Negative Lattice Expansion in an O3-Type Transition-Metal Oxide Cathode for Highly Stable Sodium-Ion Batteries

The sodium extraction/insertion in layered transition-metal oxide (TMO) cathode materials are typically accompanied by slab sliding and lattice changes, leading to microstructure destruction and capacity decay. Herein, negative lattice expansion is observed in an O3 type Ni-based layered cathode of Na_0.9 Ni_0.32 Zn_0.08 Fe_0.1 Mn_0.3 Ti_0.2 O₂ upon Na⁺ extraction. It is attributed to the weak Zn²⁺ -O^2- orbital hybridization and increased electron density of the surrounding oxygen for reinforced interlayer O-O repulsive force. This enables gliding of TMO slabs for the intergrowth phase transition of P3→OP2 to alleviate lattice strain with moderate lattice shrinkage, which exhibits general interslab spacings and volume changes as low as 2.4 % and 1.9 %, respectively. The strong Ti-O bonds accommodate the internal distortion of TMO₆ octahedra due to the flexibility of TiO₆ octahedra during cycling. These endow a high specific capacity of 144.9 mAh g^-1 and excellent cycling performance of pouch-type sodium-ion batteries with 93 % capacity retention after 3600 cycles.