Operando Monitoring of Fâ Formation in Lithium Ion Batteries

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Operando monitoring of F- formation in lithium ion batteries Christoph Bolli, Aurélie Guéguen, Manuel A. Mendez, and Erik J. Berg Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03810 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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Chemistry of Materials

Operando monitoring of F- formation in lithium ion batteries Christoph Bollia*, Aurélie Guéguena, Manuel A. Mendezb, Erik J. Berga† a

: Electrochemistry Laboratory, Paul Scherrer Institute, 5232 Villigen, Switzerland : BASF SE, RCN/BL – M311, 67056 Ludwigshafen am Rhein, Germany

b

ABSTRACT: Online electrochemical mass spectrometry (OEMS) was applied to study the influence of tris(trimethylsilyl)phosphate (TMSPa) as an additive in 1M LiPF6 (FEC/DEC) electrolyte on the gas evolution in Li-rich/NCM full cells during cycling. The results indicate that TMSPa neither influences the SEI formation on the anode nor the surface reconstruction on the cathode but acts as chemical scavenger for HF and LiF. TMSPa thus lowers the electrolyte acidity and suppresses further LiPF6 decomposition resulting in lower impedance and higher LIB performance. Furthermore the selective reactivity of TMSPa towards fluorides leads to the formation of Me3SiF enabling the additive to act as a chemical probe and to study HF/LiF formation operando by OEMS. By this methodology we were able to identify contributions from SEI formation, proton and reactive oxygen formation > 4.2 V, cross-talk between anode and cathode and the PVDF binder to the fluoride formation in LIBs.

I. Introduction Fluorinated compounds play an important role in modern lithium ions batteries (LIBs) and are commonly found among all desirably inert components, such as the electrode binders and electrolyte solvents, salts, additives, etc. LiPF6 is the most commonly employed conducting electrolyte salt providing high conductivity1-2, Al current collector passivation3-5, and facilitating the formation of a stable solid electrolyte interphase (SEI) on graphitic electrodes in carbonate solvent based electrolytes2, 6 . On the other hand, the P-F bond is thermally labile7-8 and sensitive towards hydrolysis9-10. Besides fluorine containing electrolyte salts, further fluorinated compounds can be integrated into the organic electrolyte formulation either as co-solvent or electrolyte additive. The introduction of fluorine positively shifts the oxidation and reduction potentials compared to the non-fluorinated parental compounds.11 Several fluorinated species, such as fluoroethylene carbonate (FEC), have hence been found to be more effective SEI formers because of their readiness for reduction and electrode layer forming abilities in the 1st formation cycle.12-14 Likewise, the demand for a higher oxidative stability on the LIB cathode side has led to the development of fluorinated electrode binders among which Polyvinilydene fluoride (PVDF) is most commonly employed15. Despite the enhanced anodic stability induced by the fluorination, these compounds undergo side-reactions whose products are often detrimental for the performance and lifetime of LIBs. For instance, LiF is generally found in the SEI formed from electrolytes based on fluorinated compounds.16-19 The low solubility of LiF in carbonate electrolytes20 leads to its deposition on the electrode surface with increased cell impedance as a result. Most notably, the thermal equilibrium LiPF6  LiF + PF5 (1) between the LiPF6, LiF, and PF5 is well known to shift to the right hand side as PF5 rapidly reacts with electrolyte decomposition products and/or impurities (such as water) forming HF, POF3, CO2 as well as other fluorinated species (such as organofluorophosphates or alkylfluorides)21-22. These electrolyte degradation products in turn further shift the equilibrium (eq. 1)

to the right to form more PF5,9, 23 thus autocatalytically decomposing the electrolyte (as outlined by Campion et al.22). Electrolyte decomposition and increasing HF concentrations enhance the corrosion of the cathode active materials, which additionally accelerate cell capacity loss and impedance increase. Although fluorination and electrolyte decomposition significantly impact the performance and lifetime of LIBs, there is a lack of fundamental understanding of how, why and to what extent fluorinated compounds, such as HF, form upon LIB cycling. HF is mostly monitored ex situ after cycling/storage, e.g. by 19F NMR of the electrolytes.24 Quantitative determination of HF can also be done e.g. by titration25 and/or ion chromatography, as applied by Terborg et al.26 and Kraft et al.27-28, to determine the fluoride content stemming from LiPF6 decomposition. Another approach was taken by Wilken et al. who evaluated the application of a fluoride selective electrode (FSE) for the ex situ determination of fluoride in carbonate electrolytes29. Electrolyte decomposition is however a dynamic and potentialdependent process and operando monitoring thereof during the dis-/charge is highly desired. Online electrochemical mass spectrometry (OEMS) has in this respect proven to be a powerful tool for operando investigation of the gas evolution in LIBs during cycling. Particularly the study of the SEI formation mechanism on graphite30-31 and the release of oxygen from cathode active materials32-33 has provided unique understanding in the field. Electrolyte additives are commonly used in modern LIBs with various performance-enhancing functions, such as forming a thinner and more robust SEI on the anode, acting as redox shuttles for overcharge protection, etc.34-35 Silyl ester based additives have been intensively studied during recent years, foremost in their role as HF scavengers reducing the acidity of the electrolyte.36-37 Amongst them tris(trimethylsilyl)phosphate (TMSPa) was patented as additive for LIB electrolytes by Sumitomo Chemical Company already in 200238 with the claim of improved initial discharge capacities, even in presence of residual water. A later patent by Wildcat Discovery Technologies Inc. claimed improved coulombic efficiency and LIB lifetime

