D Aidoud

D Guy-Bouyssou

D Guyomard

J Le Bideau
email: jean.lebideau@cnrs-imn.fr

B Lestriez

Photo-Polymerized Organic Host Network of Ionogels for Lithium Batteries: Effects of Mesh Size and of Ethylene Oxide Content

Solid ionogel electrolytes are rather competitive compared to liquid carbonate based electrolyte regarding ionic conductivity but they show much better safety. It is striking that the ionogels presented here are obtained within few minutes by UV curing which triggers the formation of the polymer host network confining Nmethyl,N-propylpyrrolidinium bis-(trifluoromethyl)sulfonylimide with lithium bis-(trifluoromethyl)sulfonylimide salt at the concentration of 1M. This fast and easy process allows for obtaining all-solid like micro-batteries. Herein the effect of mesh size and ethylene oxide content of various host networks is deciphered. An optimum balance between mesh size, ethylene oxide and lithium content is identified. The ionogel formed from trimethylolpropane ethoxylate triacrylate and 1,6-hexanediol diacrylate confining 90% wt of ionic liquid shows a mesh size close to 5 nm, with an enhanced fragility of the confined ionic liquid, as referred to that of the bulk one. Along with these last features, its ethylene oxide content allows a very good lithium ion diffusion. At room temperature and at C/5 cycling rate, prototype batteries LiFePO 4 | ionogel | Li metal battery can restitute 70% of the initial capacity during up to 1200 cycles.

Energy storage devices, such as lithium battery are currently facing safety issues. The conventional liquid electrolytes used in such devices, based on organic solvents, show poor thermal stability, are flammable and thus are source of hazards. In the case of overheating, the solvent can generate pressure, or leak out and catch on fire. To overcome substantially this hazard, non-flammable electrolytes are required. The use of ionic liquids (ILs) appears to be a good alternative to meet this need. They show good ionic conductivities, 1 large electrochemical stability windows, good thermal stabilities and low vapor pressure. 2 Many works have shown the compatibility of IL with different electro-active material (LiFePO 4 ,LiCoO 2 , LiMn 2 O 4 ...). [3][4][5][6] In the case of lithium batteries, higher capacity can be reached by replacing intercalating anode like graphite by metallic lithium. Nevertheless, since upon cycling dendrite growth can cause critical failure, different solid electrolytes have been developed, 7,8 as for instance polyethylene oxide (PEO) based polymer electrolytes or ceramic electrolyte (LiPON). 9 These solid materials provide resistance to dendritic growth. However, the ceramic electrolytes commercially used present low ionic conductivities (around 10 -6 S.cm -1 at R.T.), high costs of fabrication, and polymer electrolyte need to be heated up to 60 • C to be effective enough.

Another approach toward solid electrolyte involves confining a high amount (above ∼80%vol.) of IL within a solid host network, i.e. obtaining ionogels. 10,11 They combine the good ionic conductivities and thermal stabilities of the ILs, avoid leakage and show solid properties expected to prevent dendrite growth. Within ionogels, dynamics of the confined IL can be enhanced thanks to the reduction of nanostructured aggregates, to the reduction of glass transition temperatures, to the lowering of freezing or melting quantities, as well as to a better release of lithium from Li TFSI complex. 12 Confining networks may be made only of oxides within which silica 13 or of polymers 14,15 or of hybrid 10,16,17 organic-inorganic networks. The properties of the ionogels can be tuned through their composition (chemical and textural nature of the confining network, volumic ratio between IL and confining network, nature of IL).

For some ionogels, the formation of the host network could last hours or days. In the case of thermo-polymerized ionogels, no evaporation was noticeable during the polymerization process but still a long heating time was required to complete the cross-linking. Replacing thermal polymerization by photo-polymerization [18][19][20] appeared to reduce the polymerization time as well as to lower the energy cost.

