Self-Compensated and Air-Stable P2-Type Layered Cathode for Practical Sodium-Ion Batteries

Sodium-based P2-type layered cathodes are of interest for sodium-ion batteries due to their fast Na + transport and good cycling reversibility at elevated voltages. However, their low sodium content and irreversible Na loss on the anode side limit their practical use. Here, a self-compensated P2-Na 0.8 Li 0.06 □ 0.04 Ni 0.23 Mn 0.67 O 2 (P2-NLNM-Va) is developed with approximately 4% cation vacancies in the transition metal layer to regulate lattice oxygen activity during charge and counterbalance anode-side Na loss. The vacancies activate additional charge capacity without sacrificing discharge capacity, rate capability, or air/cycling stability. As a result, the full cells of P2-NLNM-Va//hard carbon exhibit an initial discharge capacity of 100.6 mAh/g and retain 83% capacity after 300 cycles, compared with 80.2 mAh/g and 62% for the vacancy-free counterpart. This self-compensation strategy raises energy density by about 20% to 245 Wh/kg (total mass of cathode and anode), offering a route toward practical P2-type oxide-based sodium-ion batteries.