Boosting the Ultrastable High-Na-Content P2-type Layered Cathode Materials with Zero-Strain Cation Storage via a Lithium Dual-Site Substitution Approach

P2-type layered transition-metal (TM) oxides, Na_xTMO₂, are highly promising as cathode materials for sodium-ion batteries (SIBs) due to their excellent rate capability and affordability. However, P2-type Na_xTMO₂ is afflicted by issues such as Na⁺/vacancy ordering and multiple phase transitions during Na-extraction/insertion, leading to staircase-like voltage profiles. In this study, we employ a combination of high Na content and Li dual-site substitution strategies to enhance the structural stability of a P2-type layered oxide (Na_0.80Li_0.024[Li_0.065Ni_0.22Mn_0.66]O₂). The experimental results reveal that these approaches facilitate the oxidation of Mn ions to a higher valence state, thereby affecting the local environment of both TM and Na ions. The resulting modification in the local structure significantly improves the Na-ion storage capabilities as required for cathode materials in SIBs. Furthermore, it induces a solid-solution reaction and enables nearly zero-strain operation (ΔV = 0.7%) in the Na_0.80Li_0.024[Li_0.065Ni_0.22Mn_0.66]O₂ cathode during cycling. The assembled full cells demonstrate an exceptional rate performance, with a retention rate of 87% at 10 C compared to that of 0.1 C, as well as an ultrastable cycling capability, maintaining a capacity retention of 73% at 2 C after 1000 cycles. These findings offer valuable insights into the electronic and structural chemistry of ultrastable cathode materials with “zero-strain” Na-ion storage.