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C: Energy Conversion and Storage; Energy and Charge Transport

Chemical “Pickling” of Phosphite Additives Mitigates Impedance Rise in Li-Ion Batteries Cameron Peebles, Juan C. Garcia, Adam P Tornheim, Ritu Sahore, Javier Bareño, Chen Liao, Ilya A. Shkrob, Hakim Iddir, and Daniel P. Abraham J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02056 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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The Journal of Physical Chemistry

Chemical “Pickling” of Phosphite Additives Mitigates Impedance Rise in LiIon Batteries. Cameron Peebles,1 Juan Garcia,2 Adam P. Tornheim,1 Ritu Sahore,1 Javier Bareño,1 Chen Liao, 1 Ilya A. Shkrob,1* Hakim H. Iddir, 2* and Daniel P. Abraham1* 1

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois, 60439, USA 2

Materials Science Division, Argonne National Laboratory, Argonne, Illinois, 60439, USA

* Corresponding authors: Ilya A. Shkrob ([email protected]), Phone: 630-252-9516; Hakim H. Iddir ([email protected]), Phone: 630-252-7820; Daniel P. Abraham ([email protected]); Phone: 630-252-4332.

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 of such electrolyte additives. However, the mechanism for its protective action (similarly 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 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. INTRODUCTION 1 ACS Paragon Plus Environment

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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, stabilization of 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)

boroxane (c-B3O3(OR)3) derivatives are among these cathode protective agents,

2, 7-14

5, 6

and

but the

mechanism of their beneficial action is not fully understood. 15, 16 The intended function of the electrolyte additive is twofold. 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 corrosion 18 and transition metal (TM) ion loss

19

) can also have considerable effect on LIB performance. The second desired 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 realtively 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 groups can react with PF6- anions in solution, yielding HF and releasing PF5 that 2 ACS Paragon Plus Environment

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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 cathode anode,

18

20-24

and, in some cases, the

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.

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PX3

OPX3

PXY2

PFX2

PF2X

OPF2X (6)

-

-

1

2

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

We are not first to suggest that this additive serves other roles than the HF "getter." E.g., it has been proposed that TMSPi (PX3, where X=OTMS and TMS is -SiMe3 group) acts as potential protective film former

35

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


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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-workers 48 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-workers 48 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 1 and 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 and 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 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 5 and 6). Put together, these experiments leave only one species as 5 ACS Paragon Plus Environment


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a possible candidate for the surface modification agent. Finally, in section 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 (SI). When referenced in the text, these materials have the designator "S", as in Figure S1.

METHODS

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 vacuum at 110 °C 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 glove box. 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 6 ACS Paragon Plus Environment

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detailed in ref.

50

and illustrated in Figure S1. Galvanostatic charge/discharge cycling was

conducted between 3.0 V 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 twenty 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 V and 4.25 V (see Figure 1a below). 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 Perkin Elmer/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, 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 V 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 oC 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 four weeks, with and without proton decoupling. Long (8-12 s) delays between the excitation spin-flip pulses were used to 7 ACS Paragon Plus Environment


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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 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 3,000 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 to 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.

Computational approach Computational studies of isolated and solvated molecules and ions were carried out using a density functional theory (DFT) method with the B3LYP functional 54, 55 and 6-31+G(d,p) basis set from Gaussian 09 suite. 56 The chemical shifts and J coupling constants for 31P and 19F 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 eV, 5.00 eV, 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 < 3 meV. After geometry optimizations within the DFT+U framework, electronic relaxation was performed 8 ACS Paragon Plus Environment

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using a single point calculation with the hybrid functional HSE06.

62

Adsorption energies (Eads)

were calculated from eq. 1 =

−

−

,

(1)

where the third term on the right side is the energy of the adsorbate in the gas phase, the second term is the energy of the bare slab, and the first term is the total energy of the molecule adsorbed on this slab. Bulk solvent effects were accounted for by using an implicit solvation model

63

implemented in the VASP suite. The ultrafine integration grid was used in all calculations. A periodically repeating slab separated by vacuum layers along the surface normal was used. A vacuum thickness of 16 Å was adopted to remove interaction between the slab layers and to accommodate the adsorbates on the surface. Each slab consisted of nine TM layers where the middle layer was fixed to the bulk equilibrium positions. The lattice parameters of the supercell were fixed at bulk value. All the ions were allowed to relax until the total energy differences were no more than 3 meV. A k-point mesh of 3x3x3 was found sufficient to get accurate electronic energies for bulk calculations on this supercell.

