Li-ion migration in Li2FeSiO4-related cathode materials: A DFT study

The orthosilicate family of materials Li2MSiO4 for M = Fe, Mn and Co are coming to be seen as potentially cheap cathode materials for large-scale Li-ion batteries, not least through the possibility for significant capacity gains if more than one Li-ion can be removed per formula unit. To gain insights into possible Li-ion migration pathways and diffusion barriers for Li-ions, model systems for LixFeSiO4 (x ≈ 1, 2) are here studied using the Density Functional Theory (DFT) approach. Li-ion and ion-vacancy migration barriers are calculated for a number of model systems. The results help explain why the Li/Fe site-mixing observed during electrochemical cycling of Li2FeSiO4 does not lead to any noticeable loss in cell performance, despite the increased tortuosity introduced into the Li-migration pathways by this ion-mixing process.