Carbonate and Ionic Liquid Mixes as Electrolytes ... - ACS Publications

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Carbonate and Ionic Liquid Mixes as Electrolytes To Modify Interphases and Improve Cell Safety in Silicon-Based Li-Ion Batteries Nicolas Dupré,*,† Philippe Moreau,† Eric De Vito,‡,§ Lucille Quazuguel,† Maxime Boniface,⊥,¶ Harald Kren,# Pascale Bayle-Guillemaud,⊥,¶ and Dominique Guyomard† †

Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2 rue de la Houssinière, BP 32229, 44322 Nantes Cedex 3, France ‡ CEA, LITEN, Minatec Campus, 17 rue des Martyrs, F-38054 Grenoble, France § Université Grenoble Alpes, F-38000 Grenoble, France ⊥ Université Grenoble Alpes, F-38054 Grenoble, France ¶ CEA-INAC-MEM, F-38054 Grenoble, France # VARTA Micro Innovation GmbH, Stremayrgasse 98010 Graz, Austria S Supporting Information *

ABSTRACT: Among the candidates as negative electrode, silicon is now one of the most attractive alternatives to graphite and has been the subject of many investigations for the past decade. The commercialization of Si electrodes is nevertheless blocked by the inability to overcome the mechanical degradation and electrolyte consumption occurring as a result of the inherent volume expansion upon silicon alloying. The unique combination of their properties renders ionic liquids very attractive and promising candidates to replace the benchmark organic carbonates and could enable an enhanced control of species constituting the solid−electrolyte interface (SEI). In the present study, evolutions of ionic liquidbased electrolytes (pure ionic liquid and ionic liquid/carbonate mixes) and the subsequently formed SEI are monitored upon aging and cycling in full Li-ion cells using nonprelithiated silicon electrodes. X-ray photoelectron spectroscopy, typically probing the first few nm of the surface of the sample, allowed monitoring of the evolution and possible degradation of the ionic liquid based electrolytes upon aging and cycling of complete Si/NMC batteries. Magic angle spinning NMR combined with scanning transmission electron microscopy-electron energy loss spectroscopy is more sensitive to changes occurring in the SEI composition. The degradation of ionic liquid components PYR13 and TFSI is evidenced and their influence on the formation of species at the surface of the silicon electrode clearly observed. However, the presence of the ionic liquid components does not prevent the degradation of carbonates in the parasitic reactions that are consuming the cyclable lithium. Therefore, the failure mechanism scenario is similar to that observed for the full cell using benchmark carbonate electrolytes. Hazard level assessments nevertheless reveal that the addition of ionic liquids is in fact able to moderate the intensity of safety relevant events and improve the cell safety.

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INTRODUCTION Because of their high energy-to-weight ratio and long cycle life, Li-ion batteries are one of the most promising energy storage technologies for transportation in the race to reduce the fossil oil dependency and CO2 emissions. Emerging applications such as electrical vehicle and power grid1 are increasingly demanding in terms of energy density and capacity retention upon cycling. Among the candidates for negative electrode, silicon is now one of the most attractive alternative to graphite due to its natural abundance, high specific gravimetric capacity (3579 mAh g−1 vs 372 mAh g−1 for graphite), and a large volumetric capacity (2081 mAh cm−3 vs 779 mAh cm−3) and has been the subject of many investigations for the past decade. Previous studies, mostly performed in half-cell configurations highlighted the continuous liquid electrolyte degradation at the surface of the Si phase and the successive swelling/shrinkage upon alloying/ © 2017 American Chemical Society

dealloying of the stack of Si particles within the confined space of the electrochemical cell as the causes of the failure mechanism of silicon based electrodes.2−4 The consecutive major geometrical changes of granular texture with a vast redistribution of interparticle contacts, the formation of cracks within the composite electrode, the increase of the amount of electrolyte-degradation products,5−9 and loss of electrical contacts at the current collector interface lead to an irreversible capacity fading.10−14 In particular, the endless exposure of Si particles surface to the liquid electrolyte results in an irreversible capacity loss at each cycle by the reduction at low potential of part of the liquid electrolyte on the exposed Received: May 12, 2017 Revised: September 19, 2017 Published: September 19, 2017 8132

DOI: 10.1021/acs.chemmater.7b01963 Chem. Mater. 2017, 29, 8132−8146

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Chemistry of Materials surface.3,7−9 In a full-cell configuration, the supply of lithium is limited and the failure mechanism as well as the chemical composition of the solid−electrolyte interface (SEI) has been found to be different.15,16 Processes of electrolyte degradation occurring in full cell configuration lead to a heterogeneous SEI composed of LiF and trapped LiPF6 in an organic matrix stemming from the degradation of the electrolyte organic solvents, in a similar way compared to Si/Li half-cell. However, the formation of non lithiated organic species at the extreme surface of the SEI for an extended cycling indicates in particular that all the lithium available for cycling has been consumed in parasitic reactions and is either trapped in an intermediate part of the SEI or in the electrolyte. In this case, this process occurs typically before the silicon electrode is irreversibly damaged. It appears then necessary to change the benchmark organic carbonates for a solvent with increased stability toward temperature and potential variation and possibly enabling an enhanced control of species constituting the SEI. The unique combination of properties such as wide electrochemical window, good chemical and thermal stability, nonflammability, negligible vapor pressure renders ionic liquids very attractive and promising candidates to replace the traditional carbonates.17 Conductivities, viscosities, thermal stability, and electrochemical stability of mixtures of ionic liquids, including imidazolium and pyrrolidinium-based materials, and carbonates, including PC, EC, DMC, and DEC, have been studied extensively by various groups over the past 20 years.18−23 More recently, the possibility of using such mixtures as electrolyte for Li-ion systems led to the thorough characterization of a wide range of compositions of LiPF6 or LiTFSI salts dissolved in ionic liquid/carbonate mixtures.24−29 Compared to classical carbonate electrolytes, it was reported in particular that mixtures of ionic liquid with alkyl carbonates enhance the thermal properties of the electrolyte, without compromising electrochemical performance. Such mixtures possess systematically a very wide electrochemical stability, exceeding 5 V, and have a good ionic conductivity at room temperature. In light of recent events concerning Li-ion batteries for smartphones, it is of utmost importance to gain knowledge on the possible safer utilization that could be provided by IL as an electrolyte component. Finally, in the search for enhanced interfaces, specially designed ionic liquid based electrolytes could combine their assets over traditional organic solvents, modify the composition of the SEI, and consequently improve long-term cycling performance. In the present study, an ionic liquid electrolyte based on 0.35 mol kg−1 LiPF 6 in N-Propyl-N-methylpyrrolidiniumbis(trifluoromethylsulfonyl)imide (PYR13-TFSI) with 10 wt % of FEC as additive ((0.35 mol kg−1 LiPF6 in PYR13-TFSI) + 10% FEC) is investigated upon aging for various times and temperatures. In a second step, evolutions of the SEI and the ionic liquid are monitored upon cycling in a full Li-ion cell configuration with silicon as the negative electrode and LiNi1/3Mn1/3Co1/3O2 as the positive electrode. No prelithiation of the silicon electrode was applied. To compare with results obtained in the case of carbonate based solvents,15 it is necessary to cycle the full Li-ion cells in the same temperature, cycling rate, and electrode loading conditions. Since a pure Pyr13-TFSI ionic liquid based electrolyte cannot sustain moderately high cycling rates such as C/2 at room temperature, ionic liquid/carbonate mixes are investigated. Even if other ILs could have led to a better behavior in our system,30−33 this choice of composition was dictated by the already sizable

