Achieving High Capacity in Nickel-Rich Cathodes via Low-Voltage Lithium Storage Expansion

Lithium-excess configurations accessed at low cutoff voltages in nickel-rich layered oxides offer a promising route to increasing cathode capacity but are typically accompanied by rapid performance degradation and poor cycling stability. To address this challenge, we propose a constant-capacity (CCap) protocol that fixes the discharge capacity at a prescribed value. By selectively accessing the low-voltage lithium-storage reservoir while limiting the extent of lithiation below the stoichiometric threshold, this strategy suppresses uncontrolled deep lithiation and mitigates structural degradation. Li||NCM811 half-cells achieve extended cycling stability, delivering up to 300, 500, and 600 cycles at 100%, 90%, and 80% capacity retention, respectively. X-ray diffraction analysis reveals that capacity extension during cycling originates from the low-voltage regime through the formation of a Li₂-NCM811 phase, which enables high-capacity output. Although the average discharge voltage decreases, the average energy density under specific discharge cycles is approximately 10% higher than that achieved with conventional voltage-controlled cycling at the same cycle number. Notably, Li||NCM811 full cells operated at a negative-to-positive capacity ratio of 4 retain full capacity over 200 cycles while maintaining a higher energy density than that obtained under standard cycling conditions for up to 180 cycles.