Fluoroethylene Carbonate as an Important Component for the

May 5, 2017 - the operation of EC-free electrolytes in Li-ion cells. Among the ..... EC- or FEC-based electrolyte solutions (Figure 2c). This conclusi...
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Fluoroethylene Carbonate as an Important Component for the Formation of an Effective Solid Electrolyte Interphase on Anodes and Cathodes for Advanced Li-Ion Batteries Elena Markevich, Gregory Salitra, and Doron Aurbach* Department of Chemistry, Bar-Ilan University, Ramat Gan 5290002, Israel ABSTRACT: The performance of lithium-ion batteries (LIBs) depends critically on the nature of the solid− electrolyte interphase (SEI) layers formed on their electrodes surfaces, which are, in turn, defined by the composition of the electrolyte solution. Here, we present a short overview and key results of a systematic study of the application of one of the recently most widely investigated components of the electrolyte solutions for LIBs, namely, fluoroethylene carbonate (FEC). We discuss the benefits of FEC-based electrolyte solutions over the most commonly used ethylene carbonate (EC)-based electrolyte solutions for different LIB systems, including the high-capacity Si anode, high-voltage LiCoPO4 and LiNi0.5Mn1.5O4, Li−sulfur, and other cathodes, as well as full Li-ion cells. Special emphasis is given to the composition and properties of the SEI that is formed on the surface of anodes and cathodes as a result of the electrochemical reduction/oxidation of FEC. organic carbonates and so-called “enablers”, additives that enable the operation of EC-free electrolytes in Li-ion cells. Among the enablers, FEC appeared to be the best for NMC/graphite cells.15 Here, we present the key results of a systematic study performed by our group on the application of FEC-based electrolyte solutions to different systems related to the most important and “hot” topics in modern electrochemistry, namely, high-capacity Si anodes, high voltage −5 V cathodes such as LiCoPO4 and LiNi0.5Mn1.5O4, and high-capacity integrated xLi2MnO3·(1−x)LiNiyMnzCo1−y−zO2 and Li−sulfur cathodes. We discuss the results obtained and compare them with the most commonly used EC-based standard electrolyte solutions. It has been shown that the use of electrolyte solutions containing FEC as a cosolvent exhibits improved performance of Li-metal anodes for rechargeable Li batteries.6−12,16−18 In addition, binder- and additive-free electrodes comprising an amorphous columnar silicon substructure demonstrate excellent low-temperature performance in 1 M LiPF6 in a FEC/DMC electrolyte solution.19 In a previous study, we demonstrated the very stable behavior of monolithic amorphous columnar silicon anodes prepared by DC magnetron sputtering in FEC-based

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he development of advanced rechargeable Li-ion batteries (LIBs) is one of the most important challenges of modern electrochemistry.1,2 Much higher capacity and energy density of the new generations of LIBs are achieved by application of electrode materials with higher capacities, such as Li alloy anodes or sulfur cathodes, or by the use of cathode materials, gaining higher red−ox potentials. The introduction of these electrode materials in LIB technology leads to increased requirements for the electrolyte solutions.2,3 High-capacity anodes such as Si−Li or Sn−Li alloys undergo dramatic volume changes during lithiation and delithiation, leading to disintegration of the electrode structure.4,5 Thus, one of the key properties of the electrolyte solution used with these anodes is the formation of a stable effective elastic solid−electrolyte interphase (SEI) that can withstand large volume/morphology fluctuation upon cycling and prevent continuous electrolyte decomposition and depletion. Fluoroethylene carbonate (FEC) has been proved to be particularly effective in enhancing the Coulombic efficiency and capacity retention of Si anodes.6−12 The development of a 5 V class of cathodes such as spinel LiNi0.5Mn1.5O4 or cobalt phosphate LiCoPO4, which operate at potentials beyond the stability limit of the alkyl carbonate-based electrolytes (4.5 V), requires not only additives but also new bulk electrolyte solvents.3,13,14 For high-voltage LIBs, Dahn et al. tested ethylene carbonate (EC) free-electrolyte solutions comprising linear © 2017 American Chemical Society

