Electrochemical Behaviors of K-Doped Na3V2(PO4)3 Cathode Materials for Na-Ion Batteries

Lithium ion batteries have been widely studied due to their superior energy and power densities to utilize large scale applications such as electric vehicles and energy storage systems. In spite of extensive researches to overcome the problem of the lithium ion battery price, the large scale battery market has been sluggishly extended primarily due to the lack of lithium resources. Alternatively, sodium ion batteries are illuminated for the large scale energy storage system because of sufficient sodium sources.

As a class of the sodium ion battery cathode materials, phosphate-based Na-insertion hosts (NaVPO₄F, NaMPO₄, Na_1.5VOPO₄F_0.5, 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-Na_3-xK_xV₂(PO₄)₃/C (x=0, 0.05, 0.10, and 0.15) are synthesized by a sol-gel method. By the doped K ion which is larger than Na ion (r_K+ (1.52 ") > r_Na+ (1.16 ")), 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.

As shown in Figure 1 (a), while undoped Na₃V₂(PO₄)₃/C and Na_2.85K_0.05V₂(PO₄)₃/C exhibited the cell failures in 200 cycles at 1 C, Na_2.90K_0.10V₂(PO₄)₃/C and Na_2.85K_0.15V₂(PO₄)₃/C showed stable cycle performances without failure. Moreover, rate capabilities of Na_2.90K_0.10V₂(PO₄)₃/C and Na_2.85K_0.15V₂(PO₄)₃/C were significantly improved compared with those of undoped Na₃V₂(PO₄)₃/C and Na_2.85K_0.05V₂(PO₄)₃/C, shown in Figure 1 (b). Especially Na_2.90K_0.10V₂(PO₄)₃/C showed the best electrochemical performances among the four samples. The enhanced electrochemical performances were originated from the low polarization resistance due to the low charge transfer resistance and the high Na ion diffusivity. This excellent kinetic behavior mainly results from the pillar effect of K ion which provides the structural stability and enough space for Na ion diffusion. This advantageous pillar effect was verified by measuring the remained K content after cycling and the volume changes between the discharged phase and the charged phase. Therefore, Na₃V₂(PO₄)₃ with the proper amount of K doping in Na site might be regarded as promising cathode materials for Na ion rechargeable batteries.