Ultra-high pseudocapacitive contribution in free-standing O-Cu₂Se/Cu electrode induces excellent kinetics and stability for sodium-ion storage

As a potential anode for sodium-ion batteries, the kinetics and stability of quasi-2D α-Cu₂Se are limited in practical applications by the large radius of Na⁺. However, the traditional single strategy has struggled to simultaneously improve these two parameters satisfactorily. Herein, a facile regulation strategy of pseudocapacitive contribution is proposed to address this challenge, that is, a hydrangea-like α-Cu₂Se with trace O element is in-situ deposited on Cu foil surface though 15-minute anion exchange process. The introduction of O offers additional surface defects and bonds, optimizing both ion diffusion and Na⁺ adsorption energy. Together with the synergistic effect of the large specific surface area and free-standing characteristic, the electrode exhibits an ultra-high pseudocapacitive contribution (97.20 % at 1 mV/s) during Na⁺ storage processes, endowing the anode with outstanding specific capacity, cyclic stability, rate performance, and initial Coulombic efficiency (ICE). When cycling at 0.1 A/g, the ICE reaches as high as 99.04 %. After 100 cycles at 2 A/g, the specific capacity remains at 893.7mAh/g. Even after 1000 cycles at 5 A/g, the electrode maintains an impressive capacity of 676.8mAh/g with a negligible average capacity loss rate of 0.0055 % per cycle. This work provides a simple and efficient perspective for achieving rapid and stable Na⁺ storage in α-Cu₂Se.