RESULTS AND DISCUSSION 1. The effect of solution aging on the impedance rise and capacity fade. Figures 1a to 1d show ASI as a function of cell voltage (the numbers of cycles for each trace is indicated next to the traces in Figure 1, panel a). As the cell ages, ASI increases at all voltages (this behavior is known as the “impedance rise”). Previous experiments indicate that in NMC532 cells the cathode makes the greatest contribution to this impedance rise.

49, 64

Modification of the cathode surface to slow down Li+ ion transport from the cathode surface into the electrolyte is the likely culprit, although irreversible changes in the lithiated oxide particles (including their subsurface modification and stress induced cracking during repeated lithiation/delithiation cycling) can also be involved. 1 In Figures 1a to 1d, the baseline ASI data (grey lines in all four panels) are compared with PX3 containing cells assembled after 0-3 weeks of ex situ chemical aging of the electrolyte. For consistency, 1 wt% PX3 was used in all of the experiments. This composition gives the initial ASI (below, all values are given at 3.7 V, which corresponds to ~50% state-of-charge) of 21.5 Ω.cm2 (vs 21.0 Ω.cm2 for Gen2 cell) and an overall increase in ASI of 9.8 Ω.cm2 after 100 aging cycles 9 ACS Paragon Plus Environment


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(vs 14.3 Ω.cm2 for Gen2), see Figure 1a. Thus, this freshly prepared PX3 electrolyte only marginally reduces the impedance rise. A dramatic change is observed after one week of chemical aging (Figure 1b). While the initial ASI is similar to the baseline cells, the change in the ASI after 100 cycles decreases from 14.3 to 4.6 Ω.cm2. Importantly, this decrease is not observed after 2-3 weeks of aging (Figures 1c and 1d). At three weeks of aging, both the initial ASI and the increase in the ASI introduced by cell cycling become greater compared to the baseline Gen2 electrolyte (Figure 1d). Thus, neither the freshly prepared electrolyte nor the electrolyte in the advanced stages of chemical aging is effective in reducing ASI rise during cell cycling; only the electrolyte that is harvested at a particular stage of chemical aging has such an effect. As shown in sections 3 and 4 (and anticipated in ref. 48) the products of this chemical evolution fall into two classes: P(III) products (phosphites) and P(V) products (phosphates). The former products are known to be volatile and can be removed by purging the electrolyte solution with an inert gas. Our NMR experiments indicate that PF2X is removed by this treatment (section 5), whereas PFX2 and PX3 are consumed after one week of aging, so they are not present even before gas bubbling (section 4). Of all major P(III) species, only PF3 remained in the solution after this treatment (sections 4 and 5). Figure S2 compares ASI plots for fresh and aged PX3 containing electrolyte before and after this Ar gas treatment. It is seen that the removal of volatile components negates the effects observed in Figure 1b, suggesting that these effects do not originate through the PF3 and P(V) species remaining in the solution. The effect of PF3 also can be assessed indirectly, as PCl3 is chemically similar to PF3 (we did not use PF3 in these experiments, as it is a hard-to-handle gas, whereas PCl3 is liquid). Therefore, we deliberately added 0.07-1.0 wt% PCl3 to Gen2 and/or PX3 solutions (Figure S3). In these experiments, slight to large increase in the ASI rise was observed depending on the PCl3 concentration, suggesting that the trihalophosphites are unlikely to be the sought agents responsible for the reduction in the ASI rise that is seen in Figure 1b. (In fact, the DFT modeling considered in section 8, already suggested that PF3 would strongly interact with the cathode by removing surface oxygen atoms, and hence, would accelerate surface degradation by promoting oxygen loss. These control experiments are in agreement with our modeling). We remind that PX3 is best known for slowing down the capacity fade.

14

Remarkably, the

trends observed for ASI rise in Figure 1 become reversed when the discharge capacity fade is considered (Figures 2 and S4). For C/3 discharge capacity (Figure S4), the fade becomes slower 10 ACS Paragon Plus Environment

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when fresh PX3 electrolyte is used (see also ref.

