Insight into the electrochemical behaviors of 5V–class high–voltage batteries composed of lithium–rich layered oxide with multifunctional additive

Journal of Power Sources xxx (2016) 1e10

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Insight into the electrochemical behaviors of 5Veclass highevoltage batteries composed of lithiumerich layered oxide with multifunctional additive Sang Hoo Lim a, Woosuk Cho a, Young-Jun Kim b, Taeeun Yim c, * a b c

Advanced Batteries Research Center, Korea Electronics Technology Institute, Seongnam-si, Gyeonggi-do, 463-816, Republic of Korea SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Gyeonggi-do, 16419, Republic of Korea Department of Chemistry, Incheon National University, Academy-ro 119, Songdo-dong, Yeonsu-gu, Incheon, 460-772, Republic of Korea

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


TMSOMs is proposed as an interfacestabilizing additive for cathode material.
TMSOMs affords an effective protection layer on cathode surface.
It greatly suppresses electrolyte decomposition and transition metal dissolution.
TMSOMs affords excellent high temperature cycling and rate performances.
Comprehensive mechanism for role of TMSOMs is clarified by systematical analyses.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 August 2016 Received in revised form 16 October 2016 Accepted 1 November 2016 Available online xxx

(Trimethylsilyl)methanesulfonate (TMSOMs), functionalized with task-specific chemical moieties, is proposed as an interface-stabilizing additive to improve the electrochemical performance of 5V-class layered over-lithiated oxides (OLOs). TMSOMs offers a great opportunity to enhance the interfacial stability of an OLO material by providing an effective protective layer composed of eSO3e and eOeSie functional groups after its electrochemical oxidation over 4.0 V (vs. Li/Liþ), which remarkably reduces the internal pressure of the cell associated with electrolyte decomposition. As a result, the cell employing TMSOMs affords excellent capacity retention (92.8% at 100 cycles) together with considerable rate performance, negligible transition metal dissolution, and stable high temperature performance based on its enhanced interfacial stability. These results are attributed to the synergistic effects of the eSO3e and eOeSie functional groups that once the sulfonic ester-based protective layer is developed on the electrode surface, it effectively mitigates decomposition of the electrolyte, while the eOeSie functional groups readily scavenge fluoride species in the electrolyte, leading to outstanding interfacial stability for the OLO material. On the basis of spectroscopic evidence, a comprehensive mechanism for the action of TMSOMs is suggested considering the specific role of each functional group. © 2016 Elsevier B.V. All rights reserved.

Keywords: Battery Over-lithiated oxide Interfacial chemistry Sulfonic ester Silyl

* Corresponding author. E-mail address: [email protected] (T. Yim). http://dx.doi.org/10.1016/j.jpowsour.2016.11.002 0378-7753/© 2016 Elsevier B.V. All rights reserved.

