density surpasses 0.10 mA cm -2 , which aims at obtaining a practical use. It's highlighted here that the specific capacity is presented as capacity per gram of carbon, which would be convenient for comparison to previous work, however it probably grossly inflates the practical capacity available. The cell voltage gap is taken from the number of 60% specific capacity (600 for 1000 mAh g -1 or 1200 for 2000 mAh g -1 et al.). Detailed fabricating and testing method or process were illustrated in the Supporting Information.

Results and discussion

Materials fabrication and characterization

Morphology of organic aerogel and carbon aerogel

Organic aerogel and carbon aerogel are synthesized through a series of complex physical and chemical processes, including sol-gel reaction, solvent exchange and nbsphigh sintering. By carefully controlling the molar ratio of resorcinol (R) to sodium carbonate (C) and the reactant mass fraction (w), various controllable pore structures varying from micron to nanometer level can be made. As can be seen in Fig. 1(a), numerous spherical-like particles aggregate to large cross-linked spatial 3D network structure for organic aerogel (RF1500-40). After calcination, it turns to carbon aerogel (CRF1500-40) and exhibits much loose state and full of nano-scale pores at the external surface (Fig. 1(b)). The TEM graphs of RF aerogel (Fig. 1(c), RF1500-40) and carbon aerogel (Fig. 1(d), CRF1500-40) also show uniform spherical particles about 100 nm and a great number of interconnected mesopores. The puffy carbon aerogel, with a low density, not only decreases the cathode coating materials mass but also provides large volume for discharge products to be accommodated.

Meanwhile, these rich pores must be convenient passages for the outside gas and anode Li + to get through electrolyte to the carbon active areas. There are two exothermic peaks and an endothermic peak in the Fig. 1 (e), corresponding to the benzene ring fracture, C-H bond split and water loss, respectively. The fracture of these bonds generates more meso-pores, promoting the transformation of macro-pores and closed pores into meso-pores and micropores.

Thus pores are greatly rich in the aerogels and the specific surface area is higher after calcination. It is also noted that the mass loss of the RF aerogel remains stable or is slowing down at temperature above 800 °C from the TG curve. In theory, higher temperature will contribute to higher conductivity at the expense of consuming more power sources. Therefore, a moderate temperature of 1050 °C is chosen, and a carbonized material yield of 27.8% is gained, with a superior conductivity of 1.218 S cm -1 . Such high conductivity, convenient for fast charge transfer in the interconnected spatial 3D network, is enough to maintain good electrochemical reaction in the lithium-air batteries Particularly, there exist hysteresis loops at the high relative pressure above 0.8, indicating the presence of a great number of meso-pores. It is known that meso-pore favors mass transport and enlarges reaction active sites, facilitating the reaction kinetics and possibility. What's more, the apparent dual-pore structure which emerges in the carbon's meso-pore scope and primarily concentrates at pore width of 3.85 nm and 17.95 nm can effectively diminish blockage stemming from the deposited discharge products and provide free path for the oxygen and Li + . Table 1 and Table S1 provide detailed porosity parameters of the carbon aerogel samples. It reveals that, for aerogel CRF1500-40, meso-porosity is dominant (83.3%) in the carbon structure and the electro-active material has a meso-porous structure with a total pore volume, a meso-pore volume and an average pore diameter of 1.02 cm 3 g -1 , 0.85 cm 3 g -1 and 8.27 nm, respectively. However, for aerogel CRF1000-30, though with very similar total pore volume, meso-pore volume and average pore size of 1.06 cm 3 g -1 , 0.80 cm 3 g -1 and 8.46 nm respectively, the meso-pores only appear around 16 nm. Here, a synergy among specific surface area pore volume and pore size distribution, which contributes to superior cycling performance, is confirmed by the later electrochemical test (see Fig. 2, Fig. 3 and Fig. 4). V total (cm 3 g -1 )

V micro (cm 3 g -1 )

V meso (cm 3 g -1 )

V meso First, the mass accessibility of the dual pore structure and the common single pore structure is evaluated. As is shown in the Fig. 2, at a current density of 0.05 mA cm -2 and a fixed capacity of 0.1 mAh, the battery with dual pore aerogel coating cathode (CRF1500-40) runs 525 cycles comparing to 332 (reach 5.0 V; 364 cycles to 2.0 V ) cycles for that of the single pore aerogel cathode (CRF1000-30). And during the galvanostatic charge/discharge cycling, the voltage gaps for the CRF1500-40 and CRF1000-30 cathode are 0.89 V and 1.16 V, respectively in the first cycle. We postulate that the difference can be ascribed to two reasons, one is the different electrical conductivity for the two different aerogels, 1.218 S cm -1 and 0.502 S cm -1 for CRF1500-40 and CRF1000-30 respectively, the other is the different pore structure. The two aerogels have very similar specific surface area, pore volume, meso-pore fraction and BJH pore size of 669.17 m 2 g -1 , 680.22 m 2 g -1 , 1.02 cm 3 g -1 ,

1.06 cm 3 g -1 , 83.30%, 75.47%, 17.95 (and 3.85) nm and 16 nm respectively (see Table 1, Table S1). For CRF1500-40, it possesses two typical meso-pores, one

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ACCEPTED MANUSCRIPT concentrating at 3.85 nm and the other at 17.95 nm, which can greatly diminish pore blockage and be convenient for mass transfer in two separate channels (see Fig. 4). Some pioneers in this area have referred that a meso-pore concentrating at 3.0~3.5 nm or 20~50 nm is beneficial for the battery cycling performance. [19,22,[56][57][58] The dual pore designed in our experimental may be just a desirable combination of these two pores. For aerogel CRF1000-30, pore only centers around 16 nm. The two aerogels, with different pore structure and electrical conductivity, thus yield different ability for mass accessibility and cell performance. And conclusions can be arrived that dual pore might improve the mass accessibility and cell performance. Thus, all the later electrochemical experiments are carried out by using the dual pore CRF1500-40 as active coating materials except when the discharge rate capability is involved in (CRF1000-30 is also discussed for comparison). For aerogel CRF1000-30, after various galvanostatic charge/discharge cycling, the discharged/charged products are also proved to involve the reversible formation/decomposition of Li 2 O 2 except for a little byproduct of LiOH, with round-shaped granular precipitations (Fig. S4). specific capacity are described as below: 0.05 mA cm -2 , 1000 mAh g -1 . The cut off voltage scope is between 2.0 V and 5.0 V for discharge and charge, respectively.

Carbon loading's affect on the cycling performance

In order to find carbon loading's effect, we compare six cathodes with different carbon loading, ranging from 0.1 mg to 0.6 mg, and discover that the cycling performance reduces as the carbon loading increases, but still keeps at least 78 cycles until the charging cutoff voltage reaches to 5.0 V for all the cases (see Fig. 3(a-b), Fig. S5 (a-f) and Table S2). Such prominent cycling stability may be associated with the carbon aerogel's inimitable dual pore, which could not be jammed simultaneously by discharged products of one kind of specific morphology, thus enhancing the reaction dynamics. We postulate that the inherent dual pore might inventively create a suitable tri-phase boundary reaction zone (see Fig. 4 (d)), facilitating oxygen and Li + diffusion in two independent pore channels. Furthermore, the large pore volume and high specific surface area can render more space for the deposition of discharged products and more active reactive sites and possibilities, respectively. All these potential advantages and characteristic lead to high energy efficiency above 65.18% and high final discharge voltage above 2.5 V of the initial 78 cycles for the batteries of all cases (see Fig. S6 and Fig. S7). LiTFSI/sulfolane is used as electrolyte; The current density is 0.05 mA cm -2 and specific capacity is 1000 mAh g -1 . The carbon loading is ranging from 0.1 mg to 0.6 mg.

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For the cycling performance deterioration, it can be explained as below: the coating becomes thicker (or denser) and part of the pore volume is blocked along with the increasing carbon loading. The former leads to longer transportation path for Li + and oxygen to travel to the active reaction sites, delaying the reaction kinetics. The later causes ineffective utilization of the total pore volume and intensifies the oxygen diffusion barrier. What's more, a great deal of precipitates that mainly consist of lithium peroxide form during discharge process, which is reported to be insoluble and insulated from granular to bulk morphology, scaling up the impedance of the whole battery (see Fig.

Discharge depth and current density's affect on the cycling performance

In this section, the discharge/charge depth and current density's function on the cycling performance are investigated. Fig. 4(a) and Fig. S11(a-c) reveal that at a current density of 0.10 mA cm -2 , when the specific capacity is set to 1000, 1500 and 2000 mAh g -1 , the lithium-air batteries ran 112, 44 and 34 cycles, respectively, during the galvanostatic charge-discharge test. To construct a realistic applied battery, the test time was simultaneously prolonged to 2.3 h when the current density exceeded 0.10 mA cm -2 . The battery cycling performance also reduces fast as the current density gets higher. Especially for a current density of 0.50 mA cm -2 , less than 1 cycle (see Though the charge voltage reaches 5 V finally, the discharge voltage decreases apparently but still remains above 2.5 V in all the cases.

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Fig. 4(b) and Fig. S11(a, d-h) also show that the discharge voltage decreases as the current density elevates from 0.10 to 1.00 mA cm -2 , but still keeps as high as 2.65

V similarly in the first cycle. Particular for a low current density of 0.02 mA cm -2 , the battery obtains an ultra-high capacity of 14.8 mAh (see Fig. S13). At a current density of 0.10, 0.20, 0.30, 0.40 and 0.50 mA cm -2 , a superior cycling performance of 112, 94, 64, 56 and 33 cycles is delivered. When the current density increases to 1.00 mA cm -2 or above (see Fig. 4(b), Fig. S11 and Table S5), there's no more than 18 cycles received. As we know, higher current density demands faster mass transportation

(including e -, O 2 and Li + ) and electrochemical reaction in the batteries [24,25,59].