48

); this benign effect becomes reversed after 1

week of ex situ aging. The slowing of the capacity fade is seen again at 2 weeks of aging only to reverse once more after 3 weeks of aging. The highest capacity retention was obtained when the electrolyte aged for 1 week was bubbled with Ar to remove volatile compounds, suggesting that slowing of the capacity fade is caused by the remaining P(V) products, while P(III) products only interfere with this action. The same effects are observed for slow-rate (C/10) diagnostic runs shown in Figure 2 (the slower cycling limits the effects of resistance on capacity loss in the cell by taking kinetically limiting transport of Li+ through SEIs out of the equation). Figure S5 demonstrates that addition of PCl3 not only causes greater impedance rise (see Figure S3) but also faster capacity fade. As the capacity fade occurs mainly due to the formation of Li+ ion trapping SEI on the graphite electrode, PX3 and (to a greater extent) P(V) products improve the quality of these SEI films on the negative electrode (presumably, via reactions involving reactions of the additive and/or the products with radical anions generated via solvent reduction on the lithiated graphite, see ref. 65), whereas the daughter P(III) product(s) that slow down ASI rise on the positive electrode have only adverse-to-slight effect on this SEI quality. For sufficiently slow cell cycling (Figure 2), the effect of ex situ chemical aging on the capacity retention becomes small whereas the effect on the ASI rise is dramatic. We conclude that from the practical perspective, one week of chemical aging appears to be the optimum regime for using PX3 as an electrolyte additive.

2. The effect of electrolyte aging on electrolyte oxidation and transition metal loss. To assess the effect of chemical aging on parasitic oxidation of the electrolyte at the cathode interface, potentiostatic hold method was used. With a single phase cathode and a lithium titanate (Li4Ti5O12) anode with a plateau voltage profile, the electric current flowing through the cell is only sensitive to oxidation reactions occurring at the cathode. 52, 53 Figure S6 shows the current as a function of hold time. In each trace, there is an initial rapid decay (< 5 h) due to polarization relaxation in the electrolyte and cathode material. At longer delay times the current is more strongly affected by electrolyte oxidation at the cathode-electrolyte interface. For fresh PX3 electrolyte, the cell current is higher than this current in the Gen2 baseline electrolyte throughout the entire 60 h hold, indicating that this electrolyte is not passivating against the cathode oxidation of the solvent. However, when the same PX3 electrolyte is aged over one 11 ACS Paragon Plus Environment


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week, the long-term current becomes lower compared to the Gen2 baseline electrolyte, indicating robust passivation of the cathode surface. This result proves that the modification of the surface by a cathode protective agent that lowers ASI rise is also preventing the oxidative electrolyte decomposition. Another indication of this surface modification is the reduced TM ion loss during the cell cycling. 14 It is known that PX3 additives can retard the loss of TM ions from the cathode 14, 35 that is commonly attributed to solvent oxidation, subsurface degradation, and cracking, as the cathode goes through repeated lithiation/delithiation cycles, see

66-70

The released TM ions (including

catalytically active Mn2+ ions) are driven towards the negative electrode where they become trapped in the SEI matrix causing more efficient reduction that facilitates additional Li+ ion trapping and capacity loss.

19

In Figure 3, we compare the concentrations of TM ions in the

negative electrodes that were harvested from Gen2 and PX3 cells before and after ex situ aging of the PX3 cells for one week. It is seen that while freshly prepared PX3 electrolyte somewhat decreases TM ion loss comparing to baseline Gen2 electrolyte, only aged electrolyte reduces this loss significantly. Furthermore, when this aged electrolyte is treated with Ar gas to remove volatile products (including the tentative cathode protective agent that impedes ASI rise, see section 1) the TM ion loss reverts to the levels observed in the freshly prepared PX3 electrolyte. This experiment suggests that the same product of PX3 decomposition can be protecting the cathode surface by inhibiting both the TM ion loss and the impedance rise. As both of these processes are caused by parasitic oxidation of the electrolyte, this product can interfere with the ability of some centers on the cathode surface to facilitate these reactions. As seen in Figure S7, the TM ion loss correlates with the ASI increase, and Mn2+ deposition into the SEI on the graphite linearly correlates with the capacity fade (Figure S8). We conclude that the cathode protection by the tentative agent or agents can be multifunctional, as it may include the impedance change, parasitic oxidation reactions, and the TM ion loss. Yet another important cue is provided by X-ray photoelectron spectra of the cathode shown in Figures S9 and S10, in which we compare the photoemission features for Gen2 and PX3 cells, after the initial (C/10 formation) cycles and 100 cell aging (C/3) cycles. For the C 1s and F 1s bands (Figure S9), these differences are small, whereas for the O 1s and P 2p bands, these differences are significant (Figure S10). The salient feature in these spectra is a prominent band that is usually attributed to lithium fluorophosphates (LixPOyFz) that are formed via hydrolysis of LiPF6; this 12 ACS Paragon Plus Environment

