Identifying Degradation Mechanism in Cathode Materials for Lithium Ion Batteries with Precession Electron Diffraction

Due to the explosive growth of the electric vehicle market, recently, attention to lithium ion batteries (LiBs) is increased. The specialty of automotive applications is promoting the development of long-life batteries based on material reliability. Developing a reliable LiB begins with understanding the mechanism of degradation of the material. In other words, based on the degradation mechanism of the material analyzed from accurate and various angles, we can propose a design method of robust material that is one step further.<br/>In this study, we tried to analyze the cause of the deterioration of cycling characteristics for Li(Ni_1/3Co_1/3Mn_1/3)O₂, which is mainly used as a cathode material for LiBs, by precession electron diffraction (PED). In other words, we wanted to know the causes of degradation in the whole system by analyzing the material properties at a single crystal in nanoscale. For the analysis of extremely small ranges of materials, we analyzed the degradation mechanism of the material in the sub-10nm range using Precession Electron Diffraction (PED) techniques.<br/>The PED method can reduce the dynamic diffraction phenomenon according to the thickness of the TEM sample, so that the diffraction intensity can be quantified within a very small range. By using the PED technique based on nano-sized electron beams, we were able to map the structural change of Li(Ni_1/3Co_1/3Mn_1/3)O₂ material in a single crystalline unit over a long period of electrochemical cycles.<br/>It is known that the main cause of the decrease in LiB’s capacity over the electrochemical cycles is the formation of an electrochemically irreversible rock salt phase inside the cathode material. However, the PED analysis showed that we could identify the existence of new metastable phase that had not discovered before. In other words, accelerated testing results revealed new unstable phases due to cation migration and found that the presence and distribution of these phases had a decisive effect on LiB’s electrochemical degradation. In addition, the quantitative relationship between the strain and phase transition also could be derived. Therefore, based on these results, we expect that PED based on nano-sized electron beams can add new perspectives to the analysis of the degradation mechanism of materials for LiBs.