However, it takes times for electron, oxygen and Li + to diffuse to the tri-phase boundary reaction zone and react with each other to produce the lithium peroxide.

Such slow transportation and reaction kinetics undoubtedly initiate reaction thirsty, undermining the whole batteries performance in high discharge rates. and accumulates on the catalytic active sites of aerogel surface; 3). Li ions migrates to the tri-phase reaction zone; 4). Electron travels through lithium peroxide layers to decompose lithium peroxides.

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Nevertheless, it is not meant that the lower the better for the batteries when it comes to current density, which is deeply rooted in the basic results that the batteries exhibit better cycling performance (0.4 mAh) at a current density of 0.10 mA cm -2 , comparing to that of 0.05 mA cm -2 , in addition to a realistic requirement of high power density. We postulate that such a discharge rate promotion might be traceable to the unique carbon structure of dual pore which facilitates the mass transport, thus resulting in a sufficient oxygen and Li + transport even in a relatively higher discharge rate and charge rate. This is further verified by the cycling performance of single pore cathode CRF 1000-30 (Fig. 4(c)). At higher current densities of 0.3, 0.5 and 1.0 mA cm -2 , cathodes with dual pore structure show 64, 33 and 18 cycles comparing to 28, 20 and 8 cycles for that of single pore, highlighting the superiority for mass transfer. In addition, the discharge voltage is higher and the charge voltage is lower in the first cycle for the dual pore CRF 1500-40 cathode in all cases, which is apparently shown in the Fig. S1; Electrochemical performance data for CRF1000-40 are not shown here)? Thus, we postulate that the mass accessibility may be the key factor to determine the electrochemical performance of the lithium-air batteries. Besides, all these demonstrate that dual pore far exceeds single pore for mass transfer. Maybe, it's a favorable combination between the two types of effective meso-pores: 3~3.5 nm and 20~50 nm [19,22,[56][57][58]. However, among the meso-pores, it needs further study to investigate whether the smaller pore (3.85 nm) or larger pore (17.95 nm) contributes more to the capacity.

Here, we postulate that the essence lies in that the distinctive dual pore carbon aerogel can successfully create a tri-phase boundary reaction zone (Fig. 4 (d)). The large pore is apt to facilitate oxygen penetration, and simultaneously yield and store discharged products at its abundant catalytically active carbon surface; while the small pore with little or non-catalytic active centre will be filled with highly conductive Li + electrolyte due to the coalition of hydrophilic carbon surface and capillary pressure, conducive to Li + transfer. Charge is prone to travel through carbon aerogel's highly conductive interconnected 3D network, and accumulates at the high oxygen concentration sites. Li-oxide precipitations will be eventually filled in the pores where house a catalytic active junction (with an enormous amount of both O 2 and Li + , complementary subsystems). In this way, common two-phase boundary reaction zone

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can be avoided, resulting in a higher discharge rate and reduced pore blockage.

However, as is shown in Fig. 4(d), a long time discharge process or large capacity still initiates serious pore blockage or cathode passivation, thus the capacity should be limited to some degree. Maybe a fixed capacity of 1000 to 3000 mAh g -1 is reasonable and meaningful, which will be in the schedule of our future task.

At last, conclusion arrives that the favorable current density should be set ranging from 0.05 to 0.20 mA cm -2 when the cell works without a catalyst, which will contribute to excellent cycling performance. And the discharge depth should also be carefully controlled, maybe in a scope of 1000 to 3000 mAh g -1 . It is worthwhile to note that most of the discharge or charge curves appear two to three potential plateaus

Reversible electrochemical reaction and electrochemical impedance spectroscopy analysis of the lithium-air batteries

The CV test is utilized to explore the lithium-air batteries' internal reversible redox reactions. Fig. 5(a) indicates that in the first cycle, a conspicuous oxidation peak and reduction peak appear at the voltage of 4.05 V and 2.30 V, respectively.

Current peak as high as 0.75-1.50 mA evinces that the carbon aerogel deposited onto CP exhibits exceptionally excellent electrochemical performance towards both formation and decomposition of discharged product, which might be derived from the high specific surface area, dual pore structure, large total pore volume and hydrophobic surface and so on. The second and third cycle performs a lower weak oxidation peak at 3.75 V and 3.87 V, depicting an electro-activation process of the cell which makes the battery more stable [6,9]. For the subsequent two cycles, the oxidation and reduction peak maintain the same or analogous shape excluding a little potential migration to the high or low voltage correspondingly which demonstrates the relative stability of the cell reaction in spite of a little cathode polarization. It also signifies that sulfolane is a favorable electrolyte solvent for the lithium-air batteries. Table S3 and Table S4 show the detailed data for the equivalent circuit and the fitting impedance spectra. The EIS test illustrates that the impedance increases with the addition of carbon loading which lowers the porosity of the air cathode and extends the diffusion path of the oxygen and Li + . What's more, it can be seen that the cell resistance grows larger as the discharge reaction proceeds and reduces

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accompanied with the corresponding charging process in most cases from the impedance test (see Fig. S8, Fig. S9 and Fig. S10). Although some impedance data is not so correlated with its carbon loading, the tendency is undoubtedly conformed, that is: cycle by cycle of galvanostatic charge-discharge test, the impedance gets larger and larger (Fig. 5(c,d), Fig. S8, Fig. S9 and Fig. S10). That can be attributed to the low conductivity and uncompleted decomposition of the discharged products, which will be clearly perceived from the SEM and XRD test in the next part (see Fig. 6 and Fig. 7).

Further more, two similar semicircles are observed in the impedance spectra, with a smaller one at high frequency and another larger one at low frequency. The low frequency semicircle exhibits larger changes as compared to that of the high frequency and there is little change for the high frequency intercept, implying that the deposition of the discharge products on the surfaces of carbon not only reduce the kinetics of oxygen reduction, but also affect the diffusion of dissolved oxygen on the electrolyte-catalyst (carbon) interface. It's highlighted that charge transfer resistance (R ct ) occupies the main part of the three resistance distribution (see Fig. S9, Fig. S10, Table S3, Table S4) through various cycles which has direct connection with the accumulation of low electrical conductivity discharged products of Li 2 O 2 in the cathode. Thus, cycle by cycle, cathode passivates seriously (see Fig. The morphology of discharged/charged products is recorded in Fig. 6, from which we can observe that granular precipitations to bulk or sheet-shaped depositions form in discharge and disappear in the subsequent charging process after 10th cycle.

At low current density, granular precipitation inclines to be generated first for its easy nucleation and then gradually growing up to large separately flake-shaped species.

However, for the high current density, mass limitation (Li + and oxygen's sluggish transport to the active electrode surface) hinders the fast electrochemical reaction, yielding only considerable medium-sized granular to bulk depositions. In the conversed process, the formed products are vanished except some scattered depositions. With regard to deep discharge, the situation lies between slow discharge and fast discharge process, tending to form enormous granular to sheet-shaped aggregates which further deposit in the internal pore and drop the close contact between the electrodes. However, the large bulk-shaped depositions are not beneficial for superior cycling performance. 6(b, g) 0.10 mA cm -2 , 1000 mAh g -1 ; 6(c, h) 0.20 mA cm -2 , 1000 mAh g -1 ;

6(d, i) 0.10 mA cm -2 , 1500 mAh g -1 ; 6(e, j) 0.10 mA cm -2 , 2000 mAh g -1 . The rod-like materials comes from separator which adheres closely to the air cathode surface after electrochemical reaction.

The following XRD test (Fig. 7 and Fig. S14) confirms that the above-mentioned precipitations are dominantly Li 2 O 2 combined with a little LiOH H 2 O (LiOH) and Li 2 CO 3 , most of which are proved to be crystalline products in the all involved cases, regardless of the difference in the morphology of the discharge products. Especially, the discharged products show a fine crystalline at a low current density of 0.02 mA cm -2 , indicating that low discharge rate favors to release lithium peroxide in crystal form (see Fig. S14), which is also in accordance with the large sheet-shaped deposits observed in Fig. 6. The formation of these byproducts may be related with: 1) the

Lithium-air batteries in air atmosphere

Finally, air's impact is also investigated. As is shown in Fig. 8, Fig. S16 and Table S6, when working in air atmosphere, the cycling number of the batteries decreases as the current density amplifies to 0.20 mA cm -2 , though with a little promotion at 0.10 mA cm -2 which also fits most of the other cases (40, 48, 8 cycles correspond to current density of 0.05 mA cm -2 , 0.10 mA cm -2 and 0.20 mA cm -2 ). We postulate that such an improvement can be further ascribed to the peculiar cathode structure, which strengthens fast mass transport. What's more, in the ambient environment, the cycling performance of the lithium-air batteries never surpasses that of the previous oxygen circumstances (see Fig.

Conclusions

In summary, a well-modified air cathode with carbon aerogel deposited onto carbon paper was synthesized to obtain an excellent cycling performance (525 to 18 cycles at a current density ranging from 1.00 to 0.05 mA cm -2 ). The dual pore carbon structure and large pore volume, combined with superior conductivity are all favorable for the fast mass transport and superior discharged products accommodation and decomposition. We postulate that such a novel nano-porous cathode structure inventively create a suitable tri-phase boundary reaction zone, facilitating oxygen and Li + diffusion in two independant pore channels, thus realizing a relatively higher discharge rate cycling performance and lower pore blockage and cathode passivation.

Experimental methods

Synthesis and characterization of carbon aerogel

Synthesis of carbon aerogel

Carbon aerogel has been traditionally synthesized through various processes including sol-gel reaction, acetic acid treatment, solvent exchange, supercritical drying and carbonization under inert atmosphere. In this article, resorcinol (R) and formaldehyde (F) were chosen as reaction precursor.