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band is enhanced in the PX3 cells compared to Gen2 cells. After the four formation cycles, this feature becomes more prominent in fresh PX3 cells than in aged PX3 cells. With further cycling the fluorophosphate signal becomes weaker in the aged PX3 cells. The formation of LiPO2F2 and other fluorophosphates is seen in our NMR spectra (see sections 3 and 4 below); as can be expected, more of these products are formed as the solution ages. Contrary to these expectations, the aged PX3 solutions yield less of the fluorophosphates on the cathode surface. This suggests that the fluorophosphates observed by XPS on the cathode surface in such solutions are structurally different from the ones that are typically formed by deposition of LiPO2F2 and like species from the bulk. Specifically, we suggest that these fluorophosphates are formed through the oxidation of certain P(III) products that are present only in the aged electrolyte solution (section 4).

3. NMR spectroscopy: the first week. We turn to the chemical changes that occur in PX3 containing electrolyte as it ages. The composition of the solution changes dramatically when the initial PX3 becomes entirely consumed (in the following, we refer to this rapid change as solution “transformation”). The exact moment of this transformation depends on the concentration of PX3 and environmental conditions, but it typically occurs 4-8 days after PX3 was added, and it can be followed by 1H NMR through the disappearance of methyl protons from PX3 and the appearance of the TMSF doublet, as shown in Figure S11. Given the dramatic nature of this change, we consider NMR spectra before and after this transformation separately (sections 3 and 4, respectively). The 19F and 31P NMR spectra of Gen2 yield only the doublet from PF6- anion; as the solution ages over 2-3 weeks, additional doublet from PO2F2- (≈0.1%) is observed. In the corresponding 31

P NMR spectra, PX3 appears as a singlet at 114.34 ppm; also seen is a phosphine impurity that

appears as an 1H-coupled doublet at -14 ppm (1.6 %, Table S1). This impurity does not appear to be involved in the solvolysis of PX3. Another impurity is OPX3 (~1.6%), which may be involved in further reactions (section 4). To obtain sufficiently strong 31P NMR signals for multiplet analyses, chemical degradation of 10 and 50 wt% PX3 solutions was studied; with the spectroscopic features of the products recognized, resonance lines in 1-5 wt% solutions can be attributed. The same products were present in all of the solutions regardless of the initial PX3 concentration; however, their relative yields depended strongly on this initial concentration. 13 ACS Paragon Plus Environment