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for carbonate based electrolytes containing TMSPa.39 Dahn et al. made in 2014 systematic investigations of silyl-based additives and found that cells with TMSPa only slightly improved the performance compared to cells with blank electrolyte solution.40-41 Despite the latter findings, several further reports (with sometimes contradictory claims) have emerged showing significantly enhanced cycling performance of cells rather including the TMSPa.38, 42-44 In addition to HF scavenging, TMSPa was claimed to electrooxidize at a lower potential (~ 4.1 V) than conventional carbonate electrolytes (~ 4.5 V)42-43 and to form an interphase layer on the cathode (similarly to a SEI) functioning as a kinetic barrier for electrolyte degradation. The cathode surface deposits in cells containing TMSPa have been found to be thinner, more homogenous, and rather contain P-O and SiO-C bonded species with less fluorinated species (e.g. LiF and no LiPF6) present42-44. These disparate results on the beneficial role of TMSPa can largely be explained by the lack of fundamental understanding of how this class of additives operate in the cell. The aim of the current study is twofold: First to provide further insights into the fundamental operation mode of this highly attractive, but debated, class of silyl ester based electrolyte additives. The second objective is to exploit the conclusions of the first part to study the formation of inorganic fluoride in LIBs during operation of the cell by OEMS. II. Experimental 1. Electrode preparation The positive electrodes were prepared by coating Celgard 2400® monolayer polypropylene with a slurry of 89 wt% Lirich NCM (BASF SE), 5 wt% polyvinylidene fluoride (PVDF Solef 5130, Solvay), 4.64 wt% Super C65 (SC65) carbon (Imerys Graphite & Carbon, Switzerland) and 1.36 wt% graphite SFG6L (Imerys Graphite & Carbon, Switzerland), dispersed in N-methylpyrrolidone (NMP, Sigma-Aldrich). The porous substrate is required for the OEMS cell configuration (wet thickness of the coating: ~ 100 µm). NMP was evaporated under vacuum at 80 °C for 8 hours and circular electrodes were subsequently punched out (15 mm diameter) and dried overnight at 80 °C before being introduced into an Ar filled glove box. The average loading for the final electrodes was in the range 132 mg/cm2 of active material. Graphite electrodes were prepared by coating Cu foil and Celgard 2400® sheet with a slurry of 95.7 wt% SFG6 (Imerys Graphite & Carbon, Switzerland), 0.5 wt% SC65 and 3.8 wt% carboxymethyl cellulose (Alpha Aesar) or PVDF binder dispersed in distilled water or NMP, respectively. The wet thickness of the electrode was ~ 300 µm in case of Cu foil and 200 µm in case of Celgard 2400® and the water was evaporated under air atmosphere at room temperature. Punched circular electrodes with diameters of 18 and 20 mm were further dried overnight at 80 and 120 °C for coating on polypropylene and Cu foils, respectively, before introduced into an Ar-filled glovebox. Self-standing LiFePO4 electrodes were prepared by mixing 80 wt% of LiFePO4 (Südchemie), 5 wt% amorphous carbon SC65 with 15 wt% polytetrafluoroethylene (Sigma Aldrich) binder in a solution of isopropanol and water (1:1) to form a viscous slurry. The slurry was sonicated and kept under mechanical stirring at 100 °C until evaporation of the solvents and