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1. Introduction In recent decades, lithium-ion batteries (LIBs) have become widely regarded as the most promising energy conversion/storage systems based on their excellent calendrical life, moderate rate capabilities, and low self-discharge behaviors [1e3]. Nevertheless, there are serious restrictions to the widespread use of current LIBs, especially in large-scale devices such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and energy storage systems (ESSs), because of their relatively low energy densities [4e6]. In this regard, layered over-lithiated oxides (OLOs, a Li2MnO3$(1-a) Li(NixCoyMnz)O2) have attracted considerable attention as innovative cathode materials because they not only afford remarkable specific capacities (>200 mA h g1) but also much higher working potentials (>4.5 V, vs. Li/Liþ) than conventional 4Veclass cathode materials such as lithium cobalt oxide (LiCoO2) and lithium nickelcobalt-manganese oxides (Li(NixCoyMnz)O2) [7e12]. However, several challenging issues remain: i) a high working potential is always accompanied by severe electrolyte decomposition during electrochemical charging/discharging [13e18], and ii) transition metal dissolution from the cathode seriously accelerates the rapid fading of cycling performance [19e21]. These concerns originate mainly from the vulnerable interfacial stability between the cathode and electrolyte, and therefore, achieving much higher interfacial stability is indispensable for ensuring the stable electrochemical performance of OLO materials. To address these problems, herein we propose the use of a multifunctional additive, (trimethylsilyl)methanesulfonate (TMSOMs), which intrinsically comprises eSO3e and eOeSie functional groups (Fig. 1a). It was anticipated that electrochemical oxidation of the sulfonic ester would effectively mitigate the oxidative decomposition of the bulk electrolyte by generating a stable protective layer on the OLO electrode surface, which would prevent continuous electron transfer from the electrolyte to the electrode [22e27]. Moreover, the eOeSie component was expected to scavenge any fluoride species (F) remaining in the electrolyte, which would greatly reduce the availability of F for reaction with the transition metal species (Ni, Co, and Mn), on the basis of the strong binding affinity of Si with F. This is important because the presence of nucleophilic F in the electrolyte would seriously accelerate deterioration of the cathode by chemical reaction with electrophilic transition metal components, resulting in readily electrolyte-soluble metal-fluoride adducts [28e34]. Note that although the simultaneous use of additives containing individual eSO3e and eOeSie species (i.e., the components achieved by the retrosynthetic analysis of TMSOMs) would seem to be an alternative approach by which to achieve this beneficial synergy, eOeSie functional groups are too hydrophobic to be miscible with the conventional carbonate-based solvents and would disturb the effective Liþ-ion migration in the cell. Thus, the structural combination of eSO3e with eOeSie in the additive was considered an attractive strategy for simultaneously stabilizing both interfacial electrolyte decomposition and transition metal dissolution behaviors. Here, we systematically explore the respective role of each functional group by focusing on its chemical/electrochemical reactivity, and demonstrate the effects of TMSOMs on the overall electrochemical performance of OLO electrodes. 2. Experimental sections The standard electrolyte (1 M LiPF6 in 1:2 ethylene carbonate (EC): ethyl methyl carbonate (EMC)) was supplied by PanaxEtec (>99.9%, battery grade). (Trimethylsilyl)methanesulfonate (TMSOMs, Aldrich, >99%) was purchased commercially. All materials were used as received without further purification. The anodic stability of each electrolyte was determined by linear

Fig. 1. (a) 2D and 3D structures of TMSOMs and material strategies, and (b) Linear sweep voltammetry (black: standard electrolyte, and blue: TMSOMs-controlled electrolyte). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

sweep voltammetry (LSV) using an electrochemical workstation (Biologic, SP-300). Three-electrode beaker-type cells with glassy carbon (7.07  102 cm2) as the working electrode and lithium foil as the counter and reference electrodes were assembled. The LSV was measured at a scan rate of 0.1 mV s1 in the 3.0e5.0 V vs. Li/Liþ range. The electrochemical behavior of the TMSOMs on the OLO electrodes was examined by the potentiostatic intermittent titration technique (PITT) using an electrochemical workstation (Series 4000, MACCOR). Three-electrode beaker-type cells assembled with the OLO electrode (as the working electrode) and lithium foil (as the counter and reference electrodes) were charge-polarized at 3.3 V (vs. Li/Liþ) until a low steady-state current was attained; then, the potential was increased to 4.6 V (vs. Li/Liþ) with a potential step of 10 mV. To investigate the effect of the additive on the internal pressure of the cell, the OLO electrode was placed into the sample station of a homemade in-situ pressure-monitoring cell equipped with a pressure sensor (UNIK5000, General Electric) and then electrolyte (with or without TMSOMs) was added into sample station. Thereafter, PE separator and Li metal anode were placed into sample station. The internal pressure was recorded as a function of the electrochemical potential while the cell potential was simultaneously increased up to 4.6 V (vs. Li/Liþ) at a current density of 0.1 C. To evaluate the effect of TMSOMs on electrochemical

