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Article Cite This: J. Phys. Chem. C 2018, 122, 9811−9824

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Chemical “Pickling” of Phosphite Additives Mitigates Impedance Rise in Li Ion Batteries Cameron Peebles,† Juan Garcia,‡ Adam P. Tornheim,† Ritu Sahore,† Javier Bareño,† Chen Liao,† Ilya A. Shkrob,*,† Hakim H. Iddir,*,‡ and Daniel P. Abraham*,† †

Chemical Sciences and Engineering Division and ‡Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: The use of high-voltage, high-capacity positive electrodes in lithium ion batteries presents a challenge, given their tendency to degrade organic electrolytes. To prevent this damage, electrolyte additives modifying the cathode surface are required. Tris(trimethylsilyl) phosphite (TMSPi) is one such electrolyte additive. However, the mechanism for its protective action (similar to other phosphite, borate, and boroxane compounds) remains not completely understood. In LiPF6 containing carbonate electrolytes, TMSPi undergoes reactions yielding numerous products. Here we demonstrate that one of these products, PF2OSiMe3, is responsible for mitigation of the impedance rise that occurs in aged cells during charge/ discharge cycling. This same agent can also be responsible for reducing parasitic oxidation currents and transition metal loss during prolonged cell cycling. Mechanistic underpinnings of this protective action are examined using computational methods. Our study suggests that this beneficial action originates mainly through inhibition of catalytic centers for electrolyte oxidation that are present on the cathode surface, by forming capping ligands on the transition metal ions that block solvent access to such centers.

1. INTRODUCTION Electrification of vehicles demands significant increases in the energy density of storage devices. Currently, the maximum capacity of Li ion batteries (LIBs) is limited by cathode materials, and high energy density (>700 Wh/L) cathodes are urgently needed. This requires a combination of high capacity (>200 mAh/g) and high voltage (>4 V vs Li/Li+) materials. While such materials are known, their use remains problematic, as such high-voltage cathodes degrade organic electrolytes in contact with the energized cathode during cell cycling.1 For this reason, the stabilization of the cathode surface by protective agents that stall the electrolyte decomposition is generally required.2,3 Phosphites (P(OR)3), phosphates (OP(OR)3),4 borates (B(OR)3),5,6 and boroxane (c-B3O3(OR)3) derivatives are among these cathode protective agents,2,7−14 but the mechanism of their beneficial action is not fully understood.15,16 The intended function of the electrolyte additive is 2-fold. First, it needs to impede Li+ ion capacity loss (also known as “capacity fade”) due to formation of solid deposits on the electrodes that are known as solid-electrolyte interphases (SEI) that trap Li+ ions and/or reduce their mobility.17 In LIBs, this capacity fade occurs in the negative (e.g., graphite) electrode, but deleterious reactions on and in the cathode (such as HF corrosion18 and transition metal (TM) ion loss19) can also have a considerable effect on LIB performance. The second desired © 2018 American Chemical Society

role of the additive is to reduce the impedance rise that occurs due to slowing down of Li+ ion migration across the interphases and/or irreversible changes in the cathode materials. The increased cell impedance prevents rapid charging and discharging of the cell, reducing both power consumption and power delivery when, e.g., the electric vehicle is driven (during acceleration and regenerative breaking, respectively). Unlike the capacity fade that involves the cathode material indirectly, this impedance rise mainly originates in the cathode due to reactions in or near the cathode, including the electrolyte oxidation.19 The ideal electrolyte additive should decrease both the capacity fade and the impedance rise. As this is rarely the case, typically more than one electrolyte additive is used in the industry. Part of the expected action of these additives is HF scavenging in the electrolyte bulk (such compounds are sometimes referred to as HF “getters”). For example, acidolysis of O−R bonds in phosphite and borate additives by HF yields relatively inactive fluorocompound products, permanently removing the corrosive hydrofluoric acid. The latter originates through hydrolysis of LiPF6 (see ref 18 and references therein) in the electrolyte. Residual moisture and/or surface hydroxyl Received: February 28, 2018 Revised: April 9, 2018 Published: April 20, 2018 9811

