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Sep 7, 2017 - ABSTRACT: Tris(trimethylsilyl) phosphite (TMSP) has received considerable attention as a functional additive for various cathode materia...
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Tris(trimethylsilyl) Phosphite as an Efficient Electrolyte Additive To Improve the Surface Stability of Graphite Anodes Taeeun Yim*,† and Young-Kyu Han*,‡ †

Department of Chemistry, Research Institute of Basic Sciences, College of Natural Science, Incheon National University, 119, Academy-ro, Yeonsu-gu, Incheon 22012, Republic of Korea ‡ Department of Energy and Materials Engineering, Advanced Energy and Electronic Materials Research Center, Dongguk University-Seoul, Seoul 100-715, Republic of Korea S Supporting Information *

ABSTRACT: Tris(trimethylsilyl) phosphite (TMSP) has received considerable attention as a functional additive for various cathode materials in lithium-ion batteries, but the effect of TMSP on the surface stability of a graphite anode has not been studied. Herein, we demonstrate that TMSP serves as an effective solid electrolyte interphase (SEI)-forming additive for graphite anodes in lithium-ion batteries (LIBs). TMSP forms SEI layers by chemical reactions between TMSP and a reductively decomposed ethylene carbonate (EC) anion, which is strikingly different from the widely known mechanism of the SEI-forming additives. TMSP is stable under cathodic polarization, but it reacts chemically with radical anion intermediates derived from the electrochemical reduction of the carbonate solvents to generate a stable SEI layer. These TMSP-derived SEI layers improve the interfacial stability of the graphite anode, resulting in a retention of 96.8% and a high Coulombic efficiency of 95.2%. We suggest the use of TMSP as a functional additive that effectively stabilizes solid electrolyte interfaces of both the anode and cathode in lithium-ion batteries. KEYWORDS: solid electrolyte interphase, lithium-ion battery, phosphite, additive, graphite

1. INTRODUCTION Graphite has become the most popular anode material since the commercialization of lithium-ion batteries (LIBs), and finding an appropriate electrolyte combination is considered one of the challenging issues limiting high cell performance.1−5 Propylene carbonate (PC) is a promising solvent for use with graphite anodes due to its high dielectric constant and wide liquid range. However, the electrochemical reduction of PC results in continuous electrolyte decomposition, which leads to poor interfacial stability of the graphite and electrolyte interfaces.6−8 In addition, the cointercalation of PC into graphite restricts its widespread application on graphite anodes because it causes an exfoliating graphite structure, leading to structural deformation of graphite.9−11 This issue was surmounted by employing a carbonate−solvent combination composed of ethylene carbonate (EC) with acyclic carbonate. The EC solvent does not cointercalate into the layered structure of the graphite; however, it forms an effective solid electrolyte interphase (SEI) layer on the graphite surface.6−8,12 In addition, the use of EC with acyclic carbonate can successfully overcome the physical limitations of EC (high melting point) as these mixtures exist in a liquid state across a wide temperature range due to the low melting temperature of acyclic carbonate. Since the introduction of carbonate-based solvents, numerous functional additives have been investigated for graphite anodes to provide further improvements in the cycle life of the cell.6−8,13 These studies have typically focused on reductiontype functional additives, which have a higher electrochemical © 2017 American Chemical Society

reduction potential than the solvent components. Most have focused on improving the SEI stability, with reduction-type organic materials such as vinylene carbonate (VC),14−16 vinyl ethylene carbonate,17,18 allyl ethyl carbonate,19 vinyl acetate,20 and divinyl adipate20 receiving significant attention as functional additives. In particular, VC has been successfully commercialized for use in LIBs, as it provides effective SEI layers on graphite surfaces by the electrochemical reduction of VC. Note that attaining effective SEI layers is very important for a graphite anode because a high interfacial stability is closely associated with a long-term cycling performance as a result of suppression of undesired surface reactions, such as electrolyte decomposition.6,7 In this sense, VC is regarded as a representative additive for graphite anodes in terms of the cycling performance. Despite its apparent advantages, VC suffers from a poor compatibility with high capacity or potential-based cathode materials, such as Ni- (NCM) or Li-rich (OLO) layered oxides.21−23 As with a graphite anode, VC also creates cathode−electrolyte interphase (CEI) layers on cathodes by electrochemical oxidation due to the presence of the reactive CC bond. However, these VC-derived CEI layers are unstable and cause fading of the cell, when compared to a cell without VC. This means that another CEI-forming additive Received: July 31, 2017 Accepted: September 7, 2017 Published: September 7, 2017 32851