Received: February 26, 2017 Accepted: May 5, 2017 Published: May 5, 2017 1337

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Figure 1. Effect of FEC on the cycling performance of monolithic amorphous Si film electrodes (on Cu foils). (a) Specific charge capacity and (b) voltage at the end of charge vs the cycle number. The surface density of the Si electrodes was 1.3 mg cm−2. The rates were 1 C with respect to a limiting capacity of 600 mAh g−1 (see the text for details). (Insets) SEM images and F 1s and C 1s XPS spectra measured from Si electrodes cycled in two electrolyte solutions for 500 cycles, as indicated, and typical voltage profiles of Si anodes measured during the initial cycling.20 Adapted with permission. Copyright 2013, The Electrochemical Society.

are shown in the inset of Figure 1a. It is clear from the figure that, in both cases, cycling of the Si electrodes leads to rifting of the uniform surface of the pristine monolithic Si film into separate islands, about 10−30 μm in diameter. In the case of the FECbased electrolyte, these islands are coated with very homogeneous and relatively thin surface films. For the electrodes cycled in the EC-based electrolyte solution, very thick irregular coating layers cover the islands and fully close the gaps between them in several locations. Obviously, the growth of these layers results in overexpansion of the islands, which leads to their pealing and separation from the copper foil current collector. Thus, growth of the surface resistance and disconnection of the separate islands from the current collector are the most probable reasons for the worse cycling behavior of Si/Li cells in EC-based electrolyte solutions compared to that for the FEC-based ones. F 1s and C 1s XPS spectra of Si electrodes after 500 cycles in the two electrolyte solutions are shown in the inset of Figure 1b. The ratio of LiF to the other F-containing species is higher for the FEC-based solution than that for the EC-based one. The C 1s spectra demonstrate a pronounced difference between the electrodes’ surface chemistry in the two electrolyte solutions in terms of the presence of oxygen-containing species in the surface films. The content of these compounds relative to C−C- and C− H-containing species (285 eV) is much higher in the surface films

electrolyte solutions (Figure 1).20 With the aim of ensuring a long cycle life for the Si electrodes, galvanostatic tests were performed with the limitation of specific capacity, which restricts the amplitude of the repeated expansion/contraction of the Si bulk. In every galvanostatic cycle, Si electrodes were discharged to 10 mV vs Li/Li+ and then charged to the specific capacity of 600 mA h g−1. As a result, we obtained very stable cycling of Si/Li cells for about 2000 cycles for Si anodes with a surface density of 1.3 mg cm−2. Figure 1b shows the dependence of the voltage at the end of the charge. The progressive formation of the surface films on the Si electrodes leads to the growth of their surface resistance. Thus, extraction of the same quantity of Li ions, corresponding to a capacity of 600 mAh g−1, from the Si/Li electrode, discharged to 10 mV, results in an increase in the cell voltage at the end of the charge to the limiting value of 1.2 V. After this point, a rapid decrease in the specific charge capacity of the cells is observed (Figure 1a) because the voltage profile of these Si electrodes in the FEC-based solutions (inset, Figure 1b) becomes stable by about the 200th cycle and does not change over more than 1800 subsequent cycles. In EC-based electrolyte solutions, the electrodes demonstrate a considerably shorter cycle life of about 500 cycles. Scanning electron microscope (SEM) images of Si electrodes cycled for 500 cycles in EC- and FEC-based electrolyte solutions 1338

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Scheme 1. Possible Surface Reactions of FEC on Si Electrodes20,a

a

Adapted with permission. Copyright 2013, The Electrochemical Society.

