Activating the Extra Redox Couple of Co<sup>2+</sup>/Co<sup>3+</sup> for a Synergistic K/Co Co-Substituted and Carbon Nanotube-Enwrapped Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> Cathode with a Superior Sodium Storage Property
Zeyi Tian, Yanjun Chen, Shiqi Sun, Xiaomei Jiang, Honglang Liu, Chao Wang, Que Huang, Changcheng Liu, Yanzhong Wang, Li Guo
Abstract
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.