Sodium carbonate (C) and deionized water were used as catalyst and solvent, respectively. The molar ratio of R/C and R/F were controlled to 1000:1-2000:1, 1:2 respectively. The reactant mass fraction was 40% (or 30%). The mixture was first stirred vigorously into a homogeneous solution, and then cured at 353K for three days in a sealed container, followed by one day trifluoroacetic acid treatment to further strengthen the gel configuration net. After that, fresh acetone was utilized to immerse the

Materials characterization

The morphology and particle size of the RF aerogels and carbon aerogels were characterized by FE-SEM using the Hitachi SU-70 Thermal Field Emission Scanning Electron Microscope by operating at 2.0 KV and TEM using the PHILIPS TECNAI-10 with an accelerating voltage ranging of 40-120

KV. The TG-DTA test was employed to determine the carbonization temperature and following degas temperature using a STA409PC Sync thermal analyzer (Germany Netzsch Instruments Manufacturing Co., Ltd.), which was performed using 10 mg RF aerogel and a heating rate of 10 K min -1 , from ambient temperature to 1673 K under N2 atmosphere. The BET surface area, porosity and pore size distribution of the sample were inspected by the analysis of static low temperature nitrogen adsorptiondesorption measurement with a NOVA1200e adsorption analyzer (Micromeritics) at 77 K. The carbon aerogel powder was degassed at 573 K for 4 h prior to analysis.

Preparation and characterization of carbon cathode

Preparation of carbon cathode

A mixture of carbon aerogel and pVdF powder (with a weigh ratio of 9:1) was grinded for half an hour using mortar/pestle and was gathered in a weighing bottle. Several drops of NMP solution were added into the above mixture, stirring for 6h more in order to get appropriate viscosity slurry. The asprepared suspension was used to coat carbon fiber paper substrate (HCP120 fuel cell carbon fiber paper; size: 100 mm×70 mm×0.21 mm; purchased from Shanghai hesen Electric Co., Ltd.)with the coating thickness of 100 μm. After drying in vacuum at 80 °C for another 6h, the above coated substrate was punched into round shape pellet (size: Φ15 mm×(0.21±0.15 mm)). At last, various carbon cathode electrodes, with carbon loading varying from 0.1 mg to 0.6 mg unequally, were

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obtained. The aerogel loading also can be shown in the unit of mg cm -2 , that is in the range of 0.057 mg cm -2 -0.340 mg cm -2 .

Cathode characterization

The electrical conductivity of the air electrode and carbon aerogel was measured using the standard electrical four-probe method by a RTS-9 twin four-probe electrical measuring instrument (purchased from Guangzhou four probe tech. Co., Ltd.).

Cell assembling and testing

Assembling the cell

In this article, the coin cell (CR2032; all the related kits and assembling tools were purchased from MTI Shenzhen Corp.) was assembled in an argon-filled glove box (MBraun) with moisture and oxygen concentration lower than 0.5 ppm. The lithium air battery, in fact a laminated configuration like a sandwich (Li anode/separator/cathode, see Fig. S1.), was fabricated as below. The stainless steel spring was put at the bottom of the cell (Anode cap) first, on top of which was the gasket (stainless steel spacer). Then a lithium disk(Size: Φ 15 mm×0.5 mm;Bought from China Energy Lithium Co., Ltd.), the separator (whatman GF/D glass microfiber filter paper; Size: Φ17 mm×1 mm; Bought from Beijing Jingkehongda Biotechnology Co., Ltd) dipped in the 1 M LiTFSI/sulfolane electrolyte (Battery grade; purchased from Shenzhen Capchem technology Co., Ltd.)and the carbon electrode fabricated previously (with the coating side facing to the separator) were placed successively on the gasket. Since the carbon fiber paper had high conductivity of 150 S/cm and perfect hydrophobility on the opposite side of the coating, the collector and hydrophobic gas permeable membrane were saved.

Finally, the positive electrode cap, containing a number of holes (Φ=1 mm) which were convenient for oxygen traveling, was covered the cell bottom and sealed under 40 MPa. Then, it aged for more than exhibit the best cycling performance. When the exposure time is more than 1 h, no significant improvement of cycling performance has been achieved. Therefore, we choose an exposing time of more than 30 min as our experimental boundaries as well as for a short testing period. Galvanostatic charge-discharge test was performed in an oxygen-filled dry container (1 atm) by adopting a CT2001A battery tester (Wuhan blue Power Electronics Co., Ltd.) at room temperature (26 °C). Herein, we mainly focused on exploring carbon loading, discharge rates, discharge depth and air's effect on the

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cycling performance. Lastly, the morphology and composition changes of the discharged or charged products were studied. After disassembling the cell and scraping the cathode surface to remove residual separator glassy fiber, the DME soaked discharged or charged products were vacuum dried for 12 h and quickly transferred to a gas-tight device to be analyzed by the FE-SEM and XRD measurement (D8 X-ray diffraction Analyzer). A.C. impedance and CV curves were carried out by a CHI660D electrochemical workstation (Shanghai Chenhua Device Company) simultaneously. Carbon aerogels (see Fig. S2 ,Fig. S3) with different pore structure were synthesized by using the sol-gel method. As is shown in the Fig. S2 and Fig. S3, all the synthesized aerogels exhibit type-IV isotherms following the IUPAC classification. Especially, there exist hysteresis loops at the high relative pressure above 0.8, indicating the presence of a great number of meso-pores. All the above six aerogels coating cathodes were used to construct lithium-air batteries, generating several hundred stable cycles (highest for CRF1500-40, 525 cycles; others, no more than 375 cycles) in the galvanostatic charge/discharge cycling. This difference can be attributed to their special nano-porous structure and electrical conductivity.

Results and discussion

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 6
Here, two kinds of carbon aerogels, CRF1000-30 and CRF1500-40, with characteristic single and dual pore size distribution, especially possessing extremely similar average mesopore diameter around 8.0 nm, were investigated in detail. At a current density of 0.05 mA cm -2 and a fixed capacity of 0.1 mAh, the battery with dual pore and single pore aerogel coating obtains 525 and 332 cycles, respectively. At higher current densities of 0.3, 0.5 and 1.0 mA cm -2 , cathodes with dual pore structure show 64, 33 and 18 cycles comparing to 28, 20 and 8 cycles for that of single pore, highlighting the superiority for mass transfer. In addition, the discharge voltage is higher and the charge voltage is lower in the first cycle for the dual pore CRF 1500-40 cathode in all cases, which is apparently shown in the Fig. 4, Fig. S11and Fig. S15. Such sufficient preponderance has a direct relation with CRF1500-40's higher electrical conductivity and better mass accessibility (dual pore).

And we postulate that the mass accessibility may be the key factor to determine the electrochemical performance of the lithium-air batteries, originating from the basic results that aerogel CRF 1000-40 owns very similar (even superior) pore parameter and electrical conductivity with (to) CRF 1500-40 but exhibits poor cycling capability (Electrochemical performance data for CRF1000-40

are not shown here.). It is worthwhile to note that the electrical conductivity of the six aerogel cathodes is very close and almost in the same level (1 S/cm), which can meet the basic use of the active coating materials. Thus, all these demonstrate that dual pore far exceeds single pore for mass transfer.

However, among the meso-pores, it needs further study to investigate whether the smaller pore (3.85 nm) or larger pore (17.95 nm) contributes more to the capacity. It is exactly right that these smaller pores may allow better conversion of discharge products during charge, however it initiates pore blockage and inefficient utilization of pore volume (see Fig. 4(d)). The smaller pores are apt to absorb and be filled with electrolyte due to capillarity, thus causing mass migrating obstacles. Conversely, the larger meso-pores would be feasible for oxygen and lithium ions transfer. As for the problem of thick layer discharge products (cathode passiviation), one can adjust it by limiting the discharge capacity. At As is shown in Fig. S6, for most cases, the discharge specific energy was above 2750 Wh Kg -1 while the charge specific energy was in the range of 3620 Wh Kg -1 to 4792 Wh Kg -1 , resulting in a relative high energy efficiency varying from 57.38% to 78.32 % even though on the condition of encountering

unstable disturbance (open the dry testing container to take out batteries for discharged or charged products analysis) or long term test above 400 cycles. Such high discharge specific energy and energy efficiency compared well with that (65 % or 62.5%) previous reported. [9,12] The final and median voltages in the discharge and charge process at various cycles were S3. Dependence of equivalent-circuit parameters upon the porosity of CRF1500-40 cathode with a carbon loading ranging from 0.1 mg to 0.6 mg after 10th discharg/charge cycles. All the meanings of the subscripts for the resistance can be seen the Table S4 below.

Carbon Loading (mg) Table S4. Dependence of equivalent-circuit parameters upon the porosity of CRF1500-40 cathode with a carbon loading of 0.4 mg at the initial state and discharged state for various cycles. Carbon (0.4 mg) Note: Re, Rint, Rct are electrolyte ohmic resistance, interfacial resistance, charge transfer resistance, respectively. CPE is constant phase element, and Cint is typical of surface films formed on the electrode.

Re(Ω cm -2 ) C int (F cm -2 ) R int (Ω cm -2 ) Cm (F cm -2 ) R ct (Ω cm -2 ) WO (Ω cm -2 ) C int-T C int-P C m-T C
Re(Ω cm -2 ) C int (F cm -2 ) R int (Ω cm -2 ) Cm (F cm -2 ) R ct (Ω cm -2 ) WO (Ω cm -2 ) C int-T C int-P C m-T C m-P WO- R WO- T WO- P CRF150 0
Cm is double-layer capacitance distributed between the ohmic and faradaic processes. Wo is finite length Warburg resistance (open-circuit terminus), on behalf of kinetic resistance for oxygen and Li + .

The subscripts of R, P, and T stand for the corresponding state and phase parameters. (d) 0.20 mA cm -2 , 1000 mAh g -1 ; (e) 0.30 mA cm -2 , 1000 mAh g -1 ; (f) 0.40 mA cm -2 , 1000 mAh g -1 ;

(g) 0.50 mA cm -2 , 1000 mAh g -1 ; (h) 1.00 mA cm -2 , 1000 mAh g -1 .

