Systematic analysis of electron energy-loss near-edge structures in Li-ion battery materials

Abstract

Electrical conductivity, state of charge and chemical stability of Li-ion battery materials all depend on the electronic states of their component atoms, and tools for measuring these reliably are needed for advanced materials analysis and design. Here we report a systematic investigation of electron energy-loss near-edge structures (ELNES) of Li-K and O-K edges for ten representative Li-ion battery electrodes and solid-state electrolytes obtained by performing transmission electron microscopy with a Wien-filter monochromator-equipped microscope. While the peaks of Li-K edges are positioned at about 62 eV for most of the materials examined, the peak positions of O-K edges vary within a range of about 530 to 540 eV, and the peaks can be categorised into three groups based on their characteristic edge shapes: (i) double peaks, (ii) single sharp peaks, and (iii) single broad peaks. The double peaks of group (i) are attributable to the d⁰ electronic configuration of their transition metal ions bonded to O atoms. The origin of the different peak shapes of groups (ii) and (iii) is more subtle but insights are gained using density functional theory methods to simulate O-K ELNES edges of group (ii) material LiCoO₂ and group (iii) material LiFePO₄. Comparison of their densities of states reveals that in LiCoO₂ the Co–O hybrid orbitals are separated from Li–O hybrid orbitals, resulting in a sharp peak in the O-K edge, while Fe–O, Li–O and P–O hybrid orbitals in LiFePO₄ partially overlap each other and produce a broad peak.