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formation of a “dough-like’ paste. Electrode sheets were obtained by working the dough with a spatula and mechanical rolling (thickness 200 µm). Electrodes were subsequently punched (20 mm diameter, ~ 60 mg) and dried (120 °C under dynamic vacuum) before being introduced into an Ar-filled glove-box. SC65/SFG6L electrodes for cyclic voltammetry (CV) experiments were prepared by dispersing SC65, SFG6L and PVDF in a 4.64:1.36:5 ratio (identical to their ratio in the Li-rich NCM electrode) in NMP. The obtained ink was coated on Celgard 2400® (200 um wet thickness). After removal of NMP at 80 °C under vacuum 18 mm diameter electrodes were punched and dried under vacuum for at least 8 h before introducing in an Ar filled glovebox. 2. Chemicals FEC/DEC (12:88, 0 and 1M LiPF6) electrolytes were used as received from BASF SE. For the preparation of EC/DEC (12:88) based electrolytes EC (99%, BASF SE), DEC (battery grade, BASF SE), LiPF6 (battery grade  99.99 %, Sigma Aldrich) and LiClO4 (battery grade  99.99 %, Sigma Aldrich) were used as received. For TMSPa containing electrolytes 1 wt% TMSPa (> 98 %, Sigma Aldrich) were prepared freshly before the cell was assembled. For the testing of the chemical stability LiF (99 %, Sigma Aldrich) and PVDF were dried in a Büchi oven at 120 °C under vacuum. The dried powders were added in a modified OEMS cell (Figure S1) and TMSPa was injected via a 1/4“ gas chromatography septum. 3. Electrochemistry A homemade cell was developed for OEMS experiments as described elsewhere.45 The assembled cells were equilibrated at open circuit potential for 4 hours prior to electrochemical cycling. Galvanostatic cycling was carried out between 0.5 and 4.7 V vs. Li+/Li at C/10 rate during the first charge and at C/5 rate for the first discharge and following cycles. For CV experiments SC65/SFG6L or SFG6 electrodes were assembled in an OEMS cell with LFP as counter and reference electrode and Celgard separator. Potentials were converted to Li+/Li reference by adding 3.45 V for the potential plateau of LFP. The cells were subjected to cyclic voltammetry at a scan rate of 55 µV/s, starting from open circuit potential (OCP, ∼ 3.2 V vs. Li+/Li) and cycling between OCP and 5 V vs. Li+/Li for anodic sweeps or OCP and 0.1 V vs. Li+/Li for cathodic sweeps. All measurements were carried out at room temperature using a computer-controlled battery cycling device (CCCC, Astrol electronic AG). 4. Online Electrochemical Mass Spectrometry (OEMS) The OEMS setup was described elsewhere45-46 and operates with a quadrupole mass spectrometer (QMS 200, Pfeiffer) for partial pressure measurements, a pressure transducer (PAA33X, Keller Druck AG) for total cell pressure, temperature, and internal volume determination, stainless steel gas pipes and Swagelok fittings (3 mm compression tube fittings, Swagelok, OH, US) to connect the OEMS cell, a set of solenoid valves (2way magnetic valve, Series 99, silver-plated nickel seal, Parker) and a scroll pump (nXDS15i, EDWARDS GmbH) for efficient flushing. The magnetic valves are electronically controlled with a Solid State Relay Module (NI 9485 measurement System, National Instruments) connected to a computer with a LabView Software (NI Labview 2013, National Instruments). For partial pressure and gas evolution rate analysis, 1.3 mL of gas are extracted from the headspace (~ 4 mL) of the cell and replaced by


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pure Ar (quality 5.0). Calibration gas bottles were utilized to relate the mass spectroscopy (MS) ion-current signals at m/z = 2, 27, 32 and 44 to known concentrations of H2, C2H4, O2 and CO2 (1000 ppm of H2, C2H4, O2 and CO2 in Ar, respectively), before and after the measurement. The calibration for Me3SiF (m/z = 77) and POF3 (m/z = 85) was performed by re-

leasing defined amount of gases by chemical or thermally reactions in tests cells and is described in the supporting information (section S2).

Figure 1 (A-C) Analysis of gas evolution of HENCM/graphite full cells containing FEC/DEC (12:88, 1M LiPF6, 0 or 1 wt% TMSPa, 4.7 V) showing the evolution rate of O2, CO2, H2, POF3 and Me3SiF during galvanostatic cycling. (D + E) Cyclic voltammograms (scan rate: 55 V s-1) for FEC/DEC (12:88, 1M LiPF6) electrolyte with 0 (blank) and 1 wt% TMSPa showing the oxidative sweep up to 5 V with a SC65/SFG6L working electrode (D) and the reductive one down to 0.1 V with a graphite working electrode (E), both with partly delithiated LFP as counter electrode. (F) Me3SiF evolution rate as a function of time after the injection (at t = 0 h) of TMSPa into DEC suspensions of PVDF, LiF and SFG6 graphite particles with (cycled) and without (pristine) preformed SEI. Evolution rates and currents were normalized with respect to the BET surface areas of the active materials.

III. Results Part 1: Reactivity of TMSPa In order to determine the reactivity of TMSPa and its influence on electrolyte decomposition in a Li-ion battery, the formation of volatile side-reaction products within a Li-rich NCM/graphite full cell was investigated during operation. Figure 1A – C shows the voltage and gas evolution profiles of the main gases during the initial formation and subsequent four cycles. Dominant gas species are CO2, H2, and O2, which compares very well with the findings in our previous reports 47-48. Most of the CO2 and H2 evolve early during the SEI formation on the graphite anode (0.2 V → 4.2 V) and predominantly derive from FEC solvent reduction49-51. Michan et al. provided a detailed discussion on the possible reduction pathways of FEC, which in short could be described by a 2-step process, first involving the reductive defluorination of FEC FEC + Li+ + e- → VC + LiF + ½H2 (2)