This article presents the study of the effect of mesh size and of chemical nature of the host network on the charge transport behavior in an ionogel, made of an IL with a lithium salt confined within a photopolymerized host network. In a first work, complementary to this one, we evaluated the interfacial stability with the lithium electrode of some of the ionogels studied here. In particular, we have shown the crucial role of the adhesion properties of ionogels on the reversibility of lithium dissolution and deposition processes at this interface. 21 Photopolymerization is well-known for being a very fast polymerization process. 22,23 A gel point can be reached after few seconds (5s, see below) under UV irradiation. Depending on the precursor formulation and the intensity of the UV source, a fully cross-linked polymeric network can be obtained after 20 minutes of irradiation. In addition to this, UV curing is energetically less demanding and cheaper than thermal polymerization; the polymerizable precursor used for photopolymerization can be cured right after deposition and do not require the addition of rheological stabilizers to avoid their creeping before being heated.

The monomers used herein (Fig. 1) have been chosen for their low molecular weight, their high content of polymerization functionalities (triacrylate and diacrylate) and also for the different chemical composition of the spacers, more specifically regarding the ethoxy group (EO) which is known for its participation to lithium ion conduction in PEO-based polymer electrolytes. The different compositions studied here allowed us to check the influence of these chemical features on the final properties of the solid electrolyte. Also, these low molecular weights monomers have a short chain length: combined with the high amount of acrylic functionalities, this results after polymerization in a highly cross-linked tridimensional polymeric network, with small mesh size. These networks are able to confine a high amount of IL with a small amount of solid host network (90% wt IL -10% wt monomers) with good mechanical properties. Compositions with comparable molecular weight and lower amount of functional monomers (diacrylate and monoacrylate) have been previously tested (not reported here), resulting in ionogels with very poor mechanical properties, not suited for the all solid state technology. Among each tested formulation, from precursors trimethylolpropane triacrylate (TMPTA), trimethylolpropane ethoxylate triacrylate (ETPTA), 1,6-hexanediol diacrylate (HDDA), three were selected and are presented here: [TMPTA/HDDA]/IL, [ETPTA/HDDA]/IL, [ETPTA 5 /PEODA]/IL with the [30/70]/90 compositions.

Experimental

Materials.-The monomers used to prepare the 3D confining network are shown Fig. 1: trimethylolpropane triacrylate (TMPTA, (H2C = CHCO2CH2)3C-C2H5, Mw = 296 g), trimethylolpropane ethoxylate triacrylate (ETPTA and ETPTA 5 ,H 2 C= CHCO2((CH2-CH2-O)nCH2)3C2H5, average n = 3 and 14 respectively, Mw = 428 and 912 g), 1,6-hexanediol diacrylate (HDDA), poly(ethylene glycol) diacrylate (PEODA, H2C = CHCO(CH2-CH2-O)nO2C2H = CH2, average n = 10, Mw = 575 g). They were all purchased from Sigma Aldrich. The photo-initiator, 2-Hydroxy-2-methylpropiophenone, was also purchased from Sigma Aldrich. The ionic liquid, N-methyl,N-propylpyrrolidinium bis-(trifluoromethyl)sulfonylimide, (PYR13 TFSI) was purchased from Solvionic (France) and the lithium salt, lithium bis-(trifluoromethyl)sulfonylimide (Li TFSI), was purchased from 3M. Each chemical was stored in an argon filled glove box.