DOI: 10.1021/acsami.7b11309 ACS Appl. Mater. Interfaces 2017, 9, 32851−32858

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Figure 1. (a) 2D- and 3D-chemical structures of TMSP and (b) LSV results for the standard electrolyte (black) and TMSP-controlled electrolyte (blue).

rather than a graphite anode, although understanding of the chemical/electrochemical reactivity of TMSP under a reductive environment is also significant in light of the full-cell level of LIBs. In this work, we have focused on clarifying the chemical/ electrochemical behaviors of TMSP on the graphite anode surface. This is the first report on the underlying reaction pathway for the TMSP reactions that considers the intrinsic chemical/electrochemical reactivity on the graphite anode surface. We found that TMSP improves the interfacial stability of a graphite anode by the formation of SEI layers, but its reaction pathway is vastly different from that initiated by wellknown SEI-forming additives such as VC. TMSP contributes the formation of effective SEI layers, combined with EC-derived radical anion intermediates by chemical reactions rather than by electrochemical reduction. We believe that the understanding of the task-specific role of TMSP based on its intrinsic molecular property and reactivity will provide new insight into the design of advanced functional additives working for both the cathode and anode surfaces in LIB systems.

should be used together with VC in the electrolyte to retain the surface stability of both the cathode and anode materials. Tris(trimethylsilyl) phosphite (TMSP) has recently received substantial attention as an effective functional additive for cathode materials.1,23−36 Bhat et al.24 reported that the cycling performance of a cell composed of LiCoO2 and LiFePO4 was greatly improved by employing TMSP among various Sifunctionalized functional additives at high temperatures. Choi et al.1 demonstrated that TMSP-derived CEI layers effectively suppress electrolyte decomposition with minimizing structural degradation of a Li1.17Ni0.17Mn0.5Co0.17O2 cathode surface, thereby affording a superior capacity retention. They also showed that the use of TMSP effectively alleviates the hydrolysis of LiPF6 by a HF scavenging reaction and increases the surface stability by the formation of a CEI layer on a LiNi0.5Mn1.5O4 cathode25,32 and on spinel lithium manganese oxide.34 Wang et al.,26 Mai et al.,27 and Sinha et al.28 also reported that TMSP enhances the long-term cycling performance of a LiNi0.33Co0.33Mn0.33O2 cathode because the protective CEI layer that develops on the LiNi0.33Co0.33Mn0.33O2 surface reduces the undesired surface reaction associated with electrolyte decomposition, as well as the dissolution of transition metals. Peebles et al.31 proved that the use of TMSP as a functional additive is effective for improving the cycling performance of a LiNi0.5Co0.2Mn0.3O2 cathode because the surface film formed by TMSP slows down the rise in internal resistance and decreases the amounts of transition-metal components during electrochemical cycling. Our group23,29,30 also reported that TMSP greatly improves the cycling retention of a cell composed of a 0.5Li2MnO3·0.5LiNi0.4Co0.2Mn0.4O2 cathode and outlined a detailed working mechanism, in combination with systematic analyses for cycled cells, as well as first-principles calculations that consider the chemical reactivity of TMSP. Kim et al.33 suggested a plausible reaction pathway for the HF scavenging reaction of TMSP and suggested, using first-principles calculations, the replacement of a methyl group of TMSP with an electron-donating group to encourage the OSi cleavage pathway for the HF scavenging reaction. Most of these studies on TMSP have focused on verifying the electrochemical aspects of TMSP limited to cathodes,

