Phase Transition Regulation Via Multi-Scale Structural Engineering Toward Robust Single-Crystalline Na-Layered Oxide...

Single-crystalline O3-type NaNi_1/3Fe_1/3Mn_1/3O₂ (S-NFM) is a promising cathode candidate for commercial sodium-ion batteries (SIBs). However, due to the extended Na⁺ diffusion path and sluggish kinetics, S-NFM suffers from severe spatially heterogeneous electrochemical reactions, resulting in rapid structure failure. Herein, multi-scale structural engineering involving the optimization of single-crystalline grains and reinforcement of lattice structure is developed to synthesize a Cu/Zr co-doped single-crystalline NaNi_0.3Fe_0.3Mn_0.3Cu_0.05Zr_0.05O₂ (S-NFMCZ), which exhibits shortened Na⁺ diffusion path, fast diffusion kinetics, boosted redox reaction activity as well as enhanced covalency of TM-O bonds. The spatial homogeneity of the phase transition in S-NFMCZ is significantly enhanced, and concurrently, the O'3 intermediate phase fully exerts its buffering effect against lattice variations to mitigate stress generation during the phase transition and its impact on single-crystal integrity, thus suppressing the chemo-mechanical degradation of the single-crystalline cathode. S-NFMCZ exhibits exceptional electrochemical kinetics and cycling stability with a high discharge specific capacity of 62.9 mAh g^-1 at 10 C and a superior capacity retention of 78.2% after 500 cycles at 1 C. This finding provides valuable guidance for the rational design of high-performance single-crystalline Na-layered oxides and advanced SIBs.