Electrolyte solution and ionogel preparation.-The electrolyte solution was prepared by dissolving Li TFSI in PYR13 TFSI with a concentration of 1M (0.2 g of Li TFSI salt in 1 g of PYR13 TFSI) at room temperature in an argon filled glove box. The resulting PYR13 TFSI Li ternary IL will be confined in the different polymeric networks presented here. The solid polymer electrolyte precursor was prepared outside the glove box, by mixing comonomers with the electrolyte solution (IL) in the [10]/90 weight ratio of [comonomers]/IL. Then 3% wt of 2-hydroxy-2-methylpropiophenone was added. 12 hours of magnetic stirring, at ambient atmosphere, ensured a good dispersion of the monomers inside the IL. The monomer ratio [triacrylate/diacrylate] was ever set to [30/70] %wt The [triacrylate/diacrylate]/IL weight composition of the resulting ionogel is thus [30/70]/90% wt The liquid precursor was drop casted between two glass plates (transmittance of 0.91 for UV light) spaced by wedges to tune the sample thickness and was irradiated using UV LED (45 mW.cm -2 ) for 20 min. Lastly, each sample was dried at 90 • C under vacuum (50 Pa) during 24 h to reduce residual water below 50 ppm in the solid electrolyte prior to any electrical and mechanical measurement. LiFePO 4 ) and battery assembly.-This was carried out as described in a previous work. 21 The positive electrode was prepared from an aqueous mixture of powder of LiFePO 4 as active material, carboxyl methyl cellulose (CMC, Mw = 250000, Aldrich) as binder, and Carbon Super PTM (Csp, Timcal) as electronic conductor additive. The ratio of LiFePO 4 /CMC/Csp was 80/8/12%wt The resulting slurry was ball milled for 2h at 700 rpm, casted on an aluminum coated carbon current collector, then dried at 60 • C during 24h followed by 24 other hours at 90 • C under vacuum (0.15 mAh.cm -2 , active mass 1 mg.cm 2 , electrode porosity 70%). Circular electrodes (1.76 cm 2 ) were cut out from the previous preparation and were used as positive porous composite electrodes. The liquid precursor of the photo-ionogel was deposited onto the electrode and UV cured for 20 min (thickness of electrolyte ∼60 mm). After drying under vacuum (50 Pa) at 60 • C, a button cell battery was assembled with a lithium foil (50 µm).

Cathode preparation (

Electrical and electrochemical characterizations.-The ionic conductivity of the different solid polymers electrolytes was measured by means of electrochemical impedance spectroscopy (EIS) between two stainless steel blocking electrodes from 185 kHz to 1 Hz with sinus amplitude of 7 mV at different temperatures from -10 • C to 80 • C with incremental steps of 10 • C. For this measurement, free samples with an average thickness of 400 µm were prepared following the previously described route. All potentials are given relatively to lithium metal. The electrochemical stability window was studied at room temperature (RT, ∼22 • C) by cyclic voltammetry (CV), between lithium and platinum electrodes, at scanning rates of 1 mV.s -1 and 0.1 mV.s -1 from OCV to 4.2 V and 5 V respectively, then down to 2V in reduction and finally ending up at the original OCV potential. The galvanostatic cycling tests were carried out at 20 • C between 2 and 4.2 V. All electrochemical experiments were monitored using a VMP biologic potentiostat/galvanostat operated with EC-Lab software and using coin cell.

Mechanical properties.-Rheological properties were measured on Anton Paar rheometer (Physica MCR 101) with a 25 mm diameter plane-and-plane geometry and analyzed with Rheoplus software. The samples were circular with a diameter of 25 mm, in agreement with the rheometer geometry, for a thickness of 500 µm. Before measurement, the samples were equilibrated at 20 • C. Storage and loss modulus, respectively G' and G", were measured for each compositions as a function of the deformation from 0.01% to 100% and from 100% to 0.01%. The measurements were led with a constant frequency of 1 Hz.

Differential scanning calorimetry (DSC).-DSC measurements

were done in a Q20 calorimeter, TA Instrument. Before measurement, samples were dried at 60 • C for 24 h and sealed in hermetic aluminum pans. The samples were quenched down to 150 • Cat-30 • C.min -1 and stabilized at -150 • C for 10 min to assure a thermal equilibrium. They were then heated from -150 to 200 • Ca t1 0 • C.min -1 . The average mass of the samples studied was 20 mg.