It is worth mentioning that each carbon paper cathode had a mass of 26.4±2 mg, which was about 250 times higher than 0.1 mg of the carbon loading described below. Therefore, it actually contributed less than 4 mAh g -1 specific capacity to the whole lithium air battery. In addition, more coating would lead to cathode pore blockage and deterioration of cycling performance, which was inconsistent with the experimental results. This strongly suggested that carbon aerogel was an effective electro-active material.

To construct a practical applied lithium-air battery, all the galvanostatic charge-discharge tests were simultaneously conducted in a long period of time ( ≧ 2.3 h). Particularly, for a perspicuous comparison, the testing time was fixed to 2.3 hours when the current density increased to 0.20 mA cm -2 or above. This discharge time comes from the case of a current density of 0.1 mA cm -2 and specific capacity of 1000 mAh g -1 (see Ⅰ in the below ) . Therefore, the discharge specific capacity differed from each other, ranging from 1000 to 5000 mAh g -1 unequally (see Table S5, Fig. 4(b), Fig. S11 and Fig. S12). All the cycling number was recorded under the cut off voltage of 5.0 V (for charge) or 2.0 V (for discharge). As is shown in Fig. S12, it revealed fast cycling performance deterioration at the same current density compared to that of a relative low specific capacity of 1000 mAh g -1 . This further demonstrated deep discharge was detrimental for the realization of highly rechargeable lithiumair batteries, consistent with the lithium-ion batteries. Meanwhile, high discharge rates also cause sharp recession of cycling performance.

Discharge time =

Note: Aerogel loading: 0.4 mg; Current density: 0.10 mA cm - Table S6. The relationship between cycling performance and current density with a CRF1500-40 cathode in the air environment.

Cathode

Fig. 1 .

 1 Fig. 1. Morphology of the RF aerogel (a, c) and carbon aerogel (b, d) and RF

3. 1 . 3 .

 13 BET surface area and BJH pore size distribution of carbon aerogel Carbon aerogels (see Fig. S2 Fig. S3) with different pore structure were M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 10 synthesized by using the sol-gel method. As is shown in Fig. 1(f), two kinds of carbon aerogels, CRF1000-30 and CRF1500-40, with characteristic single and dual pore size distribution, especially possessing extremely similar average mesopore diameter around 8.0 nm, exhibit type-IV isotherms following the IUPAC classification.

Fig. 2 .

 2 Fig. 2. Discharge-charge curves of the lithium-air batteries with (a) CRF1500-40 and

Fig. 3 .

 3 Fig. 3. Cycling life (a) and discharge-charge curves (b: 0.4 mg) of the lithium-air

5 ,Fig. 6 ,

 56 Fig.7, Fig.S8, Fig.S9and Fig.S10). To remove the deposited products, higher charging voltage is needed, causing the voltage gap to become larger (1.42 V in the Fig.3compares to 0.89 V in the Fig.2) and electrolyte to decompose.After long time's accumulation, the lithium peroxide fills up the whole pore system to separate oxygen diffusion and deform the cathode (Large amount of precipitations will cause cathode expansion which leads to a non-intimate contact between carbon particles with the CP current collector; see Fig.4(d)), and the electrolyte depletes, resulting in decreasing the discharge voltage to below 2.5 V andM A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPTgiving rise to the failure of the cell. To be highlighted, there are two to three voltage plateaus in the prolonged discharge or charge loading curves which are in directive of the probable side reactions. These parasitic reactions might involve the formation of LiOH H 2 O (or LiOH) and Li 2 CO 3 besides Li 2 O 2 , which corresponds to the voltage of 4.3 V, 4.75 V and 3.75 V, respectively, in the charging process and 4.2 V, 4.7 V and 2.5-2.75 V for that of the discharging state[5,6,[59][60][61].

Fig. 4 (

 4 Fig. 4(b), Fig. S11, Fig. S12) is obtained comparing to 33 cycles for the case of a

Fig. 4 .

 4 Fig. 4. Cycling life and its typical inset discharge-charge curves of the lithium-air

  4(b), Fig. 4(c), Fig. S11 and Fig. S15. The general voltage gap of CRF 1500-40 is 1.87 V, lower than 1.94 V for that of CRF 1000-30. Such sufficient preponderance has a direct relation with CRF1500-40's higher electrical conductivity and better mass accessibility (dual pore). Considering that the electrical conductivity of the as-prepared six aerogel cathodes is very close and almost in the same level (1 S/cm), M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 19 if the electrical conductivity plays a major role, then why the aerogel CRF 1000-40 owns even superior pore parameter and higher electrical conductivity to CRF 1500-40 but exhibits poor cycling capability (see Fig. S1, Fig. S2, Table

  which might be ascribed to the parasitic reaction of electrolyte or binder degradation, trace of water and carbon dioxide et al.. This hypothesis can be verified by the following XRD test (see Fig. 7, Fig. S4, Fig. S14), which detects the existence of LiOH H 2 O (or LiOH), Li 2 CO 3 and other carbonate-based by-products besides Li 2 O 2 [5,6,59-61].

Fig. 5 .

 5 Fig. 5. CV curves and electrochemical impedance of lithium-air batteries at different

  4(d)) which leads the cell failure. 3.3. Electrochemical reaction process analysis and air's affect 3.3.1. Morphology and composition analysis of discharged/ charged products M A N U S C R I P T A C C E P T E D

Fig. 6 .

 6 Fig. 6. Field emission scanning electron microscopic (FE-SEM) images of the

  E P T E Dcharged products (see Fig.6). Based on this phenomenon, it is assumed that the high overcharge voltage is associated with the difficulty in decomposing the discharged products, especially for the complete removal of lithium carbonates under a high potential above 4.7 V. Such high charge potential will inevitably initiate electrolyte decomposition, further accelerating the parasitic reactions which can be also discovered in the cycling performance test where several potential plateaus occur in the discharge or charge loading curves. With regard to qualitative or quantitative identification of all byproducts (particularly amorphous or low amount deposits), more advanced analysis techniques or equipment are needed, such as XPS, DEMS, FTIR, NMR, SERS et al., all of which are the task of our future research.

Fig. 7 .

 7 Fig. 7. Galvanostatic charge/discharge loading curve (a) and corresponding pristine,

  S11, Fig.S12and Fig.S16 (b, d)), for which trace of moisture and carbon dioxide, combined with the lower oxygen pressure might be blamed[23,60,63]. The cell voltage gap in air also exceeds that of the oxygen atmosphere, exhibiting of 2.16V and 1.62V respectively during the first cycle. The huge voltage gap might involve with some parasitic reactions.

Fig. 8 . 29 Fig. 9 .

 8299 Fig. 8. Discharge-charge curves of the lithium-air batteries with CRF1500-40 cathode

Hence, it is

  expected to promote the practical applications of lithium-air batteries by improving their recycling possibility with the help of relatively stable electrolyte, dry oxygen environment, feasible current density and discharge depth. Sulfolane is confirmed to be relatively stable according to our work (demonstrated by CV test and high cycle life), but its detailed changes particularly under high charge voltage still study by the DEMS, FTIR and XPS analysis (to identify the reaction deposits of amorphous state or low amount). Further more, after carefully controlling the current density and depth of discharge (DOD), and in an oxygen-filled environment which has high oxygen partial pressure and low amount of water and carbon dioxide, the lithium-air batteries can successfully run 112 and 94 cycles at a specific capacity of 1000 mAh g -1 and current density of 0.10 and 0.20 mA cm -2 , respectively.However, the cycling number reduces fast as the current density increases to 1.00 mA cm -2 or above, which may be related with the slow transport of e -, Li + and oxygen, causing limited reaction kinetics. Therefore, much work is still needed to resolve these problems. Herein, we propose a cation modified air electrode by adding the carbon with the Li + or other complexing-cations containing electrolyte in the process of fabricating nano-porous air cathode. In this way, Li + exists in the air cathode which greatly shortens the transportation path, favoring the high current density discharge/charge reaction. The complexing-cations improve the solubility of alkaline peroxides[26,65]. Meanwhile, little or no electrolyte would be added to the side of separator which faces to the cathode, minimizing blockage by electrolyte which traditionally fills in the meso-pores of the cathode.Lu, and College of Material Science and Engineering (SZU) for helpful chemical analysis and instruments support, respectively. carbon aerogel based air cathode for a highly rechargeable lithium-air battery Fang Wang a, * , Yang-Hai Xu a , Zhong-Kuan Luo

  times (one time a day) to replace water in the holes which was helpful to avoid net structure collapse. By this step, RF aerogel (ie, RF1500-40) was obtained. Finally, the wet gels were pyrolyzed in a tubular furnace under Ar atmosphere at 1323 K for 4 hours, yielding carbon aerogel (CRF1500-40). Other carbon aerogels, such as CRF1000-30 et al., were obtained simultaneously.

30 Fig. S2 .

 30S2 Fig. S2. N2 adsorption-desorption isotherms of carbon aerogel with different reactant/catalyst molar

Fig. S3 .

 S3 Fig. S3. BJH pore size distribution graph of carbon aerogels with different reactant/catalyst molar ratio

Fig. S4 .

 S4 Fig. S4. XRD patterns and SEM test of the pristine and discharged/charged cathode (CRF1000-30)

9 Fig. S5 .

 9S5 Fig. S5. Discharge-charge curves of the lithium-air batteries with a CRF1500-40 carbon cathode of

11 Fig. S6 .Fig. S7 . 13 Fig. S8 .

 11S6S713S8 Fig. S6. Specific energy and energy efficiency of lithium-air batteries that contain 1 M

15 Fig. S9 .

 15S9 Fig. S9. The variation of the resistance distribution with the increase of cathode carbon loading in the

Fig. S10 .

