Potentiostatic Activation Enables Complete Decomposition of Li2C4O4/C and Enhances Performance of NCM811 Batteries at 4.2 V

Lithium squarate (Li₂C₄O₄) is a high-capacity cathode prelithiation additive, but its full decomposition voltage (∼4.5 V) induces severe side reactions, necessitating a reduction to 4.2 V for Ni-rich cathodes. This study prepares a carbon-composited Li₂C₄O₄ prelithiation additive and investigates the impact of a 4.2 V potentiostatic activation protocol on its decomposition efficiency, interface evolution, and cell performance. A 24 h potentiostatic step at 4.2 V enables nearly complete decomposition of the additive in half cells, with a first-cycle charge capacity 64.78% higher than under galvanostatic activation. In NCM811 half cells, 100% decomposition is achieved under 4.2 V potentiostatic activation, comparable to 4.5 V galvanostatic activation, while forming a denser cathode-electrolyte interphase (CEI) that reduces impedance and suppresses side reactions. These half cells show 4.76% higher capacity retention after 150 cycles at 1C and deliver 114 mAh g^–1 at 10C. Pouch full-cell tests confirm improved additive decomposition and optimized interphases, raising capacity retention to 92.6% after 150 cycles. This study demonstrates that 4.2 V potentiostatic activation enables efficient prelithiation without high voltage side reactions, providing experimental and theoretical support for industrial applications in Ni-rich ternary systems.