Na_2.5VTi_0.5Al_0.5(PO₄)₃ as Long Lifespan Cathode for Fast Charging Sodium-Ion Batteries

Vanadium based NASICON-type cathodes are faced with the exorbitant cost and underdeveloped multi-electrons reaction of V species. In this work, a strategy of increased covalency of the NASICON framework combined with the reversible activation of V⁴⁺/V⁵⁺ couple is proposed to improve the electrochemical performance together with energy density of V-based cathodes. Making full use of V²⁺/V³⁺/V⁴⁺/V⁵⁺ and Ti³⁺/Ti⁴⁺ redox couples, Na_2.5VTi_0.5Al_0.5(PO₄)₃ exhibits admirable electrochemical performance, including a high specific capacity of 160.9 mAh g⁻¹ at 0.1 C and favorable cycling stability (a capacity retention of 88.3% at 20 C after 1000 cycles). Moreover, this cathode displays outstanding low temperature performance at 0 °C with a capacity retention of 89% after 1200 cycles at 5 C. In situ XRD and EIS analysis are conducted to reveal the Na⁺ storage mechanism. The cathode reveals a lattice volume variation of 2.16% upon cycling, which is responsible for the high structural stability during the extraction and intercalation process of Na⁺. Applying Na_2.5VTi_0.5Al_0.5(PO₄)₃ as both cathode and anode electrode, the symmetric cell is assembled and displays exceptional capacity of 59.8 mAh g⁻¹ at 20 C. The research provides an effective routine to stimulate the electrochemical potential of V-based electrode materials.