Enabling high working voltage and rate capability of NASICON cathode via moderately regulating coordination environment

Abstract

Sodium super ionic conductor (NASICON)-type Na₃V₂(PO₄)₃ (NVP) has been considered as a promising cathode candidate for sodium ion batteries (SIBs) due to its three dimensional (3D) ionic diffusion channels, high structural and thermal stability. However, such cathode usually involves a two-electron reaction with the V⁴⁺/V³⁺ couple at ~3.4 V, showing a limited specific capacity. Moreover, NVP always suffers from poor electronic conductivity and sluggish Na⁺ diffusion kinetics, resulting in poor rate capability. Herein, a V-site Mn (15 mol%) regulation strategy together with 3D interconnected porous carbon/graphene framework has been developed for constructing Na_3.3V_1.7Mn_0.3(PO₄)₃@3D porous hybrids (3DP-NVMP-Mn0.3) for not only enhancing working voltage of the V⁴⁺/V³⁺ couple but also activating electrochemical activity of the V⁵⁺/V⁴⁺ couple in NVP cathode. Compared to 3DP-NVMP-Mn0.1 (5 mol% Mn) and 3DP-NVMP-Mn0.5 (25 mol% Mn), 3DP-NVMP-Mn0.3 exhibits the highest specific capacity of 119.8 mAh g⁻¹ and the highest energy density of 413.0 Wh kg⁻¹ at the material level at 0.1C, better rate capability up to 20C (89.2 mAh g⁻¹), and a capacity retention of 75.87 % after 8000 cycles at 20C, outperforming the theoretical capacity (117 mAh g⁻¹) and energy density (397 Wh kg⁻¹) of NVP.