Two electrolyte decomposition pathways at nickel-rich cathode surfaces in lithium-ion batteries

Preventing the decomposition reactions of electrolyte solutions is essential for extending the lifetime of lithium-ion batteries. However, the exact mechanism(s) for electrolyte decomposition at the positive electrode, and particularly the soluble decomposition products that form and initiate further reactions at the negative electrode, are still largely unknown. In this work, a combination of operando gas measurements and solution NMR was used to study decomposition reactions of the electrolyte solution at NMC (LiNixMnyCo1−x−yO2) and LCO (LiCoO2) electrodes. A partially delithiated LFP (LixFePO4) counter electrode was used to selectively identify the products formed through processes at the positive electrodes. Based on the detected soluble and gaseous products, two distinct routes with different onset potentials are proposed for the decomposition of the electrolyte solution at NMC electrodes. At low potentials (<80% state-of-charge, SOC), ethylene carbonate (EC) is dehydrogenated to form vinylene carbonate (VC) at the NMC surface, whereas at high potentials (>80% SOC), 1O2 released from the transition metal oxide chemically oxidises the electrolyte solvent (EC) to form CO2, CO and H2O. The formation of water via this mechanism was confirmed by reacting 17O-labelled 1O2 with EC and characterising the reaction products via1H and 17O NMR spectroscopy. The water that is produced initiates secondary reactions, leading to the formation of the various products identified by NMR spectroscopy. Noticeably fewer decomposition products were detected in NMC/graphite cells compared to NMC/LixFePO4 cells, which is ascribed to the consumption of water (from the reaction of 1O2 and EC) at the graphite electrode, preventing secondary decomposition reactions. The insights on electrolyte decomposition mechanisms at the positive electrode, and the consumption of decomposition products at the negative electrode contribute to understanding the origin of capacity loss in NMC/graphite cells, and are hoped to support the development of strategies to mitigate the degradation of NMC-based cells.


Comparison of potential profiles for NMC811/Li and NMC811/Li x FePO 4 cells
. Comparison of the potential profiles of NMC811 electrodes in Li half-cells (data in Figure S6) and in cells with a Li 0.25 FePO 4 counter electrode (data in figure S5) using a conversion of potential that takes into account the potential of the Li 0.25 FePO 4 electrode (3.45 V vs. Li + /Li). Figure S2. The setup used to generate singlet oxygen.

Additional operando pressure measurements
To deconvolute the contribution of the EC and DMC solvents to the pressure increase observed in NMC811/Li cells made with an LP30 electrolyte solution, the measurements were repeated using a 'DMC-only' or an 'EC-only' electrolyte solution. A higher salt concentration (1.5 M vs 1 M) was used for the EC-only electrolyte to ensure that the solution was liquid at room temperature. Figure S3. Operando pressure data for an NMC811/Li cells using a 1 M LiPF 6 in DMC electrolyte. The cell was cycled between 3.0 V and a series of increasing upper cut-off potentials (4.3-4.85 V), after which it were cycled to 4.3 V again; where the potential was stepped by 0.1 V every two cycles until 4.5 V, whereafter the step-size was reduced to 0.05 V. For every cut-off potential value, the second cycle included a 2-hour potential hold at the top of charge. The internal cell pressure and potential-time data are shown in red and black, respectively. An expanded view of the data is shown on the right. The active material mass loading was 3 mg cm -2 . Figure S4. Operando pressure data for an NMC811/Li cells using a 1.5 M LiPF 6 in EC electrolyte. The cell was cycled between 3.0 V and a series of increasing upper cut-off potentials (4.3-4.85 V), after which it were cycled to 4.3 V again; where the potential was stepped by 0.1 V every two cycles until 4.5 V, whereafter the step-size was reduced to 0.05 V. For every cut-off potential value, the second cycle included a 2-hour potential hold at the top of charge. The internal cell pressure and potential-time data are shown in red and black, respectively. An expanded view of the data is shown on the right. The active material mass loading was 3 mg cm -2 .

