In Situ Wet Coating of Ammonium Phosphomolybdate for Enhancing the Kinetics and Cycling Stability of NaNi _1/3 Fe _1/3 Mn _1/3 O ₂ Cathode Material

Layered oxides for sodium-ion batteries have attracted extensive attention from researchers due to their distinctive advantages. Nevertheless, for O3-type layered oxides with a high initial capacity, critical issues including excessive surface residual sodium, sluggish reaction kinetics, and poor long-term cycling stability remain to be urgently addressed. Herein, a novel wet in situ coating strategy using ammonium phosphomolybdate is proposed. The NFM@NH-Mo material derived from the coating modification of NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O 2 (NFM) is endowed with a sodium-ion conductor group (SCG) coating layer. Various kinetic tests demonstrate that the SCG coating layer significantly facilitates the deintercalation and intercalation of Na +, leading to an approximate one-order-of-magnitude enhancement in the Na + diffusion coefficient compared with pristine NFM. In addition, the consumption of surface residual sodium on NFM during the formation of the SCG coating layer effectively reduces the polarization of NFM@NH-Mo. Combined with the electrochemical performance of NFM@NH-Mo at 55 °C, which delivers a capacity retention of 83.10% after 100 cycles against only 70.51% for NFM, the SCG coating layer is proven to suppress the anisotropic stress variation of the NFM material and the side reactions between the active material and electrolyte, thus endowing the modified material with excellent cycling stability at elevated temperatures.