Results and Discussion

Ionogel preparation.-Usually, UV curing is done under oxygen free atmosphere in order to avoid the inhibition effect of oxygen toward free radical formation. 24 Despite this, UV curing under ambient air reduces the time and cost of the process. In the case of pure monomer resin, this inhibition is effective only in the 10 µm below the surface. 25 Thus each synthesis was carried out under ambient air, and to reduce at most oxygen inhibition, each precursor was exposed to vacuum (50 Pa) during 20 minutes in order to extract potential air bubbles and dissolved oxygen. Precursors were then directly drop casted between two UV transparent glass plates before UV irradiation, where the contact surfaces with air are restricted to the edges. Finally, disc samples were cut far from the edges.

Here, we present a formulation for the electrolyte precursor made of 90% wt of electrolyte solution composed of PYR13 TFSI with 1 M of Li TFSI, and 10% wt of acrylic comonomers. The monomer ratio [triacrylate/diacrylate] was maintained at [30/70] %wt The [triacrylate/diacrylate]/IL weight composition of the resulting ionogel was thus [30/70]/90% wt Prior to UV curing, photo-initiator was added at 3% wt as referred to the precursors total mass (Fig. 2). A good dispersion of the monomers and photo-initiator inside the IL is mandatory to get a homogeneously reticulated final material. To ensure this, 12 h of magnetic stirring was applied. This precursor was then irradiated with a 40 mW.cm -2 UV LED between two UV transparent glass plates, during a time discussed hereafter. A solid material was obtained here after only 5 seconds of UV irradiation. The resulting ionogels were flexible, showed constant thickness and smooth surfaces. The typical IR-ATR spectra of the liquid precursor of the ionogel based on acrylic monomers is shown in Fig. 3.The C=C double bond of the acrylic function, which was consumed during the polymerization, corresponds to the double peaks located between 1610 and 1640 cm -1 . We can observe the decrease of this double peak over irradiation time. After 3 min of irradiation, the double peaks were greatly reduced, and after 20 min, it had totally disappeared, which ensured the completeness of the polymerization. Furthermore, it evidenced the good diffusion of the UV light even through such thick sample (500 µm) and ensured that a good polymerization could be also achieved for thinner layers. Consequently, in order to provide the maximal conversion rate, the irradiation time was set to 20 min.

The impact of the 20 min of UV irradiation on the material and the electrolyte solution was also investigated, although there was no visible modification on the PYR13 TFSI or in PYR13 TFSI Li. Stepnowskia and Zaleskab 26 have studied an ageing under UV irradiation of imidazolium based IL, and obtained after 20 min of irradiation a degradation of 10% of the initial content; it is worth to point out that dilakylimidazolium are known to possess a labile proton on the C2. Nevertheless, we checked the tolerance to the UV irradiation of the IL chosen in the present work, by comparing the IR-ATR spectra of pristine pyrrolididium IL based PYR13 TFSI and PYR13 TFSI Li to those of the same compounds after 20 min of UV irradiation. The spectra overlapped perfectly, showing that there was neither degradation nor modification.

Ionogel study.-The value of the ionic resistance of each ionogel, REIS, was measured at the intercept between the curve and the real axis in the Nyquist plan. The ionic conductivity was calculated using the following equation,