Page 14 of 48

Shortly after PX3 is introduced in Gen2 electrolyte, the concentration of PX3 begins to decrease and the parent molecules become fully consumed over a period of several days (Table S1 and Figure S11). Concomitant with the decrease in the PX3 concentration, the resonance lines of PF2X and PFX2 increase in the magnitude (Figures 4 and S12). When PX3 is still present in the solution, there is an approximate parity between the yields of these two products, but shortly before the transformation, there is ≈70% molar excess of PFX2 over PF2X; after the transformation no PFX2 is observed. The complete substitution of PX3 to PF3 also gradually sets in; at 8 days, there is 0.1% PF3 vs 2.9% PF2X and 4.8% PFX2 (see Figure 4 and S13(a)). In Table S6 we calculate the gas phase energetics for X/F substitution for several phosphites and phosphates. According to these estimates, the exothermicity of such reactions decreases considerably from PX3 to PFX2 to PF2X, which qualitatively accounts for metastability of PFX2 in the reaction mixture. For phosphates, the exothermicity decreases even more strongly from OPX3 to OPFX2 to OPF2X, which explains the relative stability of the latter species in the aged OPX3 solutions (section 4). Similar trends were found for the Y/F substitution. Among the P(V) products, PO2F2- accounts for 1.2% and product 5 (whose doublet is indicated in Figure 4) accounts for 0.1%. There are also two other species (indicated 1 and A in Figures 4 and S12) with yields of 4.8% and 1.6%, respectively (the initial PX3 concentration corresponds to 7.3%). The relative yields of 1 and A depend on the initial PX3 concentration and the aging period (Figures S12 and S14). At low concentration of PX3, species A is prevalent, whereas at higher concentration of PX3, 1 becomes prevalent. Large size (low mobility) of species 1 is suggested by the pulsed field gradient spin echo (PGSE) NMR method: the diffusion coefficient (in x10-6 cm2/s) for 1 is 1.8 vs 2.1 for PF6-, 3.97 for PF2X and 2.84 for PFX2. Species 1 and A have a PF5 subsystem with the resonance lines that arise from four equatorial fluorines (with a ddd pattern) and an axial fluorine (with a dquin pattern). Both of these multiplets exhibit the characteristic J(Feq-Fax)~50 Hz coupling between the axial and equatiorial nuclei in the octahedral PF5Z- anion (where Z is a unspecified substituting group). 71 In the 31P NMR spectrum (Figure S13, panels b and c), there are resonance lines at 13.2 ppm (with a dquin pattern) and 148.8 ppm (with a dquind pattern) with the matching J coupling constants (see Table S1), suggesting that all of these resonances originate from a single species that yields a coupled ABCD system that is shown in Figures 5 and S15. The coupling pattern in this system suggests JPP ≈1398 Hz for two phosphorus-31 nuclei. There are precedents of such large 31P-31P coupling constants 14 ACS Paragon Plus Environment

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for compounds in which PF5 is coordinated with the P(III) phosphorus.

72

The observed JPP

coupling is considerably greater than JPP in the PaIII-O-Pb VF5 compounds, 73, 74 (see Figure S15). We used DFT calculations to analyze candidate species for 1 (see the Supporting Information), and the best correspondence was obtained for X2PaIII(O-)Pb VF5 species shown in Figure 5, in which Pa is trivalent and tetragonal, Pb is pentavalent and octahedral, and the Pa-Pb bond length is 2.28 Å (Figures 6 and S15). This compound can be thought of as an isomer of X2PaIIIOPbVF5- (anion 2, Figure 6) with a Pa-O-Pb bridge. Figure S15 gives the estimated J-coupling constants for 1 and 2. According to our calculations, in the gas phase anion 1 is ~0.21 eV higher in the electronic energy than anion 2; however, this ordering can be easily reversed in solution due to ion-pairing and solvation. Using the same DFT model and conductor-like polarizable model (CPCM) of dielectric continuum as solvating medium 75, 76 we estimated that Li+ ion pairs of anion 1 are 0.23 eV lower in energy than that pairs of anion 2. Thus, 1 can be considered as a solvent stabilized PF5 complex of PX2O-Li+ (cf. Scheme S1). Importantly 1 is a strong base, so it can become easily protonated in the electrolyte (Figures 5 and 6). The J-constants for equatorial fluorines in A and 1 are fairly similar (914.8, 259.1, 48.2 Hz vs 880.8, 307.6, 55.8 Hz, respectively), and we suggest that A may be the protonated form of 1 (that is, 1H, see Figure S15), which can also be considered as the PF5 complex of PX2OH. This attribution would explain why the NMR spectra are so sensitive to the environmental conditions, as the proticity of the solution is determined by the extent of hydrolysis that in turn depends on the initial PX3 concentration and solution history. It is also possible that 1 is the protonated form and A is the deprotonated form; the two compounds have similar NMR parameters (Figure S15), which makes it difficult to make unambiguous attributions. The formation of anions 1 and 2 can occur through direct nucleophilic attack of the PF6- anion on PX3 resulting in the elimination of TMSF: PX3 + PF6- → 1 (2?) + TMSF

(2)

The complementary product of this reaction, TMSF, is observed in all aged PX3 solutions, both in 1H and 19F NMR spectra (Figure S8). We suggest that during the aging process, PF6- reacts with PX3 via reaction 2 directly, as opposed to the postulated reactions involving HF and/or PF5 (see the Introduction and refs. 38, 41, 42, 44). In solution, the two isomers (1 and 2) are likely to be in the 15 ACS Paragon Plus Environment