to form vinylene carbonate (VC), lithium fluoride, and hydrogen. In a subsequent step the reduction of VC VC + Li+ + e- → CO2 + CHCHO-∙ Li+ (3) yields CO2 and a vinyloxy radical anion, which after protonation initiates the polymerization of VC and the formation of a SEI consisting of polymeric species described as poly-VC.52 The integrated amounts of CO2 and H2 as a result of FEC reduction were 2.96 and 6.23 𝜇mol, respectively, which are close to the expected 1:2 ratio (please review Supp. Info. for estimation details). The evolution of CO2 onsetting at ~ 4.2 V is associated with the Li-rich NCM cathode as it is taking place in full-cell configuration as well as in half-cell configuration at a comparable potential (cf. half-cell data in Figure S4) and was recently shown to derive mainly from inorganic carbonate residues remaining on the cathode surface after synthesis/storage.53 Decomposition of inorganic carbonates, such as Li2CO3, most likely proceeds indirectly in an electrochemically activated



multi-step electrochemical-chemical process. Protons generated by oxidation of electrolyte impurities ROH → H+ + e- + OR (4) (such as glycols54 or traces of water) and/or NCM oxide (MOH) and/or conductive carbon (C-OH) surfaces groups on the composite electrode (with R = Csurface, MNCM, OH, O(CH2)nCH3) could subsequently partake in an acid-base reaction Li2CO3 + 2 H+ → H2O + CO2 + 2 Li+ (5) releasing CO2 and water. Water may re-oxidize thus releasing the protons again to form a self-sustaining cycle (via eq. 4 + 5) until full depletion of all Li2CO3. Protons formed at the Li-rich NCM electrode may however also diffuse and be consumed at the graphite, thus explaining the increasing H2 evolution at higher cell polarizations. The observation of two maxima at 4.3 V and 4.7 V in the CO2 evolution profile during the 1st charge indicate the initial presence of at least two proton-generating processes. Both CO2 and H2 evolution display a potential-dependent behavior in subsequent cycles with gradually diminishing intensities, which indicates depletion of available inorganic residues. The release of O2 from the oxide cathode sets in at cell voltages above 4.35 V32-33, 47, 53, 55-56 and derives from a surface reconstruction process LixMOy → Li+ + e- + MO / M3O4 + ½ 1O2 (6) leading to nm-thin disordered O-deficient rock-salt MO and spinel M3O4 type oxide surface layers as previously observed by transmission electron microscopy.48 The apparent correlation between O2 and CO2 release indicates that the evolving oxygen is sufficiently reactive to trigger electrolyte degradation. Quantum chemical calculations by Okamato propose H-abstraction from cyclic carbonates by reactive oxygen species leading to Hdeficient species which react with further oxygen to CO2 and smaller organic molecules as possible mechanism.57 A recently published experimental and theoretical study by Freiberg et al. shows singlet oxygen is formed which abstracts H-atoms from the cyclic carbonate and leads to the formation of H2O2, which will be oxidized above 3.85 V vs. Li+/Li (similar to eq. 4), resulting in a free H+.58 The observation of a H2 evolution maximum at 4.7 V (see Figure 1B), as a result of H+ reduction on the anode, is in agreement with the proposed mechanism. No O2 is observed in the subsequent cycles, thus indicating that the cathode oxide surface is passivated in the initial formation cycle. Since none of these major gases were affected by the addition of TMSPa to the electrolyte (cf. Figure S2 in the SI), the influence of TMSPa on the cell formation processes (e.g. SEI and NCM surface reconstruction) is minor. A mass scan m/z > 45 enabled the identification of two further gas species being significantly affected by the addition of TMSPa and evolving depending on potential, namely POF3 and Me3SiF. The evolution profile of POF3 resembles the oxidative CO2 profile with the onset at about ~4.1 V, a evolution peak at ~ 4.4 V and a second one at the cut-off voltage of the cell during the 1st charge (Figure 1C). The second evolution maximum reoccurs in every subsequent cycle, though with gradually diminishing intensities. The observation is in agreement with previous studies by Streich et al. for Li-rich NCM/Li half-cells, who shows that the nature of the counter electrode has a negligible influence on the POF3 formation48 and suggests that the source for POF3 is the hydrolysis of PF5 ROH + PF5 → POF3 + HF + RF (7)