of the HF elimination from the polyfluoroethylene −(CH2− CHF)n−. This suggestion was proved by the results of ToFSIMS analysis when, in addition to the high density of LiF, the authors observed intense peaks derived from the polyene structure. For the EC-based electrolyte solutions, the peaks of lithium oxides and alkoxides were dominant in the ToF-SIMS spectra. An additional polymerization route involves reactions B and D (Scheme 1), leading to the formation of saturated polyolefin (IV) and, finally, to the unsaturated polyene (V). However, formation of the fluoroethylene CHFCH2 (III) by both routes A and B is accompanied by the formation of Li2CO3 (reaction A) and lithium alkylcarbonates such as VI. The relatively low content of carbonate species in the surface films formed in the FEC-based electrolyte solutions may be explained by their reaction with HF with the formation of CO2 (reactions G, J, and I).22−25 Thus, the accumulation of LiF in the surface films should be accompanied by depletion of the carbonate species, and this trend clearly reveals itself in our XPS results. According to our NMR measurements, a possible explanation for the lower content of LiF and the higher content of the carbonate species in the surface films formed in the EC-based electrolyte solution may be the lower content of HF in the EC-based solution due to the lower rate of reaction of H2O with LiPF6 salt.20 However, examination of various mechanisms of FEC decomposition using DFT calculations and AIMD simulations does not reveal the formation of CHFCH2 as a favorable reaction product or intermediate.26,27 Therefore, the two-electron mechanism is a more favorable path for FEC reduction of the Li-containing Si clusters than the one-electron mechanism. Note that in this discussion we did not consider the presence of salt. Another reaction path of the formation of oxygen-free polymeric species during the reduction of FEC was proposed

formed in the EC-based solution than that in the FEC-based solution. A higher content of oxygen-containing surface species, namely, carbonates (ROCO2Li, C 1s peaks at 290 and 287 eV) and possibly alkoxides and polyethers (ROLi, −CH2O−) (285− 287 eV), was found in the surface films formed in EC-based solutions. In conclusion, the distinctive characteristics of the perfect thin surface films formed on Si anodes in the FEC solutions are • The predominance of LiF over all other F-containing species • The prevalence of the oxygen-free organic species (probably polymeric) over the organic and inorganic carbonates, alkoxides, and oxygen-containing organic polymers • The higher content of LixPOyFz species compared to LixPFy The formation of SEI with a high content of oxygen-free polymeric surface species and LiF and a low content of carbonate and other oxygen-containing species suggests the decarboxylation and defluorination of FEC during reduction of the Si anode with the formation of LiF and the release of CO2. Different reaction paths leading to final SEI components have been proposed in the literature. One possibility is the formation of fluoroethylene CHFCH2 as an intermediate compound.6,21 Possible surface reactions of FEC on Si electrodes proceeding according to this scenario are presented in Scheme 1. Reaction routes A, C, and E in this scheme have been proposed to explain the predominant presence of the oxygen-free species in the C 1s XPS spectra of the Si electrodes cycled in FEC-based electrolyte solutions.6 The authors presumed that the formation of the unsaturated polymer compounds −(CHCH)n− was the result 1339

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HF on the phosphate group of the delithiated (charged) form of this cathode material, as presented below.33

by Shkrob et al., with the use of radiolysis, laser photoionization, electron paramagnetic resonance, and transient absorption spectroscopy.28 The authors assumed that the reduction of FEC yields a vinoxyl radical (reaction 1) that can abstract H atoms from FEC molecules, initiating both the chain reaction causing FEC decomposition and radical polymerization with the formation of polyfluorovynilene carbonate. The resulting polymer can further defluorinate, yielding interior radicals that migrate and recombine to produce a highly cross-linked network. This feature implies that the outer SEI resulting from FEC reduction may exhibit elastomeric properties, which can account for its cohesion during the expansion and contraction of silicon particles in the course of the Li alloying/dealloying cycling. FEC + Li+ + e− → LiF + CO2 + •CH 2CHO

PO4 3 − + HF + H+ ↔ PO3F2 − + H 2O PO3F2 − + HF + H+ ↔ PO2 F2− + H 2O PO2 F2− + HF + H+ ↔ POF3 + H 2O

(2)