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performance, OLO electrodes were prepared as follows. A mixture of 0.5 Li2MnO3
0.5 LiNi0.4Co0.2Mn0.4O2 (Ecopro, Korea), poly(vinylidene fluoride) (PVdF, KF1100, Kureha), and Super P® carbon black was dispersed in N-methyl-2-pyrrolidone (NMP) at a weight ratio of 92:4:4 and stirred for more than 6 h. The slurry was coated on a piece of aluminum foil, and the resulting electrode pastes were dried in a vacuum oven at 120  C for 12 h. The loading and electrode densities were fixed at 14.63 mg cm2 and 2.98 g cm3, respectively. The 2032 coin-type cells were fabricated using an OLO cathode, lithium metal anode, a poly(ethylene) separator (PE, Asahi), and 80 mL of electrolytes controlled with or without TMSOMs. The cells were galvanostatically charged to 4.6 V (vs. Li/ Liþ), and the charging was continued until the current decreased to 0.02 C (constant current-constant voltage mode). Then, the charged cells were discharged to 2.0 V (vs. Li/Liþ) repeatedly at a constant current density of 0.1 C (constant current mode) for two cycles (formation step) and 0.5 C for one hundred cycles at room temperature, using a charge/discharge unit (TOSCAT-3100, TOYO System). To determine the rate capability, the cells were galvanostatically charged to 4.6 V (vs. Li/Liþ) at a constant current of 0.1 C, and the charging was continued until the current decreased to 0.02 C (constant current-constant voltage mode). Thereafter, the cells were discharged to 2.0 V (vs. Li/Liþ) at current densities of 0.1, 0.2, 0.5, 1.0, and 2.0 C (constant current mode). To fabricate the full cells, an anode slurry composed of graphite (7 mm, Posco Chemtech), Super P®, carboxymethyl cellulose (CMC, Cellogen, DKS), and styrene-butadiene rubber (SBR, Zeon, BM 400 B) at a weight ratio of 96:1:1:2 was prepared in distilled water. The slurry was coated onto a piece of copper foil, and the resulting electrode was dried in a vacuum oven at 120  C for 12 h. The loading and electrode densities were fixed at 8.45 mg cm2 and 1.50 g cm3, respectively. The 3450 pouch-type cells consisting of a graphite anode, an OLO cathode, and a PE separator were assembled with 500 mL of electrolytes and the cells were charged to 4.5 V and discharged to 2.0 V repeatedly with a constant current density of 0.1 C for two cycles (as a formation step) followed by 0.5 C for one hundred cycles at room temperature using a charge/discharge unit (TOSCAT-3100, Toyo). After completion of the cycling tests, each cell was disassembled in an Ar-filled glove box to avoid contamination, and the electrodes were lightly washed with dimethyl carbonate (DMC). The surface morphologies of the electrodes were analyzed by field-emission scanning electron microscopy (FESEM, Quanta 3D FEG, FEI). The chemical components corresponding to the protective layer on the electrode surface were characterized by Fourier-transform infrared spectroscopy (FT-IR, VERTEX 70, Bruker) in the attenuated total reflectance (ATR) mode and X-ray photoelectron spectroscopy (XPS, Al K-alpha, Thermo-Scientific) under N2 atmosphere in a dry room (dewpoint < 60  C). Electrochemical impedance spectroscopy (EIS) was measured with an AC signal with an amplitude of 5 mV over a frequency range of 1 MHze10 mHz using an electrochemical workstation (Biologic, SP-300). For storage performance at high temperature (60  C), the threeelectrode beaker-type cells were fully charged to 4.6 V (vs. Li/Liþ) and kept in a 60  C oven for various lengths of time. Changes in the open circuit potential (OCP) were first measured, and thereafter, the cells were fully discharged to 2.0 V (vs. Li/Liþ). The degree of metal dissolution was measured by inductively coupled plasmaemass spectrometry (ICP-MS, Aurora 60, Bruker), and recovery tests were performed after storing the cells for two weeks. To investigate the F scavenging activity of TMSOMs, ex situ NMR analysis was conducted as follows. Because of the difficulty in safely handling HF, it was produced by the chemical reaction between LiPF6 and water. In detail, the standard electrolyte (EC:EMC ¼ 1:2 þ 1 M LiPF6) was placed in a Nalgene bottle, and

