Title: High sodium content P2-type cathode for high-performance sodium-ion batteries

Abstract

P2-type layered oxides suffer from an ordered Na+/vacancy arrangement and P2→O2/OP4 phase transitions, leading them to exhibit multiple voltage plateaus upon Na+ extraction/insertion. The deficient sodium in the P2-type cathode easily induces the bad structural stability at deep desodiation states and limited reversible capacity during Na+ de/insertion. These drawbacks cause poor rate capability and fast capacity decay in most P2-type layered oxides. To address these challenges, a novel high sodium content (0.85) P2-type cathode-Na0.85Li0.12Ni0.22Mn0.66O2 was developed. In situ XRD shows both P2→O2/OP4 phase transitions and Na+/vacancy ordering can be successfully converted into a complete solid solution dominated region after Li substitution. The complete solid-solution reaction over a wide voltage range ensures both fast Na+ mobility (10-11 to 10-10 cm2 s-1) and small volume variation (1.7%). The high sodium content P2-Na0.85Li0.12Ni0.22Mn0.66O2 exhibits a higher reversible capacity of 123.4 mA h g-1, superior rate capability of 79.3 mA h g-1 at 20 C, and 85.4% capacity retention after 500 cycles at 5 C. This work highlights the importance of the solid-solution reaction mechanism in a high sodium content P2-type cathode, which ensures higher reversible capacity, superior cycling stability and remarkable rate capability.