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The Journal of Physical Chemistry C groups can react with PF6− anions in solution, yielding HF and releasing PF5 that subsequently hydrolyses to OPF3, PO2F2−, and PO3F2− (note that these fluorophosphate anions can form esters with the solvent, yielding daughter products). In each step of this sequence, additional HF molecules are generated. The released HF corrodes the cathode20−24 and, in some cases, the anode,18 causing gradual deterioration of electrochemical performance. While there are anions (e.g., imide anions) that are stable both to hydrolysis and anodic oxidation, their stability to these reactions poses a problem,25−28 as they facilitate anodic dissolution of Al current collectors in the cathode, while HF passivates the Al surface.26,29,30 Consequently, it is desired to retain LiPF6 but minimize corrosion of the active material due to HF release. Using HF “getters” can achieve this desired outcome. Since the Si−O bond in trimethylsiloxyl (-OSiMe3) group is especially easy to cleave by HF, many of the currently used additives include this group. However, the strategy of using such HF “getters” has a major logical flaw, as any reaction that consumes HF also facilitates further hydrolysis of PF6− by shifting the hydrolytic equilibria. This consideration casts doubt that electrolyte additives work exclusively or even primarily as such HF scavengers. Indeed, had this HF scavenging been the only role of the electrolyte additive, many molecular designs for such compounds would be possible; however, empirically it has been shown that the HF scavenging ability does not strongly correlate with the efficacy of an additive in retarding deterioration of electrochemical performance. In the view of such discrepancies, it has been suggested that the additives serve as sacrificial agents, either by forming thin protective layers on the cathodes or, possibly, by modifying surface centers that catalyze oxidation/degradation of the electrolyte.15,31 Importantly, such cathode modifying agents may not be the additives themselves, but rather the products of their chemical transformation in the electrolyte or on the surface.3 In this study, we demonstrate that aging of the common electrolyte additive, tris(trimethylsilyl)phosphite (TMSPi, see Scheme 1),32−36 slows down impedance rise, reduces parasitic oxidation currents, and lowers TM ion loss in LIBs through a

specific daughter product (labeled PF2X in Scheme 1) that modifies catalytically active centers on the cathode surface in a specific way. We are not first to suggest that this additive serves other roles than the HF “getter”. For example, it has been proposed that TMSPi (PX3, where X = OTMS and TMS is -SiMe3 group) acts as a potential protective film former35 and an oxygen scavenger in the electrolyte.37,38 (The elimination of reactive oxygen species generated on the cathode is thought to be one of the mechanisms for electrolyte oxidation that is competing with electrochemical oxidation involving surface centers.)39,40 The reactions of PX3 with HF and fluoride have been studied computationally.38,41−43 Two major reaction pathways for PX3 have been recognized: P−O and O−Si acidolysis, the latter being energetically preferable. Only the formation of TMSF or TMSOH through such substitution reactions was followed in the computations. However, some electrolyte additives are also known to be PF5 complexants (see Scheme S1 in the Supporting Information).44−47 The complexation slows down hydrolysis of PF5 (that yields several more HF molecules), so it is a more efficient way of preventing acidolysis than passive consumption of the HF molecules. Below we will demonstrate that compounds closely related to such complexes are indeed found among the products of PX3 reactions in LiPF6 containing electrolytes (Scheme 1). Several of such secondary products have been identified by Winter and co-workers48 who observed TMSF, TMSOH, and (TMS)2O in the headspace over aged solutions of PX3 and PF2X 1), OPFnX3−n (n = 0−2), OPHX2, and O(P(O)X2) species in the liquid phase (see Scheme 1). An elaborate mechanism for PX3 reactions leading to these end products was suggested by these authors. As shown below, our studies do not support this particular mechanism in all of detail; nevertheless, we uphold the central insight of Winter and co-workers48 pointing to poor chemical stability of phosphite additives in LiPF6 containing solutions, which raises questions as to whether the cathode protective agent is the parent compound itself. Herewith, we suggest that the actual cathode protective agent in such PX3 solutions is a specific product of PX3 degradation. To this end, we scrutinize the solution chemistry of PX3; some results are also given for OPX3 and PXY2 (Scheme 1), where Y is the methoxy group, to illustrate the generality of reactions. Using targeted experiments, we demonstrate that only a small subset of products can serve as the protective agents and then narrow this subset to a single candidate species. A computational model that rationalizes why specifically this product(s) modifies the cathode surface in the desired way is presented. The article progresses as follows. We first consider how electrochemical performance of LIBs changes with chemical aging of PX3 containing electrolyte (sections 3.1 and 3.2). We show that only at a particular stage of electrolyte aging does this reagent (or, more correctly, the products of its transformation in the solution) decrease the impedance rise, reduce the loss of TM ions, and inhibit solvent oxidation on energized cathodes. Then we consider the chemical evolution of PX3 solutions and related systems driven by their reactions with LiPF6 and hydrolytic products (sections 3.3 and 3.4). Several reaction products are identified using NMR, and their chemical evolution is followed over time. It is demonstrated that the particular aging stage identified in our electrochemistry experiments corresponds to a specific stage in this chemical