 S10 Fig. S10. Impedance spectra of lithium-air batteries at various cyclings after discharge/charge. All the

Fig. S11 .

 S11 Fig. S11. Discharge-charge curves of the lithium-air batteries with a CRF1500-40 carbon cathode at

2 ;S=1/4 πd 2 0 19 Fig. S12 .

 2219S12 Fig. S12. Discharge-charge curves of the lithium-air batteries with CRF1500-40 carbon cathode at

Fig. S13 . 20 Fig. S14 . 21 Fig. S15 .

 S1320S1421S15 Fig. S13. Discharge curves of the lithium-air batteries with CRF1500-40 carbon cathode at various

22 Fig. S16 .

 22S16 Fig. S16. Discharge-charge curves of the lithium-air batteries with CRF1500-40 air cathode at various

Table 1 .

 1 BET surface area, pore volume, pore diameter and conductivity data for

Information contains Experimental methods, results and discussion of 16 supplementary figures comprising 57 panels in total with brief legends and 6 supplementary tables.

  a, b,* , Yan Pang a , Chun-Sheng Liang a , Jing Chen a and Dong Liu a , Xiang-hua Zhang a,c

	a College of Chemistry and Chemical Engineering, Shenzhen University, 3688 Nanhai Avenue,
	Nanshan District, Shenzhen 518060, China. Fax: +86 755 26536141; Tel: +86 755 26558125; E-mail:
	wangfsz@szu.edu.cn
	b Zhejiang California International NanoSystems Institute & Department of Materials Science and
	Engineering, Zhejiang University, 38# Zhe Da Road, Hangzhou 310000, China. Tel: +86 755
	26557249; Fax: +86 755 26536141; E-mail: lzk@szu.edu.cn

c Laboratory of glasses and Ceramics,UMR CNRS 6226 "Institut des Sciences Chimiques de Rennes",Université de Rennes I, 35042 Rennes, France. Fax: +33 2 23235611; Tel: +33 2 23236937;E-mail: xzhang@univ-rennes1.fr This Supporting

Table S1 .

 S1 Pore size, pore volume, specific capacity and conductivity data for various carbon aerogels.

	Ca	Carbon	Resorcin	Reacti	SBET	Pore	Pore	Avera	Electri
	se	aerogel	ol/	on	[m 2 /g]	diamet	volum	ge	cal
			Sodium	mass		er[nm]	e[cc/g	pore	conduc
			carbonat	ratio		BJH	]	diamet	tivity[
			e [n/n]	[w%]		method		er[nm]	S/cm]
	1	CRF1000-30 1000		680.22 16.00	1.06	8.46	0.502
	2	CRF1500-30 1500	30	786.87 3.15	0.68	4.87	0.323
	3	CRF2000-30 2000		644.47 3.13	0.37	3.29	0.336
	4	CRF1000-40 1000		769.54 18.57	1.22	8.66	1.340
	5	CRF1500-40 1500	40	669.17 17.95	1.02	8.28	1.218
	6	CRF2000-40 2000		646.55 13.39	0.73	5.78	1.253

Table S2 .

 S2 The variation of cycling performance with different carbon loadings when CRF1500-40 is used as cathode active material in the oxygen environment.

	Cathode	Electrolyt	Carbon	Current	Specific	Capacity/	Cycling
		e	loading/m	density/m	capacity	mAh	number
			g	A cm -2	/mAh
					g -1
	Carbon		0.1			0.1	363
	paper
		1 M	0.1			0.1	525
		LiTFSI/	0.2	0.05	1000	0.2	230
	CRF1500-40+pVdF= 9:1	sulfolane	0.3 0.4 0.5			0.3 0.4 0.5	150 100 90
			0.6			0.6	78

Table S5 .

 S5 The relationship between cycling performance and current density as well as discharge depth with a CRF1500-40 cathode in the oxygen environment.

	Cathode	Electrolyt	Carbo	Current	Specific	Capacity	Cycling
		e	n	density/m	capacity/	/ mAh	number
			loadin/	A cm 2	mAh g -1
			mg
				0.05	1000	0.4	100
				0.1	1000	0.4	112
				0.1	1500	0.6	44
	CRF1500-40+pVdF= 9:1	1 M LiTFSI/ sulfolane	0.4	0.1 0.2 0.3 0.4	2000 2000/1000 3000/1000 4000/1000	0.8 0.8/0.4 1.2/0.4 1.6/0.4	34 24/94 10/64 3/56
				0.5	5000/1000	2.0/0.4	1/33
				1.0	10000/1000 4.0/0.4	1/18

min, 15 min, 30 min, 45 min,

min, 2 h, 3 h and 4 h, cells exposed to oxygen with 30 min to 1 h

Acknowledgements

This work was supported by Shenzhen Key Laboratory of New Lithium-Ion Battery and Mesoporous Materials, and Shenzhen Science and Technology Fund (JCYJ20130329102936684 and CXZZ20120615155541404). We thank Zheng-Wang

Cell testing and discharge/charge product characterization

The cell was then exposed to O2 atmosphere (1 atm) for a minimum of 30 min prior to the electrochemical tests. Here, a minimum of 30 minutes exposing to the oxygen had been verified best by our previous experimental which contained series of exposure time. Among all the exposing time of


Jeremy Tormos

P Piffady

Y Landry

Souchon

Kris Van Looy

Cyril Cavillon
email: cyril.cavillon@irstea.fr

Thierry Tormos
email: thierry.tormos@onema.fr

Jérémy Piffady
email: jeremy.piffady@irstea.fr

Philippe Landry
email: philippe.landry@oncfs.gouv.fr

Yves Souchon
email: yves.souchon@irstea.fr

Kris Van-

A scale-sensitive connectivity analysis to identify ecological networks and conservation value in river networks. Connectivity analysis in river networks

Keywords: Ecological Niche Factor Analysis, Integral Index Connectivity, network analysis, hydro-morphological quality, otter

A scale-sensitive connectivity analysis to identify ecological networks and conservation value in river networks

Introduction

Recent theoretical and experimental studies revealed how limits in network connectivity represent key constraints for species persistence in dendritic networks [START_REF] Fagan | Connectivity, fragmentation, and extinction risk in dendritic metapopulations[END_REF][START_REF] Carrara | Dendritic connectivity controls biodiversity patterns in experimental metacommunities[END_REF]). In the real world this essential role of connectivity still needs to be evidenced, which is hampered by the large variation in temporal and spatial scales of analysis [START_REF] Hannah | Climate Change, Connectivity, and Conservation Success[END_REF], and by the lack of knowledge on relationships between network structure and ecological patterns in dendritic networks [START_REF] Ganio | A geostatistical approach for describing spatial pattern in stream networks[END_REF]. The association of habitat quality and network structure with colonization processes in different settings and species is still poorly understood, but essential to prioritize conservation and restoration [START_REF] Robles | Influence of Habitat Quality, Population Size, Patch Size, and Connectivity on Patch-Occupancy Dynamics of the Middle Spotted Woodpecker[END_REF][START_REF] Mccarthy | Predicting Species Distributions from Samples Collected along Roadsides[END_REF][START_REF] Carranza | Connectivity providers for semi-aquatic vertebrates: the case of the endangered otter in Italy[END_REF]. For this challenge, ecological network analysis shows a remarkable expansion of methods in modelling techniques [START_REF] Thuiller | BIOMOD -a platform for ensemble forecasting of species distributions[END_REF]) and concepts like the integration of graph methods [START_REF] Minor | A Graph-Theory Framework for Evaluating Landscape Connectivity and Conservation Planning[END_REF], but little progress is made in bridging the gap between these methods and their applicability for the identification of conservation networks at large scales [START_REF] Eds | Corridors for Conservation: Integrating Pattern and Process[END_REF][START_REF] Laita | Graph-theoretic connectivity measures: what do they tell us about connectivity?[END_REF][START_REF] Carroll | Use of Linkage Mapping and Centrality Analysis Across Habitat Gradients to Conserve Connectivity of Gray Wolf Populations in Western North America[END_REF].

Large-scale analysis for connectivity often still lacks the accuracy to integrate the relevant factors in the coarse-grained maps, and the methods covering small areas cannot be applied over larger territories [START_REF] Beier | Toward Best Practices for Developing Regional Connectivity Maps[END_REF]).

To tackle this problem of scale and resolution in the design of conservation networks, we propose a scale-sensitive approach based on a connection-constrained analysis of network connectivity. We illustrate the developed approach with the distribution data of the European otter in the Loire River basin. In the last 2 decades a recovery of otter is observed for most of its West-European distribution. This recovery is recorded for Spain [START_REF] Cortés | Changes in otter Lutra lutra distribution in Central Spain in the 1964-1995 period[END_REF], Italy [START_REF] Loy | Otter Lutra lutra population expansion: assessing habitat suitability and connectivity in southern Italy[END_REF]) and France [START_REF] Janssens | Genetic pattern of the recent recovery of European otters in southern France[END_REF]) after many decades of decline [START_REF] Lodé | The decline of otter Lutra lutra populations in the region of the pays de loire, Western France[END_REF]). In the Loire river basin a core of historical population persisted and a strong re-colonization is observed [START_REF] Janssens | Genetic pattern of the recent recovery of European otters in southern France[END_REF]. We cross-examine the species' distribution with land cover and environmental variables both at the river segment and the sub-catchment scale, to identify available habitat with a graph-based method for connectivity analysis [START_REF] Saura | A common currency for the different ways in which patches and links can contribute to habitat availability and connectivity in the landscape[END_REF][START_REF] Eros | Network thinking in riverscape conservation -A graph-based approach[END_REF][START_REF] Minor | A Graph-Theory Framework for Evaluating Landscape Connectivity and Conservation Planning[END_REF]. Quantifying available habitat and connections in networks over larger scales necessitates the integration of environmental variables at

Author-produced version of the article published in Landscape Ecology (2013), vol. 28, n° 7 The original publication is available at http://link.springer.com/ doi : 10.1007/s10980-013-9869-x different scales [START_REF] Rouget | Designing Large-Scale Conservation Corridors for Pattern and Process[END_REF]. The choice of target species for prioritizing conservation determines the resolution of environmental data needed and the connectivity required [START_REF] Arponen | Effects of Connectivity and Spatial Resolution of Analyses on Conservation Prioritization across Large Extents[END_REF], both with regard to resource selection and mobility [START_REF] Moilanen | On the use of connectivity measures in spatial ecology[END_REF], as to potential biotic interactions [START_REF] Godsoe | How do species interactions affect species distribution models?[END_REF]). The otter is often presented as target species and indicator for the connectivity and quality of the river ecosystem [START_REF] Robitaille | Otter, Lutra lutra, occurence in Europe and in France in relation to landscape characteristics[END_REF]. Population dynamics of the otter plays at the river basin scale, while individuals experience the local habitat and its connectivity at the river segment scale. These two scale levels are the relevant scales to gather data for the assessment of river corridor quality and functions [START_REF] Allan | Landscapes and Riverscapes: The Influence of Land Use on Stream Ecosystems[END_REF][START_REF] Wiens | Riverine landscapes: taking landscape ecology into the water[END_REF].

