Reducing P-O bond covalency in polyanionic groups to increase the intrinsic electronic conductivity in sodium positive electrodes

Polyanionic positive electrodes with XO₄ (X = P, S, Si etc.) tetrahedra exhibit beneficial properties, making them crucial for energy storage systems. However, strong covalent X-O bonds cause electron localization, reducing intrinsic electronic conductivity and limiting rate performance. Here we show a strategy to directional design the covalency of X-O bonds for enhanced intrinsic electronic conductivity of polyanionic positive electrodes. We developed a non-equilibrium fast calcination technique to synthesize polyanionic positive electrodes inheriting weak covalent (WC) X-O bond characteristics from screened precursor. In as-synthesized NASICON-Na₃V₂O_1.6(PO₄)₂F_1.4 as proof-of-concept positive electrode, the WC P-O bonds effectively activate electron delocalization, thereby increasing electrons transport capability along V-O-P-O-V pathway. The intrinsic electronic conductivity of optimised material achieves 3.57 times enhancement, leading to 3.70 times and 3.86 times improvement in specific capacity and specific energy at 50 C. Meanwhile, the optimised material exhibits capacity retention of 85.9% after 3000 cycles at 1 C. This study potentially opens up a broad space for improving the intrinsic conductivity of polyanionic materials (such as phosphates, sulfates, silicates, etc.), with the potential to promote the development of next-generation high-power positive electrode.