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water (1000 ppm) was added to produce HF. After stirring for more than 2 h, the water was fully converted to HF by the hydrolysis of LiPF6, and the TMSOMs was slowly added into the HF-containing solution. Further stirring for 12 h allowed completion of the chemical reaction between TMSOMs and HF. Thereafter, the supernatant was analyzed by 1H NMR (ASCEND 400, Bruker). To monitor the chemical reactivity between TMSOMs and oxygenated species, ex situ NMR analyses of supernatants were again conducted after the completion of the chemical reactions. First, the reaction between TMSOMs and lithium oxide (Li2O, Aldrich) was examined. Note that the nucleophilicity of Li2O is similar to those of Li2O2 and Li2O [35,36], and therefore, this reaction can effectively elucidate the reaction outcomes between TMSOMs and oxygenated anionic species. Second, the chemical reactivity of TMSOMs with neutral oxygen (O2) was probed by the reaction of TMSOMs, LiPF6, and potassium peroxide (KO2). The combination of LiPF6 and KO2 spontaneously releases O2 by a metal exchange reaction (KO2 þ 2 LiPF6 / Li2O2 þ K(PF6)2) followed by the disproportionation of Li2O2 (2 Li2O2 / 2 Li2O þ O2[). This is a convenient way to control the O2 concentration and monitor the chemical reaction between TMSOMs and O2 [15,35,36]. After these chemical reactions were complete, the supernatants were analyzed by 1H NMR (ASCEND 400, Bruker). 3. Results and discussion To investigate the electrochemical behavior of TMSOMs, its anodic stability was examined by linear sweep voltammetry (LSV), as shown in Fig. 1b. Although the overall anodic behaviors were identical regardless of the additive up to 4.0 V (vs. Li/Liþ), the electrolyte controlled with the TMSOMs showed considerable oxidation currents in the 4.0e4.3 V (vs. Li/Liþ) range, which might be attributed to the electrochemical reaction of TMSOMs. This is consistent with our material strategy, i.e., the formation of a protective layer on the electrode surface via the electrochemical reaction of the additive. Additional electrochemical measurements of the OLO electrodes by the potentiostatic intermittent titration technique (PITT) support this explanation (Fig. S1). Initially, additional contributions to the specific capacity in the cell controlled with TMSOMs are observed in the 4.0e4.3 V (vs. Li/Liþ) range, which is in accordance with the LSV analysis. Moreover, the cell cycled with TMSOMs exhibits a less-polarized potential profile at higher potentials (>4.4 V vs. Li/Liþ), which can be ascribed to the electrochemical oxidation of TMSOMs. The effect of TMSOMs on interfacial stability can be estimated from the in-situ potential/pressure profile shown in Fig. 2. The measurement of internal pressure as a function of potential can be useful for understanding the effects of the additive on interfacial stability because changes in the internal pressure are associated with electrolyte decomposition that occurs at the electrode/electrolyte interface [37e40]. As a result, TMSOMs seems to be effective for suppressing the electrochemical decomposition of the electrolyte, because the cell cycled with TMSOMs shows a lower internal pressure than the cell with only the standard electrolyte. The TMSOMs-controlled cell achieves internal pressure stabilization after 2 h, whereas the cell with only the standard electrolyte shows continuous pressure increases until the end of measurement. This might be attributed to the enhancement of interfacial stability through the formation of a stable protective layer on the electrode surface near the de-lithiation potential of the layered Li(NixCoyMnz) O2. The TMSOMs enables the formation of a stable protective layer near 4.0 V (vs. Li/Liþ) by electrochemical oxidation, and it substantially reduces electrolyte decomposition in the cell at an early stage of the charge process. In contrast, the use of the standard electrolyte alone does not form an effective protective layer on the

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Fig. 2. In-situ pressure/potential profiles for standard electrolyte (black) and TMSOMscontrolled electrolyte (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

electrode surface, resulting in the continuous increasing internal pressure in the cell. In addition, the protective layer derived from TMSOMs seems to be effective for mitigating electrolyte decomposition, even at high potential, since the cell controlled with TMSOMs exhibits a lower internal pressure at the end of the charge process (4.6 V vs. Li/Liþ) than the cell without TMSOMs (113.8 vs 127.2 kPa), because it suppresses further electrochemical decomposition of the electrolyte at high potential. The overall electrochemical performance in terms of cycle life and rate capability was examined by varying the TMSOMs amount (Fig. 3). In the potential profile analysis, the overall shapes of the charge-discharge curves in the initial cycle are identical regardless of the additive, although the cell controlled with 5% TMSOMs shows slightly more pronounced polarization behavior (Fig. 3a). TMSOMs also affords significant enhancements in cycling performance, as shown in Fig. 3b. After 100 cycles, the cell cycled with the standard electrolyte shows a remaining discharge specific capacity of 64% of its initial value (142.1 mA h g1). In contrast, the TMSOMsadded cells show remarkable improvement in cycle retention, and all cells containing TMSOMs exhibit capacity retentions exceeding 77.0% at the end of cycling. These results accord well with preliminary studies by the Li group, which intensively demonstrated that the use of functional additives is highly effective for improving the electrochemical performance of OLO electrodes: boron-based additives were found to remarkably enhance the interfacial stability of the OLO electrode, affording excellent cycling retention even after a few hundred cycles [41e44]. Although the capacity retention of the cells employing TMSOMs is slightly inferior compared with boron-based additives, the use of TMSOMs combined with bi-functional groups is still effective as an alternative approach to improving the cycling performance of OLO electrodes. In the potential profiles after 100 cycles, although all cells show fading compared to the initial cycle with the increase in the number of cycles, the TMSOMs-controlled cells exhibit relatively less polarization than the cell cycled with only the standard electrolyte (Fig. 3a, dotted line). More polarization can be regarded as evidence of increasing internal resistance corresponding to electrolyte decomposition [45e47], and it can therefore be estimated that the cells cycled with TMSOMs suffer from fewer undesired interfacial reactions. The rate capability tests are also in agreement with the cycling performance, because even though the cell controlled with 5% TMSOMs exhibits inferior discharge specific capacity at high rates (1.0 and 2.0 C), the 1% and 3% TMSOMs-based cells still afford

