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 3-2 x 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 + . Furthermore, 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.