In this contribution we describe the proposed method of integrated network connectivity and habitat availability assessment and compare it to more commonly used distribution model approaches both in its power to predict species occurrence in the river network, and to its ability to highlight the contribution of connectivity in relationship to the observed ecological pattern of the otter recolonization over the river network. For this validation the importance of connectivity for otter was tested in a part of the river basin where the re-colonization was fully accomplished.

Author-produced version of the article published in Landscape Ecology (2013), vol. 28, n° 7 The original publication is available at http://link.springer.com/ doi : 10.1007/s10980-013-9869-x

Methods

Observational data for otter

Data of otter presence was gathered in the Loire Basin by a network of associations' volunteers gathered under the Loire Basin Mammal Network, and assembled by the services of the National Wildlife Office (ONCFS). Harmonised protocols in data collection were adopted following the internationally agreed otter census protocol [START_REF] Lodé | The decline of otter Lutra lutra populations in the region of the pays de loire, Western France[END_REF]. For the otter, presence was confirmed from the observation of spraints (otter's droppings). The otter is a highly mobile animal with home ranges of 2 -100km [START_REF] Ruiz-Olmo | Testing the surveys and visual and track censuses of Eurasian otters (Lutra lutra)[END_REF]. In accordance to guidelines for the confirmation of otter presence and to avoid overestimating accidental visits of individuals to small water courses [START_REF] Kruuk | Otters, Ecology, Behaviour and Conservation[END_REF], only main water courses in fluvial systems and rivers starting from a minimum catchment of >10km² were retained for the analysis. As otter spraints cannot provide information on otter abundance, only about presence [START_REF] Mason | Otters: ecology and conservation[END_REF][START_REF] Sulkava | Source and sink dynamics of density-dependent otter (Lutra lutra) populations in rivers of central Finland[END_REF], and furthermore the frequency of spraints may be very low when otters are at low densities [START_REF] Macdonald | The otter Lutra lutra in Southern Italy[END_REF], the species distribution analysis was limited to a presence only approach.

The consistent observation effort over the last 25 years allowed for a reconstruction in well-defined time-steps (5 survey periods) of otter presence and the re-colonization of the river basin. Although the re-colonization is surely not fully accomplished for the Loire basin -only 37% of the surveyed 17 000km are occupied -the outer bounds are all reached by now (Fig. 1). And as dispersing individuals will preferably chose the best available sites as a residence, this analysis of non-fully achieved recolonization of the river network offered good perspectives to discriminate the essential factors in dispersal and habitat selection [START_REF] Clavero | Natural, human and spatial constraints to expanding populations of otters in the Iberian Peninsula[END_REF][START_REF] Ruiz-Olmo | Testing the surveys and visual and track censuses of Eurasian otters (Lutra lutra)[END_REF]. For the analysis of the habitat selection, the presence in the most recent survey was retained (Fig. 1).

Environmental and anthropogenic stressor data of the riparian corridor

A systematic sectioning into river segments and assembling of hydromorphological and land cover data for the riparian corridor in different buffer sizes (valley floor, floodplain, 100m, 30m, 10m) was realised for the entire river network [START_REF] Chandesris | SYRAH-CE: SYstème Relationnel d'Audit de l'Hydromorphologie des Cours d'Eau A relational, multi-scale system for auditing the hydro-morphology of running waters: diagnostic tool to help the WFD Author-produced version of the article[END_REF]. Rivers were subdivided into hydromorphological units based on a semi-automatic sectioning that distinguished changes in geological

Author-produced version of the article published in Landscape Ecology (2013), vol. 28, n° 7 The original publication is available at http://link.springer.com/ doi : 10.1007/s10980-013-9869-x entities, channel form, sinuosity and valley floor width. Resulting river segments range from 1km on average for small streams and up to 20km on average for large rivers. For each of these river segments, information was collected from two spatial scales: the catchment's land cover information was gathered for regional sub-catchments (i.e. hydrological units delimited by water divides and river confluences) and locally information on land cover and hydromorphology was extracted for the individual river segment in different dimensions for the riparian corridor (Table 1).

Ecological niche factor network analysis

The restriction to presence only data was the reason to apply the ecological niche factor analysis (ENFA) in the definition of ecological networks for the species [START_REF] Hirzel | Ecological-niche factor analysis: how to compute habitat-suitability maps without absence data?[END_REF]. This method also allowed inferring key habitat factors from incomplete distribution data as was obviously the case for this colonizing species' distribution [START_REF] Clavero | Natural, human and spatial constraints to expanding populations of otters in the Iberian Peninsula[END_REF]. ENFA allowed integrating the inferred habitat factors in a network analysis to mark the potential network occupied by the species [START_REF] Basille | What shapes Eurasian lynx distribution in human dominated landscapes: selecting prey or avoiding people?[END_REF]). The marginality and specialisation of the species' presences was determined for the environmental variables over the river segments to describe the ecological niche of the species. The 95% confidence intervals of the variables with significant marginality and specialisation for occupied segments were selected as boundary values to consider segments as favourable. An extrapolation of this ecological niche over the entire river basin was carried out. Distance criteria were applied to join favourable segments based on the minimum home range values of 2km for otter, and this resulted in networks of non-fragmented habitat patches over the river basin.

Ecological network design based on connectivity analysis

The dendritic river network structure needed an adequate topological definition for the analysis of its potential as ecological network for the species [START_REF] Grant | Living in the branches: population dynamics and ecological processes in dendritic networks[END_REF]. For this purpose and in agreement with terrestrial network frameworks, the river segments were regarded as habitat nodes whereas the true river network nodes (confluences and segment junctions) were considered as connectors in the network [START_REF] Eros | Network thinking in riverscape conservation -A graph-based approach[END_REF]). As we investigated species movement both in up-and downstream direction, the full complexity of dendritic networks came in the picture. Starting from the topological definition of the river segments in GIS, scripts were developed to derive neighbourhood matrices that depicted the network structure. Figure 2 illustrates the resulting network structure that was applied in the analysis. We adopted the graph method of weighing network importance of nodes [START_REF] Urban | Modeling ecological processes across scales[END_REF], defined as river segments in our case, by a specified application of the Integral Index of Connectivity [START_REF] Pascual-Hortal | Comparison and development of new graph-based landscape connectivity indices: towards the priorization of habitat patches and corridors for conservation[END_REF]. This index offered a measure that integrates the habitat quality of nodes with the complexity and connectivity of the landscape. For our specific river network implementation, integrating the habitat suitability output of the ENFA, the general formula of the Integral Index of Connectivity was defined as follows:

IIC= Σ i Σ j [favourability i x favourability j /(1 + nl ij ) ]/ A L ²
With A L the overall sum of favourability for all the segments of the entire river basin, and nl ij the number of segments between segments i and j, as connection distance present in the neighbourhood matrix. The overall index ranges from 0 to 1 and increases with improved connectivity (Pascual-Hortal and [START_REF] Pascual-Hortal | Comparison and development of new graph-based landscape connectivity indices: towards the priorization of habitat patches and corridors for conservation[END_REF][START_REF] Pascual-Hortal | Integrating landscape connectivity in broad-scale forest planning through a new graph-based habitat availability methodology: application to capercaillie (Tetrao urogallus) in Catalonia (NE Spain)[END_REF]. The relative ranking of segments by their contribution to the overall index value is the individual segment's qualification that is useful in this conservation approach. To calculate the importance of each individual segment, we systematically removed each particular segment and recalculated the IIC index. The percentage of index value loss (delta value dIIC) measured the contribution of each segment to the overall index value [START_REF] Pascual-Hortal | Integrating landscape connectivity in broad-scale forest planning through a new graph-based habitat availability methodology: application to capercaillie (Tetrao urogallus) in Catalonia (NE Spain)[END_REF]. This dIIC can be considered a habitat availability index (Pascual-Hortal and Saura 2006). We used the recently developed Conefor Sensinode 2.2 software [START_REF] Saura | Conefor Sensinode 2.2: a software package for quantifying the importance of habitat patches for landscape connectivity[END_REF] to calculate dIIC values for each river segment.

Constraining the connection distance nl ij of number of segments, and thus the extent of the neighbourhood matrix, allowed a scale-sensitive calculation of the index. With connection distance 0, no integration took place of connectivity and habitat quality of the surrounding segments. In this way, the classical habitat network analysis was performed as described above, based on local habitat

Author-produced version of the article published in Landscape Ecology (2013), vol. 28, n° 7 The original publication is available at http://link.springer.com/ doi : 10.1007/s10980-013-9869-x quality. For a connection distance of 2 and 5, local network structure integrating the habitat quality of the segments was considered. With regard to the otter's home range this measure of 2 to 5 neighbouring segments corresponded to a relevant ecological network scale level covering river lengths from 5 up to 100km, which was in agreement with the species' home range. To infer the scalesensitivity of this network qualification approach, the calculation was also performed with constraining connection distances to 10.

