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.
Ting Jin is a professor at the State Key Laboratory of Solidification Processing and the School of Materials Science and Engineering at Northwestern Polytechnic University, China. She received her PhD in 2020 from Nankai University (China) and her B.S. in materials chemistry from Northwest University (China) in 2015. Her research interests focus on the design and fabrication of advanced electrode materials for energy storage and conversion, such as rechargeable lithium-ion and sodium-ion batteries.
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