Manganese ion doping effect on NaFe2PO4(SO4)2 for high electrochemical performances as cathode material for battery application

Abstract

Sodium-based polyanionic compounds are being widely explored as positive electrode materials for sodium-ion batteries due to the advantages of elevated operating voltage plateaus and exceptional cycling stability. This work investigates the charge/discharge characteristics of a newly developed NASICON-type polyanionic cathode, NaFe₂PO₄(SO₄)₂ (NFPS), through X-ray diffraction refinement and cyclic voltammetry. Subsequently, Mn²⁺ doping improves the structure of the material, significantly improves the electrochemical stability and conductivity, and promotes the sodium ion diffusion rate. The XPS results revealed that manganese doping generated a higher concentration of oxygen vacancies compared with the undoped samples. Electrochemical testing reveals that Na_0.84Mn_0.08Fe₂PO₄(SO₄)₂ achieves higher capacity and improved rate performance, surpassing pristine NFPS. At a current density of 25 mA g⁻¹, the initial discharge capacity of NFPS is 56.1 mA h g⁻¹ and the initial discharge capacity of NFPS-Mn_0.08 is 70.6 mA h g⁻¹. After 60 cycles, the discharge capacity of NFPS is 47.1 mA h g⁻¹ and the discharge capacity of NFPS-Mn_0.08 is 60.4 mA h g⁻¹.