Solution NMR Studies of Electrolyte Decomposition Pathways

One approach to increasing the energy density of lithium-ion batteries is to increase the operational upper voltage limit. However, higher cell voltages tend to lead to rapid capacity fade and shorter cycle life. This is typically ascribed to the loss of oxygen due to surface reconstructions of the cathode material, increased electrolyte oxidation rates, and transition metal dissolution. It is well-known that electrolyte decomposition can negatively impact cell behaviour and lifetime by causing gassing, electrochemical impedance growth, or slippage, and that extensive electrolyte oxidation at high cell voltages can induce eventual rapid capacity loss or ‘rollover’ failure.1,2 Whereas the reduction processes at the anode have been well-studied and are generally understood,3 the chemical pathways and mechanisms for oxidation at the cathode remain relatively unknown. It is desirable to develop a more detailed chemical understanding of electrolyte degradation in support of longer-lasting high voltage materials and cell designs. Herein, operando pressure measurements, liquid-state NMR, and electrochemical methods are combined to study electrolyte oxidation at multiple cell voltages. LiCoO2 (LCO)/Li cells were cycled with a lithium-ion conducting ceramic separator to identify the degradation products formed at the anode and cathode separately and to prevent further reactions of these species on the opposite electrode. The results are compared with the species generated by electrochemical oxidation of the electrolyte in a simple H-cell and with cells cycled without a separator. A major finding of the present work is that all degradation products from the cathode side of the LCO/Li cells can be explained by chemical oxidation of the electrolyte, involving oxygen evolved from the cathode surface, at least up to 4.9 V vs. Li/Li+. This result is consistent with and complementary to previously reported online electrochemical mass spectrometry results4 and has significance for nickel-containing cathode materials (e.g., NMC). Finally, a comprehensive summary of electrolyte degradation pathways was constructed by analysing a series of multinuclear and one- and two-dimensional NMR spectra. Reference s : [1] L.M Thompson, W. Stone, A. Eldesoky, N.K. Smith, C.R. M. Mcfarlane, J. S. Kim, M. B. Johnson, R. Petibon and J. R Dahn, J. Electrochem. Soc., 165 (2018), 2732-2740. [2] X. Ma, J. E. Harlow, J. Li, L. Ma, D. S. Hall, M. Genovese, M. Cormier, J. R. Dahn, S. Buteau, J. Electrochem. Soc., 166 (2019), A711-A724. [3] S. K. Heiskanen, J. Kim, B. L. Lucht, Joule, 3 (2019), 2322-2333. [4] R. Jung, M. Metzger, F. Maglia, C. Stinner, H. A. Gasteiger, J. Phys. Chem. Lett., 8 (2017), 4820-4825.