Structural enhancement of Na₃V₂(PO₄)₃/C composite cathode materials by pillar ion doping for high power and long cycle life sodium-ion batteries

Structurally stabilized Na₃V₂(PO₄)₃/C composite cathode materials with excellent electrochemical performance can be obtained by incorporating functional pillar ions into the structure. As pillar ions, K-ions have a larger ionic radius compared to Na-ions, and play an important role in enlarging the Na-ion diffusion pathway and in increasing the lattice volume by elongating the c-axis, thereby improving the rate performance. Furthermore, since the incorporated K-ions rarely participate in the electrochemical extraction/insertion reactions, they can stabilize the Na₃V₂(PO₄)₃ structure by suppressing significant lattice volume changes or structural distortion, even in a wide range of voltage windows accompanying multiple transitions of V ions and phase distortions. We investigated how the K-ion doping level affected the crystal structure and electrochemical properties of Na₃V₂(PO₄)₃ cathode materials for Na-ion batteries.