Achieving a Complete Solid-Solution Reaction in the Layered Oxide Cathode for Long-Lifespan Sodium-Ion Batteries

Sodium-ion batteries are pivotal examinees replacing lithium-ion batteries for application in energy storage. Mn-based layered oxides are the most competitive cathode materials, but the unfavorable cycling stability induced by the unavoidable phase transitions strictly inhibits practical application. Herein, we show that the complete solid-solution reaction during the charge–discharge process can be achieved in Mn-based layered oxide by precise structure regulation. The high-sodium, manganese-rich P2–Na0.8Ni0.1Ti0.1Mn0.8O2, synthesized via the sol–gel technique, exhibits a complete solid-solution reaction throughout the sodiation/desodiation cycle. This material demonstrates an exceptionally low strain of just 2.72%, as confirmed by in operando X-ray diffraction. As a result, this unique positive electrode material demonstrates a high discharge capacity of 127.2 mA h g–1, retaining 92.4% of discharge capacity at the 50th cycle within the voltage range 4.1–2.0 V at 0.1C. Apart from that, it also delivers a long lifespan of over 500 cycles at 1C while maintaining a high capacity retention rate of 83.4%, which is significantly improved compared to that of the contrast sample. The results of this study provide a new view to performance improvement of manganese-based electrode materials by constructing a solid-solution reaction type cathode.