L Chancelier

A O Diallo

C C Santini

G Marlair

T Gutel

S Mailley

C Len

Targeting adequate thermal stability and fire safety in selecting ionic liquid-based electrolytes for energy storage †

Keywords: Ionic liquids, electrolytes, thermal stability, combustion behaviour

The energy storage market relating to lithium based systems regularly grows in size and expands in terms of portfolio of energy and power demanding applications. Thus safety focused research must more than ever accompany related technological breakthroughs regarding performance of cells, resulting in intensive research on the chemistry and material science to design more reliable battery. Formulating electrolyte solutions with nonvolatile and nonflammable ionic liquids instead of actual carbonate mixtures could be safer. However, few definitions of thermal stability of electrolytes based on ionic liquids have been reported in case of abuse conditions (fire, shortcut, overcharge or overdischarge). This work investigates thermal stability up to combustion of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C 1 C 4 Im][NTf 2 ]) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([PYR 14 ][NTf 2 ]) ionic liquids, and their corresponding electrolytes containing lithium bis(trifluoromethanesulfonyl)imide [Li][NTf 2 ]. Their possible routes of degradation during thermal abuse testings were investigated by thermodynamic studies under several experimental conditions. Their behaviours under fire were also tested, including the analysis of emitted compounds.

Introduction

Environmental concerns and security of energy supply are key drivers for the promotion of a new generation of energy storage systems. These systems must be capable of addressing a number of technical challenges for the sustainable development of electromobility (terrestrial, maritime and aerial to some extent). They should efficiently store intermittent renewable sources of energy (wind, sun, sea) and be part of smart grids applications. 1 Electrochemical energy storage previously had to rely on lead-acid, nickel cadmium and nickel metal hydride technologies. Commercialized since the 90s for the consumer market, lithium-ion technology and its variants have taken the lead regarding those emerging applications as they offer significantly higher power and energy densities in practice. However safety has been identified in a number of studies as a potential market restraint. [2][3][START_REF]World Hybrid Electric and Electric Vehicle Lithium-ion Battery Market, Frost & Sullivan[END_REF] The electrolytes of lithium-ion batteries are typically a mixture of organic carbonates like ethylene carbonate or diethylcarbonate, using lithium salt such as lithium hexafluorophosphate (LiPF 6 ) (Scheme 1). Some of the advantages of these electrolytes are their high solubility of lithium salt, good cycling efficiency, good ionic conductivity and compatibility with numerous electrode materials. Unfortunately they can lead to safety issues as they are flammable, [START_REF] Yun | [END_REF] possess low flash points 6 and are not stable at high temperatures (> 60 °C). 7 The replacement of these electrolytes by room temperature ionic liquids (ILs) is now widely considered in literature [8][9][10][11][12] and in industrial projects. [13][14][15][16][17] Indeed ILs present the required properties, such as good ionic conductivity, wide electrochemical window (4 to 6 V) and good cycling efficiency in spite of their viscosity. 18 Furthermore ILs can be safe electrolytes [19][START_REF] Wasserscheid | Ionic liquids in synthesis. 1st Edition[END_REF][START_REF] Fox | [END_REF][22][23] as they have very low vapour pressure and high flash point. 24,25 They are also non-flammable according to the criteria of the Globally Harmonized System, [START_REF]Globally Harmonized System of Classification and Labelling of Chemicals[END_REF] and they are considered stable up to 300 °C. [START_REF] Maton | [END_REF] However thermal stability can be defined or interpreted differently, depending on the context of use. Currently, there is no standard to define the maximum operating temperature (T op ), even if a few attempts should be mentioned. 28,29 When the thermal instability leads to identified dangerous properties in hazardous materials regulations, they reflect the related level of instability of the concerned materials. It can be the case for self-reactive materials, water-reactive materials, flammable liquids, explosives… In all other cases, thermal stability keeps somewhat subjective and practical rules must be used according to intended use and desired overall performance of the system.

Contrarily to the current carbonate-based organic electrolytes, [START_REF] Andersson | Investigation of fire emissions from Li-ion batteries[END_REF] few definitions of thermal stability of ILs for use as electrolytes were reported in case of abuse conditions (fire, shortcut, overcharge or overdischarge). [START_REF] Stefan | [END_REF] In the vast domain of ILs, imidazolium and pyrrolidinium cations associated to NTf 2 anion were selected as they showed the highest decomposition temperatures. [START_REF] Maton | [END_REF][32][33][34][35][36] This work investigates thermal stability up to combustion of these ILs and their corresponding electrolytes (defined as solution of ILs with 1 mol.L -1 of lithium salt) for lithium-ion batteries. The possible routes of degradation of ILs and corresponding electrolytes during thermally abuse tests were investigated by thermodynamic studies under different experimental conditions.

Materials and methods

Synthesis and characterisation of the electrolytes

The ionic liquids 1-butyl-3-methylimidazoliumbis(trifluoromethanesulfonyl)imide

([C 1 C 4 Im][NTf 2 ]
) and 1-butyl-1-methylpyrrolidiniumbis(trifluoromethanesulfonyl)imide ([PYR 14 ][NTf 2 ]) were synthesised as already reported in the literature. [37][38][39] (See details in SI-1 and NMR analyses in SI-2). After drying under vacuum (10 -5 mbar) for 24 h at room temperature, the water content was lower than 60 ppm (mass ratio), as assessed by Karl Fisher coulometric titration. The absence of residual chloride (<0.5 %) was assessed by high resolution mass spectroscopy (HRMS) after several washings with water, followed by AgNO 3 tests. 40 The electrolytes were prepared by adding LiNTf 2 (1 mol.L -1 ) in a well stirred IL at room temperature, and will be referred to as

[C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ]
. Finally the neat ILs and electrolytes were vacuum-dried at room temperature for 48 h under 10 -5 mbars followed by storage in an argon-filled glove box.

1 H, 13 C, 19 F and 7 Li solution NMR spectra were recorded at room temperature on BRUKER AVANCE 300 MHz Spectrometer with a probe 5 mm BBO with gradient of shims Z. The resonance frequencies were 300. 7 Li. Deuterated dichloromethane was used as solvent. The high resolution mass spectra (HRMS) were recorded in positive and negative ion modes on a hybrid quadrupole time-of-flight mass spectrometer (MicroTOFQ-II, Bruker Daltonics, Bremen)

with an electrospray ionization (ESI) ion source. The gas flow of the spray gas was 0.6 bar and the capillary voltage was ±4.5 kV. The solutions were infused at 180 µL.h -1 . The mass range of the analysis was 50 -1000 m/z and the calibration was done with sodium formate.

Thermal stability of ILs and their electrolytes

Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed on lithium salt, neat ILs and the corresponding electrolytes.

Differential Scanning Calorimetry (DSC)

The thermal analysis of all samples was investigated using a Setaram DSC 131 apparatus. A weighed sample of 5.0 ± 1 mg was placed in a sealed stainless crucible. A standard heating rate of 5 °C.min -1 was applied from 30 up to 650 °C under 50 mL.min -1 of nitrogen. The start and the onset temperatures were determined in order to assess the thermal stability. T start is the temperature at which the sample starts to lose some mass and T onset was determined from the step tangent method. Also, the heat of decomposition of each sample was calculated based on the curve integration.

Thermogravimetric analysis (TGA)

The thermogravivetric analysis scans were collected on a Mettler Toledo TGA DSC 1. The instrument was calibrated using indium, zinc and aluminum samples. Experiments were carried out in 40 µL aluminum pans, filled and sealed in an argon-filled glove box. Samples of 10 ± 3 mg were heated under an argon flow of 30 mL.min -1 . Two types of TGA experiments were investigated. The first one called dynamic experiment was carried out using a temperature increase from 30 to 500 °C at 5 °C.min -1 heating rate. The onset temperature (T onset ) was determined from the step tangent method. 29,34,41,42 The start temperature (T start ) is the temperature at which the sample starts to lose some mass 29 and was defined as the temperature at which the first derivative of the weight loss vs. time curve is |dm/dT| > 10 -4 %.s -1 . The associated uncertainty was 5 °C and the definitions are illustrated in Figure 1. The second experiment, called isotherm experiment, consisted in a temperature increase from 30 to the selected temperature, e.g. 350 °C, at 5 °C.min -1 heating rate, followed by a plateau of 4 or 15 h at e.g. 350 °C. The slope of the curve, representing the mass loss rate, was determined between 2 and 10 h. For each experiment an argon background was subtracted.

Thermal treatment of electrolytes

3 mL of the electrolytes [C 1 C 4 Im][Li][NTf 2 ] or [PYR 14 ][Li][NTf 2 ]
were introduced in a schlenk and dried under high vacuum (10 -5 mbar) overnight before the experiment. The solution was heated by a tubular oven at 10 °C.min -1 heating rate up to 350 °C. It was kept for 2 h at this temperature while the pressure of the emitted gases was measured by a capacitance diaphragm gauge. At the end, the gas phase was analysed by infrared spectroscopy (FT-IR), by gas chromatography (GC) and by GC coupled with mass spectroscopy (GC-MS). The liquid phase was analysed by 1 H, 13 C, 19 F and 7 Li liquid NMR spectroscopy and by mass spectroscopy.

FT-IR spectra were recorded in a Nicolet 5700 spectrophotometer. 32 scans were accumulated for each spectrum (resolution 1 cm -1 ). A 25 m PORA BOND Q column was used to separate the products that were then analysed in the online GC-MS (Agilent GC 6850 MS 5975C). GC analyses were achieved on KCl alumina column.

Burning behaviour in fire conditions

The fire behaviour of the ILs and the electrolytes were determined using the Tewarson calorimeter also covered under the name of Fire Propagation Apparatus (FPA) by different standards, namely NFPA 287 [START_REF]Standard Test Methods for Measurement of Flammability of Materials in Cleanrooms Using a Fire Propagation Apparatus (FPA)[END_REF] and ISO 12136. [START_REF]Reaction to fire tests, Measurement of material properties using a fire propagation apparatus[END_REF] This equipment allows getting information about ignitability, fire propagation potential and thermal and chemical threats in fire condition. It has been described in detail by Brohez et al. [START_REF] Brohez | [END_REF]46 and has recently been used to investigate the combustion hazard profiles of several neat ILs. 47 The theoretical energies of combustion (lower heating values or LHV) were measured in an oxygen bomb calorimeter Model 1108P, Oxygen Combustion Bomb, Parr Instrument Co., Moline, Illinois.

