Sodium-ion batteries are a promising alternative to lithium-ion batteries for select applications, offering comparable performance at lower cost and reduced reliance on critical minerals. Among the cathode candidates, Fe- and Mn-rich layered oxides free of Co and Ni are particularly attractive but suffer from poor electrochemical performance due to Fe migration and irreversible structural phase transformations. Biphasic O3/P2 cathodes attempt to improve the performance by leveraging the synergistic advantages of each phase. However, they usually do not have sufficient Na content. Further, studies typically lack a design strategy to tune the O3:P2 phase fraction to optimize performance. Here, we introduce design principles to control O3:P2 phase fractions in high-Na-content NaTMO2 materials without altering the overall composition. Using Na0.78(Li0.04Mg0.02Fe0.38Mn0.5)O2 (NLMFM) as a model system, we demonstrate that tuning the calcination temperature from 600 °C to 1000 °C systematically increases the O3:P2 fraction by modifying the chemical potential of O2 (μO2) in the calcination environment, which in turn influences the O stoichiometry in the materials and changes the TM oxidation states. We show that higher temperature (lower μO2) synthesis results in a P2 phase with increased Na content, delaying detrimental P-O transitions and suppressing Fe migration. As a result, the O3-rich samples exhibit superior structural stability and electrochemical performance compared to the samples synthesized at lower temperatures (higher μO2). Our findings establish a framework for phase-engineering Fe- and Mn-rich layered NaTMO2 cathodes via processing conditions alone, providing a new pathway toward high-performance, sustainable sodium-ion batteries.
Bachu et al. (Wed,) studied this question.