literature existing for this IL, hence making it a good basis for a reference study. Following this approach and aiming at characterizing the influence of the ionic liquid components on the SEI formed at the surface of silicon particles, silicon electrodes were used instead of Si:graphite composites electrodes to get intrinsic information on the silicon SEI. Silicon electrodes were cycled using two different mixes of carbonates and ionic liquid: (50% (1 M LiPF6 in EC:DEC) + 50% PYR13 TFSI/10% FEC) and (50% (1 M LiPF6 in EC:DEC) + 50% (0.35 mol/kg LiTFSI in PYR13 TFSI)/10% FEC) at limited capacity of Si electrodes (1200 mAh g−1). Electrolytes with close composition have been previously characterized26 and tested in LiFePO4/Li and LiFePO4/ Li4Ti5O12 cells,25,28,29 confirming the attractive conductivity and thermal and electrochemical stability (>5 V vs lithium) of this family of electrolyte. In the case of both mixes, LiPF6 salt is kept as it has been identified as playing an important role in the SEI formation and with the purpose of studying the influence of the solvents alone. LiTFSI salt is then reintroduced to enhance the electrochemical performance and study a possible influence on the SEI composition. An extensive combination of techniques, 7Li, 19F magic angle spinning (MAS) NMR, X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy-electron energy loss spectroscopy (STEM−EELS), provides both a global and in-depth characterization of the SEI forming at the surface of silicon particles as well as its evolution upon cycling in a Swagelok full Li-ion cell configuration with LiNi1/3Mn1/3Co1/3O2 as the positive electrode. Such comprehensive and multitechnique characterization was previously used and allowed for a better understanding of the failure mechanism of Si-based composite electrodes for lithium batteries cycled in full-cell configuration using benchmark 1 M LiPF6 in EC:DMC electrolyte. In addition, the presence of ionic liquid in the electrolyte solvent formulation enables the use of other atoms than C, O, F (namely N and S) as XPS probes of the evolution or degradation of IL. This approach has been applied here to both aged and cycled silicon samples using ionic liquid based electrolytes. XPS, typically probing the first few nm of the surface of the sample, allowed monitoring the evolution and possible degradation of the ionic liquid based electrolytes upon aging and cycling of complete Si/NMC batteries. MAS NMR combined with STEM−EELS are more sensitive to changes occurring in the SEI. The influence of the ionic liquid components PYR13 and TFSI on the formation of species at the surface of the silicon electrode and on the lithiation mechanism of silicon particles will thus be discussed based on these two complementary techniques. The case of carbonate/ionic liquid mixes (IL/ carbonate Mix) electrolyte is investigated upon cycling in terms of SEI composition and electrochemical behavior. The failure mechanism of full Li-ion cell using an IL/carbonate mix is then discussed and compared to the case of Li-ion cell using a conventional EC:DEC carbonate electrolyte. Safety tests have been performed on pouch cells using an IL/ carbonate mix. Hazard level assessments reveal improvements on cell safety by mixing carbonate based electrolytes with ionic liquids. Although the carbonate based part of the electrolyte is primarily responsible for the safety issues of the cell, the addition of ionic liquids is in fact able to moderate the intensity of safety relevant events. 8133


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Chemistry of Materials

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each IL/carbonate ratio, ranging from 0.3 to 1 M. Corresponding details on the conductivity measurements are given in the Supporting Information. The best conductivity values were obtained for LiPF6 concentrations of 0.72 M (9.2 mS cm−1), 0.53 M (9.6 mS cm−1) and 0.32 M (9.3 mS cm−1) for 30% IL/70% carbonate, 50%IL/50% carbonate, and 70%IL/30% carbonate, respectively. All the tested electrolytes contained an additional 10 wt % FEC. Among those, the LiPF6 0.52 M in 50%IL/50% carbonate electrolyte was chosen due to its slightly higher conductivity value. In addition, a 30% IL/70% carbonate ratio seemed less interesting in view of studying the influence of the ionic liquid on the formation and evolution of the SEI. Last, although the conductivity value obtained for a LiPF6 0.32 M in 70%IL/30% carbonate seemed promising, no convincing electrochemical cycling could be obtained in full NMC/silicon cell. Therefore, the LiPF6 0.53 M in 50%IL/50% carbonate electrolyte/ 10% FEC was selected for the study. This particular electrolyte composition (denoted Mix1 in the following) corresponds to the following formulation: 50% (1 M LiPF6 in EC:DEC) + 50% PYR13 TFSI)/10% FEC. In a second step, LiTFSI salt is introduced in an attempt to enhance the electrochemical performance and study a possible influence on the SEI composition. Implementing from Mix1 composed of 50% (1 M LiPF6 in EC:DEC) + 50% PYR13 TFSI)/10% FEC, the pure ionic liquid component has been replaced by an equivalent ionic liquid electrolyte 0.35 mol/kg LiTFSI in PYR13 TFSI, corresponding to the state of the art optimized ionic liquid electrolyte.34,35 This second tested electrolyte is denoted Mix2 (50% (1 M LiPF6 in EC:DEC) + 50% (0.35 mol/kg LiTFSI in PYR13 TFSI)/10% FEC). The measured conductivity at 25 °C for Mix2 is 6.8 mS cm−1. Several electrolytes, with various compositions, were tested during safety tests. Nevertheless, solely results concerning Mix3, corresponding to a (50% (1 M LiPF6 in EC:DEC) + 50% (0.66 M LiPF6 in PYR13 TFSI)/10 wt % FEC) formulation, have been included in the present study due to its conductivity and electrochemical performances comparable to Mix2. The measured conductivity at 25 °C for Mix3 was 6.5 mS cm−1. Positive and negative electrodes were balanced so that the cycling could occur at limited capacity for the Si electrode (1200 mAh g−1). In all cases, LiPF6 salt was kept to study the influence of the solvents alone. The FEC, with a specification of below 20 ppm trace water content, was added to the electrolyte, with a specification of below 10 ppm trace water content, in the argon glovebox. Since the amount of trace water in the electrolyte containing FEC was found impossible to measure by normal Karl Fischer titration, the total water content of the as prepared electrolyte is considered to be below 20 ppm. Cells were dismantled at different cycle numbers (1st, 10th, and 100th at end of lithiation and at the end of delithiation) in an argon glovebox. Electrodes stopped at the end of lithiation or delithiation were transferred in the different characterization instruments by using airtight transfer vessels. The respective experimental conditions for solid-state NMR and XPS are identical to those presented in our previous work.15 Full Cells Prepared for Safety Tests. Stacked pouch cells (Si vs NMC) were prepared to perform safety tests using silicon and NMC electrodes described above and providing a nominal capacity of ∼0.2 Ah. The stacked cells were filled using two different electrolytes: 1 mL of carbonate based electrolyte (1 M LiPF6 in EC:DEC + 10 wt % FEC) and 1 mL of Mix3 (50% (1 M LiPF6 in EC:DEC) + 50% (0.66 M LiPF6 in PYR13 TFSI)/10 wt % FEC). Because of different wetting behavior of the investigated electrolyte systems, a polypropylen microporous film separator was used in the case of the carbonatebased electrolyte and a Freudenberg 2226 separator for the Mix3 electrolyte. For comparison, cells using Li1Ni0.8Co0.15Al0.05O2 (NCA) positive electrodes were constructed. In more detail, a description on the prepared stacked pouch cells can be found within the Supporting Information. All safety tests were performed once cells passed through a formation cycle (charging to 4.2 V; discharging to 2.0 V) with a C/ 10 rate (1 Li in 10 h), followed by a 100% SOC charging step with a C/10 rate to 4.2 V. Safety Tests. Three different safety tests were performed on cells containing both types of cathodes and electrolytes. The performed