In this work, approximately 55 g of sample were poured into a glass sample holder. An external heat flux of 50 kW.m -2 simulating fire environment or external thermal stress was set in operation by four infrared heaters in an air-flushed jacket. Thus, the sample was heated to a temperature where its vapours or flammable decomposition products can be ignited by an electric spark. The heat release rate (HRR) can be calculated by applying fire calorimetry laws, such as the oxygen consumption (OC) 48 or the carbon dioxide generation (CDG). [START_REF] Je | Physical Properties of Polymers Handbook[END_REF] The mass loss rates as well as the fire effluent concentrations were measured in situ. The related emission yields (mass ratio of pollutants per burnt product) were then deduced allowing an evaluation of fire toxicity issues. In order to maintain self-sustained combustion, after ignition the external flux was diminished to 25 kW.m -2 . 47

Results and discussion

Neat ILs and the corresponding electrolytes were studied under thermal abuse conditions such as overheating and combustion. The thermal stability was firstly determined by DSC and TGA studies. Then long term stability was studied by TGA, and a specific setup was used to analyse the decomposition products. Finally the electrolyte behaviour was tested in fire conditions, along with the identification and quantification of the emitted compounds.

DSC

The DSC results of lithium salt, ILs and their corresponding electrolytes are shown in ] the addition of lithium salt increased the height of the exothermic peak. These results suggest that the addition of LiNTf 2 lead to an improvement in thermal stability, which was not predictable from the decomposition temperatures of neat products.

Thermogravimetric analyses (TGA)

Because of the extremely low volatility of most ILs, their liquid range upper limit is defined by their decomposition temperature (T d ). [START_REF] Wasserscheid | Ionic liquids in synthesis. 1st Edition[END_REF] In the literature, the T d is generally obtained from thermogravimetric analysis (TGA). However, several parameters can vary and influence T d value, like the heating rate, the gas nature and flow rate, the nature of the pan, the apparatus and the purity of the sample. Hence results can be compared only if all the experimental parameters are the same. [START_REF] Maton | [END_REF]29,42,[START_REF] Del Sesto | [END_REF][51][52][53][54][55] Furthermore TGA reveals only degradation due to the formation of volatile species, implying that degradation occurring without mass loss is not visible. Typically, the onset temperature (T onset ) is used to define the stability of ILs, and is determined from the step tangent method. 29,34,41,42 Unfortunately, the values obtained from this method for a same IL can differ of more than 100 °C from one study to another, e.g. the

T d of [C 1 C 4 Im][NTf 2 ]
varies from 275 56 to 461 °C. 57 In literature, authors are now well aware of this when presenting TGA results and it is admitted that the thermal decomposition could occur well before T onset . 27

Dynamic TGA

The TGA spectra of

[C 1 C 4 Im][NTf 2 ], [PYR 14 ][NTf 2 ]
, LiNTf 2 and their corresponding electrolytes were reported in Figure 3, and the experimentally determined T start and T onset in Table 2. All these products were thermally stable up to 300 °C, in line with the literature. , and that this tendency was similar for the derived electrolytes. The use of T start was more relevant than T onset , in order to avoid misleading results. 60

Long term isothermal TGA

The dynamic TGA can be used as a screening method but long term stability is required for industrial applications. In order to afford information on the maximum operating temperature (T op ), isothermal long term TGA can be performed. Indeed T op is generally significantly lower than T d obtained from dynamic TGA and DSC experiments. [START_REF] Maton | [END_REF] For neat ILs and electrolytes, the temperatures between 300 and 400 °C corresponding to T start ± 50 °C were screened (Figures 4 and5). For all these products no detectable mass loss was observed at 300 °C after 4 h. ] in these analogous conditions (7.9 vs. 3.5 %.h -1 ). (Table 3)

Thermal treatment of electrolytes [C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ]

TGA reveals only decomposition due to the formation of volatile species, implying that degradation occurring without mass loss is not visible. Moreover, the temperature of 350 °C seemed appropriate to analyse the decomposition products as a mass loss comprised between 5 and 10 % was expected (Figure 5). So in this part, the long term stability (350 °C during 2 h) on larger quantities of electrolytes was studied. We aimed to quantify and characterise the volatiles and the residual liquid phase. From the TGA results vide supra, a measurable decomposition of the electrolytes occurred in these conditions.

3 mL of the solution of ILs containing 1 mol.L -1 of LiNTf 2 were introduced in a specific setup described in Figure SI-3, and dried under high vacuum (10 -5 mbar) overnight before the experiment. The reactor was closed and the solution was heated at 10 °C.min -1 heating rate, up to 350 °C. It was kept for 2 h at this temperature while the evolution of the pressure was measured. The gaseous phase was then analysed by IR, GC and GC-MS, and the residual liquid phase by solution NMR and by GC-MS.

Gas phase analysis

After 2 h the pressure levels of the gas phase were respectively 17 vs 50 mbars for

[C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ]
, with expected mass losses of 9 % and 6 % as determined from the long term TGA experiments. The results are shown in Figure 9 and the slopes of the curves were respectively 0.18 mbar.min -1 and 0.46 mbar.min -1 for

[C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ]
. The discrepancy between the two electrolytes was even higher (ratio between the slopes of 2.5) than the one found by long term TGA experiments (ratio of 1.7).

For both electrolytes the gas phases were mainly constituted of butene isomers, identified by GC and GC-MS as trans-butene (5.7 min), 1-butene (5.85 min) and cis-butene (6.01 min) (Figure 10). They could result from the cleavage of the butyl-N bond of the cationic ring.

Similarly (from the cleavage of the methyl-N bond), methane was detected at 1. The IR spectra of this gaseous phase (Figure SI-6) showed the characteristic bands of these alkanes and alkenes: ν(C-H sp3 ) at 2800-3000 cm -1 , to ν(C-H sp3 ) at 3038 cm -1 , to δ(C-H) at 1477 cm -1 and to ν(C=C) at 1649 cm -1 . The sharp peaks at 1376 and 1155 cm -1 could be assigned to ν(S=O) function.

Liquid phase analysis

After 2 h at 350 °C, the non-coloured transparent initial electrolytes turned to dark yellow to brown (Figure 11). No suspension or apparent increases of the viscosities were observed after the thermal treatment, and few changes were detected by 1 H and 13 C solution NMR (Figure SI-7).

The comparison of ESI + MS spectra of the electrolytes before and after thermal treatment at 350 °C indicated the presence of imidazolium and pyrrolidinium fragments in the thermally treated solutions (Table 4). These species resulted from alkyl chain cleavage or redistribution. [START_REF] Maton | [END_REF]57,63 In the ESI -MS spectra, fragments resulting from [NTf 2 ] fragmentation have been detected (Table 4). Note that the ESI + MS spectra of neat imidazolium and pyrrolidinium ILs treated at 220 °C for 5 h exhibited only fragments resulting from the cleavage of the hydrocarbon chain, and no species resulting from alkyl chain redistribution. 64 Here the redistribution showed a chemical evolution of the nature of the cation ring occurring at constant weight.

In conclusion, these different thermal experiments confirmed that [C 1 C 4 Im][NTf 2 ] was thermally more stable than [PYR 14 ][NTf 2 ], in agreement with the literature. [START_REF] Stefan | [END_REF][START_REF] Del Sesto | [END_REF] This result could be related to the presence of a strained five membered cycle in [PYR 14 ] cation inducing an easy ring opening. [START_REF] Del Sesto | [END_REF] LiNTf 2 exhibited a lower T d than both ILs (T start of 298 °C). However its dissolution into ILs had a positive effect on their thermal stability, especially in the case of

[PYR 14 ][NTf 2 ]
. This could be related to the fact that the dissolution of LiNTf 2 in an IL sharing the same anion [NTf 2 ] -induced a restructuration of the anions around [Li] + cations, resulting in an increase of the coordination number of [Li] + . Simulation and spectrometry results

proved that [Li] + were linked to two to four [NTf 2 ] -anions through strong oxygen -lithium bonds. [65][66][67] These [Li(NTf 2 ) n ] (n-1)-adducts 68,69 must have a higher thermal stability than solid LiNTf 2 . Note that the nature of the liquid and gas phases indicated that after 2 h both anion and cation started to decompose. Some of the characterised decomposition products were already suggested either theoretically 41 or experimentally. 47 . This unusual curve evolution observed for imidazolium-based ILs could be due to the presence of acidic C -H bonds of imidazolium cations, easily converted into NHC -carbenes under thermal or basic conditions. [73][START_REF] Nolan | N-Heterocyclic carbenes in synthesis[END_REF][START_REF] Olofson | [END_REF] This inherent reactivity could explain the presence of a second phase combustion and the acceleration of the thermal decomposition

of [C 1 C 4 Im][Li][NTf 2 ]
in the presence of lithium salt.

Toxicity of emissions

The combustion efficiency of a product can be estimated from the quantity of CO, total hydrocarbons (THCs) and soots. Indeed, lower combustion efficiency generally gives higher yields of these partially oxidised species. The combustion efficiencies of studied ILs was lower compared to carbonate solvents used in conventional lithium-ion batteries (close to 100 % in the case of fuel lean conditions, due to presence of oxygen atoms in the molecules). 6,76 The fire induced toxicity was also investigated in terms of emission factors of asphyxiant (HCN, CO) and irritant (NO x , SO 2 , and HF) gases. The emission factor is defined as the amount of pollutant emitted per mass unit of burned product. The amount of HCN released in the combustion of the electrolytes increased in comparison to the corresponding neat ILs (Table 6).

