Multifunctional-Element doping of NASICON-Structured cathode enables High-Rate and stable sodium storage

Abstract

The NASICON cathode of sodium ion battery (SIB) exhibits a stable structure and good electrochemical performance. Na3V2(PO4)3 (NVP), as a typical NASICON cathode, has been widely used in SIBs due to its high capacity and long cycle life. However, the high proportion of vanadium in NVP escalates its cost, which deviates from the initial objective of affordable sodium-ion batteries. In this work, a multifunctional-element substitution of vanadium sites in NVP is introduced by the cost-effective transition metals (Mn and Al), which optimizes the host structure, promotes the transmission of sodium ions and improves the cycle stability. The introduction of Mn can enhance its electrochemical activity and increase the reaction potential, and the doping of Al can improve structural support and cycle stability by the enhanced covalent skeleton. It was found that the Na3.7VAl0.3Mn0.7(PO4)3 cathode shows a high capacity of 87.77 mAh g−1 even at 40C (93.2 % retention compared to 1C), and the capacity retention reaches 96.33 % after 1000 cycles at 2C. In-situ XRD analysis demonstrate that the Na3.7VAl0.3Mn0.7(PO4)3 cathode exhibits a solid solution reaction mechanism instead of the previous two-phase transformation process during the Na+ (de)insertion. Furthermore, we conducted assessments on the efficacy of diverse phosphate cathodes within a coin-type full-cell configuration, which initially exhibits a high energy density of 343.9 Wh kg−1 and excellent capacity retention of 91.58 % after 150 cycles at 0.5C. These characteristics position it as a promising candidate for NASICON cathode materials in the field of research and development.