EXPERIMENTAL SECTION

In this section, a brief description of the experimental parameter that have been used is given. More details can be found in ref 15 and in the Supporting Information. Materials and Electrode Formulation. The silicon power used for this study was bought from Nanostructured and Amorphous Materials (NAM). The negative electrode was prepared by the slurry technique, out of an aqueous dispersion. The slurry of the processed electrodes contained 8 w% of binder material (sodium-carboxymethylcellulose), 80 w% of negative active material (n-Si, 30−50 nm from NAM), and 12 w% of conducting agent (Super P Carbon Black). The mass loading of the negative electrodes was 0.8 mAh cm−2 based on capacity limit: 1200 mAh g−1. Silicon electrodes were not prelithiated prior to cycling. The positive electrode was prepared by the slurry technique, in organic dispersion. The slurry contained 7 w% of polyvinylidenedifluoride (PVdF) based binder, 88 w% of cathode active material (LiNi1/3Mn1/3Co1/3O2, i.e., NMC [1/1/1]), and 5 w% of the conducting agent (Super P). For the slurry preparation N-Ethyl2-pyrrolidone (NEP) was used. The mass loading of the NMC (calendared) electrodes is 6.49 mg.cm−2 (av. theor. cap. 0.9 mAh cm−2). Full Cell Aging and Cycling. Full batteries (Si vs NMC) were prepared in three electrodes Swagelok cells using FePO4/LiFePO4 as the reference electrode. The different electrolytes used in the present study are detailed in the following paragraphs. Previous results by our group15 indicated a significant electrolyte degradation during aging under OCV conditions using a conventional 1 M LiPF6 in EC:DEC with 10 wt % FEC as additive electrolyte, considered at the time as the state-of-the-art electrolyte. This work also indicated a strong contribution from the degradation of organic carbonate solvent to the formation of the thick SEI. To investigate further the effect of the electrolyte solvent on the SEI formation and evolution, and for comparison purpose with our previous results, we decided to replace the carbonate solvents for PYR13-TFSI, a wellstudied ionic liquid. LiPF6 salt and the FEC additive are kept with the purpose of studying the influence of the solvents alone. Silicon electrodes were thus aged under OCV conditions inside an electrochemical cell with a 0.35 mol/kg LiPF6 in N-propyl-Nmethylpyrrolidiniumbis (trifluoromethylsulfonyl) imide (PYR13TFSI, Table 1) + 10 wt % FEC as additive electrolyte (hereafter

Table 1. Formula and Structure of the Ionic Liquid Used in the Present Study

“pure IL electrolyte”). A concentration of 0.35 mol of LiPF6 per kg of PYR13-TFSI corresponds to the maximum of conductivity (increasing with the amount of LiPF6 salt), keeping the viscosity low to allow for the utilization of this formulation as liquid electrolyte.34 Concerning the cycling study, to compare with results obtained in the case of carbonate based solvents,15 it is necessary to cycle the full Li-ion cells in the same temperature, cycling rate, and electrode loading conditions. Since a pure Pyr13-TFSI ionic liquid based electrolyte cannot sustain moderately high cycling rates such as C/2 at room temperature, ionic liquid/carbonate mixes are investigated. Silicon electrodes were thus cycled using two different mixes of carbonates and ionic liquid purchased from Solvionic: Mix1 (50% (1 M LiPF6 in EC:DEC) + 50% PYR13 TFSI)/10% FEC) and Mix2 (50% (1 M LiPF6 in EC:DEC) + 50% (0.35 mol/kg LiTFSI in PYR13 TFSI)/10% FEC). Electrolyte compositions have been chosen on the basis of conductivity measurements. Three IL/carbonate mass ratios were investigated (30% IL/70% carbonate, 50%IL/50% carbonate and 70%IL/30% carbonate) as well as several LiPF6 concentrations for 8134


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Chemistry of Materials safety tests were nail penetration tests, heating tests, and overcharge tests. More detailed description of the test procedures can be found within the Supporting Information. Assessment of Hazard Levels. Hazard levels are defined by EUCAR for the use of a battery in an electric vehicle, allowing a classification in 7 levels ranging from “0: No Effect” to “7: Explosion”.36−39 All investigated cells were classified according to these well-known criteria. At this point, it is essential to mention that the investigated stacked cells correspond to lap demonstrator cells, lacking security features like a safety vent to release gases or circuit interrupt devices. As a consequence, the obtained hazard levels were rather poor.

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RESULTS AND DISCUSSION Degradation of IL Part of the Electrolyte. Concerning the aging study in pure IL electrolyte (0.35 mol/kg LiPF6 in PYR13-TFSI), four samples were characterized in OCV conditions (2 h, 25 °C; 2 h, 55 °C; 1 day, 25 °C; 1 day, 55 °C). Measurements showed the extreme difficulty to sufficiently dry some of these samples before NMR and XPS analyses. Since only the first nanometers of the surface of the active material are probed in XPS and the very low saturation vapor pressure associated with ionic liquids, this technique seems particularly prone to analyze the remaining ionic liquid over the electrode solids rather than the electrode itself. XPS spectra show no drastic evolution of peaks specifically related to the ionic liquid (C 1s, S 2p, F 1s, O 1s) except for the N 1s contribution, for which the ratio N+(PYR13)/N-(TFSI) is reversed after one month at 55 °C compared to that after one month at 25 °C (Figure 1). More precisely, the at% ratio N +/N−, which is 1.06 for pure PYR13/TFSI mixture (see Supporting Information, Figure S3) decreases with temperature (from 1.1 at 25 °C to 0.75 at 55 °C). This is an indication of PYR13 degradation with higher aging temperature. This is in contrast with the aging study performed previously15 in conventional carbonate based electrolyte (1 M LiPF6 in EC:DEC + 10 wt % FEC) where degradation of the electrolyte solvents could readily be observed at room temperature after a short time of contact with the surface of the silicon particles. F 1s XPS spectra, presented in Figure 1 (bottom), display two peaks: the lower binding energy peak is related to small amounts of LiF, the intense high BE peak is related to CFx or LixPFy contributions. Quantification of XPS data (Table 2) show values of ∼7.7 at% of N and ∼21 at% of F. The ratio N/F is close to that found in the IL (8.4 at% N, 25 at% F, see Table 2). This is a clear indication that XPS experiments mainly probe the ionic liquid surrounding other components of the electrolyte, in particular LiPF6, which is then not visible from XPS analyses. The presence of CF3 groups is in quite good agreement with the −80 ppm resonance on the 19F NMR spectra (Figure 2 left). 19F NMR spectra are discussed in more details in the following section. Although it is difficult to conclude whether the detected −CF3 group belongs to nondegraded ionic liquid or to decomposition products, the evolution of the N 1s spectra upon OCV aging indicates that using a ionic liquid instead of a carbonate based electrolyte does not prevent reactions between the electrode and the electrolyte. Thus, the interface is still subjected to a very specific reactivity that might alter the functioning of the electrochemical cell. Concerning the cycling study, the evolution of the IL part of the electrolyte was monitored for both Mix1 and Mix2. However, since no significant difference could be observed from Mix1 and Mix2 analyses, electrolyte mixes will be further

Figure 1. Top: N 1s XPS spectra of electrodes aged in pure IL electrolyte (0.35 mol/kg LiPF6 in PYR13-TFSI): 1 month at 25 °C (red) and 1 month at 55 °C (blue). Bottom: F 1s XPS spectra of aged electrodes: 1 day at 25 °C (red) and 1 day at 55 °C (blue).