The combustion of ionic liquids was very specific to the nature of the anion and cation concerned. As an example, among the tested ionic liquids the formation of HCN and HF was found to be higher for DCA and NTf 2 anions respectively. 47 In addition, in the same study, heat release rates profiles were specific to each tested IL and were more versatile as compared The mass balance was also checked by performing combustion residues analysis of all tested samples (see SI-8). As can be seen, these results confirmed the mass conservation of hetero-atoms with high conversion efficiencies, which showed the smooth running of the apparatus.

Conclusion

Ionic These electrolytes exhibited good resistance to thermal stress, but fire induced toxicity can be an issue in case of an accidental fire scenario; thus it must be assessed on a case by case approach.

Scheme 1: Typical components of current electrolytes between 300 °C and 400 °C for 4 h; argon flow 30 mL.min -1 , aluminum sealed pans, 10 mg samples.

[C 1 C 4 Im][NTf 2 ] [C 1 C 4 Im][Li][NTf 2 ] [PYR 14 ][NTf 2 ] [PYR 14 ][Li][NTf 2 ]
LiNTf 2

Mass (%)

Temperature (°C)

[C 1 C 4 Im][NTf 2 ] [C 1 C 4 Im][Li][NTf 2 ] [PYR 14 ][NTf 2 ] [PYR 14 ][Li][NTf 2 ]
LiNTf 2

Figure 4:

0 1 2 3 4
Mass (%)

Time (h) [C 1 C 4 Im][NTf 2 ] 325 [C 1 C 4 Im][NTf 2 ] 350 [C 1 C 4 Im][NTf 2 ] 375 [C 1 C 4 Im][NTf 2 ] 400 [PYR 14 ][NTf 2 ] 300 [PYR 14 ][NTf 2 ] 325 [PYR 14 ][NTf 2 ] 350 [PYR 14 ][NTf 2 ] 375 [PYR 14 ][NTf 2 ] 400

Mass (%)

Time (h) Mass (%) Corresponding author: catherine.santini@univ-lyon1.fr

[C 1 C 4 Im][Li][NTf 2 ] 325 [C 1 C 4 Im][Li][NTf 2 ] 350 [C 1 C 4 Im][Li][NTf 2 ] 375 [C 1 C 4 Im][Li][NTf 2 ] 400 [PYR 14 ][Li][NTf 2 ] 300 [PYR 14 ][Li][NTf 2 ] 325 [PYR 14 ][Li][NTf 2 ] 350 [PYR 14 ][Li][NTf 2 ] 375 [PYR 14 ][Li][NTf 2 ] 400
Time (h) [C 1 C 4 Im][NTf 2 ] 325 [C 1 C 4 Im][Li][NTf 2 ] 325 [PYR 14 ][NTf 2 ] 325 [PYR 14 ][Li][NTf 2 ] 325 [C 1 C 4 Im][NTf 2 ] 400 [C 1 C 4 Im][Li][NTf 2 ] 400 [PYR 14 ][NTf 2 ] 400 [PYR 14 ][Li][NTf 2 ] 400
Time (h) [C 1 C 4 Im][NTf 2 ] at 250°C [PYR 14 ][NTf 2 ] at 250°C [C 1 C 4 Im][NTf 2 ] at 350°C [PYR 14 ][NTf 2 ] at 350°C Figure 8: [C 1 C 4 Im][NTf 2 ] [PYR 14 ][NTf 2 ] [C 1 C 4 Im][Li][NTf 2 ] [PYR 14 ][Li][NTf 2 ]

SI-1: Synthesis of the electrolytes

The 1-butyl-3-methylimidazolium chloride was synthetized following this procedure.

N-methylimidazole (99 %wt) was purchased from Sigma Aldrich and purified by distillation.

Butylchloride overnight before the experiment. A blank was collected for the Infrared (IR) spectra of the system using the connected IR cell. The entire system was put under vacuum, with 4 valves open. The reactor was then isolated closing valve 4. The solution was heated by a tubular oven at 10 °C.min -1 up to 350 °C. It was kept for 2 hours at this temperature while the pressure of the emitted gas was measured (by the pressure sensor). At the end of the two hours, valve 2 and 3 were closed and the system was cooled down to room temperature.

The system was disconnected from the vacuum line (at valve 4). The gaseous phase was analysed by infrared spectroscopy. Gas aliquots were taken up by closing valve 1 and introducing a syringe by the septum. To reproduce the analysis, the system was purged up to valve 1 (closing valve 1, open 4 th and 2 nd ), gas was brought to the antechamber by closing valve 2 and opening 1 and taken through the septum. The gas samples were analysed by GC and GC-MS. After the gas phase analysis, the schlenk was disconnected from the IR cell in a glove box, and the liquid phase was analysed by 1 H, 13 C, 19 F and 7 Li liquid NMR, and by coupled GC-MS.

SI

SI-8 Data validation from the Tewarson tests

It is a routine practice in the lab where the Tewarson apparatus is used to check validity of data from the calculation of mass balance regarding main elements contained in the test samples.

[

Carbon and sulphur balance examination

Carbon and sulphur recovery efficiencies from sample content into identified carbonated or S-containing species (in flue gas) or as C or S content (in solid residues) lied in between 84% and 103 % for carbon and 91 % to 101 % for sulphur respectively. This confirmed to our experience (several thousand test runs with the Tewarson Apparatus) that no special concern in the performance of the experiments and related data computations appeared. From dedicated analysis of fire calorimetry accuracy analysis, we know that, at lab scale, errors in yields of energy and product releases may be in the order of some 5 to 10%. Accuracy varies according to complexity of test samples (decreasing with complexity of structure and number of hetero-atom in test molecules) and operating conditions. Such error levels keep however quite reasonable for fire safety engineering purposes, owing to complexity of fire phenomena.

Fluorine balance examination

Observed conversion efficiency is thought reasonable, although classically lower than for carbon and sulphur mass balances. Although latest standards for the sampling and measurements are applied routinely (by use of ISO standards developed by ISO TC92 SC3 Committee: Fire Threat to People and the Environment), we are confronted here to potentially non identified species in the flue gas, or absorption issues in apparatus hood and ducting system and/or sampling line.

Figure

  Figure 2. All liquid samples remained stable up to 400 °C (using the start temperature) while

  31 min for both electrolytes. GC and GC-MS analyses evidenced the presence of traces of pentene isomers, ethane, ethene, propene, butane (Figure SI-4). Some differences in the nature of hydrocarbons were observed. This could be due to their origin, as for [C 1 C 4 Im][Li][NTf 2 ] hydrocarbons come only from the alkyl chains, but for [PYR 14 ][Li][NTf 2 ] they also come from the pyrrolidinium ring (Figure SI-5).[START_REF] Maton | [END_REF]51,53,62 Some compounds resulting from the anion decomposition such as CF 3 H (1.77) and HN(CF 3 )(SO 2 CF 3 ) (3.7 min) were also detected. Note that the gaseous phase was acidic (pH ~ 2).

  to conventional liquid fires. In our case the combustion of [PYR 14 ][Li][NTf 2 ] released less unburned carbon than [C 1 C 4 Im][Li][NTf 2 ]. Thus its amount of CO decreased as compared to parent ILs but still was higher than [C 1 C 4 Im][Li][NTf 2 ] (25.1 mg.g -1 vs. 21.6 mg.g -1 ). Also, the emission asphyxiant gases were very high for both electrolytes, in particular the value exceeded was close to 295 mg.g -1 of HF and 353 mg.g -1 of SO 2 for the [C 1 C 4 Im][Li][NTf 2 ]. It appears that the emission of flammable compounds was related to the cation nature, whereas the anion produces toxic species.

  liquids [C 1 C 4 Im][NTf 2 ] and [PYR 14 ][NTf 2 ] along with the electrolytes [C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ] showed high thermal stabilities. From dynamic DSC and TGA experiments, the products were stable up to 350 °C. Onset temperatures did not reveal any difference between the four solutions, but start temperatures (based on the derivative) showed that imidazolium cations form more stable ILs than pyrrolidinium. The addition of lithium salt clearly stabilised the solutions. Isotherm experiments at 350 °C confirmed and emphasised these trends. During the combustion, all the solutions were very weakly combustible, especially compared to the current electrolytes (at least five times more resistant to ignition). Lithium salt has controversial effect on thermal stability, as it acts as flame retardant for [PYR 14 ][Li][NTf 2 ] but facilitates the combustion with [C 1 C 4 Im][Li][NTf 2 ]. From a combustibility viewpoint, the addition of the lithium salt in the solutions decreased the overall complete heats of combustion. Flammable gases were emitted due to cation decomposition, and the anion decomposition formed toxic effluents.

Figure 1 :Figure 2 : 1 Figure 3 :Figure 4 :

 12134 Figure 1: Determination of the onset temperature (T onset ) and the start temperature (T start ) for [C 1 C 4 Im][NTf 2 ]; 5 °C.min -1 heating rate, argon flow 30 mL.min -1 , aluminum sealed pan, 10 mg samplesFigure2: DSC curves of [C 1 C 4 Im][NTf 2 ], [PYR 14 ][NTf 2 ], [C 1 C 4 Im][Li][NTf 2 ] , [PYR 14 ][Li][NTf 2 ]and LiNTf 2 ; 5 °C.min -1 heating rate, nitrogen flow 50 mL.min -1 Figure3:Dynamic TGA of [C 1 C 4 Im][NTf 2 ], [PYR 14 ][NTf 2 ], [C 1 C 4 Im][Li][NTf 2 ] , [PYR 14 ][Li][NTf 2 ], and LiNTf 2 ; 5 °C.min -1 heating rate from 30 to 500 °C, argon flow 30 mL.min -1 , aluminum sealed pan, 10 mg samples Figure 4: Isotherm thermogravimetric analyses of [C 1 C 4 Im][NTf 2 ] and [PYR 14 ][NTf 2 ] between 300 °C and 400 °C for 4 h; argon flow 30 mL.min -1 , aluminum sealed pans, 10 mg samples.Figure 5: Isotherm thermogravimetric analyses of [C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ] between 300 °C and 400 °C for 4 h; argon flow 30 mL.min -1 , aluminum sealed pans, 10 mg samples Figure 6: Isotherm thermogravimetric analyses of [C 1 C 4 Im][NTf 2 ], [PYR 14 ][NTf 2 ], [C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ] at 325 °C and 400 °C for 4 h; argon flow 30 mL.min -1 , aluminum sealed pans, 10 mg samples Figure 7: Long term thermogravimetric analyses of [C 1 C 4 Im][NTf 2 ] and [PYR 14 ][NTf 2 ] at 250 °C and 350 °C for 15 h; argon flow 30 mL.min -1 , aluminum sealed pans, 10 mg samples Figure 8: Long term thermogravimetric analyses of 4 solutions and lithium salt at 350 °C for 15 h; argon flow 30 mL.min -1 , aluminum sealed pans, 10 mg samples Figure 9: Evolution of the gas phase pressure during thermal treatment of [C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ] during 2 h at 350 °C Figure 10: Comparison of gas phase chromatograms of [C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ] from GC-MS apparatus Figure 11: Electrolytes before thermal treatment (left) and after 2 h at 350 °C: middle: [PYR 14 ][Li][NTf 2 ], right: [C 1 C 4 Im][Li][NTf 2 ] Figure 12: HRR profiles obtained by use of the Fire Propagation Apparatus (ISO:12136)