Validation importance of connectivity

To evaluate the merits of the proposed connectivity analysis, its outcome was compared to both the ENFA approach described above and more dynamic distribution modelling approaches. We generated five different data sets for this comparison. One set consisted of the presence-absence data directly.

Four other sets are probabilities of occurrence, generated with respectively the ENFA, a logistic regression and the IIC with connection distances 2 and 5.

The dynamic logistic regression model was obtained from the otter presences over the five surveys.

The determining habitat factors detected with ENFA were entered in a logistic regression model to derive a colonization likelihood estimate. For each time step (5 steps of 5 year) the neighbouring river segments at the colonization front were simulated and confronted with the observed progression. With this model probability of occurrence was inferred over the entire river network, and confronted with the results of the other analyses for occurrence probability. For this comparison correlations were tested for the river segments of the entire river network between the modelled ENFA habitat favourability, the regression model occurrence probability and the integrated connectivity measures.

Afterwards, the model results were confronted with the recorded colonization process for the fullycolonized upstream part of the basin, based on the detailed dataset of the regional nature park of the Livradois-Forez (PNRLF). A measure of colonization speed for the PNRLF river network was calculated over the three recent time steps in which the re-colonization took place: 1995-1999, 2000-2004, and 2005-2009. Each newly occupied river segment was attributed a value of colonization speed, by counting the number of segments traversed at first occurrence to the nearest previously occupied segment. Correlation of the different model outputs to this colonization speed over the PNRLF river network finally revealed the best predictor of otter occurrence.

Author-produced version of the article published in Landscape Ecology (2013), vol. 28, n° 7 The original publication is available at http://link.springer.com/ doi : 10.1007/s10980-013-9869-x

Results

Species niche

Only 5 out of 22 environmental variables showed discriminating scores in the analysis of marginality and specialisation (Fig. 3). The strongest explaining factor for species presence was the rate of channel normalization ('channel straightening') with significant marginality and specialisation (M = 0.12, S = 1.16). This factor showed in the niche factor analysis the highest discriminating power on the river network, as it qualifies 12% of the river segments as unfavourable. Strong marginality with little specialisation though is observed for the forest cover in the floodplain (M = -0.13, S = 0.97) and in the 30m buffer (M = -0.16, S = 0.96). Urbanisation in the 100m buffer showed a high specialisation (S=1.17) that allows a discrimination of 6% of the river basin's segments as unfavourable in the niche factor analysis due to urbanization pressure in the river corridor. The favourable river segments represent for the otter in the Loire basin 82% of the 17 000km selected river length (Fig. 4). Where favourable conditions can be found in all parts of the basin, the most unfavourable conditions are concentrated in the downstream part of the river basin.

Network analysis

The network analysis based on local habitat, without connectivity integration, determined with ENFA as favourable segments joined to ecological networks (Fig. 5a), results in 452 patches of linked favourable segments over the basin with a mean length of 29.3km but high variability (sd = 62.1km) and a dominance of smaller patches. The longest favourable patches (max= 590.2km) are mainly situated along the middle sectors of the main rivers Loire and Allier.

The network connectivity analysis based on the dIIC-connection distance 5 (Fig. 5b) shows a different picture. The favourable networks in the centre of the basin (for the main rivers of Loire and Allier and the south-western sub-catchment of the Vienne-Creuse) are also highlighted, but in this analysis the lower parts of the basin are identified as most important in the river network.

Scale-sensitivity of the method

Constraining the neighbourhood matrix up to the second neighbours (dIIC_2) allows identifying networks based on local habitat availability (Fig. 6). This result shows a higher differentiation at the local scale level. It differs from the network qualification based solely on the individual segment quality (Fig. 5a) as it integrates the favourability of the local network. It differs also from the larger scale connectivity analysis (Fig. 5b) as it reflects more strongly the local habitat availability, but not so much the larger scale network structure of the dendritic river network. With a higher connectivity of connection distance 10, the effect of local habitat availability gets completely lost for our studied network structure and only the overall river basin connectivity is highlighted with dIIC values gradually increasing from upstream to downstream.

Connectivity validation

The logistic regression model predicted the likelihood of colonization accurately in 35% of the cases.

Its modelled occurrence probability over the entire river basin showed a negative correlation with the dIIC_5 (R -0.046, p<0.05) and a positive correlation with dIIC_2 (R 0.14, p<0.05). For the 1501 segments in the upstream PNRLF-region, the modelled occurrence probability showed a positive correlation with both the dIIC_5 (R 0.2, p<0.001) and with the dIIC_2 (R 0.23, p<0.001). Still these correlations are quite weak and so the question remains for otter occurrence being more explained by local habitat factors than by connectivity.

For the PNRLF-region, colonization speed of the occupied segments was between 1 and 16, with an average of 3. This colonization speed of the segments was marginally correlated to the results of the static ENFA favourability (R 0.21, p<0.05) and of the dynamic logistic regression modelled occurrence probability (R 0.19, p<0.05). Much stronger correlation was observed for the connectivity measure of the dIIC_2 (R 0.52, p<0.001). The higher scale connectivity measure dIIC_5 on the contrary shows a negative correlation (R -0.17, p<0.05).

Discussion

Integrated approach to network connectivity analysis

The presented approach adopts the recent developments in river ecology of graph based methods [START_REF] Eros | Network thinking in riverscape conservation -A graph-based approach[END_REF]) and connectivity loss weighing [START_REF] Moilanen | A method for spatial freshwater conservation prioritization[END_REF][START_REF] Hermoso | Addressing longitudinal connectivity in the systematic conservation planning of fresh waters[END_REF]). Yet, it differs

Author-produced version of the article published in Landscape Ecology (2013), vol. 28, n° 7 The original publication is available at http://link.springer.com/ doi : 10.1007/s10980-013-9869-x from existing connectivity analysis methods to prioritize conservation networks in its integrative quantification of habitat availability and connections in the network. Furthermore, by integrating all branches and connections in the neighborhood matrices we shifted from a linear to a branched river network connectivity analysis [START_REF] Fisher | Creativity, idea generation, and the functional morphology of streams[END_REF]Grant et al. 2008). All neighbouring segments along the branches of the dendritic network both in downstream as upstream direction are embedded in this quantification. They are weighed by distance and constrained by a distance criterion to allow a scalesensitive analysis. So, although it does not provide a method that weighs up specific trajectories and connections [START_REF] Carroll | Use of Linkage Mapping and Centrality Analysis Across Habitat Gradients to Conserve Connectivity of Gray Wolf Populations in Western North America[END_REF], we do accomplish the main objective for the analysis of networks, that is to quantify connectivity in relationship with the ecological processes within the network. Most recently developed connectivity measures -even those adjusted to river networks -only count for a distance criterion of connections [START_REF] Hermoso | Addressing longitudinal connectivity in the systematic conservation planning of fresh waters[END_REF][START_REF] Moilanen | A method for spatial freshwater conservation prioritization[END_REF]. In contrast to this strong distance-based interpretation and the barrier emphasis of the least-cost path approaches [START_REF] Pinto | Beyond the least-cost path: evaluating corridor redundancy using a graphtheoretic approach[END_REF][START_REF] Morzillo | An integration of habitat evaluation, individual based modeling, and graph theory for a potential black bear population recovery in southeastern Texas, USA[END_REF], we incorporate the evaluation of the branch density of the dendritic structure in the network analysis [START_REF] Ganio | A geostatistical approach for describing spatial pattern in stream networks[END_REF], with the IIC that measures the density of connectors, that is the 'branchiness' of the river network in our example. River segments are evaluated at the same time as habitat and as connectors in our approach; which is a strong point in the conservation context of ecological networks. In this way, the IIC is generally considered a habitat availability index [START_REF] Pascual-Hortal | Comparison and development of new graph-based landscape connectivity indices: towards the priorization of habitat patches and corridors for conservation[END_REF] because it integrates topological properties (network connectivity) with habitat quantity.

Scale-sensitive ecological network analysis

The scale-sensitive integration of river segment length and resolution of the network under study is in accordance to species behaviour (Baguette and Vandyck 2007) and is the basis for the aggregation of the local habitat with the connectivity over the drainage basin, a necessary step in ecological network analyses for the otter [START_REF] Ottaviani | Modelling semi-aquatic vertebrates' distribution at the drainage basin scale: The case of the otter Lutra lutra in Italy[END_REF]). This scale-sensitive approach demands the acquisition of environmental variables from different scales according to the network structure. Most species distribution studies at large scale start from raster-based landscape representations and metrics [START_REF] Barbosa | Spatial, Environmental and Human Influences on the Distribution of Otter (Lutra lutra) in the Spanish Provinces[END_REF][START_REF] Barbosa | Otter (Lutra lutra) distribution modeling at two resolution scales suited to conservation planning in the Iberian Peninsula[END_REF][START_REF] Loy | Otter Lutra lutra population expansion: assessing habitat suitability and connectivity in southern Italy[END_REF][START_REF] Clavero | Natural, human and spatial constraints to expanding populations of otters in the Iberian Peninsula[END_REF]

that do not allow interpretation of

Author-produced version of the article published in Landscape Ecology (2013), vol. 28, n° 7 The original publication is available at http://link.springer.com/ doi : 10.1007/s10980-013-9869-x data gathered at multiple scales to the dendritic network. Only recently the geometric network analysis has been developed with the graph methods [START_REF] Urban | Landscape connectivity: A graph-theoretic perspective[END_REF][START_REF] Jordán | Characterizing the importance of habitat patches and corridors in maintaining the landscape connectivity of a Pholidoptera transsylvanica (Orthoptera) metapopulation[END_REF]. The presented integration of scale-sensitivity by confining the extent of connections allows the prioritization of conservation networks based on specific objectives, like sets of target species or ecosystem processes and functions with specific spatial demands.