Thus, this extremely unstable cathode material needs effective protection in order to be cycled in LiPF6-containing electrolyte solutions. Figure 2a demonstrates the impressive effect of the FEC-based electrolyte solutions on the cycling behavior of composite LiCoPO4 cathodes.34,35 High-resolution SEM (HRSEM) images of LiCoPO4 electrodes cycled in EC- and FEC-based electrolyte solutions (insets in Figure 2a) provide clear visual evidence of the mechanism of their capacity fading. In the EC-based solution, the particles lose mass due to dissolution in the electrolyte solution. In the FEC solutions, however, the LiCoPO4 particles keep their shape and structure when the electrodes are cycled. It is remarkable that, according to the F 1s XPS spectra and in contrast to the Si anodes, the surface films on the LiCoPO4 cathodes do not contain LiF after cycling in either EC- or FEC-based electrolyte solutions (Figure 2c). This conclusion is in line with the observation that SEI on cathodes contains a much smaller amount of LiF than the surface films on anodes in LiPF6-containing electrolyte solutions.22,36,37 Moreover, a lower content of LiF on LiFePO4 cathodes was observed when FEC was used as an additive to the EC-containing electrolytes.38,39 According to the C 1s spectra of the LiCoPO4 cathodes cycled in the two electrolyte solutions (Figure 2d), PEO-like polymer species are more abundant in the surface films formed on the cathodes in the FEC solutions. Interestingly, the cycling behavior of the LiCoPO4 cathodes in the EC-based electrolyte markedly improves when a potentiostatic step is performed for 24 h at 5.2 V before the repeated galvanostatic cycling (Figure 2b). The higher the voltage of the potentiostatic pretreatment, the higher the content of PEO-like polymer species in the SEI composition (Figure 2e) and the better the capacity retention of the LiCoPO4/Li cells (Figure 2b). Analysis of the C 1s spectra of the LiCoPO4 cathodes cycled in these two electrolyte solutions leads us to conclude that the surface films that are formed in the FEC-based electrolyte solutions contain more PEO-like polymeric species and MCO3 than those formed in the EC-based electrolyte solution. In turn, the surface films formed in the EC-based electrolyte solution seem to have a higher content of organic carbonate/polycarbonate species.34 Therefore, processes of FEC decomposition on anodes and high-voltage cathodes in Li ion cells are accompanied by the formation of highly effective compact surface films on both negative and positive electrodes of Li batteries. Our conclusion about better passivation of the surface of highvoltage LiCoPO4 cathodes in the FEC-based electrolyte solutions is reflected in work with another 5 V cathode material, LiNi0.5Mn1.5O4 spinel, whose performance is better in the FECbased solutions than that in the EC-based ones. Figure 3a shows the results of the galvanostatic tests of the LiNi0.5Mn1.5O4/Li cells performed with the two electrolyte solutions. It can be seen that, in both cases, the cells demonstrate very stable cycling behavior and the advantages of one solution composition over the other are not evident. This finding is not surprising as the LiNi0.5Mn1.5O4 spinel is much more stable in LiPF6-containing electrolyte solutions than that in LiCoPO4. However, for full LiNi0.5Mn1.5O4/Si cells, a marked improvement in the perform-

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The formation of the polycarbonate species as a result of FEC reduction on Si anodes was also proposed elsewhere.29,30 However, according to the results of the analysis of the SEI composition described above, polycarbonates are, most probably, not the final components of the surface films that are formed on the surface of the Si anodes in the FEC-based electrolyte solutions. It is worth noting that oxygen-free polymer species have been found to be main components of the SEI formed on negative electrodes when FEC was used as a main component of the electrolyte solution. For example, when FEC was utilized as an additive to the EC-containing electrolyte solution, the reduction products of FEC were found to consist mainly of LiF and −CHF−OCO2-type compounds.31 In general, the composition of the surface films formed on all kinds of lithium-related negative electrodes (Li metal, Li−C, Li− Si) in nonaqueous Li salt solutions highly depends on the cycle life and other conditions.32 As we mentioned above, cycling Si electrodes with the limitation of capacity in different potential ranges leads to a different composition of surface films.20 It should be emphasized that the SEI composition presented and discussed there relates to the prolonged cycling of Si anodes in the systems described therein.20 We emphasize that a deeper understanding of the mechanisms of FEC reduction (cells, conditions, operation regime-depend-