Scheme 1. Structural Formulas for Selected Phosphorus Compounds (X = OSiMe3)

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The Journal of Physical Chemistry C evolution that occurs near the time when PX3 becomes completely consumed. By careful exclusion, only a handful of products are related to the active component, and then two additional experiments are carried out to narrow this choice further still. In particular, an experiment implying that the agent is a volatile P(III) compound is examined, and the effects of exposure of chemically aged solutions to TM oxides are considered (sections 3.5 and 3.6). Put together, these experiments leave only one species as a possible candidate for the surface modification agent. Finally, in section 3.8 a computational model is presented that partially rationalizes our empirical observations. We suggest that the active component is not this solution species, but rather the product of its transformation on the reactive surface. Thus, the “protective action” by PX3 is shown to be a sequence of chemical transformations that gradually change PX3 to a surface species that inhibits catalytic centers for solvent oxidation on the cathode surface. The supporting tables and figures have been placed in the Supporting Information. When referenced in the text, these materials have the designator “S”, as in Figure S1.

Figure 1. Comparison of ASI plots for the fresh (a) and aged (b−d) electrolyte containing 1 wt % TMSPi (color lines). The aging time is indicated at the top of each panel. The arrows in panels b−d indicate the direction of the increasing cell aging cycles (the number of the cycles is given in panel a). The gray lines indicate the baseline electrolyte tested using identical cycling and HPPC protocols. The same symbols are used for ASI traces obtained after the same number of cycles.

2. METHODS 2.1. Experimental Approach. PX3, PXY2, and OPX3 shown in Scheme 1 were obtained from Fisher Scientific and used as received. PX3 contained ∼1.5 mol % OPX3 and OPHX2 impurity. All electrodes used in this study have been fabricated at the Cell Analysis, Modeling and Prototyping (CAMP) facility at Argonne. The positive electrode is composed of a coating of 90 wt % Li1.03(Ni0.5Mn0.3Co0.2)0.97O2 (NMC532, TODA America), 5.0 wt % polyvinylidene fluoride binder (PVdF, Solvay, Solef 5130), and 5.0 wt % C45 carbon black (Timcal) coated on an Al current collector (20 μm thick foil, 9.12 mg/cm2 coating). The negative electrode is composed of a coating of 92 wt % A12 graphite (Phillips 66 CPreme), 6 wt % PVdF binder (Kureha, KF 9300), and 2 wt % C45 carbon black coated on a Cu current collector (5.88 mg/cm2); see ref 49 for details of preparation and materials properties. Prior to cell assembly, the electrodes and the microporous polymer separator (Celgard 2325) were dried in a vacuum at 110 and 50 °C, respectively. The test cells contained ∼25 μL Gen2 (baseline) electrolyte (Tomiyama, Japan), with or without 1 wt % additive. This baseline electrolyte contains 1.2 M LiPF6 in 3:7 w/w liquid mixture of ethyl carbonate (EC) and ethyl methyl carbonate (EMC). The electrolytes were placed in sealed polypropylene containers and aged in an argon filled glovebox. All electrochemical data were collected using Maccor battery test modules and involved 2032-type steel coin cells assembled in dry, oxygen-free argon. To ensure reproducibility, multiple cells were used for each test condition. The cell assembly and testing followed standard protocols detailed in ref 50 and illustrated in Figure S1. Galvanostatic charge/discharge cycling was conducted between 3.0 and 4.4 V at 30 °C. Four initial formation cycles at a C/10 rate were followed by 100 aging cycles at a C/3 rate, which included a 3 h constant voltage hold at 4.4 V. Hybrid pulse power characterization (HPPC) tests were performed intermittently using 2C, 10 s discharge pulses at the end of each series of 20 C/3 cycles as shown in Figure S1, to obtain area specific impedance (ASI) as the function of the applied voltage.50 The impedance was determined at several set voltages between 3.50 and 4.25 V (see Figure 1a). The cell capacity at slow and fast rates (C/10 and 1C) was also