Figure 5 : 6 :Figure 7 :Figure 8 :Figure 9 :Figure 10 :Figure 11 :Figure 12 :

 56789101112 Figure 4: Isotherm thermogravimetric analyses of [C 1 C 4 Im][NTf 2 ] and [PYR 14 ][NTf 2 ] between 300 °C and 400 °C for 4 h; argon flow 30 mL.min -1 , aluminum sealed pans, 10 mg samples.Figure 5: Isotherm thermogravimetric analyses of [C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ] between 300 °C and 400 °C for 4 h; argon flow 30 mL.min -1 , aluminum sealed pans, 10 mg samples Figure 6: Isotherm thermogravimetric analyses of [C 1 C 4 Im][NTf 2 ], [PYR 14 ][NTf 2 ], [C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ] at 325 °C and 400 °C for 4 h; argon flow 30 mL.min -1 , aluminum sealed pans, 10 mg samples Figure 7: Long term thermogravimetric analyses of [C 1 C 4 Im][NTf 2 ] and [PYR 14 ][NTf 2 ] at 250 °C and 350 °C for 15 h; argon flow 30 mL.min -1 , aluminum sealed pans, 10 mg samples Figure 8: Long term thermogravimetric analyses of 4 solutions and lithium salt at 350 °C for 15 h; argon flow 30 mL.min -1 , aluminum sealed pans, 10 mg samples Figure 9: Evolution of the gas phase pressure during thermal treatment of [C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ] during 2 h at 350 °C Figure 10: Comparison of gas phase chromatograms of [C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ] from GC-MS apparatus Figure 11: Electrolytes before thermal treatment (left) and after 2 h at 350 °C: middle: [PYR 14 ][Li][NTf 2 ], right: [C 1 C 4 Im][Li][NTf 2 ] Figure 12: HRR profiles obtained by use of the Fire Propagation Apparatus (ISO:12136)

Figure 3

 3 Figure 3:

Figure 5

 5 Figure 5:

Figure 6

 6 Figure 6:

Figure 7

 7 Figure 7:

  Figure 9:

Figure 11

 11 Figure 11:

- 6 :SI- 7 : 13 C

 6713 Infrared spectra of electrolytes gas phase (after blank subtraction) Liquid phase study by 1 H, 13 C, 19 F and 7 Li solution NMR NMR spectra of [C 1 C 4 Im][Li][NTf 2 ] before (top) and after (bottom) thermal treatment

  13 MHz for 1 H, 75.48 MHz for 13 C, 282.38 MHz for 19 F and 116.64 MHz for

  29,51,58,59 The observed decomposition occurred in one step from 330 to 480 °C, leading to residual masses of16 % for [C 1 C 4 Im][NTf 2 ] and [C 1 C 4 Im][Li][NTf 2 ] , 5 % for [PYR 14 ][NTf 2 ], 8 % for [PYR 14 ][Li][NTf 2 ] and 12 % for LiNTf 2 salt.

In Table

2

, all ILs and electrolytes exhibit a T onset ∼ 428 ± 6 °C revealing no significant difference between them. The T d values determined by T start were inferior and showed that the neat [PYR 14 ][NTf 2 ] decomposed ∼ 100 °C before [C 1 C 4 Im][NTf 2 ] (277 vs. 360 °C). This trend was confirmed for their electrolytes, for which the T d difference was ∼ 20 °C (339 vs. 357 °C). LiNTf 2 was the least stable product, with a T onset of 369 °C and a T start of 298 °C. The addition of LiNTf 2 to the ILs did not have a significant effect on the T d (< 7 °C) except for T start of [PYR 14 ][NTf 2 ] (+ 60 °C). DSC and dynamic TGA results confirm that [C 1 C 4 Im][NTf 2 ] was more stable than [PYR 14 ][NTf 2 ]

  at 375 °C and 94 % after only 1 h at 400 °C. Obviously [C 1 C 4 Im][NTf 2 ] decomposed at higher temperatures and slower than [PYR 14 ][NTf 2 ]. For both electrolytes, whatever the temperature, the mass loss was lower than for the corresponding pure ILs (Figure6). Yet, at 400 °C, the mass loss reached 80 % in 170 min for [C 1 C 4 Im][Li][NTf 2 ] instead of 70 min for [C 1 C 4 Im][NTf 2 ], and 80 % in 150 min for [PYR 14 ][Li][NTf 2 ] instead of 30 min for [PYR 14 ][NTf 2 ]. Generally the mass loss rates were lower in the case of the electrolytes.

	Then longer isothermal TGA experiments were performed (15 h at 250 and 350 °C) on ionic
	liquids and electrolytes. At 250 °C no appreciable thermal decomposition of the neat ILs
	occurred even after 15 h (Figure 7). At 350 °C, a temperature significantly inferior to T onset , a
	significant mass loss was observed (Figure 8) as reported for other neat ILs. 61 For
	[C 1 C

For [C 1 C 4 Im][NTf 2 ] the mass loss was 14 % after 4 h at 350 °C. At 375 and 400 °C, the mass decreased faster to reach mass loss of 46 and 89 % respectively in 4 h. For [PYR 14 ][NTf 2 ], a fast decrease was observed after 4 h for temperatures superior to 325 °C, to reach mass loss of 17 % at 325 °C, 30 % at 350 °C, 52 % 4 Im][NTf 2 ] and [C 1 C 4 Im][Li][NTf 2 ] a similar monotonous decrease was observed, with respective slopes of 3,5 and 3,3 %.h -1 , and reaching respectively 50 and 47 % mass loss. For [PYR 14 ][NTf 2 ]

the decomposition occurred at two different rates, the first until 8 h at 7.9 %.h -1 reaching 60 % mass loss, and a second with a slope of 2.8 %.h -1 affording a mass loss of 86 %. For [PYR 14 ][Li][NTf 2 ], the same evolution was apparent but with lower rates.

The first curve had a slope of 5.5 %.h -1 corresponding to a mass loss of 43 %, and the second one had a slope of 3.0 %.h -1 yielding residual mass of 68 %. The highest stability of imidazolium-based solutions compared to pyrrolidinium was confirmed and emphasised. Indeed [PYR 14 ][NTf 2 ] decomposed two times faster than [C 1 C 4 Im][NTf 2

62, 70, 71 3.4. Fire Propagation Apparatus tests 3.4.1. Heat release rate (HRR) profiles

  PYR 14 ][NTf 2 ] were respectively 5.9 min and 4.6 min, after application of the mentioned external heat flux. These values, compared to phosphonium based IL [P 6,6,6,14 ][NTf 2 ] (ignition delay of 4.5 min),47 highlighted the highest resistance to the ignition of [C 1 C 4 Im][NTf 2 ], and the role of the cation in flammability. The combustion behaviour of ILs was also really specific to the nature of anions.47,72 As an example, for a same imidazolium-based cation, a

	general trend of NTf 2 > OTf > BF 4 > MeSO 3 > EtOSO 3 > DCA was observed when looking
	which anion released the highest heat.
	The ignition delay of [PYR 14 ][NTf 2 ] was really high compared to flammable carbonates
	mixtures ones (commercial solvents for electrolytes), that were inferior to 50 sec, with half
	the value of external heat flux (25 kW.m -2 ). 6 But [PYR 14 ][NTf 2 ] ignition delay was shorter as
	compared to [C 1 C 4 Im][NTf 2 ].
	secondary peak around time equal 7.6 min and reaching 775 kW.m -2 was observed, resulting
	in accelerated second phase combustion. It revealed that the combustion of
	[C 1 C 4 Im][Li][NTf 2 ] leads to combustible decomposition products. A similar peak of
	The results of combustion experiments performed in the Tewarson calorimeter on the ILs 475 kW.m -2 was observed for the neat [C 1 C 4 Im][NTf 2 ] near 9.3 min. Furthermore the
	[C 1 C 4 Im][NTf 2 ] and [PYR 14 ][NTf 2 ] and on the electrolytes [C 1 C 4 Im][Li][NTf 2 ] and combustion of [C 1 C 2 Im] cation associated to different anions such as DCA, OTf, MeSO 3 or
	[PYR 14 ][Li][NTf 2 ] are summarised in Table 5, and the heat release rate profiles are shown in EtOSO 3 exhibit the same heat flow profile the same profile (appearance of a peak in the
	Figure 12. The theoretical energy of combustion of LiNTf 2 confirmed that the salt was not reaction medium)

combustible as the gross heat of combustion obtained was 2.69 MJ.kg -1 . The neat ILs and their corresponding electrolytes showed good resistance to ignition under an external heat flux of 50 kW.m -2 , simulating fire conditions. The ignition delays of [C 1 C 4 Im][NTf 2 ] and [The ignition times of the electrolytes [C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ] were 4.9 min and 5.5 min respectively. It indicated that the addition of the Li salt acts as a flame retardant for the [PYR 14 ][Li][NTf 2 ]. Contrarily it facilitates the combustion with [C 1 C 4 Im][Li][NTf 2 ]. From a combustibility viewpoint, the addition of the lithium salt in the solutions decreased the overall complete heats of combustion from some 19 % in both cases (as determined from data in Table 5). These results were quite in line with overall decrease of theoretical heating value available in the mixtures. The drop in effective energy release was less important in the case of [C 1 C 4 Im][NTf 2 ] (23 %) as compared to [PYR 14 ][NTf 2 ] (35 %). The heat release rate profiles were quite similar for [PYR 14 ][NTf 2 ] and [PYR 14 ][Li][NTf 2 ] (with a peak of 475 kW.m -2 and 425 kW.m -2 , respectively), showing a plateau throughout the combustion. By contrast, in the case of [C 1 C 4 Im][NTf 2 ], the addition of the salt significantly modified the corresponding HRR profile after the first intermediate peak. Indeed a sharp

