Reduced air sensitivity and improved electrochemical stability of P2–Na_2/3Mn_1/2Fe_1/4Co_1/4O₂ through atomic layer deposition-assisted Al₂O₃ coating

Considering the abundance and low price of sodium, sodium-ion batteries (SIBs) are of particular attractiveness for large-scale electrochemical energy storage. Layered P2–Na_2/3Mn_1/2Fe_1/4Co_1/4O₂ (NMFC) is a promising cathode material for SIBs because of its large theoretical capacity, however, its high air sensitivity and poor cycling stability seriously limits the practical use. Here, we report nanometre-thick Al₂O₃ coating of NMFC electrode through atomic layer deposition (ALD) technique can effectively alleviate such deficiencies to obtain an air-insensitive electrode, which was realized by effectively preventing the direct exposure of NMFC to H₂O and CO₂ present in the air atmosphere. Interestingly, the ultrathin Al₂O₃ coating did not cause a block for sodium ion intercalation, and a high capacity of 217.9 mA h g⁻¹ at a high cut-off potential of 4.5 V was achieved. More attractively, a capacity retention rate as high as 84.8% was obtained after cycling at a current density of 0.5C over 100 cycles, demonstrating improved electrochemical stability. Based on CV and EIS characterizations, such good cycling stability is associated with the use of the Al₂O₃ coating layer as well, which effectively avoided the side reactions at the interface of the liquid electrolyte and the active material under high potential.