We emphasize that a deeper understanding of the mechanisms of FEC reduction (cells, conditions, operation regime-dependent) that results in the formation of surface films with exceptional elastic properties on negative electrodes should be the subject of further investigation. ent) that results in the formation of surface films with exceptional elastic properties on negative electrodes should be the subject of further investigation. The unique properties of FEC make it an excellent component in electrolyte solutions for Li batteries that significantly improves the cycling performance of both anodes (as discussed above) and cathodes, including those comprising 5 V LiCoPO4, which is very problematic. It has been shown that the main reason for the fast and drastic capacity fading of LiCoPO4 cathodes in standard, ECbased electrolyte solutions is their instability under the attack of 1340

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Figure 2. Effect of FEC on the cycling performance of LiCoPO4/Li cells. (a) Results of galvanostatic cycling of LiCoPO4 electrodes between 3.5 and 5.2 V at a rate of C/5 h (capacity vs cycle number curves) and SEM images of cycled electrodes (insets) in the two electrolyte solutions, as indicated. (b) Capacity vs cycle number during galvanostatic cycling of LiCoPO4/Li cells in EC-based electrolyte solution with potentiostatic steps at 4.4, 4.9, and 5.2 V performed for 24 h during the second charge. (c) F 1s and (d) C 1s XPS spectra of pristine and cycled LiCoPO4 electrodes in the two-electrolyte solutions as indicated. (e) Discharging of LiCoPO4 electrodes in EC-based electrolyte solution after potentiostatic steps performed for 24 h at 4.4, 4.9, and 5.2 V.34 Adapted with permission from the American Chemical Society, Copyright 2014.

ance with FEC-based electrolyte solutions was observed, compared to that in EC-based solutions (Figure 3b).40 In the EC-based solutions, after 20−30 charge−discharge cycles, a sudden increase in the irreversible capacity is always observed. This behavior is typical of the so-called “shuttle” mechanism, where the species that are formed on the anode as a result of side parasitic reduction processes are transferred to the cathode side and are oxidized on the cathode during charging. It is clear that good passivation of both the cathode and anode surfaces can prevent this failure scenario. Indeed, when the cells are cycled in the FEC-based solutions, they demonstrate very stable cycling, with a charge−discharge efficiency approaching 100%. Moreover, LiCoPO4/Si full cells demonstrated reasonable cycling performance for more than 100 cycles with FEC-based electrolyte solutions.20 Very stable behavior in the FEC-based electrolyte solution was also demonstrated for TiS2/Si and highcapacity integrated xLi2MnO3·(1−x)LiNiyMnzCo1−y−zO2/Si full cells.12,41 Finally, FEC also shows exceptional effectiveness as a component in excellent electrolyte solutions for Li−sulfur batteries. It is known that the most commonly used solutions

Processes of FEC decomposition on anodes and high-voltage cathodes in Li ion cells are accompanied by the formation of highly effective compact surface films on both negative and positive electrodes of Li batteries. for Li−S batteries are ethereal electrolyte solutions, based on mixtures of 1,3-dioxolane (DOL) and dimethyl ether (DME). However, the solubility of the sulfur reduction products (Lipolysulfides in them) involves a critical problem of Li−S cells, namely, the “shuttle” phenomenon, which implies that the dissolved long-chain polysulfides may be continuously reduced at the Li-metal anode during cell charging and, therefore, can never be fully oxidized at the cathode side.42 As opposed to ethereal electrolyte solutions, the electrophilic organic carbonates have been shown to be reactive toward the lithium polysulfides, which are strong nucleophiles.43 Therefore, alkyl carbonate solvents are 1341

