In-Situ TEM Investigation on Thermal Stability and Oxygen Release Behavior of Charged and Discharged LiCoO2

Since the first discovery of Li ion batteries (LIBs), enormous research has been dedicated to improve their electrochemical performance such as energy density, power density and lifetime. Yet not much research has been focused on the safety issues and thermal stability of these systems. LiCoO 2 is one of the most widely used cathode materials in LIBs. High energy density and light weight cell has made it a good candidate for first generation electric vehicles (Tesla roadster) and even airplanes (Boeing 787 Dreamliner). However, due to the poor thermal stability, its application has been limited to the consumer electronics now. It is well known that Charged LCO when exposed to higher temperatures, as in a case of a mechanical impact or overcharge, releases a portion of its oxygen. Released oxygen can react with electrolyte exothermically and trigger a hazardous thermal runaway. In order to hinder this process we need to gain more in-depth knowledge about underlying mechanisms of this harmful reaction and phase/structural transitions. In-Situ TEM is known to be a powerful method for studying materials in dynamic conditions such as high temperatures. In this study we utilized transmission electron microscopy to study Li 1-x CoO 2 (0.5<x<1) in different temperatures. High resolution imaging integrated with electron energy loss spectroscopy (EELS) reveals that cobalt valance changes from 4+ to 3+ at less than 300°C and from 3+ to 2+ at about 450°C showing a two-step oxygen release mechanism in charged LCO Simultaneously, selected area diffraction pattern (SADP) suggests formation of Li vacancy super-lattice and defects such as twins at about 300°C. Going further in temperature results in increase in density of defects and formation of grain boundaries. These findings can be helpful in improving the structural stability of these materials.