Boron Substituted Na3V2(P1-x B x O4)3 Cathode Materials with Enhanced Performance for Sodium-Ion Batteries

Abstract

The development of excellent performance of Na-ion batteries remains great challenge owing to the poor stability and sluggish kinetics of cathode materials. Herein, B substituted Na₃V₂P_3-x B _x O₁₂ (0 ≤ x ≤ 1) as stable cathode materials for Na-ion battery is presented. A combined experimental and theoretical investigations on Na₃V₂P_3-x B _x O₁₂ (0 ≤ x ≤ 1) are undertaken to reveal the evolution of crystal and electronic structures and Na storage properties associated with various concentration of B. X-ray diffraction results indicate that the crystal structure of Na₃V₂P_3-x B _x O₁₂ (0 ≤ x ≤ 1/3) consisted of rhombohedral Na₃V₂(PO₄)₃ with tiny shrinkage of crystal lattice. X-ray absorption spectra and the calculated crystal structures all suggest that the detailed local structural distortion of substituted materials originates from the slight reduction of V-O distances. Na₃V₂P_3-1/6B_1/6O₁₂ significantly enhances the structural stability and electrochemical performance, giving remarkable enhanced capacity of 100 and 70 mAh g^-1 when the C-rate increases to 5 C and 10 C. Spin-polarized density functional theory (DFT) calculation reveals that, as compared with the pristine Na₃V₂(PO₄)₃, the superior electrochemical performance of the substituted materials can be attributed to the emergence of new boundary states near the band gap, lower Na⁺ diffusion energy barriers, and higher structure stability.

Keywords: DFT calculation; Na3V2(PO4)3; Na‐ion battery; cathode materials; doping.

Figure 1

a) XRD patterns of Na₃V₂P_{3 –x}B_xO₁₂ (x = 0, 1/10, 1/6, 1/3) powders, b) magnified view, and c) calculated crystal structure of Na₃V₂P_{3 –x}B_xO₁₂ (x = 1/6).

Figure 2

HR‐TEM images of Na₃V₂P_{3 –x}B_xO₁₂: a) x = 0 and b) x = 1/6.

Figure 3

a) XANES and b) Fourier transformed EXAFS spectra of the Na₃V₂P_{3 –x}B_xO₁₂ (x = 0 and x = 1/3). Local distortion around the doping site after structural optimization of Na₃V₂P_{3 –x}B_xO₁₂ c) (x = 0) and d) (x = 1/6).

Figure 4

Calculated a) total density of states (DOS) of Na₃V₂P_{3 –x}B_xO₁₂ (x = 0, 1/6, 1/3) and b) partial spin up density of states of Na₃V₂P_3‐1/6B_1/6O₁₂. E_f represents the Fermi energy level.

Figure 5

a) Rate capability of the Na₃V₂P_{3 ‐x}B_xO₁₂ (x = 0, 1/10, 1/6, 1/3) electrodes at various current density, and charge‐discharge profile of the Na₃V₂P_{3 ‐x}B_xO₁₂ b) (dotted lines for x = 0, solid lines for x = 1/6), cycling stability of the samples at c) 1 C and d) 5 C.

Figure 6

CV curves of the pristine a) Na₃V₂(PO₄)₃ and b) Na₃V₂P_3‐1/6B_1/6O₁₂ at various sweep rates; b) I _p versus υ^1/2 and linear fitting curves of CV.

Figure 7

The calculated diffusion trajectory of Na ion along a) pathway 1 and b) pathway 2 in Na₃V₂(PO₄)₃, and the corresponding diffusion energy barriersalong conduction path c) Na2‐Na1 and d) Na2‐Na2 in Na₃V₂P_{3 –x}B_xO₁₂ (x = 0 and x = 1/6). The light green balls, connecting the two Na atoms, emphasize the tracks of diffusion. The blue dashed lines represent the bottleneck triangle.