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which derives from the LiPF6 electrolyte salt (eq. 1). However, a recent study by Solchenbach et al. showed that thermally released PF5 is also detected as POF3, showing that OEMS is monitoring a mix of POF3 and PF5.23 We therefore have to keep in mind that we not only observe the hydrolysis product POF3 but also PF5 by OEMS, the latter being formed upon acidification of the electrolyte. Plakhotnyk et al. showed that the increase in H+ concentration promotes the hydrolysis of LiPF6 by the increased formation of PF5 in two steps:9 First, in presence of Li+, the equilibrium in the equation HF + Li+ ⇌ LiF + H+ (8) is shifted towards the right due to the low solubility of LiF20. Second, in presence of H+ ion pairs are formed with PF6- anions, leading to the formation of HF and PF5 H+ + PF6- ⇌ H-F-PF5 → HF + PF5 (9) A self-sustaining catalytic cycle is thus formed, unless the protons are scavenged (e.g. by the negative electrode). 9 This was also confirmed by Solchenbach et al. who added a strong acid into their electrolyte and observed enhanced formation of POF3/PF5 by OEMS. The release of protons would, in effect, accelerate both the decomposition of PF6- anions as well as the inorganic carbonate residues, hence explaining the apparently coordinated electrochemically activated formation of POF3 and CO2 between 4.1 V – 4.4 V in Figure 1B + C and Figure S4. Additionally, due to electrolyte oxidation by released reactive oxygen (maximum >4.4 V), an evolution maxima for POF3/PF5 are observed as result of reactions (8 + 9) as well as (7). Addition of TMSPa is observed to effectively suppress the POF3/PF5 formation (Figure 1C), proposedly by scavenging HF HF + (Me3SiO)3PO → Me3SiF + HO(Me3SiO)2PO (10) as formed e.g. in equation 7, which in turn could reduce the aforementioned H+ catalyzed PF5 formation (eq. 8 + 9) and thus decreasing the subsequent PF5 hydrolysis (eq. 7). Such a chemical reaction mechanism would also explain the operando observation of gaseous Me3SiF during charge (Figure 1B) and is supported by general thermodynamic considerations (bond energies: Si-F (595 kJ mol-1) > Si-O (444 kJ mol-1))59 as well as previous studies showing the general HF scavenging ability of the related tris trimethylsilyl phosphite (TMSPi) additive in LIB electrolytes.60-61 Reaction of TMSPa with HF according to equation 10 leads to the formation of the phosphoric acid HOPO(OSiMe3)2. Despite one order of magnitude higher dissociation in aqueous media (cf. pKa values: 2.16 for HF62 vs. 3.2 for the first proton of H3PO462 used as reference substance for HOPO(OSiMe3)2 (please note that a –SiMe3 group is causing a slight +I effect, which would result in a slightly lower acidity of the acid HOPO(OSiMe3)2)) the dissociation in LIB electrolytes will be strongly affected by the presence of Li+. For HF the presence of Li+ leads to a reaction to LiF which possesses low solubility in the electrolyte, thus increasing the concentration of H+ in the electrolyte as shown in equation (8) and increasing its acidity. On the other hand, the deprotonated LiOPO(OSiMe3)2 can be expected to be soluble in organic carbonates (c.f. the monoanion LiO2PF2 has good solubility in organic carbonates (≥ 1 wt% for LiO2PF263 vs. 6 ppm for LiF20)), therefore the formation of the Li+ salt of the conjugated base is in equilibrium with the acid. HOPO(OSiMe3)2 + Li+ ⇌ H+ + LiOPO(OSiMe3)2 (11) The presence of TMSPa is therefore reducing the concentration of the free protons in the electrolyte, and thus the acidity of the


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electrolyte, resulting in the formation of less PF5/POF3 according to equation 8 as observed by OEMS. Although the addition of TMSPa neither alters the SEI formation nor the surface reconstruction process of the NCM, cyclic voltammetry was performed in order to assess the electrochemical stability of TMSPa (Figure 1D + E). Despite the application of a broad potential window (0.1 − 5 V vs. Li+/Li), in none of the experiments a distinct peak for oxidation or reduction of the additive was observed which is in agreement with recent theoretical studies from Kim et al. that show the reduction potential of TMSPa is clearly below 0 V vs. Li+/Li (~ − 1.1 V) and its oxidation potential to be well above 5 V vs. Li+/Li (~ 6.4 V).64 Please note that the side reactions observed for the cathodic sweep in Figure 1E take place in the both electrolytes and result from PF6- intercalation into the working electrode.65 In order to exclude any direct chemical reactivity between the additive and any of the fluorinated cell components, a dedicated cell setup for in situ injection of TMSPa (or other liquids) to the cell compartment was developed. Any volatile side-products would immediately be recorded upon contact of the reactants. Figure 1F shows for instance no significant Me3SiF evolution once TMSPa injected into a suspension of PVDF in DEC. In fact, no indication of side-reactions between TMSPa and any of the pristine cell components were observed within the typical experimental measurement time (~ 24 h). However, a significant evolution of Me3SiF was observed when TMSPa was brought into contact with a pre-cycled graphite electrode (as compared to a pristine graphite electrode, Figure 1F), thus indicating that TMSPa does not exclusively react with HF. Indeed, after screening of several possible cell degradation products, we observed that TMSPa readily reacts also with LiF, thus leading us to update eq. 10 accordingly Li-/HF + (Me3SiO)3PO → Me3SiF + Li-/HO.(Me3SiO)2PO (12) Clearly, TMSPa is able to abstract fluoride from strongly ionic fluorides such as HF or LiF, whereas the reaction with the more covalent C-F or P-F bond does not occur or is rather slow, despite the lower E-F bond energies for the latter (489 kJ mol-1 for C-F compared to 565 kJ mol-1 for H-F). In conclusion, based on the above findings, we propose that the electrolyte additive TMSPa solely acts as a chemical scavenging agent of inorganic fluoride, such as HF and LiF, with no electrochemical activity within the voltage windows of the Li-ion cell. The lower cell impedance in cells containing TMSPa observed in previous studies40, 43-44, 66 and the decrease of the F1s peak for LiF in XPS spectra42-43, 66 could hence be explained simply by the removal of LiF from LIB electrodes. Part 2: TMSPa for operando monitoring of LiF and HF The findings that TMSPa is (a) electrochemically stable within the applied cell voltages, (b) non-reactive towards all pristine cell components, but (c) reactive towards HF and LiF, opens up the exploitation of TMSPa as an operando analytical probe for the formation of fluorides during operation of LIBs. Both LiF and HF are believed to play a critical role in Li-ion battery cell chemistry, particularly during the initial formation cycle of the cell, and further understanding thereof is highly desired.51-52, 6768

