Manipulating O3/P2 phase ratio in bi-phasic sodium layered oxides via ionic radius control

Abstract Bi-phasic O3/P2 sodium layered oxides have emerged as leading candidates for the commercialisation of next-generation sodium-ion batteries. However, beyond simply altering the sodium content, rational control of the O3/P2 ratio in these materials has proven particularly challenging despite being crucial for the realization of high-performance electrode materials. Here, using abundant elements, we manipulate the O3/P2 ratio using the average ionic radius of the transition metal layer and different synthesis conditions. These methods allow deterministic control over the O3/P2 ratio, even for constant Na contents. In addition, tuning the O3/P2 ratio yields high-performing materials with different performance characteristics, with a P2-rich material achieving high rate capabilities and excellent cycling stability (92% retention, 50 cycles), while an O3-rich material displayed an energy density up to 430 Wh kg −1 , (85%, 50 cycles). These insights will help guide the rational design of future high-performance materials for sodium-ion batteries.