Rational Design of a Complete Solid-Solution Reaction P2-Type Na-Ion Layered Oxide Cathode with High Performance

P2-type Na-ion layered oxides have received wide consideration as cathodes for Na-ion batteries (SIBs) attributing to their easy syntheses, high theoretical capacity, and convenient Na+ diffusion paths. However, the different Na+/vacancy orderings and large-volume phase transitions at high voltages cause poor cycling performance and rate capability, limiting the development and application of P2-type layered oxides. Herein, we propose a feasible strategy for rationally adjusting transition metals and combining different functionalities of multiple elements. Results showed that multiple redox couples provided sufficient charge compensation and moderately inactive Ti could help suppress occurrence of Na+/vacancy ordering and hinder the phase transitions from P2 to O2 with large volume variations. Consequently, the designed Na0.7Ni0.25Cu0.05Fe0.1Mn0.35Ti0.25O2 cathode presented a complete solid-solution reaction with small volume change (∼1%) upon charging and discharging. The highly reversible solid-solution behavior and inhibited P2–O2 phase transformation enabled this cathode to deliver a capacity of 121 mAh/g in a voltage between 2.0 and 4.2 V, outstanding cycling stability with a capacity retention of 89% after 1000 cycles, and rate performance (62% conservation rate at 10 C). This study provides a highly available approach to design high-performance layered oxide cathodes for advanced SIBs.