Figure 1B shows that the formation of LiF and HF (as monitored by the evolution of Me3SiF via eq. 12) displays essentially three potential-dependent maxima during the 1st cycle of the cycling of a Li-rich NCM full cell: (I) directly after the start of the

charging process when the dominant process is the SEI formation on the anode, (II) during the voltage plateau from 4.2 V – 4.6 V, and (III) when the cell reaches its cut-off voltage of 4.7 V. Additionally the evolution rate of Me3SiF decreases in a rather slow exponential decay during discharge whereas the evolution of CO2, O2 and POF3 decreases rapidly. This indicates that the formation of Me3SiF is rather a chemical reaction and thus limited by factors such as LiF solubility or diffusion of TMSPa. In subsequent cycles only a maximum at the upper cut-off voltage is observed, indicating potential-activated proton formation judging from the concomitant H2 and CO2 evolution (see Figure 1B), although with diminishing intensities. As several possible sources for the formation of fluoride can be imagined, we studied systematically the influence of the following conceivable processes: 1. SEI formation on the graphite anode 2. Conducting salt decomposition 3. Electrode cross-talk 4. Degradation of the cyclic carbonate 5. Defluorination of the binder To study these contributions we replaced individual components of the cell and compared the evolution of Me3SiF during the 1st charge either with the reference full cell system containing FEC/DEC and 1M LiPF6 or with another suitable reference. The results of this comparison experiments are presented in Figure 2 and discussed in the following sections. 1. SEI formation on the graphite anode In order to determine the fluorides formed due to the SEI on the graphite anode we replaced the NCM cathode by LFP, which operates at a potential close to 3.45 V vs. Li+/Li and therefore does not undergo any side reactions with the electrolyte or induce any noticeable gassing. Figure 2 shows the potential profile (panel A) and the Me3SiF evolution rate (panel B) for the graphite/LFP cell as compared to the Li-rich NCM/graphite cell. Both cells show a similar immediate increase of the Me3SiF formation when the cell is charged with maxima at the FEC reduction potential plateau and then exponentially decaying signal, analogous to the reaction of TMSPa with pure LiF (Figure 1F). The dominating electrochemical process at this stage is the formation of the SEI on the graphite electrode involving reduction of FEC with LiF as one of its major products (according to eq. 2). The total amount of detected Me3SiF (8.59 mol m-2) is however lower than the total amount of CO2 (36.64 mol m-2), thus indicating that not all LiF is being scavenged. In summary, the reduction of FEC can be identified as the main fluoride source < 4.2 V. 2. LiPF6 conductive salt decomposition Apart from the initial Me3SiF evolution maximum due to FEC reduction and SEI formation on the graphite, Figure 2B shows that the cell with LiClO4 as salt displays only one Me3SiF evolution maximum at the upper cut-off voltage compared to two maxima > 4.2 V for the reference cell based on LiPF6. The fluoride formation at high voltages originates mostly from sidereactions at the cathode side, since none of these processes occur in the graphite/LFP cell. The main source of fluoride formation is the LiPF6 salt as the LiClO4 based cell displays significantly less Me3SiF (cf. 4.05 mol m-2 for 1M LiClO4 in contrast to 17.71 mol m-2 Me3SiF for 1M LiPF6 ).



Figure 2 Potential profile (A) and Me3SiF evolution (B-D) for different cells containing 1 wt% TMSPa: Li-rich NCM/graphite cells with FEC/DEC electrolyte with 1M LiPF6 or LiClO4 andgraphite/LFP/LFP with 1M LiPF6 (B); subtracted Me3SiF profiles (r(LiPF6) – r(LiClO4)) for Li-rich NCM/graphite, Li-rich NCM/Li and graphite/LFP cells in FEC/DEC electrolyte (C); Lirich NCM/graphite cells with EC/DEC electrolyte (1M LiPF 6 or 1M LiClO4) compared to an FEC/DEC (1M LiPF6) electrolyte with subtracted Me3SiF contribution from the SEI formation (D).