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unsuitable for Li−S battery applications. However, it was found that for several types of carbon hosts sulfur−carbon (S/C) composite electrodes with sulfur encapsulated inside of the carbon pores may be reversibly lithiated in organic carbonate electrolyte solutions, demonstrating a voltage response very similar to that of solid-state Li−S batteries.44−46 This type of behavior is associated with the reaction of desolvated Li ions with sulfur inside of the pores of the carbon host and is known as a quasi-solid-state (QSS) reaction.47 We show that the formation of effective SEI on the surface of S/C composite electrodes makes the operation of S/C composite electrodes possible and effective via the QSS mechanism. It is remarkable that in this case too FEC-based electrolyte solutions remarkably outperform the EC-based solutions.48 This result was achieved for S/C composite electrodes prepared with different carbon hosts and is attributed to the ability of FEC to form more effective protective surface films on the surface of the composite electrodes, as well as to the lower desolvation energy of Li ions in FEC-containing solutions.49 The positive effect of FEC on the formation of SEI on S/C electrodes has also been observed previously.50 An example of excellent cycling performance of Li− S cells demonstrating thousands of cycles of S/C composite cathodes operating according to the QSS mechanism in FECbased electrolyte solution is shown in Figure 4a.51 The carbon for these electrodes was prepared by carbonizing polyvinylidene dichloride (PVDCDC). Surface films formed on S/PVDCDC

Figure 3. Curves of charge (full dots) and discharge (hollow dots) capacity vs cycle number obtained upon galvanostatic cycling of LiNi0.5Mn1.5O4/Li (a) and LiNi0.5Mn1.5O4/Si (b) cells at a current rate of C/8 in the EC-based and FEC-based electrolyte solutions, as indicated (30 °C).34 Adapted with permission. Copyright 2014, American Chemical Society.

Figure 4. (a) Galvanostatic cycling results of Li/S-PVDCDC cells (a typical capacity vs cycle number curve). (b) SEM images and EDS results measured with composite S-PVDCDC electrodes cycled for 320 cycles. (c) F 1s XPS spectra of a pristine composite S-PVDCDC electrode and a S-PVDCDC electrode cycled for 320 cycles. The electrolyte solution was 1 M LiPF6/FEC/DMC.51 Adapted with permission. Copyright 2016, The Electrochemical Society. 1342

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current collector of positive electrodes, we believe that progress in the development of the electrolyte solutions will relate to modification of organic components. Thus, not only FEC but also other fluorinated solvents deserve deep further investigation for the design of advanced Li ion batteries.

