Sustainable layered cathode with suppressed phase transition for long-life sodium-ion batteries

Abstract

Sodium-ion batteries are among the most promising alternatives to lithium-based technologies for grid and other energy storage applications due to their cost benefits and sustainable resource supply. For the cathode—the component that largely determines the energy density of a sodium-ion battery cell—one major category of materials is P2-type layered oxides. Unfortunately, at high state-of-charge, such materials tend to undergo a phase transition with a very large volume change and consequent structural degradation during long-term cycling. Here we address this issue by introducing vacancies into the transition metal layer of P2-Na0.7Fe0.1Mn0.75□0.15O2 (‘□’ represents a vacancy). The transition metal vacancy serves to suppress migration of neighbouring Na ions and therefore maintain structural and thermal stability in Na-depleted states. Moreover, the specific Na−O−□ configuration triggers a reversible anionic redox reaction and boosts the energy density. As a result, the cathode design here enables pouch cells with energy densities of 170 Wh kg−1 and 120 Wh kg−1 that can operate for over 600 and 1,000 cycles, respectively. Our work not only suggests a feasible strategy for cathode design but also confirms the possibility of developing a battery chemistry that features a reduced need for critical raw materials. The poor structural stability of cathode materials is responsible for the rapid capacity loss of sodium-ion batteries during cycling. This work addresses the instability by introducing vacancies into the transition metal layers and realize long-life pouch cells.