referred as IL/carbonate mix and discussed without further discrimination in the following. XPS N 1s and S 2p spectra are particularly worth detailing (Figure 3 and 4). Please see the Supporting Information for further details on F 1s and C 1s spectra. XPS spectra of N 1s show that the degradation of the ionic liquid components (PYR13 and TFSI) appears from the first cycle. In Figure 3, new contributions in N 1s spectra appear with the first cycle. The increased number of components and global broadening for each step of lithiation and delithiation reveal a partial degradation of ionic liquid molecules. Although identifying solely with XPS energy shifts the precise compounds formed, several papers suggest the formation of NSxOy species through the cleavage of S−C bonds in TFSI: NSO2−, NS2O4−, or NH2SO3− (from FSI in the latter case).40,41 Moreover, in ref 41 the degradation of SO2CF3− into SO2− is proposed, which may explain the high binding energy peaks observed for S 2p in Figure 3. Other species like 1methylpyrrolidin, CF3H, or NSO2CF3− may appear as well. A recent modeling study concerning Si electrodes confirmed these assumptions and concluded that FSI tends to quickly form LiF and smaller inorganic compounds like SO2, while 8135


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Table 2. XPS Quantification Table (at%) for Aged Samples in Pure IL Electrolyte (0.35 mol/kg LiPF6 in PYR13-TFSI) sample aged at 25 °C

aged at 55 °C

PYR13/TFSI

2 1 1 2 1 1

time

Li

Si

F

S

N

P

O

C

h day month h day month

1.5 1 0 0 0 0

1.4 1.9 1.7 5.4 7.6 0

21.3 20.5 20.2 19.8 18.6 19.3 25

12.1 11.9 12 10.7 12.1 10.8 8.3

7.7 7.5 7.4 6.9 6.6 8.8 8.4

1 0.9 0.6 0.8 0.3 0.2 -

12.7 12.7 12.4 13.3 12.4 11.9 16.7

42.3 43.6 45.7 43.1 42.4 49 8.3

Figure 2. 19F (left) and 7Li (right) MAS NMR spectra for two aging conditions in pure IL electrolyte (0.35 mol/kg LiPF6 in PYR13-TFSI).

Figure 3. N 1s (top) and S 2p (bottom) XPS spectra for the IL/carbonate mix electrolyte as a function of number of cycles, at the end of 1st, 10th, 100th lithiations and at the end of 1st, 10th, and 100th delithiations. Spectra of N 1s and S 2p for pure PYR13/TFSI mixture are reported as a reference at the bottom of the figure. 8136


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produce significant negative effects because of their low concentration (a fraction of some at%, see Table 2). In the present case, their presence seems to be mainly a result of TFSI degradation with cycling. It should also be mentioned that other degradation products like the dithionite and thiosulfate anions are well-known corrosion agents and could explain some degradation observed in some of the used Swagelok cells. Extended cycling in IL/carbonate mix electrolyte therefore shows unambiguously a degradation of both compounds (PYR13 and TFSI). It should be mentioned here that since the results are identical for Mix1 and Mix2, this degradation is intrinsic to the PYR13 and TFSI ions and is also expected to occur when TFSI is used simply as a replacement salt. These results are also in clear contrast with those obtained in the aging study as they demonstrate that an electrochemically triggered degradation of the IL part of the mixes also occur. Characterization of SEI, Comparison with the Case of Carbonate-Based Electrolyte. The 7Li NMR spectra of the silicon electrodes aged in pure IL electrolyte (0.35 mol/kg LiPF6 in PYR13-TFSI) are compared in Figure 2 (right) for two different aging times and temperatures. The analyses performed on the 2 h, 25 °C and 1 day, 55 °C samples show that, like in the case of the conventional carbonate based electrolyte (1 M LiPF6 in EC:DEC + 10 wt % FEC),15 the surface of the silicon material reacts spontaneously with the electrolyte components as three resonances can be observed in the ex situ 7Li MAS NMR spectra of the electrode recovered after 2 h of exposure to the pure IL electrolyte at 25 °C (Figure 2 right). The resonance at 0.3 ppm can be assigned to lithiated non fluorinated products, while resonances at −1.3 and −2 ppm are assigned to fluorinated products, possibly including non degraded LiPF6 (−2 ppm). The resonance assigned to lithiated nonfluorinated product in the 7Li NMR spectrum (0.3 ppm) clearly decreases with aging time and temperature to the benefit of lithiated fluorinated species. Then, in contrast with the aging process in carbonate based electrolytes,49−54 a larger contribution of the lithiated species precipitated on the surface of the electrode seems to be fluorinated. Nonreacted LiPF6 salt is also spotted by a sharp and well-defined doublet at −72/−74 ppm in the corresponding 19F spectrum (Figure 2 left), confirming its presence, trapped in the porosity of the SEI. The −80 ppm resonance in the 19F NMR spectra, usually attributed to fluorophosphates such as LixPFy, LixPOyFz or organic fluorophosphates such as R-PO2F2 and R-POF3,55−61 seems to contain also a contribution of CF3 groups, typically rising in the −80/ −90 ppm range, from the TFSI anions or coming from their decomposition products, as mentioned in the previous section. After 1 day at 55 °C, a broad signal at −145 ppm is detected. This resonance, also observed in the case of the aging of silicon electrodes in carbonate-based electrolyte containing FEC additives was assigned to decomposition products of the FEC. CF2H2 and species containing − CH2F groups are expected in the −140/−145 ppm range. Typically, 19 F NMR shifts for CC−F groups rise in the −100 to −150 ppm range, in agreement with the experimentally observed chemical shift. Nevertheless, the formation of a carbon−carbon double bond is not likely to happen from the ring opening reaction of FEC.62 No traces of LiF, known to be one of the main SEI components54,63−65 in the case of carbonate based electrolytes, have been observed by NMR. It seems that one of the main influences of the use of an ionic liquid as electrolyte on the SEI chemical composition is the much lower amount of detected LiF. The strong contribution of fluorinated species

Figure 4. Deconvolution of the S 2p spectrum after one cycle at the end of lithiation. Li2SO4 is in purple, TFSI in green, Li2S in red, and Sn2− in blue.

TFSI is more robust, leading to bigger decomposition products (such as SO2CF3−) at slower time scales.42 Another older study, concerning lithium−sulfur batteries cycled in various electrolytes containing LiTFSI as a salt, concluded on the formation of degradation species, which are in agreement with those mentioned above (LiF, LixCFy, Li2NSO2CF3, or Li2SO2CF3).43 S 2p XPS spectra (Figure 3, bottom) show high Li2S (S reduced form at 160.5 eV) and Li2SO4 (S oxidized form at 168.3 eV) contributions. The deconvoluted S 2p spectrum at the end of lithiation 1 (Figure 4) clearly reveals several contributions in addition to that of TFSI: Li2S, Li2SO4 and also polysulfides at ∼162.1 eV. With extended cycling, it seems that the degradation of TFSI goes on: the original peak decreases at the expense of these new contributions. The related degradation products are close to those that are supposed to form on lithium or copper electrodes.44,45 Below are a couple of redox reactions that may lead to reduced (Li2S) or oxidized (Li2S2O4) forms for sulfur: LiN(SO2 CF3)2 + ne− + n Li+ → Li3N + Li 2S2 O4 + LiF + C2Fx Li y Li 2S2 O4 + 10e− + 10Li+ → 2Li 2S + 4Li 2O Li 2S2 O4 + 4e− + 4Li+ → Li 2SO3 + Li 2S + Li 2O