electrodes cycled in FEC-based solutions are visible in SEM images (Figure 4b). As clearly seen in Figure 4b, all of the PVDCDC particles are uniformly coated by surface films with attached bright spherical droplets of various sizes up to about 300 nm in diameter. This type of surface film morphology is described in the literature for surface films that are formed on graphite composite electrodes in LIBs.52,53 The agglomerates thus observed were assigned to LiF crystallites. This conclusion is in line with the results of the energy dispersive spectroscopy (EDS) measured from the surface of the spherical agglomerates (insets in Figure 4b) and the XPS spectra of cycled electrodes (Figure 4c). The sites marked in red in Figure 4b obviously originate from the occasional removal of LiF crystals. These traces provide additional evidence of the existence of surface films on the smooth part of the S−C composite particles, which include polymeric matrixes in which the LiF crystals are embedded. Thus, SEI-type surface films that are formed on the surface of the S/C composite electrodes during the initial discharge play a key role in the operation of the S/C electrodes via the QSS mechanism. The surface films thus formed prevent the encapsulated sulfur from detrimental direct contact with the liquid electrolyte solution and facilitate desolvation of the Li ions before they react with the sulfur. We can now conclude that FEC is a very promising component in electrolyte solutions for rechargeable Li batteries. FEC is sufficiently versatile to improve the protective properties of surface films formed on negative electrodes due to its reduction at low potentials (down to 0 V vs Li/Li+) and on highvoltage cathodes due to its oxidation. This ability of FEC to protect the surface of electrodes in advanced Li batteries over a wide potential range enables elaboration of high-voltage full cells with improved characteristics. The mechanism of decomposition of FEC in different potential regions in real Li battery systems is not fully understood yet and requires further investigation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Doron Aurbach: 0000-0002-1151-546X Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. Biographies Elena Markevich (Ph.D.) is a Senior Scientist in the Electrochemistry Group in the Department of Chemistry at Bar Ilan University. Her main research interests are in the development of advanced Li batteries. Gregory Salitra (Ph.D.) is a Senior Scientist in the Electrochemistry Group in the Department of Chemistry at Bar Ilan University. His main research interests are in the development of advanced Li batteries. Doron Aurbach is a full professor in the Department of Chemistry, leading the electrochemistry group at Bar Ilan University (BIU). He is a MRS, ECS, and ISE fellow. He leads INREP  Israel National Research center for Electrochemical Propulsion (22 research groups). His team studies the electrochemistry of active metals and nonaqueous electrochemical systems, develops spectroscopic methods (in situ and ex situ) for sensitive electrochemical systems, studies electrochemical intercalation processes, electrochemical water desalination, and electronically conducting redox polymers, and develops rechargeable high-energy-density batteries and EDL capacitors. The group collaborates with several academic groups throughout the world and with a number of leading industries in Israel and abroad (e.g., BASF Germany and GM U.S.A.).

SEI-type surface films that are formed on the surface of the S/C composite electrodes during the initial discharge play a key role in the operation of the S/ C electrodes via the QSS mechanism. The surface films thus formed prevent the encapsulated sulfur from detrimental direct contact with the liquid electrolyte solution and facilitate desolvation of the Li ions before they react with the sulfur.

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ACKNOWLEDGMENTS Partial support for this work was obtained by the Israel Science Foundation (ISF) and Israel Committee for High Education (CHE) in the framework of the INREP project.

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REFERENCES

(1) Armand, M.; Tarascon, J.-M. Building Better Batteries. Nature 2008, 451, 652−657. (2) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the Development of Advanced Li-ion Batteries: a Review. Energy Environ. Sci. 2011, 4, 3243−3262. (3) Xu, K. Electrolytes and Interphases in Li-Ion Batteries and Beyond. Chem. Rev. 2014, 114, 11503−11618. (4) Kasavajjula, U.; Wang, C.; Appleby, A. J. Nano- and Bulk-SiliconBased Insertion Anodes for Lithium-Ion Secondary Cells. J. Power Sources 2007, 163, 1003−1039. (5) Su, X.; Wu, Q.; Li, J.; Xiao, X.; Lott, A.; Lu, W.; Sheldon, B. W.; Wu, J. Silicon-Based Nanomaterials for Lithium-Ion Batteries: A Review. Adv. Energy Mater. 2014, 4, 1300882. (6) Nakai, H.; Kubota, T.; Kita, A.; Kawashima, A. Investigation of the Solid Electrolyte Interphase Formed by Fluoroethylene Carbonate on Si Electrodes. J. Electrochem. Soc. 2011, 158, A798−A801.

It is worth noting that currently not only FEC but also other fluorinated solvents attract considerable attention as components of the electrolyte solutions for Li batteries. The reason is that many of such solvents, including fluorinated organic carbonates and ethers, have a wide electrochemical stability window, low flammability, and the ability to form effective surface films on both negative and positive electrodes and current collectors.54−56 These properties of the fluorinated solvents are manifested by outstanding cycling performance of Li cells reported in the last years. Taking into consideration the unique properties of LiPF6 salt, which ensure excellent conductivity of the electrolyte solutions and effective passivation of the Al 1343

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