measured (see Figure S1), with the slow rate capacity being the least affected by impedance changes in the aged cell. After 100 cycles the cells were disassembled, and the electrodes were extracted and lightly washed with dimethylcarbonate to remove the electrolyte. The graphite material was scraped off the current collector, ashed, digested in an acid mixture, and analyzed for Li, P, Si, Al, and TM ions using a PerkinElmer/ Sciex ELAN DRC-II spectrometer calibrated with standards prepared from NIST traceable solutions as described in ref 19. X-ray photoelectron spectroscopy of the cathodes was carried out as described in refs 14, 18, and 51. In the potentiostatic hold experiments,51−53 we used the coin cells with the anodes composed of 87 wt % Li4Ti5O12 (NEI), 8 wt % PVdF (Solvay) binder, and 5 wt % carbon on an Al current collector (26.1 mg/cm2 coating) and a cathode with a lower loading of 4.08 mg/cm2. These cells were cycled at 27 mA/goxide for one cycle and 85 mA/goxide for 10 cycles from 2.85 to 1.45 V before being charged at 85 mA/goxide to a terminal voltage of 3.05 V (which corresponds to 4.6 V vs Li/ Li+). The cells were held at this terminal voltage for 60 h with the current recorded at 5 min intervals. Spectroscopic studies of the aging electrolyte solutions containing PX3, PXY2, and OPX3 were conducted by adding 1, 10, or 50 wt % of these additives to Gen2. The solutions were placed in a polypropylene container (all such experiments were conducted at 27 °C in argon). After aging, these solutions were then dispensed into a fluorinated ethylene propylene inserts (Bel-Art SP Scienceware, Wilmad) and sealed with a Teflon plug. The filled inserts were placed in the standard borosilicate NMR tubes containing acetonitrile-d3 or acetone-d6 (no additional NMR standards were used). 1H, 19F, and 31P NMR spectra were obtained using a Bruker Avance III HD 300 MHz spectrometer. The spectra were obtained daily over the period of 4 weeks, with and without proton decoupling. Long (8−12 s) delays between the excitation spin-flip pulses were used to ensure accurate integration. In some experiments, the aged solutions were treated additionally before spectroscopic examinations. In particular, we purged these solutions with ∼4 L of argon over 15 min to remove volatile components. In 9813

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to get accurate electronic energies for bulk calculations on this supercell.

other experiments, the solutions were placed in contact with the NMC532, MnO2, or PbO2 oxides for 1−3 h. These materials were introduced as fine mesh crystalline powders (1:5 w/w) in a stoichiometric excess, and the resulting suspensions were agitated vigorously using a magnetic stirring rod. The solids were removed by centrifuging the suspensions at 3000 rpm for 5 min, and the supernatant was passed through a 0.2 μm pore Teflon filter. All concentrations (in mol %) are given vs PF6− anion that was used as the internal standard. The chemical shifts (δ) for 1H, 19F, and 31P nuclei are given in parts per million (ppm) vs SiMe4, CFCl3, and H3PO3, respectively, and the J (spin−spin) coupling constants are given in Hz. In the following, some products (the progenitors of multiplets in the 19F and 31P NMR spectra that are listed in Tables S1−S5) are numbered as indicated in these tables. Only some of these reaction products have been structurally identified using NMR spectroscopy; however, we were able to classify the observed resonance lines as belonging to certain types of P(III) and P(V) species. 2.2. Computational Approach. Computational studies of isolated and solvated molecules and ions were carried out using a density functional theory (DFT) method with the B3LYP functional54,55 and 6-31+G(d,p) basis set from the Gaussian 09 suite.56 The chemical shifts and J coupling constants for 31P and 19 F nuclei were estimated using Gaussian 09 for geometry optimized gas phase species; these are listed in the spreadsheet in the Supporting Information. All calculations of the cathode surface were carried out using spin-polarized DFT as implemented in the Vienna Ab Initio Simulation Package (VASP).57,58 The exchange-correlation potentials were treated by the generalized gradient approximation (GGA) parametrized by Perdew, Burke, and Ernzerholf (PBE).59 The interaction between valence electrons and ion cores was described using the projected augmented wave (PAW) method.60 Furthermore, the GGA+U scheme was used for applying the on-site correlation effects among 3d electrons of the TM ions, where the parameter of (U-J) was set to 5.96, 5.00, and 4.84 eV for Ni, Co, and Mn, respectively,61 The wave functions were expanded in the plane wave basis up to a kinetic energy of 500 eV. The structure was allowed to relax until the total energy differences were POPF5−