The first Me3SiF evolution maximum at ~4.4 V (NCM full-cell – LiPF6, Figure 2B) most likely results from proton release due to surface group oxidation (eq. 4) and the release of reactive singlet oxygen (eq. 6), due to the onset of the surface reconstruction process of the NCM active material, abstracting protons from the electrolyte FEC/DEC solvents.58 The release of oxygen and protons promotes LiPF6 decomposition (eq. 7 − 9), which cannot take place if LiClO4 is the conducting salt.23 The formation of H+ at the cathode at this potential was discussed

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above and is supported by the concomitant evolution of CO2 as a result of the dissolution Li2CO3 residues (according to eq. 5). The second process occurring when the cell voltage increases steeply >4.5 V is potential-activated and the generated fluorides most likely derive from several sources as the evolution maximum occurs regardless of the LiPF6 salt. The prevalent Me3SiF, CO2, H2, and POF3 evolution in subsequent cycles, although with gradually diminish intensities, is evidence of the decomposition of both the electrolyte salt and solvent, most likely triggered by proton abstraction and release (e.g., as described by Shkrob et al.)69. For the LiClO4 based electrolyte, only FEC solvent and the PVDF binder are the available fluoride sources, but the contribution of these components to LiF/HF formation will be discussed in detail in the subsections 4 and 5. 3. Electrode cross-talk A significant decomposition of the electrolyte occurs during the SEI formation, which leads to the question if there is any contribution of SEI species diffusing from the anode to the cathode on the fluoride formation in the cell. In order to decouple the fluorides simply stemming from the FEC reduction, we subtracted the Me3SiF evolution profile of the LiClO4 based electrolyte (Figure 2B, curve II) from the evolution profile of the LiPF6 reference system (Figure 2B, curve I). The subtraction was applied for Li-rich NCM/graphite, Li-rich NCM/LFP as well as Li-rich NCM/Li cells, thus eliminating the contributions from FEC, and shown in Figure 2 (curve I – II). The gas evolution sets in at about 4.25 V for the Li-rich NCM/graphite cell and at about 4.35 V for the cells with Li or LFP counter electrodes. For the latter two, the evolution profile mimics a potential-activated process (cf. detailed explanation in Supp. Info section S4) and is likely related to the onset of oxygen release from the NCM oxide and the acidification of the electrolyte. The slightly lower Me3SiF evolution rate for the half-cell can be attributed to the ability of the metal Li anode to reduce the protons in the electrolyte to H2 and thus reduce the formation of PF5 and HF, as described above (equations 8 + 9). In case of the full-cell, the additional process between 4.25 V and 4.55 V displays an evolution maximum at ~ 4.42 V with almost twice the amounts of Me3SiF (10.11 mol m-2) compared to a half-cell (5.53 mol m-2) in the same potential window (see colored areas Figure 2, panel C). The higher surface area compared to Li metal and the lower operating potential compared to LFP leads to larger extents of FEC solvent reduction for graphite during the formation cycle. The FEC reduction products (e.g. alkoxides, semicarbonates49 or formate52) would migrate and oxidize at the cathode > 4.2 V (similar to eq. 4) and increase the amount of formed protons and subsequently HF (eq. 8 and 9). However, the migration of anode decomposition products to the cathode and vice versa is so far not fully understood, making a final assignment of the observed feature difficult. 4. Cyclic carbonate The results of the previous subsection indicate that FEC could contribute not only to the fluoride formation on the anode side of a full cell but also on the cathode at high cell voltages. In order to study the contribution of the cyclic carbonate, we repeated the experiments with a comparable EC based electrolyte. The Me3SiF evolution in EC/DEC with LiPF6 (Figure 2, panel D) shows also three evolution maxima of which the first results from LiPF6 salt decomposition during the SEI formation. LiF is typically found in the SEI of EC and LiPF6 based electrolytes.70


