Simultaneous modification of Na-rich and Ca2+/Ni2+ dual-substitution boosting superior electrochemical performance of Na3V2(PO4)3

Abstract

The lower intrinsic electronic conductivity of Na3V2(PO4)3(NVP) has seriously limited its further development. Herein, Ca2+/Ni2+ co-doped and carbon nanotubes (CNTs)-coated Na3+x+0.04V1.96-xCa0.04Nix(PO4)3/C@CNTs (CaNi0.07@CNTs) system is presented. Both Ca2+ and Ni2+ are substituted for V3+, triggering charge compensation and producing p-type doping effect, generating abundant hole carriers to improve electronic conductivity. Furthermore, the ionic radius of Ca2+ is significantly larger than that of V3+, so introduction of Ca2+ can support NVP crystal structure and improve the stability. Furthermore, the introduction of Ca2+ can increase the lattice spacing, thus expanding the transport channels for sodium ions. The introduction of Ni2+ reduces the resistance suffered during charge transport and optimizes the chemical properties. Meanwhile, due to low valence of Ca2+ and Ni2+, more Na+ are designed to be introduced to the NVP system for charge balance. The Na-rich strategy induces excess active Na+ participating in the de-intercalation process to supply more reversible capacities. Furthermore, the CNTs wrapped around the active grains serves to buffer deformation of the crystal and to establish a conductive network connecting the particles. The after-cycling XRD/SEM/XPS further confirms the improved crystal stability of CaNi0.07@CNTs. Comprehensively, CaNi0.07@CNTs possess superior sodium storage in half and full cells.