Fig. 3. (a) Potential profiles at 1 cycle (solid line) and 100 cycles (dotted line), (b) cycling performance at room temperature, and (c) rate capabilities (black: standard electrolyte, red: 1% TMSOMs-controlled electrolyte, blue: 3% TMSOMs-controlled electrolyte, and green: 5% TMSOMs-controlled electrolyte). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

excellent rate performance compared to the cell cycled with only the standard electrolyte (Fig. 3c). This indicates that the use of 5%

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TMSOMs may be excessive, and that 1% and 3% TMSOMs are appropriate. Additional SEM analyses for the cycled electrodes provided more evidence to clarify the effect of TMSOMs on interfacial stability (Fig. 4aed). After 1 cycle, the surface morphology of each electrode seems identical regardless of the additive. In contrast, the surface morphologies are easily distinguishable after 100 cycles, as the TMSOMs-controlled electrode has a cleaner and more uniform morphology than the electrode without TMSOMs, which seems to be covered with thick layers of electrolyte decomposition products. Moreover, the EDS analysis provides additional informative clues that help to clarify the specific role of TMSOMs: both S and Si, as key elements in the molecular structure of TMSOMs, are detected in the electrodes after both 1 and 100 cycles (Fig. S2). This indicates that TMSOMs allows uniform coverage on the electrode surface during the initial formation step, and this effect remains even after longterm cycling, which seems to be responsible for the improved electrochemical performance. EIS analyses further clarified the effect of TMSOMs on interfacial stability (Fig. S3). After the initial cycle, the electrode cycled with TMSOMs shows a slightly larger semicircle at middle-to-high frequencies, which corresponds to the resistance of the solid electrolyte interphase because of the formation of a protective layer on the electrode surface (RSEI: 21.96 U for the standard electrolyte and 22.39 U for the TMSOMs-controlled electrolyte) [48,49]. This leads to a huge difference in the resistance behaviors after 50 cycles, as the electrode cycled with TMSOMs shows a relatively low value of RSEI (27.18 U) and RCT (30.11 U), whereas the electrode cycled without TMSOMs displays significant increases in all resistance levels (RSEI: 43.39 U and RCT: 31.95 U). Note that these remarkable differences in internal resistance are mainly associated with the interfacial stability between the electrode and electrolyte [50,51], and are in line with our electrochemical and analytical results. Therefore, it can be concluded that TMSOMs is effective in

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mitigating (or suppressing) undesired chemical/electrochemical reactions at the electrode/electrolyte interface through the formation of a protective layer on the OLO electrode surface. The effectiveness of TMSOMs was also observed in hightemperature storage tests, as shown in Fig. S4. During selfdischarge behavior measurements at 60  C, the TMSOMscontrolled cells exhibit smaller decreases in the open circuit potential (OCP) for every week of storage (Fig. S4a). Self-discharge behaviors are highly associated with interfacial stability, and if an electrode with a deep state-of-charge (SOC) is exposed to a hightemperature environment, the electrode becomes more unstable. At that stage, the loss of electrons in the electrolyte is encouraged and decreases the overall electrode potential, which allows the electrode to become more stable. The electrolyte is therefore easily decomposed at the surface, resulting in a decrease in the OCP of the cell [32,52]. Thus, TMSOMs is effective at minimizing electrolyte decomposition at the electrode surface. Similar behaviors were observed in additional electrochemical storage tests performed at 60  C with variations of the storage time (Figs. S4b and S4c), in which the TMSOMs-added cell retained 87.9% of its capacity after six weeks, whereas the cell without TMSOMs continuously suffered from the loss of discharge specific capacity, showing only 78.9% capacity retention. Therefore, it can be concluded that TMSOMs effectively stabilizes the interfacial chemistry at the OLO surface, leading to excellent electrochemical performance. To better understand the properties of the protective layer that develops on the electrode surface, each cycled electrode was analyzed by FT-IR (Fig. 5). Each electrode contained common chemical components made of electrochemical adducts derived from the bulk solvents, such as polycarbonate (C]O, 1767 cm1), alkyl carbonate (ROCO2R, 1,557, 1,524, and 1447 cm1), and lithium carbonate (Li2CO3, 1339 cm1) [14,32,53]. Interestingly, additional signals are observed for the TMSOMs-controlled electrode at 1 cycle: eSO3e (1310 cm1) and eOeSie functionalities (1,177, 1,155,