The comparison of C 1s spectra after cycling with those obtained for a fresh PYR13/TFSI mixture (see Supporting Information) leads to similar conclusions, particularly for the PYR13 molecule. Except for the first cycle after delithiation and the 100th cycle after lithiation, the relative distribution of sulfur compounds seems stable. This particular behavior is yet unexplained but may be related to the dissolution of some of superficial degradation compounds discussed above. They are probably in the external part of the SEI and could participate in the dynamics of the SEI between lithiation and delithiation. Polysulfides, commonly found in Li−S systems, are known to have a detrimental effect on the durability of the cells due to the so-called polysulfide-shuttle behavior.46−48 However, in our case, the presence of these compounds is not expected to 8137


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Chemistry of Materials observed in the 7Li NMR spectra suggests that species such as LixPFy and LixPOyFz are preferentially formed over LiF. The reactivity of the FEC part is in good agreement with recent results.66 Interesting information can also be obtained from the comparison of 7Li NMR spectra for samples aged in pure IL electrolyte, in carbonate based electrolyte (1 M LiPF6 in EC:DEC + 10 wt % FEC) and in IL/carbonates mix electrolytes (Figure 5).

the electrolyte. Increasing the carbonate content of the electrolyte leads also to a progressive broadening of the signal. The presence of the ionic liquid could thus be viewed as a way of limiting the formation of lithium organic nonfluorinated species at the surface of the silicon nanoparticles, at least in OCV conditions. Electrolytes based on Li salts dissolved in pure ionic liquid, such as the pure IL electrolyte tested in the present work, are known for their high viscosity and low conductivity compared to carbonate based electrolytes. This might impede their use in practical Li-ion batteries. Indeed, a pure Pyr13-TFSI ionic liquid based electrolyte cannot sustain moderately high cycling rates such as C/2 at room temperature. Nevertheless, their expected influence on the nature of species formed in the SEI makes their use as cosolvent of the electrolyte salt promising. In particular, they seem to promote the formation of fluorinated species and lead to a decrease of the lithiated organic species identified as cyclable lithium consumers.15 The evolution of the SEI upon electrochemical cycling has then been monitored in the case of the IL/carbonate mix. The formation of the SEI at the silicon surface is detected during the first lithiation process as in the case of pure carbonate electrolyte.15 Similar 7Li MAS NMR resonances and overall evolution are detected. Minor differences are observed though, in particular at the end of the successive lithiations, and are detailed in the Supporting Information. At the end of the first lithiation, 19F MAS NMR (Figure S4, Supporting Information) clearly indicates the presence of LiF at −204 ppm, along a doublet assigned to PF6− groups (−72/−74 ppm). The signal at −80 ppm already observed in the case of the aging in OCV conditions (see also Figure 6, displaying the 19 F NMR spectrum at the end of the 10th lithiation) is also observed here with probable contributions from fluorophosphates such as LixPFy and LixPOyFz as well as CF3 groups from the TFSI anions or their decomposition products. This particular resonance was not observed during the cycling with conventional carbonate based electrolyte (Figure 6, left) and is therefore a specificity of electrolyte decomposition reactions involving ionic liquid or influenced by the presence of the ionic liquid. It is also interesting to note that the relative amount of LiF is lower in the case of the IL/carbonate electrolyte mix compared to the conventional carbonate electrolyte.

Figure 5. Comparison of 7Li MAS NMR spectra of silicon electrodes aged in pure IL electrolyte (red), in carbonate-based electrolytes (green) and carbonates/IL mix (black).

The broader and less resolved line shape observed for conventional carbonates based electrolyte suggests a wider distribution of Li local environments. The Li local environments appear more defined in the case of the aging in pure IL electrolyte, suggesting that a better control of the species formed at the surface of the silicon electrode would be possible when using an ionic liquid based electrolyte. It also shows that (i) the composition of the surface layer is clearly influenced by the solvents of the electrolyte salt and (ii) the ionic liquid is involved in the formation of decomposition products detected at the surface of the silicon electrode. By comparing the ratio of lithiated nonfluorinated (0.3 ppm) and lithiated-fluorinated species (negative shifts) for the pure IL, mix, and carbonate electrolytes, the contribution of Li (organic) nonfluorinated species clearly increases as the carbonate content increases in

Figure 6. (left) 19F MAS NMR spectra of silicon electrodes cycled in conventional carbonate and the IL/carbonates mix electrolytes and stopped after the 10th lithiation. (right) 7Li MAS NMR spectra of silicon electrodes cycled in the IL/carbonates mix electrolyte and stopped after the 1st, 10th, and 100th delithiations. 8138


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Figure 7. Composite thickness maps computed from STEM−EELS analysis on electrodes cycled in the IL/carbonates electrolyte mix. Silicon (white), LixSi (yellow to red), carbon black (teal), and the IL (blue) are represented in the top row, while the main SEI components, LiF (green), carbonates and organics (purple), are on the bottom row. Color intensity quantitatively scales with thickness. (a,d) 1st lithiation: conformal carbonate/organic SEI layer and large LiF deposits on lithiated nanoparticles. (b,e) 10th lithiation: the carbonate component slowly accumulates while LiF amount is stable. (c,f) 100th lithiation: limited carbonate buildup and no noticeable LiF accumulation.

is similar in width at the end of the first delithiation (Figure S5) but is even wider in the case of IL/carbonates mix after 100 cycles (Figure 8). It confirms that the wider distribution of Li

Upon cycling, the 7Li spectra at the end of delithiation (Figure 6 right) are centered at −0.6 ppm for the first and 10th cycle. The apparent signal is again probably composed of contributions from fluorinated species and organic lithiated species, in agreement with the STEM−EELS mapping of the SEI (Figure 7). After 100 cycles, the signal shifts slightly to −0.3 ppm suggesting an increasing contribution of lithiated organic species. At the same time, the signal also tends to broaden. Since a broad and unresolved signal seems to be the signature of decomposition products when a conventional carbonate electrolyte is used, the broadening observed here seems to be a sign of the degradation of the carbonate part of the IL/carbonates mix electrolyte. In addition, the LixSi alloy component at 1.3 ppm detected in the spectra at the end of lithiations (Figure S4) seems to decrease upon cycling (approximately 45%, 30%, and 25% of the total integrated intensity for the 1st, 10th, and 100th lithiations, respectively) to the benefit of components with lower chemical shift. This suggests that a lower amount of lithium−silicon alloy is formed upon cycling, in agreement with the fading of the electrochemical performance, as more SEI forms. The presence of some intensity in the 1.5−10 ppm range, even after the 100th lithiation, suggests that some Li can be present in the alloy, in agreement with the nonzero specific capacity. Nevertheless, from the electrochemical data, this capacity is reversible. A contribution from lithium trapped in lithium−silicon alloys to the broadening of 7Li NMR spectra at the end of delithiations (Figure 6 right) can therefore be ruled out. The comparison of the 7Li NMR signals for electrodes cycled with IL/carbonates mix and carbonate-based electrolytes clearly shows that the NMR signal corresponding to species in the SEI

Figure 8. 7Li MAS NMR spectra of silicon electrodes cycled in the IL/ carbonates mix and carbonates based electrolytes and stopped after the 100th delithiation.

local environments attributed to the decomposition of the carbonate solvents is also present in the case of mixes, suggesting that the presence of the ionic liquid in the electrolyte mix influences the composition of the SEI upon a long-term cycling, leading to an even wider distribution of lithium local environments. The 19F NMR spectra evolution upon cycling (Figure S4) show no significant differences: contributions from LiPF6 (−72/−74 ppm), fluorophosphates and CF3 groups (−80 ppm), and LiF (−204 ppm) are observed with comparable 8139


