Unveiling the Influence of Formation Voltage on Li-Rich Layered Oxide Cathode.

Lithium-rich layered oxide (LRLO) cathodes are recognized for their high energy densities, primarily driven by oxygen-related anionic redox activities, yet substantial activation of this process simultaneously induces structural instability. The typical voltage range in academic studies spans 2.0-4.8 V. Although 2.5-4.5 V are generally considered in industrial applications for enhanced capacity retention and electrolyte compatibility, this moderate voltage window leads to reduced capacity. To address energy density limitations, several top battery suppliers propose to separately increase the formation voltage during the initial cycle to enhance capacity, while other companies (e.g., Contemporary Amperex Technology Co., Ltd., CATL) claim that this high-voltage formation protocol would exacerbate cycling capacity fading. Herein, we systemically demonstrate that high-voltage formation promotes substantial Li⁺ extraction from the transition metal (TM) layers, creating vacancies (in TM layer) that drive in-plane TM migration. This migration triggers a transformation in the OM₆ (M, cation) configuration from O4 (OLi_xTM₂) to O5 (OLi_yTM₁). Such evolution simultaneously enhances both anionic and cationic redox activity, collectively boosting capacity. Nonetheless, the induced in-plane TM migration would further aggravate out-of-plane TM migration, leading to progressive structural degradation, which has been elucidated as the main reason for cycling capacity fading.