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

Motofumi Saitoh,Yumi H Ikuhara,Takafumi Ogawa,Yuichi Ikuhara,Akihide Kuwabara,Xiang Gao,Craig A J Fisher,Shunsuke Kobayashi

doi:10.1039/c8cp03390k

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.

Highlights

Li-ion batteries (LIBs) are widely used in a myriad of applications such as portable devices, electric vehicles and renewable energy storage systems
In this work we focus on a number of actively investigated oxide LIB materials, some of whose energy-loss spectroscopy (EELS) spectra have not yet been reported
When an electron is elevated from an O-1s shell in LiFePO4, the projected density of states (PDOS) of O-p, Fe-d and P-p orbitals at the bottom of the conduction band change dramatically, while their PDOSs at higher energies undergo relatively little change

Summary

Introduction

Li-ion batteries (LIBs) are widely used in a myriad of applications such as portable devices, electric vehicles and renewable energy storage systems. In an effort to develop superior LIBs, many candidate electrode materials have been investigated in terms of energy density, output voltage, charge–discharge rates and cycle lifetimes.[1,2,3] At the same time, efforts to improve battery safety, compactness and stability have led to the development of solid electrolytes that exhibit good Li-ion conductivities and structural integrity after many charge–discharge cycles when used in all-solid-state LIBs.[4]. Take place during battery operation is of fundamental importance in the development of improved LIB materials. Energy-loss near-edge structure (ELNES) spectra of these elements, obtained by performing electron energy-loss spectroscopy (EELS) within a transmission electron microscope (TEM), can provide useful data about their energetics and coordination environments with a spatial resolution of a few nanometres.[5,6,7,8,9,10,11,12] In this work we focus on a number of actively investigated oxide LIB materials, some of whose EELS spectra have not yet been reported

Methods

Results

Conclusion