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Chemistry of Materials relative intensity, suggesting a quite stable fluorinated part of the SEI. Moreover, no significant variations of the relative amount of LiF could be observed for the successive lithiations and delithiations, supporting the stability of the formed LiF in the SEI. This supports an evolution of the SEI chemistry concerning mostly nonfluorinated, carbonated species simultaneously with the cyclable lithium supply decrease. After 10 cycles, analyses of the STEM−EELS data (Figure 7) indicate that a new unidentified compound is present in large deposits and is interpreted as the PYR13-TFSI, which does not evaporate in the vacuum of the microscope (similarly to XPS). Mapping of amorphous and crystalline silicon areas reveals the same nanostructures that were observed and discussed for cycling in pure carbonate electrolytes. The lithium appears again to be diffusing across grain boundaries and produce an amorphous matrix with crystalline domains.67 In these images, the contributions of LiF and carbonate-like species are clearly evidenced, LiF appearing as patches and carbonates as a conformal layer around the particles. These observations are very similar to those made for the pure carbonate electrolyte study.15 The main difference consists in the presence of a new contribution (in light blue in the images), essentially concentrated within the aggregates. By comparison with reference spectra relative to the IL part of the electrolyte, this contribution is assigned to the non evaporated IL part of the mixes. STEM−EELS spectra are not precise enough or the quantity is not large enough to allow confirming the degradation of the IL part of the electrolyte during cycling, as observed by XPS. Additionally, a closer look at the STEM− EELS spectra corresponding to carbonates shows a clear fading of the Li−K edge, in particular after 100 cycles. This depletion of lithium was also previously observed in the study of the SEI with carbonate-based electrolyte.15 Combined NMR and STEM−EELS results suggest strongly that the carbonate part of the SEI does not seem to form in lesser extent than for electrodes cycled in a conventional carbonate based electrolyte. The presence of ionic liquid does not prevent the degradation of the carbonate part of the IL/ carbonate mix electrolyte upon cycling. The failure mechanism for full Li-ion cells using silicon as the negative electrode and conventional carbonate based electrolytes is due to a lack of cyclable lithium, consumed in parasitic reactions occurring before the accumulation of electrolyte degradation products clogs the porosity of the composite electrode or disconnects the active material particles. For extended cycling, all the lithium available for cycling ends up either trapped in an intermediate part of the SEI, mostly in organic compounds, or in the electrolyte. This nevertheless does not prevent the further degradation of the organic electrolyte solvents, leading to the formation of lithium free organic degradation products at extreme surface of the SEI.15 Results obtained here, in the case of an IL/carbonate mix electrolyte point out that introducing an ionic liquid in a carbonate based electrolyte will not stop the cyclable lithium consumption leading to Li-ion failure. It would be then reasonable to expect a comparable electrochemical behavior when silicon electrodes are cycled in full cell configuration with the IL/carbonates mix electrolyte. Cycling Behavior. Figure 9 displays the typical specific capacity versus cycle number obtained for full Si/NMC Li-ion cells at a C/2 rate (1 Li in 2 h) cycled using conventional carbonate based (1 M LiPF6 in EC:DEC + 10 wt % FEC) electrolyte and the two IL/carbonates based electrolytes (Mix1 and Mix2) investigated in the present study (Mix1 and Mix2)).

Figure 9. Comparison of specific capacities obtained for full Li-ion silicon/NMC cells cycled with conventional carbonates based electrolyte (black) and IL/carbonate electrolyte Mix1 (red) and Mix2 (blue). The specific capacities are expressed in mAh per gram of silicon.

In all cases, the delivered capacity drastically decreases after only a few cycles (less than 10). Conventional carbonates based electrolyte and Mix2 lead to very similar behavior with a specific capacity reaching approximately 200 mAh per gram of silicon or less after 100 cycles. The electrochemical cycling performed with Mix1, although displaying a slower decrease in specific capacity (400 mAh per gram of silicon after 100 cycles), does not show a significant improvement. These results indicate that the minor modifications brought by the modification of the electrolyte formulation to the SEI composition and the presence of ionic liquid in the electrolyte mix has only a minor influence on the capacity retention of the Li-ion cell and the cycle life. The evolution of the galvanostatic profile of the full Si/NMC cell (Figure S7) for each electrode is also similar to that observed in the case of the cycling performed using carbonate based electrolyte.15 Again, after 100 cycles, the silicon electrode reaches a much higher potential (0.25 V) at the end of the cell charge (lithiation of Si) compared to the early stage of the electrochemical cycling. On the other hand, the NMC electrode reaches its cutoff potential of 4.4 V. This behavior indicates clearly that no lithium ions are available anymore from the NMC electrode to lithiate the silicon electrode. In addition, the overall high potentials of both NMC and silicon suggest that both electrodes are in delithiated or almost delithiated states after 100 cycles. Overall, the results obtained here are consistent with our recently published results, where we propose upon cycling a progressive shift of the potential at which the silicon electrode operates (toward higher values). A higher potential then corresponds to distinct, lithium-poor, SEI products formed. The observed behavior of each electrode indicates therefore that the failure mechanism of a full Si/NMC Li-ion cell will be the same whether it is cycled with IL/carbonates mix electrolytes or with a conventional carbonate based electrolyte: the loss of cyclable lithium is the cause of the failure mechanism, rather than the degradation of positive or negative electrode or the blocking of the electrode porosity by electrolyte decomposition products. Such a comparable failure mechanism is in good agreement with the minor changes observed in the composition of the SEI and its evolution upon 8140


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addition, it can be observed that the electrochemical performance, in particular the capacity retention, appears to be better when tested in Pouch cells compared to the Swagelok configuration. This is in agreement with results obtained by B. Vortmann−Westhoven et al. indicating that the lower lithium loss in the pouch cells set up compared to other set-ups would make them more appropriate to study Li batteries in full cell configuration.68 Within the formation cycle at C/10 rate, discharge capacities of 207 mAh (carbonate based electrolyte) and 185 mAh (Mix3) are obtained, in agreement with the expected nominal capacity. Subsequent attempts in cycling at charge/discharge rates higher than C/2 reveal reduced discharge capacities, attributable to kinetic effects. Because of the increased viscosity and reduced conductivity of Mix3, the discharge capacity drops significantly compared to the carbonate based electrolyte system. Moreover, cycles 2 to 50 reveal the drastic impact of continuous mobile lithium losses on the cycle stability of such a Li-ion cell, as the discharge capacity strongly declines with every subsequent cycle. For the carbonate based electrolyte system the discharge capacity of 185 mAh in cycle 2 is reduced by 52.1% when reaching the 50th cycle, while the achievable discharge capacity of cells containing Mix3 is reduced by 48.5% (cycle 2 discharge capacity: 128 mAh). This last result is in good agreement with previous results obtained with Mix1 and Mix2, confirming minor influence on the capacity retention of the Li-ion cell brought by the introduction of 50% of ionic liquid. Improvement of Security. The 0.2 Ah stacked pouch cells were used to investigate the influence of conventional carbonate based electrolyte (1 M LiPF6 in EC:DEC + 10 wt % FEC) and IL/carbonate electrolyte (Mix3) on cells safety. All safety tests were performed with cells containing NMC and NCA positive electrodes by performing repeat determination for each investigated cell setup (Table 3). Figure 11 displays the behavior of the cells obtained by nail penetration tests, while Figure 12 pictures the cells after completing the test and provides a representative time sequence for cells with both electrolytes. For the carbonate based electrolyte, the penetration of the nail resulted in a slight drop of the cell potential followed by heat generation. Although the temperature tends to increase only slightly in the beginning, it is followed by a very quick increase of the temperature (thermal runaway), resulting in a hazard level of 5: fire/flame. The behavior of the cells after the nail penetrated was strongly dependent on the chosen electrolyte/separator system, as for cells containing Mix3 the penetration of the nail resulted in the preferable immediate drop of the cell voltage and almost no heat generation.

