W-doped LiWxNi0.5Mn1.5−xO4 cathodes for the improvement of high rate performances in Li ion batteries

► LiWxNi0.5Mn1.5−xO4 is investigated as cathodes in Li ion batteries. ► The best electrochemical performance is obtained when x=0.005. ► LiW0.005Ni0.5Mn1.495O4 showed significant improvement in capacity retention. ► The change of inter-atomic distance with W-doping produces superior performance. W-doped LiWxNi0.5Mn1.5−xO4 (x=0.00–0.10) are synthesized via a sol–gel method, and their electrochemical properties are investigated. During synthesis, W4+ is converted to W6+, the amount of which significantly affects the charge–discharge behaviors of LiWxNi0.5Mn1.5−xO4. When limited to x≤0.005, W-doping enhances the electrochemical activity of cathodes, leading to a greater discharge capacity and less capacity fading than LiNi0.5Mn1.5O4 at high C-rates. This is interesting since lowering the average valence state of Mn ions by incorporating W6+ can introduce structural instability, extending a 4V plateau (Mn3+/4+). The reasons for such behavior associated with W-doping are examined by electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and impedance spectroscopy. It is disclosed that the simultaneous increase in LiO bond length and decrease in transition metal-oxide bond length, while the unit cell volume is maintained almost invariant, provides LiW0.005Ni0.5Mn1.495O4 with optimal high rate performance. When W-doping exceeds x=0.01, 3 factors (intrinsically low electronic conductivity of W6+, the presence of tungsten oxide impurities, and an increase in the inter-metallic distance) aggravates electrochemical performance of LiWxNi0.5Mn1.5−xO4. The last factor also induces structural instability during repeated cycling because of the expansion of a unit cell volume.