Revealing the surface-to-bulk degradation mechanism of nickel-rich cathode in sulfide all-solid-state batteries

Layered nickel-rich materials (LiNi1-y-zCoyMnzO2, 1–y–z ≥ 0.8) are regarded as promising cathode candidates for all-solid-state batteries (ASSBs); however, nickel-rich cathodes exhibit low Coulombic efficiency and poor cycle stability at high cutoff potentials (E ≥ 4.2 V vs. Li+/Li). To interpret this, much attention has been focused on the study of interface reactions, while ignoring the bulk structure evolution of active materials during cycling. Herein, we thoroughly investigate the bulk structure evolution of single-crystal LiNi0.8Co0.1Mn0.1O2 in ASSBs at different cycles, further correlated with its interface reactions and electrochemical performance. Operando X-ray diffraction detects the emergence of sluggish phase in ASSBs during the first charge process, which accumulates significantly as the cycle progresses, corresponding to the limited delithiation and rapid performance decay. Our results reveal that the surface chemistry has a great effect on the bulk structure evolution and such a surface-to-bulk degradation mechanism is critical to the cathode design toward high-performance ASSBs. From this novel perspective, we demonstrate that the enhanced performance employing the surface coating on the nickel-rich materials is attributed not only to the suppression of interfacial side reactions but also to the elimination of sluggish phase.