Probing Into the NiO Segregation Mechanism for Optimized Synthesis of High-Capacity Sodium-Ion Layered Cathodes

Developing high-capacity transition-metal layered oxide cathodes is crucial for building high-energy sodium-ion batteries. Increasing the redox-active nickel content in O3-type layered cathodes effectively boosts the output capacity, yet a high Ni content (>40%) accounts for the formation of NiO impuritie during the high-temperature solid-state synthesis and compromises the reversible capacity of the cathode. Herein, we revealed the underlying mechanism of NiO formation during sintering, which was driven by the lattice sodium volatilization at an elevated temperature. We further proposed a low-temperature annealing method coupled with an excessive amount of Na to eliminate NiO impurities in the layered cathodes. The optimized NiO-free NaNi_0.41Zn_0.01Fe_0.11Mn_0.32Ti_0.1Al_0.05O₂ cathode delivers a high reversible capacity of 156 mAh g^-1 at 0.1C and 144 mAh g^-1 at 1C in a voltage range of 2.0-4.2 V vs. Na⁺/Na, with a capacity retention of 91.3% after 100 cycles, showing promise to practically realize high-energy Na-ion batteries.