>P(O)OPF5−

X X F

X F F

−0.030 −0.117 −0.207

0.183 (0.002) 0.220 (−0.034) 0.165 (−0.162)

0.087 (−0.146) 0.134 (−0.206) 0.312 (−0.244)

a

Electronic energy gain (eV) in a gas phase reaction (B3LYP/631+G(d,p) model); R1R2P(x)X + PF6− → R1R2P(x)OPF5− + TMSF (x = O or none) bIn parentheses: B3LYP/6-31+G(d,p) and conductor-like polarizable model (CPCM) of dielectric continuum that includes Li+ ion pairing and solvent interactions are shown.

expect this substitution reaction to occur for these two phosphites, too. However, only P−O−P isomers (PF5Z− anions) can form in these reactions. When the solution aging is performed for PXY2 (where Y = MeO) instead of PX3, additional structural information becomes available, as there is JPH of 8−12 Hz for the protons in the methoxy groups, which makes it easy to recognize the extent of the methoxy substitution from the multiplet patterns obtain with and without proton decoupling. PF3, PF2X, PF2Y, PFXY, PY2OH, and PFY2 resonances were found (Tables S2 and S3), with the latter species (PFY2) being the main P(III) product. Thus, while P−X bond is the easiest to undergo the fluorosubstitution, it can also involve the alkoxy groups (Table S4). The main difference appears to be in the reaction rates. For PX3, the complete decay of the parent compound ( OPF5− anion. This reaction is exergonic by 0.26 eV. The OPF5− terminus of the Mδ+··· Oδ‑PF2OPF5− species can also form such a P−O bond thereby providing additional anchoring to the surface. Even further decomposition of this intermediate species can be envisaged eventually yielding (OPF2)2O that is attached to one or two TM ions. We suggest, therefore, that PF2X may not be the passivating agent per se but rather serve as the last precursor that is observed in the electrolyte solution: the actual agent is a product of transformation of the chemisorbed PF2X on the cathode surface. Given the facility of reaction 2 for this chemisorbed species, we hypothesize that this “final” agent is a PV−O−PV ligand whose strong interaction with the TM ions makes it impossible to displace this ligand, so it permanently caps and deactivates the reaction centers on the surface preventing solvent oxidation by these centers. We hope that further advances in computational chemistry will allow us to prove or disprove this scenario.

Table 2. Computed Adsorption Energies (Eads) for Phosphites on the Partially (80%) Delithiated (012) Surface of an NMC Cathode

a

adsorbed molecule

Eads, eV

PX3 PFX2 PF2X PF3 PX2OH PX2O− PX(OH)2 PXFOH P(OH)3

−0.08 −0.61 −1.26 a −0.85 −1.20 −1.28 −1.19 a

Oxidizes to OPF3.

promoting an electron from the lone pair in the sp3-hybridized orbital of the phosphorus to form a π-bond with the oxygen. While the fluoro-substitution strengthens the P−O bond in such TM complexes, it weakens the M−O bond. For PF3, the interaction of OPF3 with the TM ion becomes so weak that it is released from the TM ion, becoming physisorbed on the surface. In the electrocatalysis paradigm suggested in ref 15 (see Figure 9a), the electrolyte (S) oxidation is carried by charged

4. CONCLUSION In his comprehensive review of electrolyte chemistry, Kang Xu observed that “one should always keep in mind that what really exists in an electrolyte system might not be taken at its face value’”.3 Our study provides a striking example of this maxim. We demonstrate that the commonly used cathode protective electrolyte additive for high-voltage Li+ ion batteries, P(OTMS)3, has a dual role in the mitigation of the capacity fade and impedance rise in cells containing LiPF6 based electrolytes. The observed reduction in the impedance rise correlates with the suppression of parasitic oxidation currents on the cathode surface at high voltages and the suppressed loss of transition metal (TM) ions from the cathode, suggesting that all three beneficial actions can be caused by the same agent or agents. We explain all three of these beneficial features through inhibition of reaction centers for electrolyte oxidation on the cathode surface (Figure 9a) by an agent present in the chemically aged electrolyte.15 From previous research,14,48 we know that P(OTMS)3 itself cannot serve as this cathode protective agent. Indeed, P(OTMS)3 is unstable in LiPF6 containing electrolyte solutions, where it reacts with the hexafluorophosphate anion and/or the products of its hydrolysis yielding multiple products. Only such chemically aged (“pickled”) solutions contain the cathode protective agent that decreases the impedance rise; still more remarkable is that this product is observed only at a particular stage of the aging process. This chemical aging and the resulting substitutions in the phosphite molecules are not peculiar to P(OTMS)3; what makes this compound special is the high rate of the fluorosubstitution that allows in situ

Figure 9. A schematic representation of (a) oxidation of the solvent by charged M−O reaction centers on the cathode surface and (b−e) inhibition of these M−O centers by certain phosphite molecules.