Fig. 4. SEM images for OLO electrode (a) cycled without TMSOMs (1 cycle), (b) cycled with TMSOMs (1 cycle), (c) cycled without TMSOMs (100 cycles), and (d) cycled with TMSOMs (100 cycles).

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Fig. 5. Fourier-transform infrared spectra for OLO electrode at (a) 1 cycle and (b) 100 cycles (black: standard electrolyte; and blue: TMSOMs-controlled electrolyte). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

1,128, and 1070 cm1) are evident, in contrast to the electrode without TMSOMs (Fig. 5a) [53]. This observation is well consistent with the XPS results (Fig. S5), which showed spectroscopic evidence for the presence of eSO3e (166.9, and 169.4 eV) [54e56] and eOeSie functionalities (101.9 eV) [57,58] in the TMSOMscontrolled electrodes. It should be noted that these functional groups form the molecular structure of the TMSOMs additive. Therefore, it can be supposed that the electrode surface is mainly modified by the electrochemical reaction of TMSOMs. After 100 cycles, the electrode surfaces show additional noticeable changes. For example, the transmittance signals corresponding to eOeSie connectivity nearly disappear after the 100 cycles, whereas the sulfonic ester is still present (Fig. 5b). Note that these functional groups originate from the single molecular structure of TMSOMs. This implies that the eSO3e group is responsible for forming the backbone of the protective layer. If the eOeSie group acted as the main backbone of the surface film, the eSO3e species would not be observed on the electrode surface after 100 cycles, because most of film components do not remain on the electrode surface as the scavenging reaction proceeds. In practice, when the recovered TMSOMs-based electrolyte (after the storage test at high temperature) is re-assembled with pristine cathodes and re-evaluated by electrochemical testing, superior electrochemical performance is observed compared with the pristine electrolyte (Fig. S6). Note that, if films derived from TMSOMs are decomposed in the cell, the physicochemical and electrochemical properties of the recovered electrolytes are directly affected, leading to poor electrochemical performance. This means that the TMSOMs-derived eSO3e group still imparts a certain level of surface stability, although the eOeSiefunctional group selectively reacts with nucleophilic species in the cell. These results can be rationalized by the task-specific role of the eOeSie functional group, i.e., the scavenging of the F species. It is assumed that, even though the concentration of F species inevitably increases in the electrolyte as the number of cycles increases [25,59,60], the eOeSie functional group (which has excellent binding affinity with F) can continually remove the generated F species, leading to a more stable interface. Preliminary studies using additives functionalized with the eOeSie moiety well support this explanation: 1) an additive having a eOeSie functional group effectively scavenged F species remaining in the cell and led to remarkable cycling performance for several kinds of cathode materials [15,61,62] and 2) even for an anode material, the eOeSie group greatly enhances surface stability by capturing HF species which can seriously disturb the

