Rational design of a tunnel-monoclinic intergrowth cathode based on surface energy chemistry modulation for sodium-ion batteries.

The tunnel-type oxide cathode material Na0.44MnO2 (NMO) and the sodium-based ferricyanide Na x Mn[Fe(CN)6]1-δ ·nH2O (FMHCF) are considered among the most promising cathode materials for sodium-ion batteries. However, the poor structural stability of FMHCF and the low sodium content in the NMO cathode limit their practical production and application. Guided by theoretical calculations, this work proposes an in situ growth strategy based on surface energy regulation mechanisms. The growth mechanism of the NMO-FMHCF intergrowth material was investigated by tracking its real-time morphological evolution at varying phase ratios and aging times. Furthermore, through electrochemical testing combined with theoretical calculations, we elucidate the synergistic enhancement effects of the NMO-FMHCF intergrowth structure in terms of structural stability, electrochemical performance, and sodium replenishment during the first cycle. The optimized intergrowth cathode material demonstrated significantly improved first-cycle charge-discharge efficiency, with a capacity retention rate of 70.46% after 950 cycles at 5C, indicating a promising new approach for designing high-performance sodium-ion battery cathode materials.