2. EXPERIMENTAL SECTION A mixture of EC and ethyl methyl carbonate (EMC) (1:2, vol %) with 1 M lithium hexafluorophosphate was used as a standard electrolyte (PanaxEtec), and TMSP (Aldrich) was added into the standard electrolyte in change of wt % ratio. The cathodic stability was measured by linear sweep voltammetry (LSV) using an electrochemical workstation (Biologic, SP-300). Glassy carbon was used as the working electrode, and lithium foils served as the counter and reference electrodes. The scan rate was 1 mV s−1 with the voltage range being 3.0−0.0 V (vs Li/Li+). The electrochemical performance was evaluated by preparing a graphite anode as follows: a mixture of graphite (PoscoChemtech), Super P (carbon black), carboxymethyl cellulose (CMC, Cellogen, DKS), and styrene−butadiene rubber (SBR, Zeon, BM 400B) was prepared with a weight ratio of 96:1:1:2 and subjected to fine stirring for 3 h. The resulting slurry was then coated on Cu foil, and the electrode was dried in a vacuum oven at 100 °C for 3 h. The loading density of the electrode was controlled at 8.75 ± 0.09 mg cm−2. Coin cells were assembled from graphite, lithium metal, a poly(ethylene) separator (Asahi), and electrolyte, with or without TMSP. The cells were discharged to 0.01 V (vs Li/Li+) and charged to 1.5 V (vs Li/Li+) at a constant rate of 0.1 C for two initial cycles (formation step) and 32852

DOI: 10.1021/acsami.7b11309 ACS Appl. Mater. Interfaces 2017, 9, 32851−32858

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ACS Applied Materials & Interfaces

Figure 2. Electrochemical performance for a graphite anode: (a) Coulombic efficiencies, (b) potential profiles (line = at 1 cycle, dot = at 100 cycles), and (c) cycling performance. (d) Cycling performance for a cell composed of a graphite anode and an OLO cathode (black = standard electrolyte, red = 0.5% TMSP-controlled electrolyte, blue = 1.0% TMSP-controlled electrolyte, green = 3.0% TMSP-controlled electrolyte, and orange = 3.0% VC-controlled electrolyte). 1.0 C for 50 cycles. The cycle life was examined using a charge/ discharge unit (TOSCAT-3100, TOYO) at room temperature. After the cycling measurements, the cells were disassembled in a glovebox to recover the cycled electrodes, which were quickly washed with dimethyl carbonate (PanaxEtec). The top morphology of the electrodes, with atomic compositions developed on the graphite surface, were analyzed by field-emission scanning electron microscopy (FESEM, Quanta 3D FEG, FEI). The chemical composition on the surface of the cycled graphite was verified by analyzing the recovered electrodes by solid-state nuclear magnetic resonance spectroscopy (NMR, ASCEND 400, Bruker) and X-ray photoelectron spectroscopy (XPS, K alpha, Thermo-Scientific). All analytical experiments were performed at room temperature. The ground-state structures of the molecules have been fully optimized within the C1 symmetry by means of density functional theory (DFT) methods. The Kohn−Sham equation was calculated with the B3PW91 functional37,38 and 6-311G(d,p) basis sets of triple-ζ quality. This study employs the conductor-variant polarized continuum model (CPCM),39 which places the solute in a molecular-shaped cavity imbedded in a continuum dielectric medium. A dielectric constant of 31.9 was adopted as a weighted average value between the dielectric constants of ethylene carbonate (EC = 89.2) and ethyl methyl carbonate (EMC = 2.9) because an EC/EMC = 1:2 (vol %) solution was used as the solvent in this work. Han and coworkers40−42 demonstrated that the CPCM calculations are effective for evaluating various electrochemical properties in a battery electrolyte. All of the DFT and CPCM calculations were performed with the program package Gaussian 09.43

Figure 3. SEM analyses for the graphite anode cycled with the standard electrolyte at (a) 1 cycle and (b) 50 cycles and with 3% TMSP-controlled electrolyte at (c) 1 cycle and (d) 50 cycles.