σ EIS = d/ (S × R EIS )
where d and S are respectively the thickness and the surface of the samples. Fig. 4 shows the temperature dependency of the ionic conductivity of (i) the three ionogels with 1 M of Li TFSI and, for the sake of comparison, (ii) the ionic conductivity of a carbonate liquid electrolyte (LP 30, 1 M LiPF6 in ethylene carbonate and dimethyl carbonate, EC:DMC 1:1 v/v) and (iii) a solid ceramic electrolyte, LiPON (lithium phosphorous oxynitride, LixPOyNz). 27 Ionogel [ETPTA/HDDA]/IL presented the highest measured ionic conductivity, followed by [ETPTA 5 /PEODA]/IL and then [TMPTA/HDDA]/IL. Ionogels [ETPTA/HDDA]/IL and [ETPTA 5 /PEODA]/IL exhibit conductivities very close to that of the bulk IL. Of course, the values calculated here correspond to the ionic conductivity of the whole ionogel, i.e. taking into account the confining network along with the confined IL. Despite the fact that they are solid materials, all compositions showed ionic conductivities close to that of the non confined liquid electrolyte, such as IL or LP 30, and considerably higher conductivities than that of the solid ceramic electrolyte such as LiPON. In addition to this, regarding the ionic conductivity dependence to temperature, each ionogel presented slope breaks (Fig. 4), due to the liquid-solid phase transition upon cooling. Compared to the bulk (non confined) IL, it corresponds to a lowering of the liquid-solid transition temperature by 10 and 20 • C for [ETPTA/HDDA]/IL and [ETPTA 5 /PEODA]/IL respectively. The DSC measurements (Fig. 5), allowed to determine the fusion enthalpy (endothermic phenomena) resulting from the solid-liquid transition of the IL, confined or non confined (bulk): these values were reduced upon confinement, accordingly with the quenching of the cold crystallization (exothermic phenomena) (Table I). The bulk-like ratio in Table I represents the part, as referred to 90% wt IL content, of the enthalpy attributable to the amount of confined IL that undergoes phase transition. The DSC diagrams also showed, for non confined IL, several endothermic peaks. The peaks, before the last one which corresponds to melting, can be imputable to solid-solid transition 28 and do not appear for the different ionogels diagrams, thus showing that the number of allowed solid phases is reduced upon confinement. The conductivity behavior (Fig. 4) does not correspond to true Arrhenius behavior and should be better described by the Vogel-Tamman-Fulcher (VTF) equation,

σ = σ 0 exp[-B/ (T-T 0 )]
where σ 0 is related to the number of charge carriers in the electrolyte and corresponds to the conductivity at infinite temperature, B is related to the activation energy of associated ion transport, and T 0 is the ideal glass transition temperature (Vogel temperature) at which no molecular mobility remains. Based on this model, all the different parameters were extracted from a VTF fitting of the data plots, far above the liquid-solid transition, and are presented in Table II.TheB parameter can also be related to the fragility index D. 29,30

D = B/T 0
The more the liquids are fragile (lower D) the more they are sensitive to perturbation leading to fast structural and dynamical molecular re-arrangement (short relaxation times). It has been observed that confinement impacts the fragility of ILs. 31 Indeed, the D index measured for silica-based ionogels (biphasic solid/liquid materials, i.e. host network/confined IL) is even sometimes lower than that of pure IL.

For silica-based ionogels, the D index decreases with pore size, thus corresponding to an increasing fragility. Herein the average mesh size of the different polymeric confining networks used was evaluated. In a first approach, according to Flory theory, the average mesh size (ξ) of polymers in solvents is linked to the storage modulus by the following relation, 32

ξ = 3 [(k B T) /G ′ ]
where k B is the Boltzmann constant, and T the absolute temperature. The measurement of G' was done with minimum pressure applied on the sample to be as close as possible of the freestanding condition. Then, the value of the storage modulus was obtained by extrapolating the plateau at low deformation (Table II). Flory equation provides an approximate value, nevertheless sufficient to describe a relative tendency.

Consistently with the results obtained for silica based ionogels, the fragility index is lower for the confined IL, and its decrease is correlated with the decrease of the average mesh size of the polymeric network (Table II). This could be explained by the fact that the addition of a confining network disrupts the short range interaction between ions within pairs or aggregates, and appeared to break them down. 31 Furthermore, polar host (PEO or silica) would generate layer organization of the ions at the interface. [START_REF] Lee | [END_REF][34][35] The lithium transference number (t Li+ ) was also determined for each ionogel using symmetrical cells (Li|ionogel|Li) and following Bruce and Evans method. 36 The t Li+ was calculated for each ionogel using the following equation,

t Li+ = [I s ( V-R 0 I 0 )] / [I 0 ( V-R s I s )]
where R 0 and R S , are respectively the interfacial resistance of the symmetrical cell, before and after polarization. I 0 and I S , are the initial and steady state current, and V (20 mV here) is the potential of polarization of the cell. The values of t Li+ are reported in Table III.

