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

The lower intrinsic electronic conductivity of NaV(PO)(NVP) has seriously limited its further development. Herein, Ca/Ni co-doped and carbon nanotubes (CNTs)-coated NaVCaNi(PO)/C@CNTs (CaNi0.07@CNTs) system is presented. Both Ca and Ni are substituted for V, triggering charge compensation and producing p-type doping effect, generating abundant hole carriers to improve electronic conductivity. Furthermore, the ionic radius of Ca is significantly larger than that of V, so introduction of Ca can support NVP crystal structure and improve the stability. Furthermore, the introduction of Ca can increase the lattice spacing, thus expanding the transport channels for sodium ions. The introduction of Ni reduces the resistance suffered during charge transport and optimizes the chemical properties. Meanwhile, due to low valence of Ca and Ni, 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.