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The second evolution maximum is again found during the potential plateau and a third one at the cut-off voltage of the first charge. In order to facilitate the comparison between the EC and FEC based electrolytes at high voltages, we subtracted the contribution from fluorides resulting from FEC reduction during SEI formation (Figure 2, panel D, curve I - III). Clearly, there is a qualitative agreement between the Me3SiF evolution profiles > 4.2 V for both electrolytes containing LiPF6, even though significantly more fluorides form with EC (0.084 mol min-1 m-2) than with FEC (0.038 mol min-1 m-2). Two sources of the larger amounts of fluorides in EC based electrolytes can readily be conceived: First, the reactive oxygen species formed at the surface of HE-NCM during its activation attack and decompose the organic carbonates48, which leads to the acidification of the electrolyte and subsequently increased formation of fluorides from the conducting salt (eq. 7 - 9). Previous DFT studies from Okamoto have shown that EC is much more prone to H abstraction than FEC.57 Secondly, it has been shown theoretically71 and experimentally on Si anodes72 that EC forms during the SEI formation thicker SEI layers than FEC. Therefore EC would conceivably also lead to larger amounts of species (e.g. alkoxides, semicarbonates49 or formate52) migrating to and oxidizing at the cathode. 5. PVDF binder Evaluating the EC electrolyte with the LiClO4 salt surprisingly also revealed a minor potential dependent Me3SiF evolution conceivably originating from the PVDF binder, which is the only remaining fluorinated compound in the cell (Figure 2D). Since no direct defluorination of the PVDF binder by the TMSPa was observed (c.f. injection cell experiments shown in Figure 1F), the PVDF decomposition must occur during electrode cycling. Indeed, previous works from Andersson et al. and Edström et al. have shown that cathodes prepared with PVDF binder contain a small ratio of LiF formed due to dehydrofluorination reaction induced by LiOH residues from the cathode active material.73-74 -CF2-CH2- + LiOH → -CF=CH- + LiF + H2O (13) Most likely, Brönsted bases are formed in NCM/graphite cells during cycling (e.g. alkoxides during the SEI formation49), as side products from reactions of reactive oxygen species released during the surface reconstruction of the Li-rich NCM or potentially by decomposition of LiClO4 at high voltages. Cyclic voltammetric experiments with conductive carbon SC65 electrodes with or without PVDF clearly show a distinct Me3SiF evolution > 4.4 V (see Figure S7) in the presence of the PVDF binder. The exact process of this reaction will be subject of further studies as it has strong implications for the mechanical integrity of the high-voltage cathodes for LIBs. IV. Conclusions Silyl ester based electrolyte additives have been extensively investigated and applied in recent years for improving the performance of Li-ion batteries. We investigated the role and operating mechanism of one of the most prominent members, tris(trimethylsilyl)phosphate (TSMPa), by studying the gas evolution in advanced Li-rich NCM/graphite cells based on a next-generation FEC/DEC based electrolyte. Both SEI formation on the graphite anode (leading to H2 and CO2 evolution) as well as surface reconstruction of the cathode (resulting in O2/CO2 evolution) are essentially unaffected by presence of TMSPa during the initial formation cycles of the cell. Our results rather show that the role of TMSPa is to act as a chemical scavenger of two

of the most detrimental side-products, HF and LiF, forming during operation of Li-ion cells. Removal of HF lowers electrolyte acidity and suppresses further electrolyte LiPF6 decomposition (less PF5/POF3 evolution), which - along with LiF scavenging explain the lower impedance and higher performance of Li-ion batteries containing TMSPa.

Figure 3 Graphical conclusion and proposed working mechanism for the TMSPa additive in a full-cell with FEC/DEC LiPF6 electrolyte showing the reduction of FEC (I), electrochemical oxidation of surface groups (II) or of solvent molecules by reactive oxygen species (III) both leading to the release of protons and subsequently the formation of HF and LiF in a catalytic loop (IV). In presence of TMSPa the formed HF is scavenged (V) leading to reduced acidity in the electrolyte. The application of TSMPa as chemical probe further allows to monitor contributions from cross-talk (VI) and fluoride formation from the PVDF binder (VII).

The selective reactivity of TMSPa towards fluorides by forming Me3SiF provides an excellent chemical probe for operando monitoring of HF/LiF formation during Li-ion cell operation. The nature, extent and onset potentials of several side-reactions, as overviewed in Figure 3, were investigated by systematically analyzing the individual contribution of several commonly employed Li-ion battery components to fluoride generation. The developed method shows that: 1. Through chemical probing with TMSPa, previous assumptions that the SEI formation on the graphite anode is the major early contributor to LiF (along with CO2) are confirmed and the formation of LiF continues to evolve during the whole 1st cycle as a result of FEC reduction. 2. Proton and reactive oxygen formation at the cathode surface occurs at higher cell voltages > 4.2 V and leads to the formation of HF. The concomitant evolution of Me3SiF and POF3 (and the suppression of the latter when including TMSPa) further demonstrates the intimate relationship between H+ formation and LiPF6 decomposition. 3. The stronger Me3SiF evolution from the cathode surface in cells containing graphite anodes compared to



Li or LFP as anode shows that cross-talk between anode and cathode enhances HF formation and acidification of the electrolyte. 4. The minor, but still clearly detectable, Me3SiF evolution in a fluorine free electrolyte indicate that also the most commonly employed PVDF binder may be releasing fluoride during the cell operation, which warrants further critical review of the inertness of PVDF under cell operation at high voltages. The methodological approach developed herein paves the way for further exploration of Li-ion battery electrolyte additives not only to directly improve cell performance by chemical action, but also to improve the same by acting as highly selective analytical probes, thus deepening our fundamental understanding and guiding the development of future Li-ion battery cell chemistries.

ASSOCIATED CONTENT SUPPORTING INFORMATION Details for the POF3 and Me3SiF calibration. Methodology for the determination of gas amounts from SEI formation. Experimental data of additional CV experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

Present Addresses † Department of Chemistry, Ångström Laboratory, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Funding by BASF SE is gratefully acknowledged. E.J.B. acknowledges Swiss National Science Foundation (SNSF) under the “Ambizione Energy” funding scheme (Grant No. 160540).

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