High voltage sulphate cathodes Li2M(SO4)2 (M = Fe, Mn, Co): atomic-scale studies of lithium diffusion, surfaces and voltage trends

Abstract

The search for high voltage cathodes for lithium-ion batteries has led to recent interest in the monoclinic Li₂Fe(SO₄)₂ material which has a voltage of 3.83 V vs. lithium, the highest recorded for a fluorine-free iron-based compound. Here we investigate the defect, surface and lithium migration properties of the Li₂M(SO₄)₂ (M = Fe, Mn, Co) materials using combined atomistic modelling and density functional theory (DFT) techniques. All intrinsic defect types including Li/M antisite disorder are found to be of high energy, suggesting insignificant concentrations. Low activation energies are found for lithium migration along the a-axis channels giving rise to long-range 1D diffusion, which are supported by molecular dynamics (MD) simulations. For the crystal morphology a significant surface area is exposed to these 1D diffusion channels, which would allow facile Li insertion and extraction. Using DFT simulations we reproduce the high voltage of the Li₂Fe(SO₄)₂ material in accord with electrochemical data and also examine local structural distortions on lithium extraction.