Understanding the Oxidative Stability of LiPF6 at Charged Ni Rich Cathode Interfaces

Current commercial liquid electrolytes for Li-ion batteries consist of a mixture of linear and cyclic carbonates coupled with a lithium salt (possessing a weakly coordinating anion) and additives. However, these electrolytes are limited by poor solvent stability, in particular at high potentials. 1,2 To mitigate against these issues, recent attention has been paid to highly concentrated electrolytes owing to their improved electrochemical and thermal stabilities. 3-5 In recent work we have revealed the formation of an inorganic rich passivating interphase at high potentials on Ni rich cathodes for higher salt concentrations of LiPF 6 in EC: EMC v: v 3:7. 6 The inorganic rich interphase supresses cathode degradation methods such as transition metal dissolution. 7 Here we focus on uncovering the origin of this passivating interphase in these more highly concentrated electrolytes using both spectroscopic and computational techniques. Classical molecular dynamics (MD) is a computationally efficient means to investigate solvation structures that form in electrolytes on the molecular scale. It provides insight into how the Li + ion coordination environments, the number of non-Li + associating solvent species and the activity factors of the solvent change with increasing salt concentration. Density functional theory (DFT) can then be used to investigate how the electronic structure, and consequently the electrochemical stability, of the LiPF 6 in the electrolyte changes with increasing salt concentration. Simulated results from MD and DFT are compared with experimental Raman and X-ray absorption spectra from the electrolytes to verify the results. MD simulations and Raman measurements reveal almost all EC and EMC are involved in direct Li + association in the more highly concentrated electrolytes. More PF 6 - is found in the first solvation sheath of Li + ions in the more highly concentrated electrolytes as the EC and EMC are shared amongst a larger number of Li + ions. The increased involvement of EC and EMC in direct Li + association is accompanied by a decrease in their activity factors in the more highly concentrated electrolytes. The activity factors of LiPF 6 are found to increase with salt concentration in agreement with previous work. 8 Density of states (DOS) calculations on MD snapshots reveal the increased presence of PF 6 - contributions to the highest occupied electronic states, indicating the oxidation of PF 6 - competes with that of EC and EMC in the more highly concentrated electrolytes accounting for the observations in our earlier work. X-ray absorption measurements exhibit similar trends to those observed in the DOS calculations with increasing salt concentration, supporting the simulated results. [1] Dose, W.M., Temprano, I., Allen, J.P., Björklund, E., O’Keefe, C.A., Li, W., Mehdi, B.L., Weatherup, R.S., De Volder, M.F. and Grey, C.P., 2022. Electrolyte reactivity at the charged Ni-rich cathode interface and degradation in Li-ion batteries. ACS Applied Materials & Interfaces, 14(11), pp.13206-13222. [2] Dose WM, Li W, Temprano I, O’Keefe CA, Mehdi BL, De Volder MF, Grey CP. Onset potential for electrolyte oxidation and Ni-rich cathode degradation in lithium-ion batteries. ACS Energy Letters. 2022 Sep 22;7(10):3524-30. [3] Wang, J., Yamada, Y., Sodeyama, K., Chiang, C.H., Tateyama, Y. and Yamada, A., 2016. Superconcentrated electrolytes for a high-voltage lithium-ion battery. Nature communications, 7(1), p.12032. [4] Suo, L., Borodin, O., Gao, T., Olguin, M., Ho, J., Fan, X., Luo, C., Wang, C. and Xu, K., 2015. “Water-in-salt” electrolyte enables high-voltage aqueous lithium-ion chemistries. Science, 350(6263), pp.938-943. [5] Suo, L., Borodin, O., Wang, Y., Rong, X., Sun, W., Fan, X., Xu, S., Schroeder, M.A., Cresce, A.V., Wang, F. and Yang, C., 2017. “Water‐in‐salt” electrolyte makes aqueous sodium‐ion battery safe, green, and long‐lasting. Advanced Energy Materials, 7(21), p.1701189. [6] Phelan, C.M., Björklund, E., Singh, J., Fraser, M., Didwal, P.N., Rees, G.J., Ruff, Z., Ferrer, P., Grinter, D.C., Grey, C.P. and Weatherup, R.S., 2024. Role of salt concentration in stabilizing charged Ni-rich cathode interfaces in Li-Ion batteries. Chemistry of Materials, 36(7), pp.3334-3344. [7] Björklund, E., Xu, C., Dose, W.M., Sole, C.G., Thakur, P.K., Lee, T.L., De Volder, M.F., Grey, C.P. and Weatherup, R.S., 2022. Cycle-induced interfacial degradation and transition-metal cross-over in LiNi0. 8Mn0. 1Co0. 1O2–graphite cells. Chemistry of Materials, 34(5), pp.2034-2048. [8] McEldrew, M., Goodwin, Z.A., Bi, S., Kornyshev, A.A. and Bazant, M.Z., 2021. Ion clusters and networks in water-in-salt electrolytes. Journal of The Electrochemical Society, 168(5), p.050514.