Additional on-line electrochemical mass spectrometry (OEMS) measurements
To confirm that the evolution of C 2 H 4 and CO observed at low cell potentials in NMC811/Li cells originates from electrolyte reduction reactions at the lithium metal electrode, the OEMS measurements were repeated using a partially delithiated Li x FePO 4 (LFP) electrode.
The amount of oxygen produced in the NMC811/LFP cell is twice as high as that produced in the NMC811/Li cell (100 vs 50 ppm). We attribute this difference to the consumption of the generated oxygen by the lithium metal electrode to form lithium oxides. The formation of CO 2 or H 2 at low potentials, < 4.3V vs. Li + /Li, is negligible in the NMC811/Li cells ( Figure 3 in the main text and Figure S6). Previous work has shown that the reduction of water traces at the anode produces H 2 and hydroxide ions, which then promote the decomposition of the organic electrolyte forming CO 2 . 10,63,67 These processes are absent in the NMC/LFP cell in Figure S5, as expected, because the potential of the LFP counter electrode is too high to induce water reduction. However, these processes are also

Three electrode measurements
The potential the NMC electrode reaches in NMC/delithiated-LFP cells was determined using a three-electrode cell, where lithium metal was used as the reference electrode. Figure S7. Galvanostatic charge and discharge curve for a three-electrode NMC811/Li/delithiated LFP cell, cycled at a C/5 rate between 0.2-1.53 V cell , which was found to correspond to a potential window of 3.7-4.9 V vs Li/Li + for the NMC811 electrode.

DMSO-d 6
To help distinguish between 1 H NMR signals arising from electrolyte decomposition products and those originating from impurities, 1 H NMR spectra of each batch of DMSO-d 6 were acquired. The DMSO-d 6 used in this work was 99% chemically pure. Individually sealed ampoules of DMSO-d 6 (0.75 mL) were used to avoid any absorption of moisture or further deterioration of the solvent.

Comparison of glass fibre with polypropylene
The cell made with a polypropylene (PP) separator (Celgard 3501; b) shows the presence of presence of methanol, formic acid, acetals and OPF 2 (OCH 3 ). This indicates that DMC is hydrolysed, and thus demonstrates that water is also formed in cells without a glass fibre (G/F) separator (and so the water cannot originate from reactions of HF with the separator, but instead must be formed in a different way). We therefore attribute the formation of water to the chemical oxidation of EC by singlet oxygen. No 19 F NMR signal is seen for SiF x and only a small signal is seen for BF 4when a PP separator is used instead of a G/F one. The small amount of BF 4most likely comes from the S12 glass NMR tube or the glass vial the separator is placed in to extract the electrolyte solution with DMSO. The 19 F NMR spectra show that the SiF x and BF 4species observed in cells assembled with a G/F separator originate from the glass fibre, and the 19 F NMR signal intensity of SiF x /BF 4can be used as a measure for the amount of HF formed in the cell.

Reactions between singlet oxygen and the carbonate solvent
To confirm that 1

Assignment of the 1 H NMR signal at 3.98 ppm
The incorrect assignment of the 1 H NMR signal at 3.92 ppm in previous work by some of the authors 2 was discovered after comparison with a 1 H NMR spectrum of glycolic acid in DMSO-d 6 ( Figure S30). The peak at 3.98 ppm is now assigned to the fluorophosphate ester OPF 2 (OCH 3 ), based on the appearance of a signal at 3.98 ppm in the 1 H NMR spectra of LP30 with 2 vol.% methanol added (Figure 10 in main text). Figure S30. 1 H NMR spectrum of glycolic acid in DMSO-d 6 ; the 1 H NMR signals are observed at 3.92 ppm (s) and 3.36 ppm (broad), the peaks being assigned to the CH 2 and -OH groups, respectively.