cycling. This conclusion is supported by the post-mortem cycling performed on both electrodes and electrolyte/separator. Silicon and NMC electrodes were cycled separately with fresh electrolyte and fresh Li metal electrodes and could deliver performance comparable to that of pristine silicon and NMC electrodes, respectively. Similar experiments were done in the case of the carbonate electrolyte leading to similar results.15 Results obtained in pouch cell configuration are typically closer to the performance of commercial batteries. Since pouch cells are used in the next section concerning safety tests (vide inf ra), it is interesting to compare their electrochemical performance with those obtained in Swagelok configuration. In addition, several electrolytes mixes, with various formulations, have been used for safety test. We chose to include results on Mix3 in the present study due to the fact that Mix2 and Mix3 possess similar conductivities and similar electrochemical behaviors upon cycling. Figure 10 displays the

Figure 10. Achievable discharge capacities in stacked silicon/NMC pouch cells (nominal capacity of ∼0.2 Ah) with a carbonate based electrolyte (black) and an IL/carbonate electrolyte Mix3 (red). Right ordinate displays recalculated values of the obtained specific silicon capacity in mAh per gram of silicon.

discharge capacity versus cycle number obtained for stacked pouch cells (Si vs NMC; nominal capacity: ∼0.2 Ah) cycled at C/2 rate, using a conventional carbonate based electrolyte (1 M LiPF6 in EC:DEC + 10 wt % FEC) and IL/carbonate based electrolyte (Mix3). In addition, the specific silicon capacity was recalculated using the values of negative electrode area, 217 cm2, mass loading negative electrode, 0.83 mg cm−2. Although the main objective of these cells was to perform safety tests, cycling data confirm the operational readiness of the cells and exhibit further confirmation of previously discussed results. In

Table 3. Summary of Performed Safety Tests with Observed Hazard Levels for Each Cell

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Figure 11. Nail penetration test results on 0.2 Ah stacked silicon/NMC pouch cells with carbonate based (left) and IL/carbonate electrolyte Mix3 (right). (Arrows on the abscissa axis correspond to time sequence pictured in Figure 12). Cell potential (in black), temperature of cell surface (in red), temperature of the nail (in blue).

Figure 12. Pouch cells (0.2 Ah stacked silicon/NMC) with carbonate based electrolyte (a) and Mix3 (f) after nail penetration test. Time sequence (corresponding to data presented in Figure 11) for carbonate based electrolyte: before the nail penetrates (b), after the nail penetrated and heat is generated (c), within and after the moment of the thermal runaway (d, e). Time sequence for Mix3: before the nail penetrates (g), after the nail penetrated (h, i).

Additional measurements revealed that the difference in the cell behavior and the type of short circuit produced can not only be attributed to the electrolyte composition but is strongly dependent on the used separator. Because of different wetting behavior of the investigated electrolyte systems, polypropylene microporous film separator was used for carbonate based electrolyte and Freudenberg 2226 for Mix3. While the polypropylene microporous film separator seems to initially keep the electrode stack separated and electrically isolated after the penetration of the nail, Freudenberg separator leads to an immediate short-circuit. Because of this influence of the separator, nail penetration tests do not allow assessing the influence of the chosen electrolyte systems on cell safety. Comparable results were found for the cells with NCA positive

electrode material, indicating that the selection of separator had a higher impact on cell safety than the choice of positive electrode materials. The second safety test (heating test) did not show any significant differences in the tested cells irrespective of the electrolytes or the positive electrode materials (NMC, NCA) used. For this reason, the results are only provided as Supporting Information. They illustrate that the electrochemical process is deeply involved in the safety behavior of these cells rather than the pure chemical reactivity of electrolytes with the materials, even at high temperature. In the case of the overcharge tests, the examined cells were significantly destroyed, providing serious hazard issues. It must be noted that these tests cells are applicable for material testing 8142


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Freudenberg 2226 separator acted more safely compared to carbonate based electrolyte with polypropylen microporous film. Obviously, the influence on cell safety caused by these electrolytes is higher than the effect of the investigated cathode materials, although cells containing NCA positive material tend to act even more unsafely compared to NMC positive electrodes.

but not for consumer applications, as they do not offer a safety vent to release gases or a circuit interrupt device (CID), which would prevent such massive events. Each cell continuously expanded during the test; however, the temperature in cells containing Mix3 increased significantly quicker. The significant difference between the two electrolyte solutions clearly becomes evident when comparing the cells after the performed test in Figure 13. Cells containing carbonate based electrolyte

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CONCLUSION A combination of 7Li, 19F MAS NMR, XPS, and STEM−EELS provided an in-depth characterization of the SEI formation on the surface of silicon and its evolution upon aging in pure IL based electrolyte and cycling in IL/carbonates mixes electrolytes in a full Li-ion cell configuration with no prelithiation of the silicon electrode. In contrast with the aging in 1 M LiPF6 in EC:DEC + 10 wt % FEC electrolyte where degradation of the electrolyte solvents could be observed at room temperature after only a short time of contact, partial degradation of ionic liquid was detected only after 1 month at 55 °C. XPS spectra N 1s and S 2p allowed monitoring the evolution of the IL/carbonate based electrolytes upon cycling of complete Si/NMC batteries and clearly indicated that a degradation of the ionic liquid components (both PYR13 and TFSI) occurs. New contributions on N 1s spectra appearing with the first electrochemical cycle along broadening of the spectra for each step of lithiation and delithiation revealed a partial electrochemical degradation of ionic liquid molecules. S 2p XPS spectra showed, in addition, high Li2S (S reduced form) and Li2SO4 (S oxidized form) contributions. The degradation of both PYR13 and TFSI is confirmed with extended cycling. Changes were observed in the SEI upon the aging process using MAS NMR combined with STEM−EELS. The use of pure ionic liquid as the electrolyte solvent of LiPF6 leads to a much lower amount of detected LiF. The strong contribution of fluorinated species observed in the 7Li NMR spectra suggests that species such as LixPFy and LixPOyFz are preferentially formed over LiF. The Li local environments corresponding to species in the SEI, as observed by NMR, are more defined in the case of the aging in ionic liquid, suggesting that a better control of the species formed at the surface of the silicon electrode would be possible when using ionic liquid. It also shows that (i) the composition of the surface layer is clearly influenced by the solvents of the electrolyte salt and (ii) the ionic liquid is involved in the formation of decomposition products detected on the surface of the silicon electrode. However, the presence of carbonates in the electrolyte mix rules the SEI behavior and lead to a failure mechanism scenario similar to that observed in the case of cycling using a conventional carbonate based electrolyte such as 1 M LiPF6 in EC:DEC + 10 wt % FEC. Indeed, the increasing 7Li NMR contribution from lithiated organic species upon cycling indicates that the presence of the ionic liquid components does not prevent the degradation of carbonates in the parasitic reactions that are consuming the cyclable lithium. As a matter of fact, quite similar electrochemical behaviors are observed for full Si/NCM Li-ion cells using pure carbonates or ionic liquid/ carbonate mix as electrolyte solvents. In both cases, the failure mechanism is assigned to the loss of cyclable lithium rather than the degradation of positive or negative electrode or the blocking of the electrode porosity by electrolyte decomposition products.

