A phosphite-based layered framework as a novel positive electrode material for Na-ion batteries

Abstract

Na-ion batteries (SIBs), perceived as the most promising alternative energy storage technology, are attractive for large-scale stationary applications due to the cost effectiveness and global abundance of sodium. One of the formidable challenges in the way of their extensive commercialization is the development of suitable low-cost positive electrode materials with high energy density and long cycle life. In this study, a phosphite-based layered polyanionic material with the formula, Na₂[(VOHPO₃)₂(C₂O₄)]·2H₂O, has been investigated as a novel positive electrode for SIBs. The material was synthesized through a room temperature precipitation method and undergoes reversible Na⁺-ion insertion at an average discharge voltage of 3.65 V which is higher than that of the same V⁴⁺/V⁵⁺ redox couple of NaVOPO₄ (3.4 V) in a non-aqueous Na cell. The material exhibits a high discharge capacity of ∼101 mA h g⁻¹ at 0.1C rate in Na half-cells. Capacity fading encountered by the pristine material was overcome with the help of ball-milling with carbon. The layered material facilitates the migration of large Na⁺ ions, resulting in a superior rate performance (∼80 mA h g⁻¹ at 10C rate). In addition, a long-term cycling stability over 1000 cycles was demonstrated at 2C rate with 62% capacity retention. Operando XRD studies reveal that the reversible Na⁺-ion insertion in the framework happens via a bi-phasic mechanism. The feasibility of full cells was demonstrated using NaTi₂(PO₄)₃ as the negative electrode and the full cell exhibited a reversible capacity of 71 mA h g⁻¹ at 0.1C rate and 65 mA h g⁻¹ at 1C rate with good capacity retention.