Surface and bulk properties of Li-ion electrodes – A surface science approach

Abstract

Lithium-Ion batteries are important devices for present and future energy storage, offering high energy density and durability. Positive electrode (cathode) materials are predominantly transition metal oxides containing lithium, such as layered oxides or oxides with spinel structure. The electronic/ionic structure of these materials in the bulk as well as at the phase boundary of the solid-electrolyte interface is a key for their properties, such as electrode potential, degradation and reactivity.This contribution gives examples of surface science investigations on battery materials, addressing especially electronic structure of layered-oxide cathode materials and cathode-electrolyte interface formation. In a surface science approach, well-defined surfaces/interfaces are prepared by thin film deposition techniques and analyzed with surface sensitive analytical methods such as photoelectron spectroscopy (XPS, UPS) and X-ray absorption spectroscopy (XAS) before and after polarization in liquid electrolyte or contact with electrolyte components. Surface science offers the possibility to analyze the electronic and chemical structure of surfaces and interfaces, allowing conclusions on the electronic and ionic structure of the bulk, on reactivity with other phases, and on electrochemical interface formation.After an introduction into the methodology and fundamental phenomena, the contribution focuses on the role of the electronic structure and electric double layer for electrode degradation and surface layer formation. Charge transfer (electrons and ions) and defect formation is discussed on basis of energy level diagrams extracted from experimental data.