Navigating low state of charge phase transitions in layered cathodes for long-life sodium-ion batteries

Abstract

Sodium-ion batteries (SIBs) are attracting significant attention as a cost-effective and sustainable alternative to lithium-ion batteries. However, challenges related to achieving long-term cycling stability and high energy density persist. This study elucidates the phase transitions in layered cathodes NaNi_1/3Fe_1/3Mn_1/3O₂ at low states of charge (SOCs), which are unavoidable during practical usage but remain poorly understood regarding their impact on cycling stability at realistic rates. Using operando synchrotron X-ray diffraction on full cells at a 1C rate, we demonstrate that the low SOC O3–P3 phase transition involves layer gliding and significant lattice mismatch, the latter worsening with greater depths of discharge (DODs). This transition leads to substantial morphological and interfacial degradation, rapidly degrading performance, as shown by cycling cells at varying DODs. Through a combination of theoretical calculations and operando X-ray diffraction, we reveal that substituting sodium with calcium effectively buffers structural changes and minimises lattice mismatch. As a result, cycling stability is notably enhanced, with cells retaining 80% capacity beyond 1300 cycles at full DOD. This work sheds light on the pivotal role of low SOC phase transitions and highlights calcium substitution's potential to significantly improve sodium-ion batteries’ stability and commercial viability.