Activating the Extra Redox Couple of Co²⁺/Co³⁺ for a Synergistic K/Co Co-Substituted and Carbon Nanotube-Enwrapped Na₃V₂(PO₄)₃ Cathode with a Superior Sodium Storage Property

Na₃V₂(PO₄)₃ (NVP) materials have emerged as a promising cathode for sodium ion batteries (SIBs). Herein, NVP is successfully optimized by dual-doping K/Co and enwrapping carbon nanotubes (CNTs) through a sol-gel method. Naturally, the occupation of K and Co in the Na1 sites and V sites can efficiently stabilize the crystal cell and provide the expanded Na⁺ transport channels. The existence of tubular CNTs could restrict the crystal grain growth and effectively downsize the particle size and provide a shorter pathway for the migration of electrons and ions. Moreover, the amorphous carbon layers combined with the conductive CNTs form a favorable network for the accelerated electronic transportation. Furthermore, the ex situ XPS characterization reveals that an extra redox reaction pair of Co²⁺/Co³⁺ is successfully activated at the high voltage range, resulting in superior capacity and energy density property for KC0.05/CNTs composites. Comprehensively, the optimized KC0.05/CNTs electrode exhibits a distinctive electrochemical property. It delivers an initial reversible capacity of 119.4 mA h g^-1 at 0.1 C, surpassing the theoretic value for the NVP system (117.6 mA h g^-1). Moreover, the KC0.05/CNT electrode exhibits the initial capacity of 113.2 mA h g^-1 at 5 C and 105.8 mA h g^-1 at 10 C, and the maintained capacities at 500 cycles are 105.8 and 100.8 mA h g^-1 with outstanding retention values of 96.6 and 95.3%. Notably, it releases capacities of 99.8 and 84.5 mA h g^-1 at 50 and 100 C, and the capacity retention values at 2500 cycles are 66.2 and 58.8 mA h g^-1, respectively. What is more, the KC0.05/CNTs//Bi₂Se₃ asymmetric full cell exhibits a high capacity of 191.4 mA h g^-1 at 2.65 V, with the energy density being as high as 507 W h kg^-1, demonstrating the eminent practical application potential of KC0.05/CNTs in SIBs.