3. RESULTS AND DISCUSSION The cathodic stability of electrolytes was measured by LSV (Figure 1). All the electrolytes exhibited similar cathodic stability during cathodic polarization: electrochemical reduction

occurred at around 0.65 V (vs Li/Li+) regardless of the presence of TMSP. Note that the electrochemical reduction potential for EC initiates at around 0.65 V (vs Li/Li+).44,45 This 32853

DOI: 10.1021/acsami.7b11309 ACS Appl. Mater. Interfaces 2017, 9, 32851−32858

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Figure 4. 31P NMR analysis for cycled graphite for (a) all ranges and (b) expanded ranges from 50 to −20 ppm (black = standard electrolyte and blue = 3% TMSP-controlled electrolyte).

eV) < EC (1.05 eV) < EC−EC (1.16 eV). Interestingly, the natural population analysis indicates that electron entry into EC and TMSP is 94% and 6%, respectively, which implies that the EC molecule is reduced in the EC−TMSP complex. Lastly, we tried to calculate the RP value of the EC−Li+−TMSP complex. The natural population analysis showed that the EC, Li+, and TMSP moieties receive 94%, 4%, and 2% of the electrons, respectively, implying that EC is reduced in the complex. Our DFT calculations imply that TMSP is quite stable under the reductive condition and does not create SEI layers solely by electrochemical reduction. This is very interesting, in that the electrochemical reductive decomposition of the SEI-forming additives is regarded as a natural mechanism for forming SEI layers on a graphite anode surface.6,8 The electrochemical performance was significantly improved in the cell cycled with the TMSP-controlled electrolyte (Figure 2). The cell cycled with the standard electrolyte showed a Coulombic efficiency of 93.1%. The cell cycled with the VCcontaining electrolyte had a slightly lower Coulombic efficiency of 92.9%. The lower Coulombic efficiency of the VC-controlled cell is attributed to the electrochemical reduction of VC, which is responsible for forming SEI layers on the graphite surface. Conversely, the cell cycled with the TMSP-containing electrolyte exhibited improved Coulombic efficiency: the electrolyte controlled with 3.0% TMSP gave a remarkable Coulombic efficiency of 95.2%. For the cycling performance, the cell cycled with the standard electrolyte showed 69.7% specific capacity remaining after 50 cycles. The addition of VC to the standard electrolyte greatly improved the cycling performance (97.4%), as VC creates effective SEI layers on the graphite surface. Interestingly, the cell cycled with TMSP also exhibited a comparable cycling performance: the cell cycled with 3.0% TMSP retained 96.8% of its cycling performance. Note that the cell cycled with TMSP exhibited a high initial Coulombic efficiency and high capacity retention comparable to the VC case. This is a clear indication of the effectiveness of TMSP in terms of cell design in battery manufacturing. If the initial Coulombic efficiency is low owing to the electrochemical reactions of the electrolyte (which participates in the formation of the SEI layer), the use of excess cathode material will be necessary to balance the ion storage between the anode and the cathode. In this case, the use of excess cathode material increases the weight of the cell, decreasing the overall energy density of LIBs. However, the use of TMSP allows a high Coulombic efficiency and high capacity retention to be retained at the first cycle. This can reduce the required weight of the

Figure 5. XPS analyses of cycled graphite for (a) C 1s, (b) F 1s, (c) P 2p, and (d) Si 2p (black = standard electrolyte and blue = 3% TMSPcontrolled electrolyte).

indicates that the observed reductive currents in all the electrolytes are attributed to the reductive decomposition of EC, rather than TMSP reduction. This explanation is well supported by a first-principles calculation study for the reduction potential of TMSP: the reduction potential of TMSP (0.34 eV) was calculated to be ∼0.7 V lower than the EC value (1.05 eV).18 Herein, we calculated the RP values of Li+−EC, Li+−TMSP, (EC)2, (EC)(TMSP), and EC−Li+− TMSP because the incorporation of the ion and solvent can be important in redox potential calculations.46−50 The RP calculation procedure was described elsewhere.18 The optimized structures are depicted in Figure S1. First, the attachment of a Li+ ion causes an increase in the RP values for both EC and TMSP, but the RP value of Li+−EC (1.26 eV) is 0.33 eV larger than the Li+−TMSP value (0.93 eV), indicating a more readily reduced nature of EC. Second, the calculated RP values increase in the order EC−TMSP (1.00 32854