formation of an SEI layer on the anode surface [63]. In addition, computational studies have indicated that additives containing the eOeSie functional group seem to be effective for decreasing the chemical reactivity of F species, as the formation of the SieF chemical bond is strongly and thermodynamically preferred [19,64]. To demonstrate this explanation, we conducted accelerated dissolution tests for fully charged electrodes, varying the amount of additive and storing the cells at high temperature (60  C) for two weeks. After this time, the metal components dissolved in the electrolyte were quantified by ICP-MS (Fig. S7). Whereas the degree of dissolution is negligible in the TMSOMs-controlled cell (Ni ¼ 115.1 ppm, Co ¼ 2.0 ppm, and Mn ¼ 4.4 ppm), the cell cycled with only the standard electrolyte has large amounts of dissolved metal components (Ni ¼ 5629 ppm, Co ¼ 2.7 ppm, and Mn ¼ 234.7 ppm). Note that metal dissolution is greatly accelerated in the presence of F [65e67]. Therefore, the much lower metal concentrations in the TMSOMs-controlled electrolyte can be rationalized by the scavenging capability of the eOeSie functional group. To support this conjecture, we investigated the chemical reactivity of the eOeSie groups with F ions, by subjecting the electrolyte to accelerated decomposition followed by treatment with TMSOMs. Note that TMSOMs has a eOeSie functional group in its molecular structure. TMSOMs, which is a chemical equivalent of eOeSie, chemically reacted with F at elevated temperatures (60  C). The resulting supernatant was analyzed by 1H NMR spectroscopy to investigate the changes in the molecular structure of the eOeSie moiety (Fig. 6). 1H NMR can afford informative spectroscopic evidence for interpreting the scavenging pathway of the eOeSie functional group, and confirm whether additional chemical compounds are formed from the decomposition of TMSOMs, including methanesulfonic acid (CH3SO3H, s, 3.01 ppm, identified with commercially available CH3SO3H), fluorotrimethylsilane ((CH3)3SiF, TMSF, s, 0.14 ppm) [66,68,69], and trimethylsilanol ((CH3)3SiOH, TMSOH, s, 0.05 ppm) [70,71]. These additional chemical compounds are observed in the supernatant after completion of the reaction. On the basis of such spectroscopic evidence, a plausible reaction mechanism for the scavenging pathway is suggested in Fig. S8. Because the eOeSie component is electrophilic, nucleophilic F can attack the most electrophilic reaction site of Si (nucleophilic substitution), affording the decomposition products (CH3SO3H, (CH3)3SiF, and (CH3)3SiOH). This implies that the chemical reactivity of F is greatly reduced, and therefore, any

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Fig. 6. 1H NMR spectrum of supernatant after completion of chemical reaction between TMSOMs and F.

undesired reaction between F and the cathode material can be effectively suppressed, resulting in negligible dissolution of the metal components in the TMSOMs-controlled cell. We additionally analyzed reaction supernatants by ex situ NMR to evaluate the chemical reactivity between oxygenated species and TMSOMs. Since oxygenated species can be produced during the electrochemical charging process of an OLO electrode, these ex situ NMR analyses could provide informative clues toward elucidating the underlying role of TMSOMs in the cell (see details in the experimental section). Interestingly, spectroscopic results similar to those from the ex situ NMR analysis of the reaction between TMSOMs and HF are observed: both methanesulfonic acid and trimethylsilanol (the main decomposition products from TMSOMs) appear after the chemical reaction between the oxygenated species and TMSOMs (Fig. S9). These results indicate that the eOeSie functional group can also scavenge oxygenated species in the cell. In practice, the internal pressure of the OLO-based cell is greatly reduced when the cell is cycled with the TMSOMs-controlled electrolyte (Fig. 2). Therefore, we can conclude that TMSOMs is effective for not only removing F species but also scavenging oxygenated species in the OLO-based cell, resulting in improved safety characteristics. With these results in hand, we then fabricated full cells composed of graphite and OLO electrode materials and evaluated their electrochemical performance (Fig. 7). Dissolution of the transition metals, especially Mn, significantly decreases the cycling performance of the graphitic anode because nucleation of the dissolved Mn species on the anode surface severely disturbs the formation of a stable SEI layer, which results in poor cycling performance [72e75]. In this respect, examination of the full-cell performance is an effective way to evaluate the F scavenging performance of TMSOMs. In the initial formation/cycle voltage profile (Fig. 7a and b), the TMSOMs-modified cell shows lesspolarized behavior, in agreement with the result of its half-cell test. The TMSOMs-controlled cell also exhibits remarkable cycling retention behavior even after 100 cycles (89.1% (retention ratio was calculated based on highest discharge capacity of the cell) vs. 51.9%). In addition, the TMSOMs-controlled cell exhibits a much higher average Coulombic efficiency (99.8%) than the cell cycled with the standard electrolyte (97.9%). These results imply that, as proposed, TMSOMs effectively enhances the interfacial stabilities of not only OLO cathode but also the graphitic anode.