M−O centers on the cathode surface; a cathode protection agent reacts with these M−O centers (before or after charging) and inactivates them. If the M−O center remains active, it can oxidize organic molecules in the electrolyte either by H abstraction or through oxygen loss, as illustrated in Figure 9a. In the latter case, the TM ion can be subsequently lost to the electrolyte. This paradigm bodes well with our experiments suggesting that neither PX3 nor PF3 can serve as the cathode protection agent: the parent molecule (Figure 9b) only weakly associates with the M−O centers present at the surface (which explains inefficiency of oxidation of PX3 on the TM oxides in our NMR experiments), whereas PF3 removes the M−O oxygen entirely (Figure 9c), resulting in the subsequent hydrolysis of the released OPF3 to PO2F2− (generating corrosive HF in the process) and facilitating TM ion loss to the electrolyte. As the PX3 solution ages, the -OTMS groups in PX3 become fluoro-substituted, and one of the resulting products (PF2X) yields a strong complex (Mδ+···Oδ‑PF2X) with the reactive centers at the surface (Figure 9d). The same complex is formed when OPF2X (that we also observe in the 9821

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the Applied Battery Research (ABR) for Transportation Program. This research used the computer facilities of LCRC at Argonne National Laboratory and the National Energy Research Scientific Computing Center (NERSC). NERSC is a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

generation of the cathode protective agent to be achieved in a relatively short period of time and in sufficient concentration. In a series of targeted experiments, we demonstrate that PF2OTMS is either this agent or the last precursor for this agent in solution. Our computational studies suggest that PF2OTMS strongly binds to reaction centers on the cathode surface without causing the removal of oxygen from the surface (which is the case for PF3). This surface bound molecule can further react with PF6− transforming into a still stronger binding ligand that permanently caps the reaction center. In this way, parasitic oxidation of the solvent and TM ion loss (resulting in the formation of new reaction centers) become delayed, resulting in the stabilization of interfacial resistance. Now that the mechanism for the cathode protective action by the phosphite additives has been better understood, new ways of achieving and/or improving of this action can be considered, and we encourage readers to follow their own chemical imagination.