Table 1 :

 1 Start and onset temperatures with heats of decomposition of lithium salt, ionic liquids and electrolytes as determined from DSC experimentsProduct [C 1 C 4 Im][NTf 2 ] [PYR 14 ][NTf 2 ] [C 1 C 4 Im][Li][NTf 2 ] [PYR 14 ][Li][NTf 2 ] LiNTf 2

	T onset	451	451	456	452	395
	(°C)
	T start	425	403	420	400	355
	(°C)
	Heat	1644	1579	1406	1470	854
	(J.g -1 )

Table 2 :

 2 Onset and start temperatures for electrolyte components decomposition as determined from dynamic TGA experiments

	Product [C 1 C 4 Im][NTf 2 ] [PYR 14 ][NTf 2 ] [C 1 C 4 Im][Li][NTf 2 ] [PYR 14 ][Li][NTf 2 ] LiNTf 2
	T onset (°C)	427	423	434	427	369
	T start (°C)	360	277	357	339	298

Table 3 :

 3 Slopes of mass loss curves in % per h between 2 and 10 h at 350 °C. * slope during 30 first minutes then during 8 last hours

	Product	Mass loss rate (%/h)	R²
	[C 1 C 4 Im][NTf 2 ]	3,54503	0,99984
	[PYR 14 ][NTf 2 ]	7,77356	0,99894
	[C 1 C 4 Im][Li][NTf 2 ]	3,33617	0,99994
	[PYR 14 ][Li][NTf 2 ]	5,61302	0,99962
	LiNTf 2	75 then 0.98 *	0.992 then 0.991*

Table 4 :

 4 Cationic and anionic fragments in the liquid phase identified by MS

	Type of decomposition	Imidazolium fragments	Pyrrolidinium fragments
	Alkyl chain removal
	Redistribution
	Anion fragments

Table 5 :

 5 Burning behaviour of [C 1 C 4 Im][NTf 2 ], [PYR 14 ][NTf 2 ], [C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ] [C 1 C 4 Im][NTf 2 ] [PYR 14 ][NTf 2 ] [C 1 C 4 Im][Li][NTf 2 ] [PYR 14 ][Li][NTf 2 ]

	External flux (kW.m -²)	50 then 25	50 then 25	50 then 25	50 then 25
	Initial mass (g)	59.3	51.3	61.9	65.1
	Mass lost (%)	85.5	94.3	85.1	92.6
	Time to ignition (s)	357	280	294	330
	Average mass
	loss rate	38	24	46.6	36
	(g.m -².s -1 )
		Heat of combustion (MJ.kg -1 )
	Lower heating value (LHV)	13.3	14.8	10.78	12.05
	Effective heat of
	combustion (fire calorimetry	10.0	12.6	7.7	8.2
	techniques)
	Energy efficiency (%)	75.2	85.1	71.4	65.6

Table 6 :

 6 Emission factors of different gases emitted during combustion of neat ILs and corresponding electrolytes

	Emission factor (mg.g -1 )	[C 1 C 4 Im][NTf 2 ] [PYR 14 ][NTf 2 ] [C 1 C 4 Im][Li][NTf 2 ] [PYR 14 ][Li][NTf 2 ]
	CO 2	746	875	552	531
	CO	12.6	29.6	21.6	25.1
	Soot	9.7	43.9	11	37.7
	THCs	3.3	11.2	4.8	0.6
	SO 2	313	310	353	317
	NO	6.9	5.1	4.9	3.0
	N 2 O	1.3	2.2	1.9	1.3
	HF	246.1	229.5	294.8	216.6
	HCN	2.8	6.7	6.9	8.3

NMR characterisation of the ILs [C 1 C 4 Im][NTf 2 ]

  The structure was checked by1 H and13 C liquid NMR Bruker Avance 300 with probe BBO 5 mm at 27 °C, using deuterated dichloromethane as solvent. Standard 5 mm borosilicate NMR tubes were used. Chemical shifts were reported in ppm (singlet = s, doublet = d, and multiplet = m). Impurity levels of the ILs were then measured. Their water content was lower than 60 ppm (mass ratio), as assessed by Karl Fisher coulometric titration. The chloride content was lower than 0.5 % as shown by mass spectroscopy.The electrolytes were prepared by adding LiNTf 2 (1 mol.L -1 ) in a well stirred and dried IL at room temperature, and will be referred to as[C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ].Finally the neat ILs and electrolytes were vacuum-dried at room temperature for 48 h under 10 -5 mbars followed by storage in an argon-filled glove box.

	SI-2: 1 H NMR
	δ = 0.990 (t, 3H, N-(CH 2 ) 3 -CH 3 ), 1.395 (m, 1.99H, N-(CH 2 ) 2 -CH 2 -CH 3 ), 1.880 (m, 2.00H, N-
	CH 2 -CH 2 -), 3.949 (s, 2.98H, N-CH 3 ), 4.190 (t, 2.03H, N-CH 2 -), 5.356 (s, CD 2 Cl 2 ), 7.339 (s,
	1.93H, N-CH=CH-N), 8.640 (s, 0.97H, N=CH-N)
	13 C NMR
	(1.3 equivalent, Sigma Aldrich, 99 %wt) and N-methylimidazole (1 equivalent) δ=13.321 (N-(CH 2 ) 3 -CH 3 ), 19.649 (N-(CH 2 ) 2 -CH 2 -CH 3 ), 32.220 (N-CH 2 -CH 2 -), 36.601 (N-
	were mixed without solvent at 70 °C during 3 days. The resulting IL was a white solid and CH 3 ), 50.301 (N-CH 2 -), 53.867 (m, CD 2 Cl 2 ), 120.221 (m, N-(SO 2 CF 3 ) 2 ), 122.760 (C4H),
	was washed and recrystallized in toluene at 0 °C. Finally, this [C 1 C 4 Im][Cl] was dried under 124.101 (C5H), 136.079 (N=CH-N)
	primary vacuum at ambient temperature for 24 h. The chemical structure and the purity of the
	[C 1 C 4 Im][Cl] were checked by 1 H and 13 C NMR. The 1-butyl-1-methylpyrrolidinium chloride
	(98 %) was purchased from Solvionic and used as received.
	The NTf 2 -based ILs were prepared by mixing 1 equivalent of the chloride IL with
	1.1 equivalent of lithium bis(trifluoromethanesulfonylimide) (LiNTf 2 ) salt (Solvionic,
	99.5 %wt, dried at 120 °C under high vacuum for 48 h) in water at ambient temperature for
	24 hours. After several extractions with dichloromethane (Aldrich, 99.8 %), the absence of
	chloride anion was tested using silver nitrate. It was then filtered through silica, and the
	solvent was evaporated under vacuum. The NTf 2 -based ILs [C 1 C 4 Im][NTf 2 ] and

[PYR 14 ][NTf 2 ] were vacuum-dried at room temperature for 48 h under 10 -5 mbars followed by a storage in an argon-filled glove box.

[PYR 14 ][NTf 2 ]

	1 H NMR
	δ = 1.013 (t, 3H, N-(CH 2 ) 3 -CH 3 ), 1.4375 (m, 2.01H, N-(CH 2 ) 2 -CH 2 -CH 3 ), 1.770 (m, 2.21H,
	N-CH 2 -CH 2 -), 2.268 (m, 4H, N-CH 2 -CH 2 -CH 2 -CH 2 -N), 3.036 (s, 3.01H, N-CH 3 ), 3.298 (m,
	2.04H, N-CH 2 -), 3.506 (m, 4.05H, N-CH 2 -CH 2 -CH 2 -CH 2 -N), 5.368 (s, CD 2 Cl 2 )
	13 C NMR 1
	δ=13.674 (N-(CH 2 ) 3 -CH 3 ), 19.908 (N-(CH 2 ) 2 -CH 2 -CH 3 ), 21.895 (N-CH 2 -CH 2 -CH 2 -CH 2 -N),
	26.037 (N-CH

2 -CH 2 -), 48.877 (N-CH 2 ), 53.871 (m, CD 2 Cl 2 ), 65.053 (N-CH 3 and N-CH 2 -CH 2 -CH 2 -CH 2 -N), 120.255 (quadruplet, N-(SO 2 CF 3 ) 2 ) SI-

3: Setup to analyse the decomposition products formed during thermal treatment of the electrolytes [C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ] during 2 h at 350 °C In

  the glove box, 3 mL of the electrolytes [C 1 C 4 Im][Li][NTf 2 ] and [PYR 14 ][Li][NTf 2 ] were introduced in an apparatus depicted below. They were dried under high vacuum (10 -5 mbar)

  C 1 C 4 Im][NTf 2 ] [PYR 14 ][NTf 2 ] [C 1 C 4 Im][Li][NTf 2 ] [PYR 14 ][Li][NTf 2 ]

	Initial mass (g)	59,3	51,3	61,9	65,1
	Residual mass (g)	8,6	2,9	9,2	4,8
			% of element in the residue
	Carbon	49,8	60,9	50,1	55,6
	Fluorine	8,7	9,9	11,1	10,1
	Sulphur	5,5	7,1	4,4	3,2
			Mass of residue (g)
	Carbon	4,28	1,77	4,61	2,67
	Fluorine	0,75	0,29	1,02	0,48
	Sulphur	0,47	0,21	0,40	0,15
		Conversion efficiency without residue (%)
	Carbon	66	92,5	59,1	69,3
	Fluorine	73,5	76,1	81,3	65,6
	Sulphur	86,9	98,2	91,8	89,7
		Conversion efficiency with residue (%)
	Carbon	91,5	103,6	88,6	84,3
	Fluorine	78,2	78,2	86,9	68,1
	Sulphur	92,1	100,9	95,8	91,1

Physical Chemistry Chemical Physics

Acknowledgements

The authors wish to thank O. Boyron for the help with the TGA measurements and K. Szeto for the GC-MS experiments from C2P2. We thank F. Albrieux, C. Duchamp and N. Henriques, from the Centre Commun de Spectrométrie de Masse (CCSM) of Université Lyon 1, for the assistance and access to the mass spectrometry facility. Special thanks are addressed to Jean-Pierre Bertrand for having conducted the experiments on the Tewarson calorimeter. This work was performed, in partnership with the SAS PIVERT, within the frame of the French Institute of Excellence in the field of Low-Carbon Energies (IEED) P.I.V.E.R.T. (www.institut-pivert.com) selected as an Investment for the Future ("Investissements d'Avenir"). This work was supported, as part of the Investments for the Future, by the French Government under the reference ANR-001.