The lithium transference number showed a clear improvement, compared to that of pure IL, for [ETPTA/HDDA]/90 and [ETPTA 5 /PEODA]/90, with a maximal value for the [ETPTA/HDDA]/90 of 0.30. For the [TMTPA/HDDA]/90, the t Li+ was similar to that of the pure IL. Bruce and Evans method of t Li+ measurement depends on the interfacial resistance during the DC polarization. [ETPTA/HDDA]/90 and [ETPTA 5 /PEODA]/90 presented a stable interface over time (Fig. 6 and values of R 0 and R S in Table III). For [TMTPA/HDDA]/90, the interfacial resistance showed a sharp increase during the DC polarization, which explained the strong diminution of the current over time. In addition, we performed Raman spectra for each ionogel (Fig. 7). For pure PYR13 TFSI, an intense symmetrical peak can be observed at around 742 cm -1 . For non confined PYR13 TFSI Li, a shoulder is present at around 748 cm -1 , due to TFSI interaction with Li + . [37][38][39] As shown in Fig. 7, we can observe a decrease of the intensity of the peak at 748 cm -1 for each composition, showing the decrease of the TFSI-Li + coordination, due to the presence of the confining network. This was also observed on ionogels with silica or hybrid host networks. 31,40 For [ETPTA/HDDA]/90 and [ETPTA 5 /PEODA]/90, the decrease of the shoulder is greater than for [TMTPA/HDDA]/90, showing that the additional presence of EO groups in the confining network is directly correlated with the decrease of the Li-TFSI coordination. Indeed, the peak at 748 cm -1 is hardly visible for the two ETPTA based ionogels. This implies that very few Li + ions are still coordinated to TFSI anions. This modification should contribute to improve the lithium mobility, accordingly with the higher t Li+ for [ETPTA/HDDA]/90 and [ETPTA 5 /PEODA]/90 than for non confined IL. Moreover, the difference between the t Li+ value of [ETPTA/HDDA]/90 and [ETPTA 5 /PEODA]/90 could be attributed to the different amount of EO groups in the structure of the confining network. Actually, the Li + ion could be trapped onto the more numerous repeated EO chain present along the IL/confining network of the [ETPTA 5 /PEODA]/90 ionogel. Nevertheless, the best t Li+ is obtained for [ETPTA/HDDA]/90, which seems to result from an optimized balance between the content of Li + and EO (thus similarly to silica based ionogels), a small mesh size and a high fragility (low D value), as summarized Fig. 8.

Electrochemical characterization.-The ionogel [ETPTA/

HDDA]/90 showed the best interfacial resistivity stability over time, attributed to a better adhesion, which results in a higher restituted capacity at C rate. 21 In addition of the properties presented herein, [ETPTA/HDDA]/90 was thus chosen for further study of all-solid state lithium battery.

The electrochemical stability of the ionogel [ETPTA/HDDA]/90 was measured by cyclic voltammetry (CV) in a Pt | [ETPTA/HDDA]/90 | Li cell. The measurement was done between 2 and 4.2V, for different time of UV irradiation at 1 mV.s -1 .Theresults showed the effect of the irradiation time on the electrochemical stability. A better stability was obtained for 20 min of UV irradiation than for 3 min. As explained previously, the polymerization was not totally completed after 3 min of UV curing; thus the measured current could be attributed to the remaining free monomers and photo-initiator. Indeed, this was confirmed by the very good stability obtained after 20 min of UV curing, time after which the polymer- ization is completed. A low scan rate (0.1 mV.s -1 ) measurement on the ionogel UV cured for 20 min was carried out between 2 and 5V. We observed a decrease of the current along with the decrease of the scan rate; moreover, the current did not exceed 2 µA.cm -2 ,e v e na t 5V. This confirmed the very good compatibility of this ionogel with respect to high voltage application.

