Evolution of the Electrode–Electrolyte Interface of $\mathrm{LiNi_{0.8}Co_{0.15}Al_{0.05}O_{2}}$ Electrodes Due to Electrochemical and Thermal Stress

Abstract

For layered oxide cathodes, impedance growth and capacity fade related to reactions at the cathode–electrolyte interface (CEI) are particularly prevalent at high voltage and high temperatures. At a minimum, the CEI layer consists of Li$_2$CO$_3$, LiF, reduced (relative to the bulk) metal-ion species, and salt decomposition species, but conflicting reports exist regarding their progression during (dis)charging. Utilizing transport measurements in combination with X-ray and nuclear magnetic resonance spectroscopy techniques, we study the evolution of these CEI species as a function of electrochemical and thermal stress for $\mathrm{LiNi_{0.8}Co_{0.15}Al_{0.05}O_{2}}$ (NCA) particle electrodes using a LiPF$_6$ ethylene carbonate:dimethyl carbonate (1:1 volume ratio) electrolyte. Although initial surface metal reduction does correlate with surface Li$_2$CO$_3$ and LiF, these species are found to decompose upon charging and are absent above 4.25 V. While there is trace LiPF$_6$ breakdown at room temperature above 4.25 V, thermal aggravation is found to strongly promote salt breakdown and contributes to surface degradation even at lower voltages (4.1 V). An interesting finding of our work was the partial reformation of LiF upon discharge, which warrants further consideration for understanding CEI stability during cycling