Insight into Ca-Substitution Effects on O3-Type NaNi_1/3 Fe_1/3 Mn_1/3 O₂ Cathode Materials for Sodium-Ion Batteries...

O3-type NaNi_1/3 Fe_1/3 Mn_1/3 O₂ (NaNFM) is well investigated as a promising cathode material for sodium-ion batteries (SIBs), but the cycling stability of NaNFM still needs to be improved by using novel electrolytes or optimizing their structure with the substitution of different elements sites. To enlarge the alkali-layer distance inside the layer structure of NaNFM may benefit Na⁺ diffusion. Herein, the effect of Ca-substitution is reported in Na sites on the structural and electrochemical properties of Na_1-x Ca_x/2 NFM (x = 0, 0.05, 0.1). X-ray diffraction (XRD) patterns of the prepared Na_1-x Ca_x/2 NFM samples show single α-NaFeO₂ type phase with slightly increased alkali-layer distance as Ca content increases. The cycling stabilities of Ca-substituted samples are remarkably improved. The Na_0.9 Ca_0.05 Ni_1/3 Fe_1/3 Mn_1/3 O₂ (Na_0.9 Ca_0.05 NFM) cathode delivers a capacity of 116.3 mAh g^-1 with capacity retention of 92% after 200 cycles at 1C rate. In operando XRD indicates a reversible structural evolution through an O3-P3-P3-O3 sequence of Na_0.9 Ca_0.05 NFM cathode during cycling. Compared to NaNMF, the Na_0.9 Ca_0.05 NFM cathode shows a wider voltage range in pure P3 phase state during the charge/discharge process and exhibits better structure recoverability after cycling. The superior cycling stability of Na_0.9 Ca_0.05 NFM makes it a promising material for practical applications in sodium-ion batteries.