For battery testing, we used composite porous electrode incorporating LiFePO 4 (theoretical capacity of 170 mA.h.g -1 ) as active material. The average areal capacity and porosity of the electrodes were respectively 0.15 mA.h.cm -2 and 70%. The liquid precursor of the ionogel [ETPTA/HDDA]/90 was directly poured over the electrode. A vacuum step was then carried out in order to obtain a good filling of all the porosity. Furthermore, the vacuum step limited the dissolution of the O 2 contained in the porosity inside the precursor and thus limited the inhibition of the polymerization, thereby allowing a homogeneous solidification of the precursor inside the electrode.

The SEM images, after UV curing and removal of current collector, showed the good precursors spreading inside the electrode up to the current collector. Additionally, the same aspect was observed at the surface of the [ETPTA/HDDA]/90 and around the grains, confirming that the precursor had solidified throughout the thickness of the electrodes.

After UV curing through glass plates spaced by 100 µmt h i c k wedges, a parallel all-solid state electrode/electrolyte stacking was obtained. A further drying at 90 • C under vacuum for 24h was carried out. The ionogel thickness was measured afterward at 50µm. Finally, inside a glove box, a lithium disk (50 µm, 1.76 cm 2 ) was stacked onto the ionogel. The resulting lithium metal battery was placed between two stainless steel electrodes inside a coin cell (LIR2032). The battery was charged and discharged at RT at the constant rate of C/5 (representing a complete theoretical charge or discharge in 5 h) during 1200 cycles between 2 and 4.2V. Fig. 9 shows the capacity in charge and discharge, as well as the efficiency. At this rate, the initial reversible discharge capacity of the battery was 100.6 mA.h.g -1 . Upon cycling, we observe that the battery can restitute 80% of the initial capacity during 380 cycles, and 70% during up to 1200 cycles. Furthermore, after 600 cycles, the capacity was stabilized around 70 mA.h.g-1 for the next 400 cycles with efficiency higher than 99.9%. The stability of the polarization upon cycling showed that the ionic conductivity of the ionogel [ETPTA/HDDA]/90 and the resistance of the battery were constant all along the 1000 cycles. The cycling rate was reduced to C/20 then C/10 after 1000 cycles and the battery was still able to restitute, respectively, 117 mA.h.g -1 and 104 mA.h.g -1 with efficiencies of 99% and 99.5%. The cycling rate was then restored to its initial C/5 rate for 200 more cycles. The delivered capacity was still around 70 mA.h.g -1 and reached its final value after 1200 cycles at 68.5 mA.h.g -1 . It is worth to point out that fading was only around 10% of its initial capacity during the last 800 cycles.

Conclusions

We have presented herein a comparative study of ionic liquid confined in different photo-polymerized organic networks. The process by UV curing appears simple, efficient, fast and cheap. As compared to conventional polymer electrolytes, these ionogels present improved ionic conductivity and lithium transference number. This improvement was attributed to (i) the reduction of the nanostructured domains of the confined ionic liquid expressed by an increased fragility, (ii) to the lowering of the strong coordination of Li + by TFSI and (iii) to preferential path of diffusion created along the interface between the ionic liquid and the confining network. The influence of mesh size and of ethylene oxide of the confining network were evidenced. It suggested that an optimal balance between ethylene oxide and lithium cation amounts should exist: adding few ethylene oxide groups improved ionic conductivity and Li + mobility, but too many ethylene oxide groups appeared to reduce the mobility of lithium cations along the chains of the host network, i.e. at the interface between ionic liquid and polymer. Among the three photopolymerizd ionogels presented herein, the one with 90 wt% of ionic liquid confined in a network obtained from trimethylolpropane ethoxylate triacrylate (ETPTA, H2C = CHCO2(CH2-CH2-O)3CH2]3C2H5) and hexanediol diacrylate (HDDA) showed the best properties as solid electrolyte for all solid state lithium battery. This ionogel presented a good ionic conductivity, a high t Li+ , a good electrochemical stability and a very good cyclability. Lithium battery testing showed up to 1200 cycles with 70% of the initial capacity, without critical failure. The ionogels presented herein thus appear as highly competitive solid electrolytes for microdevices, given the easy and fast processability, good performances, long life cyclability, and high safety.