Major improvement achieved by increased resistance to ignition when replacing carbonates by ionic liquids as electrolyte solvents


Valentina Carbone
email: vcarbone@escpeurope.eu

Corinne Blanquart
email: corinne.blanquart@ifsttar.fr

Examining Collaborative Green Supply Chains Through the Lens of Proximity Economics

Keywords: Collaborative practices, environment, proximity

The aim of this paper is to examine collaborative supply chain practices and their related ways of integrating an environmental dimension. Seven case studies were analysed relating to different stages of the supply chain (supply side, forward logistics, reverse logistics). We draw on "proximity economics", a theoretical framework at the crossroads of spatial and industrial economics. Starting with a sample of (alleged) green collaborative initiatives taken from the specialised press in the SC field, and further studied through in-depth interviews, a typology of collaborative practices was developed and characterised in terms of forms of proximity. This typology enabled us to identify the preconditions for environmental collaborative practices in terms of the forms of proximity among the actors involved.

Introduction

Integrating the environmental dimension into supply chain (SC) management can take different forms, beyond the traditional approach of a modal shift from highly polluting transport modes (road transport) to cleaner ones. Following [START_REF] Srivastava | Green supply-chain management: a state-of-the-art literature review[END_REF], green supplychain management has been defined as 'integrating environmental thinking into supply chain management, including product design, material sourcing and selection, manufacturing processes, delivery of the final product to consumers, and end-of-life management of the product after its useful life'.

Increasingly, collaborative practices are evoked in the specialised press as promising avenues for integrating the environmental dimension in the supply chain. In the past few decades, scholars have acknowledged the role of inter-organisational initiatives for the greening of the supply chain [START_REF] Walton | The Green Supply Chain: Integrating Suppliers into Environmental Management Processes[END_REF]. Their importance has further increased recently as "the company-internal view on corporate environmental issues is being challenged as customers and legislation alike broaden a company's environmental responsibility to include organisations upstream as well as downstream in the supply chain" [START_REF] Kovacs | Corporate environmental responsibility in the supply chain[END_REF]. Several scholars have shown the possibility of effectively combining the introduction of collaborative practices and the pursuit of an environmentally friendly supply chain, in particular with regard to the following SC stages: procurement, distribution, and reverse logistics [START_REF] Preuss | In dirty chains? Purchasing and greener manufacturing[END_REF][START_REF] Bowen | Horses for courses: explaining the gap between the theory and practice of green supply[END_REF][START_REF] Sarkis | Greening the supply chain[END_REF][START_REF] Brito | Towards a sustainable fashion retail supply chain in Europe: organisation and performance[END_REF].

Others have mainly emphasised the increasing number of green collaborative practices among actors belonging to the same SC segments, such as two manufacturers competing in the same marketplace or two logistics service providers sharing transport and logistics facilities and solutions [START_REF] Cruijssen | Horizontal cooperation in transport and logistics[END_REF]. In France, and particularly in the textile and clothing industry, the Durlog project findings [START_REF] Blanquart | La place du transport dans les organisations logistiques durables[END_REF] confirm that companies initiate environmentally friendly supply chains as much in an individual way, through transport modal shift, as in a collaborative way, via voluntary sharing of logistics assets and transport modes. Other research [START_REF] Roure | Logistique collaborative: une question d'avenir[END_REF] highlights the short and long-term environmental advantages of collaborative practices, with emphasis placed on collaborative freight transport initiatives [START_REF] Pan | The reduction of greenhouse gas emissions from freight transport by merging supply chains[END_REF].

In reality, however, in many cases, companies proclaim their concern for the environment without actually implementing it at the supply chain level. Such an attitude opens the door to criticism. Many companies are accused of 'greenwashing', i.e. green communication rather than green action [START_REF] Carbone | Towards greener supply chains: an institutional perspective[END_REF].

In the face of such rising concerns and scepticism of green collaborative SC practices, the aim of this paper is twofold: first, it aims to critically analyse alleged green collaborative practices, in order to establish whether some environmental action has actually followed the communication campaign about the initiative; second, it aims to examine the diverse forms of collaborative practices and their related ways of integrating an environmental dimension, in order to identify the different preconditions for greening the supply chain through collaborative initiatives.

The initial set of cases to study was selected by screening the specialised supply chain press. Articles mentioning collaborative initiatives both between actors at different stages of the SC (vertical collaboration) and between actors at the same stage in the chain (horizontal collaboration) were chosen. Face-to-face interviews were then conducted with the actors involved in the collaborations. We drew on "proximity economics", a theoretical framework at the crossroads of spatial and industrial economics. First, a typology of the collaborative practices was developed and characterised in terms of forms of proximity. Then, using this typology, we identified the different preconditions for environmental collaborative practices, leading to a more nuanced comprehension of the link between collaborative practices and the environmental dimension.

Collaborative practices: how to define and analyse them

We adopted a definition of "collaborative supply chain" with the following characteristics "Two or more independent companies working jointly to plan and execute SC operations with greater success than when acting in isolation" [START_REF] Simatupang | The Collaborative Supply Chain[END_REF]. There is agreement in the literature about the dynamic nature of collaborative practices which have been shown to evolve through time both in terms of scope and governance structures [START_REF] Lambert | Developing and Implementing Supply Chain Partnerships[END_REF][START_REF] Fawcett | A three stage implementation model for supply chain collaboration[END_REF]. [START_REF] Simatupang | The Collaborative Supply Chain[END_REF] show that collaborative practices are shaped over time in a life cycle running from engagement to (eventually) disengagement. Such a life cycle covers four main processes: (1) the engagement process aiming at identifying the rationales for a collaboration and finding the right partners;

(2) contract deployment, i.e. forward-looking planning to manage the interdependencies of resources, tasks and capabilities; (3) the implementation process through daily operations; and finally (4) the evaluation process to assess the collaborative practices and decide on the next steps of the agreement.

However, we assume that in addition to the "time" dimension there needs to be a "space" dimension to effectively characterise the relationships among the actors involved in the collaborative practice. Such an assumption is derived from the "proximity dynamics" literature stream, which deals with the coordination of economic and social activities by explicitly integrating their spatial dimension and emphasising the plural nature of the forms of proximity [START_REF] Pecqueur | L'économie de proximités[END_REF][START_REF] Rallet | Choix de proximité et processus d'innovation technologique[END_REF][START_REF] Torre | Economie de la Proximité et Activités Agricoles et Agroalimentaires[END_REF], involving intellectual and cultural proximity between individuals, the use of tools to connect physically distant actors (telephone, internet, etc.), or actors having fairly close ties with each other through common networks.

Specifically, the plural notion of proximity comprises three different dimensions:

Geographical proximity, which refers to the physical distance between the actors, due to their different locations and areas of activity; Organisational proximity, concerning economic interactions between actors with complementary resources and participating in a same finalised activity through an inter-organisational arrangement; Institutional proximity, which is based on adhering to a common system of values and beliefs [START_REF] Dupuy | Entreprises et territoires; Les nouveaux enjeux de la proximité[END_REF], as a result of repeated interactions, either of a business or of a social nature.

Drawing on the proximity dynamics literature, we conducted a comprehensive study of seven cases to better characterise the interactions within (alleged green) collaborative practices. All together, ten different cases of collaborative initiatives in the SC showing a clear environmental orientation were identified in the specialised SC press. An effort to establish direct contact with the actors involved was done by email and telephone. We managed to make contact with seven of the initial cases, relating to different stages of the supply chain (supply side, forward logistics, reverse logistics) and analysed them in detail, extending the initial contact to other partners. Consequently, the distinctive feature of our approach is to have involved most of the players participating in the collaboration, thus going beyond the limitations of a dyadic analysis applied to multi-actor inter-organisational arrangements. We carried out 23 semi-structured interviews (total length of the interviews: 29 hours) as many as the number of actors involved in the collaborative initiatives (cf. Table1). The interviews allowed us to compare different representations of the collaboration features and individual perceptions of the importance given to environmental issues, beyond what had been diffused through the press. All the interviewees belonging to the same initiative had direct contact with each other and had participated in the collaborative project since its engagement phase. The interview guidelines (available on request) contained two different sections: the first aiming at describing the collaborative practice (actors involved, resources, activities, length of the contractual arrangement, perception of the rationales and performance of the collaboration), and the second aiming at analysing the environmental dimension (role of the environmental issue, objectives, results and monitoring system specific to the environmental dimension).

Case

We analysed the "time" [START_REF] Simatupang | The Collaborative Supply Chain[END_REF] and "space" dimensions of the inter-organisational interactions within each case, defining a typology of collaborative practices. Using this typology, we were able to identify the specific preconditions for the inclusion of the environmental dimension within the collaborative initiative, in terms of different forms of interactions between actors.

