O3-type NaNi Fe Mn O (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 Ca NFM (x = 0, 0.05, 0.1). X-ray diffraction (XRD) patterns of the prepared Na Ca NFM samples show single α-NaFeO type phase with slightly increased alkali-layer distance as Ca content increases. The cycling stabilities of Ca-substituted samples are remarkably improved. The Na Ca Ni Fe Mn O (Na Ca NFM) cathode delivers a capacity of 116.3 mAh g 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 Ca NFM cathode during cycling. Compared to NaNMF, the Na Ca 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 Ca NFM makes it a promising material for practical applications in sodium-ion batteries.

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Commercially available Na 4 Fe(CN) 6 and Fe 4 [Fe(CN) 6 ] 3 are two cheap compounds that have ever been investigated as positive electrode materials for sodium-ion batteries. However, poor electronic conductivity of Na 4 Fe(CN) 6 and sodium deficiency of Fe 4 [Fe(CN) 6 ] 3 prevent these two materials from being used in practical rechargeable sodium-ion batteries. In this paper, a NaFeFe(CN) 6 cathode material was synthesized by ball milling the Fe 4 [Fe(CN) 6 ] 3 / Na 4 Fe(CN) 6 mixture. The obtained NaFeFe(CN) 6 demonstrated a single cubic phase indexed to Fm3m space group similar to Fe 4 [Fe(CN) 6 ] 3 but with larger lattice parameter due to the existence of Na + in the lattice framework. The NaFeFe(CN) 6 electrode delivered first desodiation capacity of 119.4 mAh g −1 and first sodiation capacity of 153.6 mAh g −1 at 0.05C rate. The NaFeFe(CN) 6 showed excellent cycling stability with reversible capacities of 118.2 mAh g −1 and 96.8 mAh g −1 at 0.1C and 1C rate, respectively. In-situ XRD analyses demonstrated a single cubic phase process during charge-discharge of NaFeFe(CN) 6 electrode. Low water content benefited from the solid reaction method and the homogenous single phase process during charge/discharge assured the stable long term cycling performance of the NaFeFe(CN) 6 product.

Cobalt phosphide embedded in a graphene nanosheet network can be developed by a versatile strategy for advanced Li-ion battery anodes.