Understanding non-ideal voltage behaviour of cathodes for lithium-ion batteries

Abstract

One of the most relevant negative characteristics of high-capacity cathodes for lithium batteries is indeed non-flat voltage–time (V–t) or voltage–capacity (V–C) behaviour during charge–discharge. A clear rationale for this behaviour has not yet been addressed in the literature. Here, by means of density functional theory (DFT) calculations corroborated by basic experimental electrochemical characterization, we investigated both the thermodynamic and the kinetic aspects relevant to voltage variations during charge–discharge of Li₂FeSiO₄ intended as a model case. A simple physical model allowed us to take into account all the experimental evidence. We suggested that voltage deviations from its theoretical value are due to the formation of undesired delithiated structures caused by concentration gradients. We also related voltage behaviour to relevant quantities such as reaction energies, lithium diffusion coefficients, and particle size, so suggesting some strategies for optimization of materials.