Effects of K-Ion Doping on Electrochemical Performance of Na3V2(PO4)3 Cathode Materials for Na-Ion Batteries
As a class of the sodium ion battery cathode materials, phosphate-based Na-insertion hosts (NaVPO4F, NaMPO4, Na1.5VOPO4F0.5, Na2FePO4F, NaTi2(PO4)3, etc.) with strong polyanion networks have received great attention due to their excellent structural and thermal stabilities. NASICON-Na3V2(PO4)3 is the one of the promising cathode materials for the sodium ion batteries. Na3V2(PO4)3 shows 117.6 mAh·g-1 theoretical capacity at 3.4 V vs. Na/Na+ in V4+/V3+ redox couple. It has strong PO4 polyanion networks which improve the safety of the large scale applications. Unfortunately, Na3V2(PO4)3 shows severe capacity fading at high rate because of the low electrical conductivity and ion diffusivity. In the phosphate-based active materials for Li or Na ion batteries, the addition of conductive materials and the substitution of transition metal or alkali metal with alien ions have been conducted to overcome these problems. Especially the substitution of Li ion with Na ion or K ion for the Li ion batteries shows better electrochemical performances due to the lattice expansion and narrowing the band gap.
In this work, NASICON-Na3-xKxV2(PO4)3/C (x=0, 0.05, 0.10, and 0.15) are synthesized by a solid state reaction. By the doped K ion which is larger than Na ion (rK+ (1.52 Å) > rNa+ (1.16 Å)), the structure becomes enlarged and it makes Na ion migrate easily. Moreover the doped K ion in the structure even after charging contributes to the structural stability by reducing volume changes during cycling.