Introduction of High-Valent Metal in Transition Metal Layer as a Structural Reinforcement for a O3-Type NaCrO2 Sodium-Ion Battery Cathode

Abstract

O3-type layered sodium chromium oxide (O3-NaCrO2) is a promising cathode material for cost-effective and practical sodium-ion batteries (SIBs). However, achieving a high energy density in SIBs with NaCrO2 as the cathode remains challenging owing to chromium migration and irreversible phase transition at voltages above 3.6 V (vs Na/Na+). Herein, a substantially improved high-voltage stability of O3-NaCrO2 (O3-NCO) as a cathode through the implementation of high-valent Nb5+ substitution strategies is reported. The strong interaction between Nb5+ and O2- ions alleviates structural stress during repeated charge and discharge processes. The introduced Nb5+ enhances the disorder in the transition metal layer and structural stability, increasing the reversibility by suppressing Cr disproportionation reactions and irreversible Cr migration. Additionally, Nb5+ creates Na+/vacancies in the Na layer owing to charge compensation, thereby facilitating the Na-ion diffusion kinetics. Consequently, 3 mol% of Nb-substituted O3-Na0.94Cr0.97Nb0.03O2 cathode demonstrates superior reversible capacity, cycle life, and rate capability. Furthermore, the high-valent Nb substitution strategy improves water stability and shows good compatibility with all-solid-state battery systems, highlighting its excellent practical applicability.