In line with previous research [START_REF] Mckinnon | Analysis of Transport Efficiency in the UK Food Supply Chain[END_REF], the different types of collaborative practices differ in terms of the following criteria: their belonging to the same or to different SC (vertical collaboration or horizontal collaboration) and the extent to which they are complementary or in competition for products and services. Four "types" of collaborative practices are described by our life-cycle analysis framework and characterised by different combinations of proximity types (geographical, organisational, and institutional).

A typology of SC collaborative practices

From our analysis of the cases we were able to identify different types of collaborative practices (Figure 1), which share similar "time" and "space" dimensions. These cases also reveal different ways of taking the environmental dimension into account in collaborative SC initiatives. This supports the idea that the answer to the environmental challenge is neither univocal, nor simplistic and is related to different forms of interaction between actors. It is thus possible to illustrate different stories of green collaborative SC initiatives, each of them associated with one or more priority forms of proximity.

Our typology of SC collaborative practices, to which we will later add the time, space and green dimensions, comprises the following types: a) Type "A" includes two horizontal collaboration initiatives (i.e. among actors belonging to different supply chains) in which several producers of consumer goods, faced with increasing logistics constraints imposed by large retailers, switch from dedicated distribution networks to shared logistics solutions, and where the common physical parts of the supply chain are means of transport and warehouses. These two initiatives rely on flow consolidation for compatible finished products in terms of common departure and destination points for final delivery to retailers. A neutral logistics service provider coordinates the consolidation scheme, thus enabling them to overcome the operational constraints and challenges of the "co-opetition" arrangement set up by the producers [START_REF] Bengtsson | Coopetition in Business Networks -to Cooperate and Compete Simultaneously[END_REF]. Very limited sharing of information between producers is needed. A specific and detailed contract is put in place to handle governance issues.

b) The common feature in type "B", in which there are also still two horizontal collaborative initiatives as well as a vertical collaborative initiative, is the proactive strategy of the actors, aiming either at taking advantage of the increasing retailing constraints, or at meeting the constant innovation requirements of some markets. The vertical collaboration concerns the co-design of new products by a specialised brandretailer and its supplier, whereas in the horizontal collaboration the shared logistics solution (cf. type A) is associated with a Vendor Managed Inventory approach (the retailer entrusts the management of its store inventories to the suppliers who had decided to consolidate their deliveries). In type "B", partners belong to a single chain or distribution channel. However, unlike previous cases, their products or services are complementary, allowing them to extend the scope of the collaboration. Accordingly, the essential difference with the first type is the greater degree of information sharing between partners. c) Types "C" and "D" only have one case each. Type "C" refers to the creation of an eco-organisation in order to cope with the logistics complexity brought on by an update of Waste Electrical and Electronic Equipment Directive (WEEE), which came into force in January 2007. In this case, an inter-organisational arrangement between the main actors of the industry entrusts a logistics service provider with the management of return flows on a Europe-wide basis. d) Finally, type "D" refers to a flexible associative structure, with variable scope, including companies, academic and local institutions belonging to different domains and with complementary competencies. This inter-organisational arrangement stands as an innovative model for the provision of new services in a specific territory. The involvement of institutional partners contributes to the project with the aim of promoting local development The "time" and the "green" dimensions in the collaborative practices Each type of collaborative initiative can be analysed in relation to its life cycle (cf. Table 2), which spans from engagement to disengagement and, as previously stated, covers four main processes: engagement, management of interdependencies, implementation and finally evaluation [START_REF] Simatupang | The Collaborative Supply Chain[END_REF]. During the interviews, our analysis of the different steps of the collaboration life cycle was conducted without explicitly mentioning the environmental dimension, so that the interviewees were free to mention (or not) such a dimension in any of the life cycle steps. This was done with the aim of seeing whether there is any validity to the claim made in the press that an environmental dimension is present within collaborative practices. As is evident in the following table summarising the different life cycle steps within each type of collaborative practice, the environmental dimension was not a key element in each case. In type A initiatives, started as a defensive approach to a market or channel constraint, the environment seems to be taken into account only as a "by-product" of the optimisation objective aimed at reducing costs and optimising flows (lean-green natural convergence).

Belonging to the same SC

Belonging to a different SC Type B initiatives can be considered to have built their raison d'être on the environmental concern, proactively, from the beginning of the relationship and with a clear long-term objective. In type C, we are faced with a kind of paradoxical situation: on the one hand the substance of the initiative concerns reverse flows for recycling (green economy), thus emphasizing the environmental dimension, but on the other hand, this is done exclusively to reduce the overall costs of the regulatory obligation introduced by new legislation. None of the interviewees foresees any green strategic orientation for the initiative in the long term as far as process management is concerned.

Finally, type D does not seem to cover a sufficiently developed environmental dimension, probably due to its young age. The interviewees even seemed surprised that the press article mentioned such a dimension. Accordingly, we decided to drop this case in the following effort to identify the preconditions for an effective inclusion of the environmental dimension.

We are then confronted with four different shades of (alleged) green practices: from a lack of green (type D) to increasing levels of environmental engagement, going from type C to type A and finally type B, which represents the most substantial type of environmentally friendly collaborative practice. We find sufficient evidence to place under scrutiny the very optimistic message emerging from the specialised press on green collaborative practices. Some of these articles show that the aim of certain companies is pure window dressing, lending credence to scepticism about such initiatives.

The "space" and the "green" dimensions in the collaborative practices Each type of collaborative practice is based on different forms of proximity (cf. Figure 1).

For type A, the interactions between the actors rest on various forms of geographical and organizational proximity, which in turn generate indirect positive environmental impacts. On the one hand, interdependencies between partners are made possible by their spatial proximity. The three initial partners had warehouses located close to each other, which initially made it possible to use the "multipick" system, and the choice of the last partner depended on the very close location of its warehouse to the one jointly selected by the initial partners. On the other hand, in terms of organisational proximity, all the partners had to meet a certain number of conditions: common points of delivery, compatible frequencies, compatible sizes of batches, as well as a common logistics service provider. The interviewees in type A cases mentioned the importance of geographical proximity for setting up horizontal collaboration, which requires traditional competitors to share warehouses, distribution centres and transport means for freight consolidation. Geographical proximity is needed to avoid detours that will weaken the consolidation scale effect [START_REF] Ballot | Reducing greenhouse gas emissions through the collaboration of supply chains: lessons from French retail chains, Production[END_REF]. Freight consolidation is also able to produce positive environmental impacts. Environmental concern is not one of the triggers of the collaborative initiative, but it is one of its indirect effects, in particular in terms of the reduction of CO 2 emissions. Although the initial aim of the partners is to reduce their economic and financial costs, they declare: "We assume that if savings are realised, then it will also generate a positive environmental impact". Increasingly, manufacturers and logistics service providers acknowledge the benefit of integrating an environmental dimension in their freight organisation, both to reduce emissions and improve the "green" image of their business. The natural convergence between transport optimisation and green orientation is one of the most promising triggering factors for the enlargement of horizontal collaboration in the SC. As for the environmental impact, one of the collaborative freight consolidation projects made it possible to halve the number of trucks on the road compared to the former situation, (292 000 km and 241 tons of CO 2 saved). However these results are to be nuanced. One of the partners declared that "the environmental impact of freight consolidation is however a very meagre result… in relation to the 4.6 million km/year for all our flows".

In the case of type B, the collaboration seeks to take advantage of the constraints and specificities of the distribution channel and is built on a combination of the three forms of proximity: geographical, organisational, and also institutional. An "optimum" shared warehouse had to be defined. One of the partners was already storing its goods at the selected site and the other actors were able to transfer their stock to the new warehouse. As for organisational proximity, the same compatibility criteria as found in type A were present. Moreover, the partners either share a common history or common values: in one case, two companies were taken over by the same group; in another case, two logistics managers from two different companies had worked for the same former employer; in the third case, the founders of the two partner companies were both oriented toward environmental values, both in their private and business lives. Concerning this third type B case, we observe that a high level of institutional proximity is the main precondition for a strong environmental orientation. In this case, of the three forms of proximity, the collaboration mainly concerns institutional proximity, in the sense that partners have shared a whole set of common values and beliefs for a long time, which facilitates their collaborative initiative as supplier and customer involved in the co-design of new products. An environmental orientation is indeed one of the core values of these two companies. Consequently, their customer-supplier relationship is nourished by the same orientation, also because its objective is to design products that comply with stringent environmental criteria concerning the production process, the type of raw materials to be used (biological agriculture and fair trade), and the quality and traceability of the products. The supplier's benefits derive from the visibility gained over the year on orders from its customer; this enables the supplier to optimise its own production and supply scheduling. Other advantages are as follows: its customer provides support for the purchase of environmentally friendly raw materials as well as a guarantee of favourable payment conditions. Thanks to this collaboration, the supplier has developed a carbon footprint assessment project, relying on the customer's expertise in this field. The supplier has actively contributed to the development of a software application that makes it possible to obtain an overall assessment of the ecological footprint of a product following a lifecycle approach, simplified in five stages (raw materials, manufacture, distribution, use and end of life). The customer is progressively extending the use of this software to other privileged suppliers to assess the ecological footprint of existing products and, in two years, it aims to apply this screening tool as a preliminary step for the introduction of any new product. The application provides a detailed picture of each supplier (structure, environmental and social dimensions, assets, certifications, etc.) and of each product, through an evaluation of its social and environmental performance. It consists of a multiple-choice questionnaire adapted to each family of products (games/toys, wellbeing, art of living, garden, excursion, stationery products, jewellery), and is centred around five impact indicators: the consumption of non-renewable resources, energy consumption, environmental pollution and health impact, degree of recyclability, and social risks related to the products' country of manufacture. In this manner, the specialised retailer is anticipating the forthcoming obligation concerning environmental labelling. Each product evaluation is summarised in a score for each stage of product life cycle, which will make it possible to identify the main possibilities for improvement (reduction of packaging, replacement of plastic with renewable materials, etc.). The customer promises to assist suppliers in this improvement phase (obtaining certifications, carrying out the carbon footprint assessment, etc.). The tool will also enable the retailer to select the more innovative suppliers for new product development in order to emphasise the environmental orientation of its catalogue.