DOI: 10.1021/acsami.7b11309 ACS Appl. Mater. Interfaces 2017, 9, 32851−32858

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Figure 6. (a) Binding energies of TMSP and EC with Li+ and (b) decomposition Gibbs free energies (ΔG in kcal mol−1) for the Li+−TMSP and CH3O− reaction and the Li+−EC and CH3O− reaction.

recovered graphite cycled with TMSP exhibited a relatively uniform surface that seemed to be well covered with an SEI layer. Interestingly, the atomic compositions related to TMSP were observed in the cycled graphite with TMSP, when analyzed by EDS (Figure S1). The EDS results, combined with the LSV analyses, indicate that TMSP contributes to the formation of the SEI layer by a reaction pathway (vide infra) other than an electrochemical reaction. After 50 cycles, the graphite cycled with the TMSP-containing electrolyte exhibited a relatively clean and uniform surface state, whereas the surface of the graphite cycled with the standard electrolyte seemed to be covered with decomposed adducts originating from the electrochemical decomposition of the electrolyte. This result strongly indicates that TMSP improves the interfacial stability of the graphite anode.

cathode material in LIBs when compared with LIBs assembled with typically used electrolytes, thereby contributing to an increased energy density of the cell, together with remarkable cycle life, which is an attractive point in terms of cell engineering. We also demonstrated the effectiveness of TMSP in an OLO/graphite full-cell. The cell cycled with the standard electrolyte showed a continuous fading of cycling performance: only 41.8% of capacity remained after 100 cycles. By contrast, the cell cycled with 3% TMSP-controlled electrolyte exhibited a much improved retention: 83.3% of capacity still remained when compared with its initial cycle. The SEM analyses supported the effectiveness of TMSP for improving the cycling performance (Figure 3). After one cycle, the surface of the graphite anode cycled with the standard electrolyte showed an irregular morphology. By contrast, the 32855