Figure 1 .

 1 Figure 1. Monomers used herein: a) trimethylolpropane triacrylate (TMPTA) or trimethylolpropane ethoxylate triacrylate (ETPTA), with l+m+n=0f o r TMPTA, average l+m+n=3 and 15 for ETPTA and ETPTA 5 respectively; b) 1,6-hexanediol diacrylate (HDDA); c) poly(ethylene glycol) diacrylate (PEODA) (average n=10).

Figure 2 .

 2 Figure 2. Ionogel synthesis and preparation.

Figure 3 .

 3 Figure 3. Top: typical FTIR-ATR spectra of acrylic ionogel depending on the UV curing time; bottom: storage modulus evolution depending on UV curing time and temperature.

Figure 4 .

 4 Figure 4. Temperature dependency of the ionic conductivity of the different ionogel, LP30 and LiPON; PYR13 TFSI Li appears in black symbols and solid line.

Figure 5 .

 5 Figure 5. DSC measurement of the pure PYR13 TFSI LI 1M and of the three ionogel.

Figure 6 .

 6 Figure 6. Chronoamperometry curves of the ionogels; bottom: EIS measurements before and after DC polarization.

Figure 7 .

 7 Figure 7. Raman spectra of the pure PYR13 TFSI, PYR13 TFSI Li 1M and ionogels.

Figure 8 .

 8 Figure 8. Summary of the three ionogels studied here, along with the confined IL; mesh size increases from left to right, as well as oxygen content on the host network (blue symbols).

Figure 9 .

 9 Figure 9. Top: cyclability of [ETPTA/HDDA]/90 ionogel as electrolyte in lithium battery; bottom: charge and discharge profiles of the same battery at selected cycles.

Table I . Glass transition temperatures (Tg) and melting enthalpies of non confined and confined ILs.

 I

		Tg (K)	m H(J.g -1 )	Bulk like m H(%)
	PYR13 TFSI Li 1M	200	26.2	100
	[TMTPA/HDDA]/90	195	14.8	50
	[ETPTA/HDDA]/90	194	15.9	54
	[ETPTA 5 /PEODA]/90	194	5.1	17

Table II . VTF prefactor σ 0 , Vogel temperature T 0 , activation energy E a , fragility index D of non confined ILs and of ionogels, and storage modulus G' and mesh size ξ of ionogels.

 II Value obviously (too high) tainted by error, but appears here as a raw result because it is obtained under the same conditions as the other parameters.

		σ 0	T 0	E a	G'	ξ (nm)
		(S.cm -1 ) (K) (eV) D (kPa) (25 • C)
	PYR13 TFSI Li 1M	0.784	182 0.34 4.2	-	-
	[TMTPA/HDDA]/90	0.700	245 * 0.23 0.7	50	4
	[ETPTA/HDDA]/90	0.203	198 0.26 2.3	25	5.5
	[ETPTA 5 /PEODA]/90	0.363	189 0.30 3.2	10	7
	*

Table III . Initial and steady state current I0 and IS and interfacial resistance R0 and RS, respectively before and after polarization, Li+ transference number tLi + and ratio of number of lithium cations on ethylene oxide in the ionogels.

 III

		I 0	I S	R 0	R S		n(Li)/
		(µA) (µA)	( )( )t Li+ n(EO)
	PYR13 TFSI Li 1M					0.06	-
	[TMTPA/HDDA]/90	26.3	3.9	581	2652 0.07	1.7
	[ETPTA/HDDA]/90	20.4	13.9	774	762	0.30	1.2
	[ETPTA 5 /PEODA]/90	16.6	8.7	1025 1031 0.14	0.3

Acknowledgments

Financial funding from the project TOURS 2015 n • 012590-418393 (Programme d'Investissement d'Avenir -FSN -AAP Nanoélectronique n • 1) is acknowledged. ORCID J. Le Bideau https://orcid.org/0000-0003-3959-3694