Page 16 of 48

equilibrium, with 1 or 1H being the prevalent forms. In one of these compounds, a bond rearrangement occurs, resulting in the X/F substitution and elimination of fluoride and OPF3 (that rapidly hydrolyzes to PO2F2-). From the structural perspective (Figure 3), 1H is the most likely intermediate to react in this fashion:

X2Pa(OH)··PbF5 → OPF3 + PFX2 + HF

(3)

Analogous reactions may occur for PFX2 and PF2X; however, our calculations suggest that P-O-P isomers become increasingly favored over P-P isomers (0.21 eV vs 0.34 eV vs 0.37 eV in the gas phase, see also Table 1) with further X/F substitution, which may explain why only one such P-P compound is observed in the NMR spectra. According to our calculations in Table 1, the heat of reaction 2 for PFX2 and PF2X does not change considerably compared to PX3, i.e. we expect this substitution reaction to occur for these two phosphites, too; however, only P-O-P isomers (PF5Zanions) 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.

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 highvoltage Li+ ion batteries, P(OTMS)3, has 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 knew 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 24 ACS Paragon Plus Environment

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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 to achieve in situ generation of the cathode protective agent 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 the readers to follow their own chemical imagination.

ASSOCIATED CONTENT Supporting Information: A PDF file containing additional schemes, figures and tables and an Excel spreadsheet containing DFT calculations of NMR parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

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 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 25 ACS Paragon Plus Environment


Page 26 of 48

of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. 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-AC02-06CH11357. 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.


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Table 1. Energetics for PF6- Substitution of the TMS Group in Phosphites PXR1R2 and Phosphates OPXR1R2 (X=OTMS). a,b

R1 X

R2 X

X

F

>P(O-)PF5 -0.030 -0.117

F

F -0.207

>POPF50.183 (0.002) 0.220 (-0.034) 0.165 (-0.162)

>P(O)OPF50.087 (-0.146) 0.134 (-0.206) 0.312 (-0.244)

a) electronic energy gain (eV) in a gas phase reaction (B3LYP/6-31+G(d,p) model) R1R2P(x)X + PF6- → R1R2P(x)OPF5- + TMSF (x = O or none) b) in 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.



Page 28 of 48

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

Eads,

Molecule

eV

PX3

-0.08

PFX2

-0.61

PF2X

-1.26

PF3

a

PX2OH

-0.85

PX2O-

-1.20

PX(OH)2

-1.28

PXFOH

-1.19

P(OH)3

a

a) Oxidizes to OPF3.


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Figure captions.

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 to d indicate the direction of the increasing cell aging cycles (the number of the cycles is given in panel a). The grey 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.

Figure 2. Capacity fade as determined during C/10 diagnostic cycles for C/3 cycling over 120 cycles for the baseline electrolyte (Gen2) and electrolyte solutions containing 1 wt% PX3 and aged 0-3 weeks. To reduce clutter, the traces are separated into two panels, a and b. Panel b illustrates the effect of Ar bubbling on the capacity fade. Figure 3. Concentration of transition metal (TM) ions (Mn, Ni, and Co) lost from NMC532 electrode and deposited into the graphite negative electrode after 100 cycles, for the cells containing Gen2 baseline electrolyte (far right) and the cells containing Gen2 electrolyte with 1 wt% PX3 (from left to right) immediately after PX3 addition, after one week of aging, and after one week aging and Ar bubbling to remove volatile components.

Figure 4. Excerpts from the

19

F NMR spectrum of 10 wt% PX3 solution aged over 8 days, before the

transformation. Only traces of PX3 are present in this aged solution. In addition to PFX2 and PF2X, doublets of PF3 and 5 appear in the NMR spectrum. Also shown are the resonance lines from species 1 and A, as explained in the text. Figure 5.



Page 30 of 48

The ABCD spin system of product 1 according to the 19F and 32P NMR spectroscopy. Chemical shifts (in ppm) are given in red, multiplet patterns are given in green, and J-coupling constants (in Hz) are given in blue. Shown below is the postulated equilibrium between the protonated and deprotonated 1 (X=OSiMe3). Figure 6. Space models for 1, 1H and 2 according to our gas phase DFT calculations with some interatomic distances indicated in the sketch. Figure 7. 19

F NMR spectra (three regions are shown) of aged 10 wt% PX3 (aged one to three weeks) after

the transformation. At three weeks, almost no P(III) species remain in the solution. Figure 8. Molecular configuration for chemisorbed (a) PX3 and (b) PF2X molecules on partially delithiated NMC111 (012) surface, with the adsorption energy Eads indicated at the top of the panels. The spheres represent Ni (silver), Co (dark blue), Mn (magenta), Li (green), Si (light blue), P (grey), C (carbon), O (scarlet) and H (white) atoms. Lower adsorption energy corresponds to a stronger interaction with the surface. Figure 9. A schematic representation of (a) oxidation of the solvent by charged M-O reaction centers on the cathode surface and (b-d) inhibition of these M-O centers by certain phosphite molecules.


