A New P2-Type Layered Oxide Cathode with Extremely High Energy Density for Sodium-Ion Batteries

Herein, a new P2-type layered oxide is proposed as an outstanding intercalation cathode material for high energy density sodium-ion batteries (SIBs). On the basis of the stoichiometry of sodium and transition metals, the P2-type Na _0.55 [Ni _0.1 Fe _0.1 Mn _0.8 ]O ₂ cathode is synthesized without impurities phase by partially substituting Ni and Fe into the Mn sites. The partial substitution results in a smoothing of the electrochemical charge/discharge profiles and thus greatly improves the battery performance. The P2-type Na _0.55 [Ni _0.1 Fe _0.1 Mn _0.8 ]O ₂ cathode delivers an extremely high discharge capacity of 221.5 mAh g ⁻¹ with a high average potential of ≈2.9 V (vs Na/Na ⁺ ) for SIBs. In addition, the fast Na-ion transport in the P2-type Na _0.55 [Ni _0.1 Fe _0.1 Mn _0.8 ]O ₂ cathode structure enables good power capability with an extremely high current density of 2400 mA g ⁻¹ (full charge/discharge in 12 min) and long-term cycling stability with ≈80% capacity retention after 500 cycles at 600 mA g ⁻¹ . A combination of electrochemical profiles, in operando synchrotron X-ray diffraction analysis, and first-principles calculations are used to understand the overall Na storage mechanism of P2-type Na _0.55 [Ni _0.1 Fe _0.1 Mn _0.8 ]O ₂ .