Pillar-beam structures prevent layered cathode materials from destructive phase transitions.

Energy storage with high energy density and low cost has been the subject of a decades-long pursuit. Sodium-ion batteries are well expected because they utilize abundant resources. However, the lack of competent cathodes with both large capacities and long cycle lives prevents the commercialization of sodium-ion batteries. Conventional cathodes with hexagonal-P2-type structures suffer from structural degradations when the sodium content falls below 33%, or when the integral anions participate in gas evolution reactions. Here, we show a "pillar-beam" structure for sodium-ion battery cathodes where a few inert potassium ions uphold the layer-structured framework, while the working sodium ions could diffuse freely. The thus-created unorthodox orthogonal-P2 K0.4[Ni0.2Mn0.8]O2 cathode delivers a capacity of 194 mAh/g at 0.1 C, a rate capacity of 84% at 1 C, and an 86% capacity retention after 500 cycles at 1 C. The addition of the potassium ions boosts simultaneously the energy density and the cycle life.