Fig. 7. (a) Voltage profiles for graphite/OLO full cells at 1 cycle (solid line) and 100 cycles (dotted line). (b) Capacity retention at room temperature (black: standard electrolyte, and blue: TMSOMs-controlled electrolyte). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The comprehensive effects of TMSOMs on the electrode surface chemistry are presented in Fig. 8, and a mechanism is proposed for the formation of the protective layer on the basis of the analytical results (Fig. S10). If the electrochemical reaction of TMSOMs occurs, the oxygen in the eOeSie functional group may be first oxidized owing to its vulnerable electrochemical stability [22,24,25], resulting in the formation of an oxidized intermediate. Although it could substantially decompose via pathway 1 or 2, pathway 1 would likely be preferred, because the larger sulfur atom is better able to stabilize the positive charge than the smaller oxygen atom in terms of charge delocalization [33,34]. After formation of cationic sulfone (2) through pathway 1, it may further react with TMSOMs remaining in the electrolyte. Note that the oxygen atoms in S]O (in TMSOMs) have partial negative charge by the formation of a resonance structure (S]O 4 SþeOe). Therefore, these nucleophilic O can attack the positively charged electrophilic sulfur atom in cationic sulfone (2), giving new intermediates (3-1 and 3-2) that are stabilized by the formation of additional resonance structures. Thereafter, the generated silanol radical species (counterparts of cationic sulfone, 2) can react with the S]O bond in intermediate (3) to afford new silyl-substituted radical intermediate (4). Since

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S.H. Lim et al. / Journal of Power Sources xxx (2016) 1e10

Fig. 8. Scheme for the role of TMSOMs during the electrochemical process.

there is a high concentration of cationic sulfone (2) on the electrode surface as a result of the electrochemical oxidation of TMSOMs, the unstable radical intermediate (4) can react with cationic sulfone (2) on the electrode surface. Through these repeating processes, TMSOMs might form the protective layer on the surface of the OLO electrode, although the overall reactions occurring in the cell are likely more complicated than the suggested mechanism. The electrochemical oxidation of TMSOMs affords a sulfonic ester (eSO3e)-based protective layer on the electrode surface, which effectively reduces the decomposition of the bulk solvents and helps improve the long-term cycling behavior of the cells. The sulfonic ester (eSO3e)-based protective layer is effective for not only increasing the electrode stability but also for increasing the electrolyte stability. An additional chemical component containing a eOeSie functional group develops on the protective layer, which effectively suppresses the dissolution of transition-metal components. This is because the eOeSie functional group greatly reduces the F concentration in the cell owing to a chemical scavenging reaction, which is attributed to the high binding affinity of Si with F. Therefore, TMSOMs, which contains task-specific functional groups, remarkably enhances the overall interfacial stability of the OLO electrode, leading to stable electrochemical performance. 4. Conclusions To overcome critical restrictions to the application of OLO electrodes, we proposed a bi-functional additive, (trimethylsilyl) methanesulfonate, to enhance the interfacial stability between the electrode and electrolyte. TMSOMs, which consists of eSO3e and eOeSie functionalities, forms a protective layer on the electrode surface upon its electrochemical oxidation at 4.0 V (vs. Li/Liþ). The eSO3e component effectively mitigates the decomposition of the bulk solvent as the main backbone of the protective layer, considerably reduces the internal pressure and resistance, and maintains

a clean surface morphology. In addition, the eOeSie functional group effectively scavenges nucleophilic F and additional oxygenated species generated in the cell, reducing both transition metal dissolution and the internal pressure. TMSOMs imparts excellent electrochemical performance, and all TMSOMs-controlled cells showed excellent capacity retention at the end of cycling along with remarkable rate capability. In addition, the cell cycled with the TMSOMs exhibited superior long-term storage performance at elevated temperature (60  C), less pronounced self-discharge behavior, and much higher capacity retention even after six weeks, which are strongly associated with interfacial stability. The ICP-MS analyses showed that the cell controlled with TMSOMs showed considerably reduced amounts of dissolved metals in the electrolyte (Ni: 115.1 ppm, Co: 2.0 ppm, and Mn: 4.4 ppm), and a scavenging mechanism was established based on spectroscopic evidence, considering the chemical reactivity of the ROSiR3 functional group. Therefore, TMSOMs, proposed as a bi-functional material strategy, effectively modulated the overall interfacial stability with respect to electrolyte decomposition and metal dissolution, which increased the applicability of the OLO. We believe that this combinatorial material strategy, based on a comprehensive understanding of the specific roles of the functional groups, could be extended to not only LIBs but also to diverse energy conversion/ storage systems. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2016R1C1B1009452). Appendix A. Supplementary data Supplementary data related to this article can be found at http://

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