Li2MnSiO4 cathodes modified by phosphorous substitution and the structural consequences

Abstract

Li1.8MnSi0.8P0.2O4 prepared by a citrate acid assisted sol–gel route followed by calcination at 800 °C is compared to a sample of standard Li2MnSiO4 prepared under the same experimental conditions. The particle size of the P-substituted material is 20–40 nm compared to 50–100 nm for the P-free sample. Rietveld refinement of crystallographic models with synchrotron powder X-ray diffraction data shows that both samples consist of a mixture of the Pmn21 and Pmnb polymorphs of Li2MnSiO4 and a minor MnO impurity (< 5%). In the P-substituted material, P is found to partially replace Si in the Pmn21 polymorph accounting for the PO43 − bands observed in the FTIR spectra. The lattice parameters of the P-substituted material differ only slightly from those of the P-free material. Electrochemical tests show that Li/Li1.8MnSi0.8P0.2O4 cells have better performance than Li/Li2MnSiO4 cells in terms of reduced polarisation and a 66% better initial discharge capacity at 40 mA g− 1. P-substituted samples show some loss in long-range order with cycling but less than that observed in P-free samples. Significant reflections are observed in P-substituted cathode samples in the discharged state after the first cycle, in contrast to P-free material. This shows that P-substituted cathodes are less prone to amorphisation than standard P-free cathodes.