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(1) Schipper, F.; Erickson, E. M.; Erk, C.; Shin, J.-Y.; Chesneau, F. F.; Aurbach, D. Review - Recent Advances and Remaining Challenges for Lithium Ion Battery Cathodes I. Nickel-Rich, LiNixCoyMnzO2. J. Electrochem. Soc. 2017, 164, A6220−A6228. (2) Zhang, S. S. A Review on Electrolyte Additives for Lithium-Ion Batteries. J. Power Sources 2006, 162, 1379−1394. (3) Xu, K. Electrolytes and Interphases in Li-Ion Batteries and Beyond. Chem. Rev. 2014, 114, 11503−11618. (4) Yan, G.; Li, X.; Wang, Z.; Guo, H.; Wang, C. Tris(Trimethylsilyl)Phosphate: A Film-Forming Additive for High Voltage Cathode Material in Lithium-Ion Batteries. J. Power Sources 2014, 248, 1306−1311. (5) Wang, Z.; Xing, L.; Li, J.; Xu, M.; Li, W. Triethylborate as an Electrolyte Additive for High Voltage Layered Lithium Nickel Cobalt Manganese Oxide Cathode of Lithium Ion Battery. J. Power Sources 2016, 307, 587−592. (6) Li, J. X. L.; Zhang, R.; Chen, M.; Wang, Z.; Xu, M.; Li, W.; Xing, L. Tris(Trimethylsilyl)Borate as an Electrolyte Additive for Improving Interfacial Stability of High Voltage Layered Lithium-Rich Oxide Cathode/Carbonate-Based Electrolyte. J. Power Sources 2015, 285, 360−366. (7) Zygadło-Monikowska, E.; Florjanczyk, Z.; Tomaszewska, A.; Pawlicka, M.; Langwald, N.; Kovarsky, R.; Mazor, H.; Golodnitsky, D.; Peled, E. New Boron Compounds as Additives for Lithium Polymer Electrolytes. Electrochim. Electrochim. Acta 2007, 53, 1481−1489. (8) Burns, J. C.; Sinha, N. N.; Jain, G.; Ye, H.; VanElzen, C. M.; Lamanna, W. M.; Xiao, A.; Scott, E.; Choi, J.; Dahn, J. R. Impedance Reducing Additives and Their Effect on Cell Performance II. C3H9B3O6. J. Electrochem. Soc. 2012, 159, A1105−A1113. (9) Ping, P.; Wang, Q. S.; Sun, J. H.; Xia, X.; Dahn, J. R. Studies of the Effect of Triphenyl Phosphate on Positive Electrode Symmetric LiIon Cells. J. Electrochem. Soc. 2012, 159, A1467−A1473. (10) Zhu, Y.; Li, Y.; Bettge, M.; Abraham, D. P. Electrolyte Additive Combinations That Enhance Performance of High-Capacity Li1.2Ni0.15Mn0.55Co0.1O2 - Graphite Cells. Electrochim. Acta 2013, 110, 191−199. (11) Freiberg, A.; Metzger, M.; Haering, D.; Bretzke, S.; Puravankara, S.; Nilges, T.; Stinner, C.; Marino, C.; Gasteiger, H. A. Anodic Decomposition of Trimethylboroxine as Additive for High Voltage LiIon Batteries. J. Electrochem. Soc. 2014, 161, A2255−A2261. (12) Sinha, N. N.; Burns, J. C.; Dahn, J. R. Comparative Study of Tris(Trimethylsilyl) Phosphate and Tris(Trimethylsilyl) Phosphite as Electrolyte Additives for Li-Ion Cells. J. Electrochem. Soc. 2014, 161, A1084−A1089. (13) Zhu, Y.; Li, Y.; Abraham, D. P. Mitigating Performance Degradation of High-Capacity Lithium-Ion Cells with Boronate-Based Electrolyte Additives. J. Electrochem. Soc. 2014, 161, A1580−A1585. (14) Peebles, C.; Sahore, R.; Gilbert, J. A.; Garcia, J. C.; Tornheim, A.; Bareño, J.; Iddir, H.; Liao, C.; Abraham, D. P. Tris(Trimethylsilyl) Phosphite (TMSPi) and Triethyl Phosphite (TEPi) as Electrolyte Additives for Lithium Ion Batteries: Mechanistic Insights into Differences During LiNi0.5Mn0.3Co0.2O2 - Graphite Full Cell Cycling. J. Electrochem. Soc. 2017, 164, A1579−A1586. (15) Shkrob, I. A.; Abraham, D. P. Electrocatalysis Paradigm for Protection of Cathode Materials in High-Voltage Lithium-Ion Batteries. J. Phys. Chem. C 2016, 120, 15119−15128. (16) Martinez de la Hoz, J. M.; Balbuena, P. B. Reduction Mechanisms of Additives on Si Anodes of Li-ion Batteries. Phys. Chem. Chem. Phys. 2014, 16, 17091−17098.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b02056. Additional schemes, figures and tables (PDF) Excel spreadsheet containing DFT calculations of NMR parameters (XLSX)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(I.A.S.) E-mail: [email protected]. Phone: 630-252-9516. *(H.H.I.) E-mail: [email protected]. Phone: 630-252-7820. *(D.P.A.) E-mail: [email protected]. Phone: 630-252-4332. ORCID

Cameron Peebles: 0000-0002-0062-8645 Juan Garcia: 0000-0002-5911-8850 Ritu Sahore: 0000-0002-2390-9570 Javier Bareño: 0000-0003-1230-9278 Chen Liao: 0000-0001-5168-6493 Ilya A. Shkrob: 0000-0002-8851-8220 Hakim H. Iddir: 0000-0001-5285-6474 Daniel P. Abraham: 0000-0003-0402-9620 Notes

The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC0206CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the U.S. Department of Energy’s Vehicle Technologies Program (DOE-VTP), specifically from Peter Faguy and Dave Howell, is gratefully acknowledged. The electrodes and electrolytes used in this article are from Argonne’s Cell Analysis, Modeling and Prototyping (CAMP) Facility. Both facilities are supported within the core funding of 9822

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