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Figures.

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 to d indicate the direction of the increasing cell aging cycles (the number of the cycles is given in panel a). The grey 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.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

specific discharge capacity, mAh/g oxide


C/10 cycling 1 wt% TMSPi

190

Page 32 of 48

1 week Ar bubbled

180

(a) 170

(b)

aging, weeks 1 2 3

Gen2 fresh

160 0

40

80

120 0 cycle #

40

80

120

Figure 2. Capacity fade as determined during C/10 diagnostic cycles for C/3 cycling over 120 cycles for the baseline electrolyte (Gen2) and electrolyte solutions containing 1 wt% PX3 and aged 0-3 weeks. To reduce clutter, the traces are separated into two panels, a and b. Panel b illustrates the effect of Ar bubbling on the capacity fade.


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Figure 3. Concentration of transition metal (TM) ions (Mn, Ni, and Co) lost from NMC532 electrode and deposited into the graphite negative electrode after 100 cycles, for the cells containing Gen2 baseline electrolyte (far right) and the cells containing Gen2 electrolyte with 1 wt% PX3 (from left to right) immediately after PX3 addition, after one week of aging, and after one week aging and Ar bubbling to remove volatile components.



10 wt% TMSPi, 8 days

PFX2

PFX2

PO2F2-

1 (eq)

x20

PF2X

PF2X

NMR signal, arb. u.

5

5 1 (ax)

x5

-32

A

PF3

PF3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 48

-36

-56

-60 19

-72

-76

-80

-84

-88

δ( F), ppm

Figure 4. Excerpts from the 19F NMR spectrum of 10 wt% PX3 solution aged over 8 days, before the transformation. Only traces of PX3 are present in this aged solution. In addition to PFX2 and PF2X, doublets of PF3 and 5 appear in the NMR spectrum. Also shown are the resonance lines from species 1 and A, as explained in the text.


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13.2 dquin

31 P

-80.5 0 19 F ax

a

dquin

307.6 56.9

1398.3 790 31 P

dquind

b

x4 880

19 F eq

ddd

-58.5

-148.8 4eq

- H+

b

a

ax

+ H+

1

1H

Figure 5. The ABCD spin system of product 1 according to the 19F and 32P NMR spectroscopy. Chemical shifts (in ppm) are given in red, multiplet patterns are given in green, and J-coupling constants (in Hz) are given in blue. Shown below is the postulated equilibrium between the protonated and deprotonated 1 (X=OSiMe3).



Page 36 of 48

Figure 6. Space models for 1, 1H and 2 according to our gas phase DFT calculations with some interatomic distances indicated in the sketch.


PF3

PF3

(a)

NMR signal, arb. u.

week 1 2 3

PF2X

PF2X

10 wt% TMSPi

3

4

-30

-35

3

4

-40

-45

PF5, eq b

a

d

b

a

d

e

e

c

c

(b) f

h

g

g

-56

NMR signal, arb. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NMR signal, arb. u.

Page 37 of 48

-60

-64 PO2F2c

PF6-

5

8

8

PF5, axial

(c) 5

a? b?

-70

-75

-80 19 δ( F), ppm

-85

Figure 7. 19F NMR spectra (three regions are shown) of aged 10 wt% PX3 (aged one to three weeks) after the transformation. At three weeks, almost no P(III) species remain in the solution.



Page 38 of 48

Figure 8. Molecular configuration for chemisorbed (a) PX3 and (b) PF2X molecules on partially delithiated NMC111 (012) surface, with the adsorption energy Eads indicated at the top of the panels. The spheres represent Ni (silver), Co (dark blue), Mn (magenta), Li (green), Si (light blue), P (grey), C (carbon), O (scarlet) and H (white) atoms. Lower adsorption energy corresponds to a stronger interaction with the surface.


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



Page 40 of 48

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