Ultra-stable, high-rate solid-state sodium batteries with bulk-interface engineering of NaCrO₂ cathode

Solid-state sodium batteries (SSBs) have attracted great interests due to their high energy density, good safety and cost-effectiveness. However, their cycling stability and rate capability remain limited. Here, we report a synergistic modification strategy of Li⁺ bulk doping and carbon surface coating on the O3-type layered oxide NaCrO₂ (NCO). Li⁺ doping enhances sodium ion diffusion coefficient (from 8.30*10^-11 cm² s^-1 to 1.22 * 10^-10 cm² s^-1) and improves structural reversibility during cycling. Meanwhile, the uniform carbon coating effectively inhibits the side reactions on the surface and reduces the electrode volume change to 0.7 %. The modified NCO-1.5 L@8 %P material shows excellent cycle stability and rate performance with conventional organic electrolyte (80.8 % retention after 1600 cycles, and 113.5 mAh g^-1 at 10C current rate). Leveraging enhanced solid-solid interfacial compatibility, this material was further used to assemble NCO-1.5 L@8 %P/0.2 Mg-NZSP/Na solid state batteries. Under a high current rate of 2C, the SSB cycles stably with a capacity retention rate of 95.5 % after 200 cycles. When the current is increased to 5C/10C, a capacity of 102.7/96.1 mAh g^-1 can still be obtained, effectively overcoming the rate capability limitations typically associated with conventional solid-state batteries. Through a comprehensive design strategy encompassing bulk-phase structural regulation, interfacial modification, and solid-state adaptation, this work elucidates the regulation mechanism by which synergistic modifications enhance electrochemical kinetics and interfacial stability.