DOI: 10.1021/acsami.7b11309 ACS Appl. Mater. Interfaces 2017, 9, 32851−32858

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ACS Applied Materials & Interfaces The cycled cell was investigated by 31P NMR spectroscopy to analyze the chemical compounds in the SEI layers (Figure 4). The cycled graphite with the TMSP-controlled electrolyte exhibited several new peaks in the downfield region of the 31P NMR spectroscopy scan. All the observed peaks corresponded to phosphonate functional groups derived from the chemically decomposed products of TMSP.51,52 The detection of the phosphonate groups can be considered to indicate that TMSP participates in the SEI layer as a chemically modified form. If the graphite surface contains low-oxidation-state phosphorus elements derived from phosphonate groups via a surface reaction, these elements would improve the surface stability as lithiated phosphorus forms.53,54 The XPS studies revealed distinct chemical compositions of the cycled graphite electrodes depending on the use of TMSP (Figure 5). The typical SEI components, such as RCH2OLi and RCH2OCO2Li formed by EC reduction, were commonly observed for cycled graphite in C 1s, but their intensity was lower in the cycled graphite with the TMSP-controlled electrolyte than with the standard electrolyte. In addition, the peaks for solvent decomposition became smaller in the cycled graphite without TMSP than that cycled with TMSP/LiF (687.6 eV in F 1s) and LixPFy (137.4 eV in P 2p). Instead, the peak intensity of LixPOyFz, which can be regarded as one of the effective SEI components to improve surface stability, was increased (685.3 eV in F 1s and 134.8 eV in P 2p), and new peaks were observed at 130.9 eV (P 2p) and 101.9 eV (Si 2p), which are ascribed to TMSP. The XPS analyses also supported that TMSP participated in the formation of the SEI layer. Several reaction pathways for the possible reactions between TMSP and a nucleophile are calculated, and their plausibility is demonstrated via first-principles calculations (Figure 6). A simple CH3O− group, among the alkoxy (RO−) functional groups that can be generated by the reductive decomposition of EC, is selected as a nucleophile to simplify the calculations. Comparison of the reactivity between TMSP and the nucleophile with that between EC and the nucleophile is very useful for understanding the initial reactivity of a TMSP and EC mixed electrolyte under the reductive condition. The Li+coordinated EC is known to execute a nucleophilic attack to form a ring-opened linear structure on the graphite anode surface. Interestingly, our calculations indicate that the Li+ affinity is greater for TMSP than for EC, as shown in Figure 6a. This implies that TMSP is likely to form an association with Li+ in the presence of EC. The three O atoms of TMSP are calculated to capture a Li+ ion. The reactions of Li+−TMSP with CH3O− via the PO bond and OSi bond cleavages are considered. Figure 6b shows that the reactions are highly thermodynamically favorable. The PO bond (−43.8 kcal mol−1) and OSi bond (−52.3 kcal mol−1) cleavages are both permitted, but the OSi bond cleavage reaction is more exoergic, leading to the formation of ((CH3)3SiO)2POLi and (CH3)3SiOCH3. This reaction is ∼10 kcal mol−1 more favorable than the reaction of Li+−EC with CH3O−. Even if Li+ is not coordinated, the reactivity of TMSP with CH3O− exceeds that of the EC case (Figure S3). This suggests that when TMSP and EC coexist, TMSP is highly likely to undergo an attack by the nucleophiles on the graphite anode surface, in agreement with our NMR and XPS results indicating that TMSP participates in the SEI layer.

4. CONCLUSIONS The effect of TMSP on a graphite anode was investigated by considering the intrinsic chemical/electrochemical aspects of TMSP. The formation of the SEI layers by electrochemical reduction of TMSP is difficult because the TMSP is stable under cathodic polarization. Instead, the electrophilic TMSP participates in the formation of the SEI layers in decomposed forms by chemical reactions with nucleophilic radical anion intermediates derived from the electrochemical reduction of EC. NMR, XPS, and first-principles analysis data suggest that the TMSP additive participates in the SEI layer as a chemically modified form, as phosphonate functional groups derived from the chemically decomposed products of TMSP were observed in the 31P NMR spectrum at 25.3 and 9.9 ppm and in the XPS spectrum at 130.9 eV (P 2p) and 101.9 eV (Si 2p). The cell cycled with the electrolyte controlled with 3.0% TMSP provides a remarkable Coulombic efficiency of 95.2% at the initial cycle and 96.8% specific capacity retention after 50 cycles. The high Coulombic efficiency of TMSP is an attractive point in terms of cell engineering because it contributes to an increased energy density of the cell, thereby reducing the required weight of the cathode material in LIBs. We believe that understanding the underlying distinct mechanism of TMSP would provide novel insights for leading the development of functional additives for battery anode materials. Our findings also pave a way for developing new types of highperformance additives that effectively stabilize solid electrolyte interfaces of both the anode and cathode in LIBs.

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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/acsami.7b11309. Optimized structures of Li+−EC, Li+−TMSP, (EC)2, (EC)(TMSP), and EC−Li+−TMSP; SEM-EDS analyses for cycled electrodes; and calculation results for the decomposition reaction energies of the TMSP and CH3O− reaction and the EC and CH3O− reaction (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Young-Kyu Han: 0000-0003-3274-9545 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea, by the Korean Government (Grant nos. NRF2016R1C1B1009452, 2016R1A2B4013374, 2017M2A2A6A01020938, and 2017R1A6A1A06015181), and by the Materials and Components Technology Development Program of MOTIE/KEIT (10076731). Y.K.H. thanks Mr. Jaeik Yoo and Mr. Keon-Joon Lee for their technical support.

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DOI: 10.1021/acsami.7b11309 ACS Appl. Mater. Interfaces 2017, 9, 32851−32858