Synthesis and In-Situ Investigation of Olivine LiMnPO4 Composites Substituted with Tetravalent Vanadium in High-Rate Li-Ion Batteries

LiMnPO4 substituted with tetravalent V is synthesized by a polyol method with wide ranges (0 ≤ x ≤ 0.3) of precursor ratios (Mn:V = 1–2x:x and 1–x:x). V4+ is found to suppress the grain growth and promote particles exposing (020) facets. As V-substituted LiMnPO4 is extensively analyzed by XRD, ICP-OES, and XANES, the accommodation of V4+ on the Mn site is accompanied by Mn vacancy for charge compensation. During the synthesis process, the higher V4+ concentration will cause the precipitation of trace amounts of Li3PO4 and amorphous VO2, but the carbon-coated V-substituted LiMnPO4 (x = 0.2) still exhibits a superior rate capability for a capacity of 157 mAh g–1 at 0.1 C and 106 mAh g–1 at 20 C. V4+ improves the electrochemical kinetics by enhancing the Li-ion diffusivity and charge-transfer ability. In-situ XANES reveals that continuous V3+/V4+ and V4+/V5+ redox reactions occur in the ranges 2.0–3.5 and 3.5–4.3 V. They promote the solid-solution reaction of V-substituted LiMnPO4 and provide additional capacity below 3.5 V. Furthermore, in-situ XRD shows that the phase-transition mechanism in V-substituted LiMnPO4 is dominated by a fast crystalline-to-amorphous process. The formation of the amorphous phase can relieve the interfacial strain during delithiation. The amorphization is promoted with the V4+ increases, which explains why the Li-ion can quickly insert/extract through the olivine structure.