<i>Operando</i> X‐Ray and Postmortem Investigations of High‐Voltage Electrochemical Degradation in Single‐Crystal‐LiNiO ₂ –Graphite Cells

Abstract Single‐crystalline LiNiO 2 (SC‐LNO), a high‐energy‐density Li‐ion cathode material, suffers from poor long‐term electrochemical performance when cycled above 4.2 V (vs Li + /Li). In this study, this degradation is evaluated using SC‐LNO–graphite pouch cells electrochemically aged within a stressful voltage window (2.5–4.4 V) using a constant‐current constant‐voltage (CC‐CV) protocol. Notable capacity fade is observed after one hundred cycles at C/3 rate, in addition to an increase in the overall electrochemical cell impedance. Operando X‐ray diffraction data reveal that, despite no significant long‐range bulk structural changes, (de‐)lithiation of the aged SC‐LNO becomes kinetically hindered after 100 cycles. Aging‐induced changes in the short‐range structure and charge compensation are evaluated through a multi‐model quantitative analysis of the operando X‐ray absorption spectroscopy data. While the electrochemical aging does not result in particle cracking, soft X‐ray absorption spectroscopy data revealed the reconstruction of the cathode surface to a dense rock salt‐like layer after long‐term cycling, which acts as a kinetic trap for Li + diffusion. Therefore, even under stressful conditions, it is the surface reconstruction that dominates the overall cathode degradation by reducing the Li + mobility and leading to the capacity fade. Cathode surface engineering will therefore be key to improving the long‐term electrochemical performance of SC‐LNO cathodes.