Electrochemical Investigation of Calcium Substituted Monoclinic Li3V2(PO4)3 Negative Electrode Materials for Sodium- and Potassium-Ion Batteries

Herein, the electrochemical properties and reaction mechanism of Li_3‒2xCa_xV₂(PO₄)₃/C (x = 0, 0.5, 1, and 1.5) as negative electrode materials for sodium-ion/potassium-ion batteries (SIBs/PIBs) are investigated. All samples undergo a mixed contribution of diffusion-controlled and pseudocapacitive-type processes in SIBs and PIBs via Trasatti Differentiation Method, while the latter increases with Ca content increase. Among them, Li₃V₂(PO₄)₃/C exhibits the highest reversible capacity in SIBs and PIBs, while Ca_1.5V₂(PO₄)₃/C shows the best rate performance with a capacity retention of 46% at 20 C in SIBs and 47% at 10 C in PIBs. This study demonstrates that the specific capacity of this type of material in SIBs and PIBs does not increase with the Ca-content as previously observed in lithium-ion system, but the stability and performance at a high C-rate can be improved by replacing Li⁺ with Ca²⁺. This indicates that the insertion of different monovalent cations (Na⁺/K⁺) can strongly influence the redox reaction and structure evolution of the host materials, due to the larger ion size of Na⁺ and K⁺ and their different kinetic properties with respect to Li⁺. Furthermore, the working mechanism of both LVP/C and Ca_1.5V₂(PO₄)₃/C in SIBs are elucidated via in operando synchrotron diffraction and in operando X-ray absorption spectroscopy.