Electrochemical Investigation of Calcium Substituted Monoclinic Li$_3$ V$_2$(PO$_4$)$_3$ Negative Electrode Materials for Sodium‐ and Potassium‐Ion Batteries

Herein, the electrochemical properties and reaction mechanism of Li$_{3‒2x}$Ca$_x$V$_2$(PO$_4$)$_3$/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, Li$_3$V$_2$(PO$_4$)$_3$/C exhibits the highest reversible capacity in SIBs and PIBs, while Ca$_{1.5}$V$_2$(PO$_4$)$_3$/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 Ca$^{2+}$. 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$^+$. ... mehrFurthermore, the working mechanism of both LVP/C and Ca$_{1.5}$V$_2$(PO$_4$)$_3$/C in SIBs are elucidated via in operando synchrotron diffraction and in operando X-ray absorption spectroscopy.