Organisational proximity supports collaboration in type C. Companies are European or global actors, without any particular geographical proximity. They voluntarily set up the conditions for their collaboration by creating an eco-organisation and involving a leading logistics service provider able to optimise reverse flow management on a European scale. The shared desire to anticipate new regulations that the collaboration is founded upon is not sufficient to be described as institutional proximity, given that the collaboration henceforth suffers from a strategic lack of vision in the long run. Instead, we would speak of organisational proximity, in search of optimisation, faced with a regulatory constraint. In the collaborative reverse logistics case, the environmental dimension was suggested by the promulgation of the European WEEE Directive. The WEEE Directive aims to reduce the amount of electrical and electronic equipment being produced and to encourage everyone to reuse, recycle and recover it. The WEEE Directive also aims to improve the environmental performance of businesses that manufacture, supply, use, recycle and recover electrical and electronic equipment. The environmental impact is thus implicit in such an arrangement since the collaboration is developed to ensure the European management of waste electronic and electrical components! Nevertheless, the creation of an eco-organisation for the management of return flows aimed to optimise the recycling process on a global scale. This organisational proximity was created voluntarily by competitors who decided to anticipate the regulation in progress. They also decided to work with a leading logistics provider on a European scale, although that decision has been recently questioned by the eco-organisation. While it is the reduction of the economic and financial costs that is sought by partners, the environmental vocation of the regulation inevitably leads to some environmental effects. The logistics provider had to sign the sustainability charter provided by each partner of the eco-organisation and has to make sure that waste is properly recycled. Ninety percent of the collected waste products are thus either refurbished, dismantled or partially or totally recycled. Although the WEEE Directive imposes a minimal collection rate, the logistics provider has an incentive to improve its collection performance through the chosen reward system (proportional to the tonnage of waste collected). However, there is no specific objective for the environmental impact, beyond the accidental convergence between transport optimisation and a positive environmental impact. On an initiative of the French Ministry of Ecology, Sustainable Development and Energy, with the support of ADEME (French Environment and Energy Management Agency), a specific charter on voluntary reductions of CO 2 emissions was drawn up in 2010 to raise awareness of these issues among road freight companies. The logistics provider working for the eco-organisation wanted to demonstrate its commitment by signing this charter alongside other major transport and logistics players. Specific measures were adopted: updating the fleet with Euro 5 standard models, limiting vehicles' top speed, training drivers in rational driving techniques with or without on-board IT systems, modal transfer and optimisation of transport plans. This case can be interpreted as a confirmation of the Porter and Van der Linde thesis (1995) -quite controversial in the literature -defending the idea that regulation is a potential source for environmental innovation, when companies manage to transform a regulatory constraint (which usually elicits a low compliance attitude from companies) into an opportunity for both business innovation and environmental protection.

Finally, type D presents a double ambition: to offer complementary services in order to find new markets and, in so doing, to generate a dynamic of local development. This case combines two forms of proximity: on one hand there is a high level of geographical proximity between the actors, all located in the same area (industrial actors, institutional actors and a competitiveness cluster 1 ) and on the other hand, organisational proximity, as actors deliberately set up the arrangement for their collaboration, i.e. creation of the associative structure, definition of the rules of operation, and financial support from an industrial and innovation cluster. Although the majority of the companies had already collaborated or formed interpersonal relations before joining the associative structure, institutional proximity

1 In France, a competitiveness cluster is an initiative that brings together companies, research centres and educational institutions in order to develop synergies and cooperative efforts. (http://www.industrie.gouv.fr/poles-competitivite/brochure-en.html ) nevertheless remains to be built around the project, which only started less than two years ago. As already mentioned, this case lacked any type of concrete environmental consideration.

Conclusion

This paper contributes to highlighting the diversity of forms that collaborative practices take in the supply chain, both in terms of life cycle development (time dimension) and types of proximity among the actors (space dimension). It shows how these forms lead to different degrees and forms of environmental integration within the collaborative initiative, and provides a preliminary analysis of the preconditions for environmental collaborative practices in terms of the forms of proximity between the actors involved. Accordingly, a more nuanced understanding of the link between the environment and collaborative practices has been achieved. Not all the recent SC collaborative practices include a green dimension, and when they do, it can reflect very different attitudes (from defensive to a very engaged attitude; from early to late introduction in the collaborative initiative), and pursue diverse objectives (optimisation, new market development, innovation and creativity orientation).

This study has been carried out drawing on "proximity economics", a theoretical framework which seems particularly suitable for analysing collaborative issues, especially in the SC field, as it builds upon the three founding dimensions of Supply Chain Management: geographical, organisational and institutional. The first dimension takes into account the specific spatial and operational activities linking the different nodes of the chain. Through the second we scrutinise the inter-organisational interfaces between the different actors. And the third allows us to introduce norms and values that are shared by the SC actors.

A typology of collaborative practices has been developed and characterised in terms of forms of proximity. On the one hand, geographical proximity alone is not a sufficient condition to foster collaborative practices integrating an environmental dimension, except for the natural convergence between transport optimisation and a positive environmental impact. On the other hand, organisational and institutional forms of proximity appear to be preconditions for effective collaborative environmental practices to a wider extent, each with peculiar characteristics. Within the organisational form of proximity, we have examined the peculiar role of logistics service providers as "intermediation" actors and the importance of dedicated tools to implement the co-opetition arrangements.

Overall, the environmental dimension does not seem to be naturally integrated in voluntary collaborative practices in SC, on a broad basis. Environmental collaborative practices have been put in place principally with the objective of reducing costs (lean-green attitude). Only one case (type B) definitely recognises the environmental dimension as one of the core elements of the collaboration, allowing collaborative green value creation. Finally, it is worth observing that most of the actors declare that they are aware of the positive impact of environmentally friendly initiatives on their own image in the marketplace. Accordingly, they have now adopted a "green attitude", i.e. proclaiming their concern for the environment. Green-washing or green supply chain? Our research started with the same question that we have used to end this paper…

Collaborative practice Industry/Sector Interviews Interviewees and length of the interviews

		distribution of	Consumer		-SC Director (1 hour)
		finished product	Goods
	5	New product	Specialised	-1 Specialised retailer	-Purchasing Vice Director (1hour 30)
		development,	distribution	-1 Supplier	-Key account manager (1 hour
		purchasing			/telephone)
	6	Collaborative	Logistics	-2 Industrial companies	-Operations manager (1 hour)
		provision of	services	-2 LSPs	-SC Director (1 hour 30)
		complementary		-1 Scholar	-Operations manager (1hour)
		industrial services,			-Head of IT (1 hour)
		namely			-Professor (2 hours)
		postponement,
		reverse logistics,
		refurbishing.
	7	Collaborative reverse	Reverse	-1 LSP	-Global Solutions Manager (2 hours)
		logistics, comprising	logistics	-2 Manufacturers	-Manufacturing Director (1hour 30)
		collection, recycling,			-Assistant to Manufacturing Director (1
		refurbishing of			hour)
		electrical and
		electronic products.
	1	Collaborative	Fast Moving	-1 Logistics Service	-Head of logistics (1 hour)
		distribution of	Consumer	Provider	-IT manager involved in the project (1
		finished product	Goods	-1 Information System	hour)
				Provider	-Production manager (1hour 30)
				-2 Manufacturer	-Head of Logistics (1hour)
	2	Collaborative	Fast Moving	-2 Manufacturers	-SC Manager (1 hour)
		distribution of	Consumer	-1 LSP	-SC Manager (2 hours)
		finished product	Goods	-1 Retailer	-Key account manager (1hour 30)
					-Logistics Director (1 hour)
	3	Collaborative	Fast Moving	-2 Manufacturers	-Head of logistics (1 hour 30)
		distribution of	Consumer	-1 LSP	-Operations manager (1hour)
		finished product	Goods		-IT manager involved in the project (1
					hour)
					-
	4	Collaborative	Fast Moving -2 Manufacturers	-Operations Director (2 hours)

Table 1 :

 1 List of case studies and interviews

Table 2 :

 2 The time and green dimensions in the green collaborative practices

	Type of	Engagement	Contract deployment/	Implementation	Evaluation	Environmental
	practice		Management	of		process /	dimension
			interdependencies
	Type A	-Defensive, in	-Pooling logistics		-Pre-allocation of	-Mainly cost	-Late
		reaction to an	resources		"space" in shared	reduction	introduction
		initial constraint	"horizontally"		logistics and	-Flow	in the
		-Choice of	-Limited		transport	Optimisation	collaboration
		compatible	information sharing	resources	-Improvement	-The lean-
		products/volum	-Key role of an LSP	-Shared deliveries	of customer	green
		e/origins and				service	convergence
		destinations
	Type B	-Proactive, for	-Pooling logistics		-Sharing of	-Long-term	-At the core of
		Mkt expansion	resources		information about	market	the agreement
		-Personal and	"horizontally"		inventory and	development	-Distinctive
		long-lasting	-Vendor		allotted space in	-Improvement	motivation for
		relationships	Management of		the shared log. &	of customer	starting the
			Inventory		transp. resources	service	collaboration
			-High information		-Joint inventory	-Improvement
			sharing		mgmt and	of the
					deliveries	innovation
					-Co-development	process
					of new products	-Flow
						Optimisation
	Type C	-Proactive to	-Set-up of an eco-		-Sharing of	-Reduction of	-The purpose
		anticipate	organisation among	information to	the overall	of the
		regulations	the actors		plan collection,	logistics costs	collaboration
			-Key role of an LSP	sorting and	for the	is a green
					treatment of waste	mandatory	practice
					-LSP executes all	reverse SC	(recycling)
					the operations		-Lack of a
							green
							strategic view
							of the process
	Type D	-Proactive for	-Flexible associative	-Internal call for	-New market	-Too early to
		business	structure		projects for each	development	consistently
		diversification			order	-Revision of the	address the
		-Local actors			-Leadership role	internal	green
		within the same			played alternately	functioning	dimension
		business			by different actors	process	-A political
		ecosystem					will

22nd IPSERA Conference, Nantes, France, 24-27 mars 2013