Substituting inert phosphate with redox-active silicate towards advanced polyanion-type cathode materials for sodium-ion batteries

Abstract

Polyanion-type phosphate materials with Na-super-ionic conductor structures are promising for next-generation sodium-ion battery cathodes, although the intrinsically low electroconductivity and limited energy density have restricted their practical applications. In this study, we put forward substituting an inert phosphate with a redox-active silicate to improve the energy density and intrinsic electroconductivity of polyanion-type phosphate materials, thus enabling an advance in sodium-ion battery cathodes. As a proof of concept, some of the phosphate of Na₃V₂(PO₄)₃ was replaced by silicate to fabricate Na₃V₂(PO₄)_2.9(SiO₄)_0.1, which exhibited a higher average discharge voltage of 3.36 V and a higher capacity of 115.8 mA h g⁻¹ than pristine Na₃V₂(PO₄)₃ (3.31 V, 109.6 mA h g⁻¹) at 0.5 C, therefore improving the energy density. Moreover, the introduced silicate enhanced the intrinsic electroconductivity of Na₃V₂(PO₄)₃ materials, as confirmed by both theoretical simulation and electrochemical measurements. After pairing with a commercial hard carbon anode, the optimized Na₃V₂(PO₄)_2.9(SiO₄)_0.1 cathode enabled a stable-cycling full cell with 90.1% capacity retention after 300 cycles at 5 C and a remarkable average coulombic efficiency of 99.88%.