Figure 13. Pouch cells (0.2 Ah stacked silicon/NMC and silicon/ NCA) comprising carbonate based and IL/carbonate Mix3 electrolyte after the overcharge test.

resulted in a complete destruction of the cell, with large parts of the electrodes that have left the cell housing. Such behavior is attributable to a hazard level of 6/7: Rupture/Explosion. In contrast, investigated cells containing IL/carbonate Mix3 reacted more safely and showed a hazard level of 5: Fire/ Flames, as no considerable electrode parts left the cell housing. The measurement data obtained for these overcharge safety tests are provided within the Supporting Information. The latter cells open due to the overcharging, but they tend to burn and keep the active materials within the cell. The addressed difference in the vehemence of the event becomes evident when comparing the time sequence in Figure 14. Table 3 summarizes the data of the performed safety tests and assignable hazard levels, allowing the conclusion that cells containing IL/carbonate electrolyte Mix3 in combination with

Figure 14. Time sequence of 0.2 Ah stacked silicon/NMC cell containing carbonate based electrolyte and a 0.2 Ah silicon/NCA stacked cell containing IL/carbonate Mix3 during the overcharge test. 8143


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lithium batteries and fine soils micromechanics. Electrochim. Acta 2014, 120 (20), 319−326. (4) Xu, K. Electrolytes and Interphases in Li-ion Batteries and Beyond. Chem. Rev. 2014, 114, 11503−11618. (5) Nguyen, C. C.; Song, S. W. Interfacial structural stabilization on amorphous silicon anode for improved cycling performance in lithiumion batteries. Electrochim. Acta 2010, 55, 3026−3033. (6) Baggetto, L.; Niessen, R.; Notten, P. On the activation and charge transfer kinetics of evaporated silicon electrode/electrolyte interfaces. Electrochim. Acta 2009, 54, 5937−5941. (7) Winter, M. The Solid Electrolyte Interphase - The Most Important and the Least Understood Solid Electrolyte in Rechargeable Li Batteries. Z. Phys. Chem. 2009, 223 (10−11), 1395−1406. (8) Oumellal, Y.; Delpuech, N.; Mazouzi, D.; Dupre, N.; Gaubicher, J.; Moreau, P.; Soudan, P.; Lestriez, B.; Guyomard, D. The failure mechanism of nano-sized Si-based negative electrodes for lithium ion batteries. J. Mater. Chem. 2011, 21 (17), 6201−6208. (9) Mazouzi, D.; Delpuech, N.; Oumellal, Y.; Gauthier, M.; Cerbelaud, M.; Gaubicher, J.; Dupré, N.; Moreau, P.; Guyomard, D.; Roué, L.; Lestriez, B. New insights into the silicon-based electrode’s irreversibility along cycle life through simple gravimetric method. J. Power Sources 2012, 220 (0), 180−184. (10) Liu, W.; Yang, M.; Wu, H.; Chiao, S. M.; Wu, N. Enhanced Cycle Life of Si Anode for Li-Ion Batteries by Using Modified Elastomeric Binder. Electrochem. Solid-State Lett. 2005, 8 (2), A100− A103. (11) Ryu, J. H.; Kim, J. W.; Sung, Y.; Oh, S. M. Failure Modes of Silicon Powder Negative Electrode in Lithium Secondary Batteries. Electrochem. Solid-State Lett. 2004, 7 (10), A306−A309. (12) Park, S.; Kim, T.; Oh, S. M. Electrochemical Dilatometry Study on Si-Embedded Carbon Nanotube Powder Electrodes. Electrochem. Solid-State Lett. 2007, 10 (6), A142−A145. (13) Reyter, D.; Rousselot, S.; Mazouzi, D.; Gauthier, M.; Moreau, P.; Lestriez, B.; Guyomard, D.; Roué, L. An electrochemically roughened Cu current collector for Si-based electrode in Li-ion batteries. J. Power Sources 2013, 239 (0), 308−314. (14) Lee, S. W.; McDowell, M. T.; Choi, J. W.; Cui, Y. Anomalous Shape Changes of Silicon Nanopillars by Electrochemical Lithiation. Nano Lett. 2011, 11, 3034−3039. (15) Dupré, N.; Moreau, P.; De Vito, E.; Quazuguel, L.; Boniface, M.; Bordes, A.; Rudisch, C.; Bayle-Guillemaud, P.; Guyomard, D. Multiprobe study of the Solid Electrolyte Interphase on silicon based electrode in full cell configuration. Chem. Mater. 2016, 28 (8), 2557− 2572. (16) Delpuech, N.; Dupre, N.; Moreau, P.; Bridel, J. S.; Gaubicher, J.; Lestriez, B.; Guyomard, D. Silicon electrode aging mechanism upon cycling in full Li-ion battery. ChemSusChem 2016, 9 (8), 841−848. (17) Armand, M.; Endres, F.; MacFarlane, E.; Ohno, H.; Scrosati, B. Ionic-liquid materials for the electrochemical challenges of the future. Nat. Mater. 2009, 8, 621−629. (18) Appetecchi, G. B.; Montanino, M.; Zane, D.; Carewska, M.; Alessandrini, F.; Passerini, S. Effect of the alkyl group on the synthesis and the electrochemical properties of N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquids. Electrochim. Acta 2009, 54, 1325−1332. (19) Koch, V. R.; Dominey, L. A.; Nanjundiah, C.; Ondrechen, M. J. The intrinsic anodic stability of several anions comprising solvent-free ionic liquids. J. Electrochem. Soc. 1996, 143, 798−803. (20) McEwen, A. B.; Ngo, H. L.; LeCompte, K.; Goldman, J. L. Electrochemical properties of imidazolium salt electrolytes for electrochemical capacitor applications. J. Electrochem. Soc. 1999, 146, 1687−1995. (21) Suarez, P. A. Z.; Selbach, V. M.; Dullius, J. E. L.; Einloft, S.; Piatnicki, C. M. S.; Azambuja, D. S.; de Souza, R. F.; Dupont, J. Enlarged electrochemical window in dialkyl-imidazolium cation based room-temperature air and water-stable molten salts. Electrochim. Acta 1997, 42, 2533−2535.

Unless pure ionic liquid can be found with sufficient conductivity to avoid the presence of carbonate solvents and its subsequent degradation, the possibility of enhanced control of electrode/electrolyte interface will be impeded and the tailoring of interface will rely on carbonate additives. Performed safety tests revealed the beneficiary influence of mixtures of carbonate based electrolytes with ionic liquids on cell safety. Considering necessary changes of the separator due to wetting reasons and reduced electrolyte kinetics, battery hazard levels could clearly be reduced in the case of mixes of ionic liquid and carbonate based electrolytes. Moreover, the presented work reveals that the difference in safety behavior caused by the two electrolytes is higher than the effect of a different cathode material (NMC vs NCA).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01963. Details on silicon pristine materials, electrode formulation; electrolyte composition and conductivity measurements; details on parameters used for electrochemical cycling of full cells; experimental details on NMR, XPS, and STEM−EELS experiments; experimental details for safety tests; F 1s XPS spectra of lithiated electrodes, corresponding deconvolutions; quantification of XPS data for carbonate/ionic liquid mix electrolyte; XPS spectra of reference PYR13/TFSI; C 1s spectra of PYR13/TFSI mixture and of SEI; 7Li and 19F MAS NMR spectra of silicon electrodes; potential of NMC electrode and silicon electrode as function of lithium content; evolution of temperature and cell potential as function of time for silicon/NAC pouch cells; evolution of state of charge and temperature of cell surface as function of time for silicon/NMC pouch cells (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nicolas Dupré: 0000-0002-0687-9357 Philippe Moreau: 0000-0002-1691-1592 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the European Research Council (ERC), EU FP7 Energy.2013.7.3.3 program on Understanding interfaces in rechargeable batteries and supercapacitors through in situ methods, Grant Award No. 608491 on project “Battery and Supercapacitors Characterization and Testing (BACCARA)”.

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REFERENCES

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