From analysis to operating strategy: Dealing with phase transitions in sodium-ion batteries

Sodium-ion batteries are emerging as a sustainable and cost-effective alternative to lithium-ion batteries for large-scale stationary energy storage. However, a key challenge of layered oxide cathodes, such as , is their structural instability under high-voltage operation. In particular, it is suspected that a voltage-induced phase transition accelerates degradation and limits long-term performance. This creates a central trade-off: expanding the voltage window is highly attractive from an application perspective, as it enables higher capacities and energy densities. However, it may also compromise cycle life by promoting structural degradation. In this study, we directly address this issue by linking the evolution of high-voltage phase transitions to the practical performance limits of sodium-ion layered oxide cathodes. Operando X-ray diffraction combined with electrochemical cycling analysis is first used to establish a mechanistic understanding of how these phase transitions develop during cycling. We then systematically evaluate their occurrence as a function of temperature and C-rate, quantifying their impact on capacity, energy, and power capability. Long-term cycling further reveals the effect of high-voltage operation on cycle life. Based on these findings, we propose an operating strategy that balances the gains in initial capacity, energy output, and power capability with the degradation risks associated with high-voltage operation.