Electrochemical and Structural Investigation of Calcium Substituted Monoclinic Li 3 V 2 (PO 4 ) 3 Anode Materials for Li‐Ion Batteries

In this work, the effect of Li + substitution in Li 3 V 2 (PO 4 ) 3 with a large divalent ion (Ca 2+ ) toward lithium insertion is studied. A series of materials, with formula Li 3−2 x Ca x V 2 (PO 4 ) 3 /C ( x = 0, 0.5, 1, and 1.5) is synthesized and studied in the potential region 3–0.01 V versus Li + /Li. Synchrotron diffraction demonstrates that Li 3 V 2 (PO 4 ) 3 /C has a monoclinic structure (space group P 2 1 / n ), while Ca 1.5 V 2 (PO 4 ) 3 /C possesses a rhombohedral structure (space group R ‐3 c ). The intermediate compounds, Li 2 Ca 0.5 V 2 (PO 4 ) 3 /C and LiCaV 2 (PO 4 ) 3 /C, are composed of two main phases, including monoclinic Li 3 V 2 (PO 4 ) 3 /C and rhombohedral Ca 1.5 V 2 (PO 4 ) 3 /C. Cyclic voltammetry reveals five reduction and oxidation peaks on Li 3 V 2 (PO 4 ) 3 /C and Li 2 Ca 0.5 V 2 (PO 4 ) 3 /C electrodes. In contrast, LiCaV 2 (PO 4 ) 3 /C and Ca 1.5 V 2 (PO 4 ) 3 /C have no obvious oxidation and reduction peaks but a box‐type voltammogram. This feature is the signature for capacitive‐like mechanism, which involves fast electron transfer on the surface of the electrode. Li 3 V 2 (PO 4 ) 3 /C undergoes two solid‐solution and a short two‐phase reaction during lithiation and delithiation processes, whereas Ca 1.5 V 2 (PO 4 ) 3 /C only goes through capacitive‐like mechanism. In operando X‐ray absorption spectroscopy confirms that, in both Li 3 V 2 (PO 4 ) 3 /C and Ca 1.5 V 2 (PO 4 ) 3 /C, V ions are reduced during the insertion of the first three Li ions. This study demonstrates that the electrochemical characteristic of polyanionic phosphates can be easily tuned by replacing Li + with larger divalent cations.