The presented case study shows some restrictions for the attribution of habitat quality to the observations as the data of otter occurrence only allowed for a presence-only approach with its weaknesses [START_REF] Tsoar | A comparative evaluation of presenceonly methods for modelling species distribution[END_REF]). Still we believe this dataset for the Loire basin is one of the best documented examples of otter colonization globally, especially in view of its time span and the size of the basin. The unique large scale dataset on environmental quality of the riparian zones proved appropriate to infer the relevant ecological factors in the colonization process. Restrictions of this environmental dataset concern aspects of water quality, food resources and the height of dams and dikes. Yet, from earlier analyses in the Loire basin no relationship between otter expansion and water quality, prey abundance or fish community could be inferred [START_REF] Lemarchand | Organochlorine compounds (pesticides and PCBs) in scats of the european otter (Lutra lutra) from an actual expanding population in central france[END_REF][START_REF] Janssens | Genetic pattern of the recent recovery of European otters in southern France[END_REF], neither proved the height of dams a significant obstacle to the species' progress (Varray 2011).

Therefore and also based on the limited data at hand for these variables, we did not include them in the analysis.

The ecological potentials and targets for the river system in question will determine the scale of connectivity to incorporate in the designation of conservation priorities. In our case for the Loire River basin, a well-preserved aquatic biodiversity together with a high potential for conservation and restoration of mobile riverine species like otter, beaver, salmon, sturgeon, osprey and river terns, calls for the larger scale connectivity approach and identifies priorities principally in the downstream reaches.

Connectivity in river networks

Our results confirm the conclusions of Moilanen and colleagues that realistic modelling of connectivity has a major influence on the areas proposed for conservation, pointing out different parts of river basins as high priority conservation targets [START_REF] Moilanen | A method for spatial freshwater conservation prioritization[END_REF]

. Same as several other

Author-produced version of the article published in Landscape Ecology (2013), vol. 28, n° 7 The original publication is available at http://link.springer.com/ doi : 10.1007/s10980-013-9869-x recent publications [START_REF] Carranza | Connectivity providers for semi-aquatic vertebrates: the case of the endangered otter in Italy[END_REF][START_REF] Hermoso | Integrating multidirectional connectivity requirements in systematic conservation planning for freshwater systems[END_REF], we supply evidence and methods to integrate different scales and dimensions in connectivity measures, although not focussed on interbasin connectivity as these authors. The proposed method for the integrated connectivity measure proves even more accurate in predicting species occurrence than the dynamic probability modelling that sorted as best available method from earlier comparisons [START_REF] Van Teeffelen | Connectivity, probabilities and persistence: Comparing reserve selection strategies[END_REF].

The more classical habitat-based network analysis corresponds to the result without node network connections (nl ij = 0) in our approach. This result clearly deviates from the results with integration of functional connections. For the analysis of ecological networks for the otter the local networkfocussed dIIC_2 provides the most accurate predictor for otter occurrence probability. Still, for the long time population expansion and survival, as well as for the conservation prioritization, the dIIC_5 surely brings important additional information as well. The comparison to the observed colonisation speed in the upstream part of the basin shows that the dIIC_5 measure is probably not the best predictor for otter colonisation, but it might well be the best prospection for an ecological network to assure otter survival in the long run. This assumption of course needs another type of analysis, or a repeated analysis by the time the colonisation process is fully accomplished over the river basin. The validation of the connectivity analysis with the comparison to the static and dynamic distribution modelling approaches, showed the strength of our approach as it predicted better the otter colonization. Where our approach starts from a static distribution modelling approach, this result proves the strength of the proposed approach, as it even gives a better result than the dynamic habitatbased colonization model that is based on the extended dataset on otter presence over time in the river basin. We didn't integrate this more dynamic distribution model in the proposed method for connectivity analysis, as obviously such a dynamic model needs to be based on temporally explicit data and an exceptionally rich dataset as presented here is mostly not available.

In conclusion we can say that the presented method meets our goal to quantify connectivity in the river network, considering both the complexity of the dendritic network structure and the scale-sensitivity of relationships between network structure and ecological patterns in rivers. It even showed more

Author-produced version of the article published in Landscape Ecology (2013), vol. 28, n° 7 The original publication is available at http://link.springer.com/ doi : 10.1007/s10980-013-9869-x reliable results in predicting species colonization in the network than distribution modelling techniques. The outcome proves the importance of connectivity for the otter, as the movement and occurrence in the river network was more explained by connectivity measures than just with habitat quality aspects. In this way it confirms the otter as a guiding species for connectivity and not solely for habitat quality.

With the presented method insight can be gained into the choices for river restoration and conservation prioritization. Allocation of financial resources for conservation and restoration can be based on this kind of analyses with the integration of multi-species approaches [START_REF] Schwenk | A Multispecies Framework for Landscape Conservation Planning[END_REF].

Cross-examining resulting ecological networks for multiple species with multiple relevant scales for connectivity can reveal priorities for conservation [START_REF] Williams | Planning for climate change: Identifying minimum-dispersal corridors for the Cape proteaceae[END_REF]. The here presented approach can also be used for fully aquatic species in river systems with integration of flow-directed connectivity. Author-produced version of the article published in Landscape Ecology (2013), vol. 28, n° 7 The original publication is available at http://link.springer.com/ doi : 10.1007/s10980-013-9869-x Author-produced version of the article published in Landscape Ecology (2013), vol. 28, n° 7 The original publication is available at http://link.springer.com/ doi : 10.1007/s10980-013-9869-x

Figure 1 .Figure 2 .

 12 Figure 1. Presence of the European otter in the Loire river basin in 2011

Figure 3 .

 3 Figure 3. Ecological Niche Factor Analysis for the European otter.

  version of the article published in Landscape Ecology (2013), vol. 28, n° 7 The original publication is available at http://link.springer.com/ doi : 10.1007/s10980-013-9869-x

Figure 4 .

 4 Figure 4. Otter favourability of river segments in the network of the Loire basin (favourability values present contribution of individual segments in percentage to total network).

  Author-produced version of the article published in Landscape Ecology (2013), vol. 28, n° 7 The original publication is available at http://link.springer.com/ doi : 10.1007/s10980-013-9869-x (a).Author-produced version of the article published in Landscape Ecology (2013), vol. 28, n° 7 The original publication is available at http://link.springer.com/ doi : 10.1007/s10980-013-9869-x (b).

Figure 5 .

 5 Figure 5. The ecological networks based on local habitat quality -with connection distance 0 -(a), and based on the Integral Index of Connectivity calculation for connection distance 5 (b). The figure 5a presents length (km) of continuous favourable

Figure 6 .

 6 Figure 6. The ecological network qualification based on the Integral Index of Connectivity calculation for connection distance 2 for the central and upstream part of the Loire River basin.

Table 1 .

 1 Environmental variables from the different spatial scales that were attributed to the river segments. forest patches for the 30m riparian buffer of the river segment riparian forest canopy percentage cover of forest patches for the 10m riparian buffer of the river segment Infrastructure percentage cover of infrastructure over riparian buffer of three river widths of the segment urbanisation percentage cover of urbanisation for the 100m riparian buffer of the river segment

	Watershed
	urbanisation	percentage cover of urban land use class in CORINE land cover data of the sub-catchments
	intensive agriculture	percentage cover of intensive agricultural CORINE land cover classes of the sub-catchments
	Natural	percentage cover of near-natural CORINE land cover classes data of the sub-catchments
	River segment
	Altitude	elevation at downstream point of river segments
	river slope	slope of the river bed over the segment
	valley slope	valley slope perpendicular to the river
	Discharge	mean annual discharge for gauging station or model prediction at river segment level
	Sinuosity	sinuosity of the river bed over the segment
	channel straightening percentage of straight reaches over the segment, weighted by river type
	density of bars	number of lateral bars over the segment, divided by river length
	density of weirs/dams number of weirs/dams per segment, divided by river length
	density of bridges	number of river crossing bridges per segment, divided by river length
	Alluvial plain
	alluvial forest cover	percentage cover of forest patches over the alluvial plain of the river segment
	Infrastructure	percentage cover of infrastructure over the alluvial plain of the river segment
	urbanisation	percentage cover of urbanisation over the alluvial plain of the river segment
	density of dikes	length of flood protection levees per segment, divided by river length
	connected waters	percentage cover of connected standing waters over the alluvial plain of the river segment
	Disconnected waters	percentage cover of disconnected standing waters over the alluvial plain of the river segment
	River bank
	riparian forest cover	percentage cover of

Author-produced version of the article published in Landscape Ecology (2013), vol. 28, n° 7 The original publication is available at http://link.springer.com/ doi : 10.1007/s10980-013-9869-x

Acknowledgments

This work was only possible thanks to the strong observation effort of the volunteers of the Loire Basin Mammal Network and the agents of the National Wildlife Office (ONCFS) since 1985. Nadine Nogaret of the Regional Nature Parc of the Livradois-Forez provided detailed information on the species' re-colonization. The river data collection in the hydro-morphology audit system SYRAH is financed by the French Ministry of Research and the Ministry of Environment and Sustainable Development. We are indebted to Sandrine Ruette (ONCFS) and Charles Lemarchand (University Clermond-Ferrand) for their constructive comments on the analysis approach and interpretation of the species' inventories. The editor and two anonymous reviewers offered useful suggestions to improve the manuscript.

Author-produced version of the article published in Landscape Ecology (2013), vol. 28, n° 7 The original publication is available at http://link.springer.com/ doi : 10.1007/s10980-013-9869-x

Author-produced version of the article published in Landscape Ecology (2013), vol. 28, n° 7 The original publication is available at http://link.springer.com/ doi : 10.1007/s10980-013-9869-x