This article comes from the WeChat public account: In order to meet people's growing needs, it has become one of the frontier hot spots of scientific research. At present, the development of lithium-ion batteries is largely restricted by cathode materials. The development of high-performance cathode materials has important scientific research and application value. Layered cathode materials have attracted much attention due to their high theoretical capacity and high energy density. Especially under high voltage, the energy density of the materials has been greatly improved, and the prospects are better. However, cycling under high voltage will cause bulk phase change in the layered cathode material, and the cathode-electrolyte interface (CEI) will continue to deteriorate, resulting in poor cycle performance. Generally speaking, CEI is one of the key factors for the stability of cathode materials under high voltage. Therefore, it is of great significance to study the formation mechanism of CEI and effectively regulate it, but it still faces great challenges.
In view of this, Academician Chen Jun of Nankai University and Professor Cheng Fangyi combined theoretical calculations and experimental studies to propose that CEI components are largely affected by dominant species in the Inner Helmholtz Layer (IHL). The inner Helmholtz layer serves as the interface between the electrode and the electrolyte, where solvent molecules and characteristic adsorbed anions exist. They first found through theoretical calculations that the dominant species in the IHL of the original layered cathode (here, the layered lithium cobalt oxide is taken as an example) adsorbs anions. However, after delithiation, the dominant species in the IHL of the cathode material changed from the original properties. The adsorbed anions are converted into solvent molecules , and the Zeta potential measurement results also support the changes of the dominant species in the IHL before and after charging. Organic solvent molecules will derive CEI rich in organic components, and as charge and discharge progress, the dominant species in the IHL continue to change, which is not conducive to the formation of stable CEI and maintenance of interface stability.
Based on the above discussion, they imagined that IHL can be controlled by regulating the surface reactivity of the material, thereby achieving interface stability. By designing an inert spinel surface coating layer, it was found that the dominant species in the IHL on the material surface before and after charging can be stably maintained as characteristic anion adsorption. IHL in which inorganic anions are the dominant species contributes to the formation of CEI in which inorganic substances are the main component.
Figure 1. Changes in dominant species within the IHL. Image source: Angew. Chem. Int. Ed.
Further studies such as XPS etching and AFM have shown that the CEI mainly composed of organic matter is unevenly distributed on the cathode surface and has a low Young's modulus, while the CEI mainly composed of inorganic matter is distributed unevenly on the cathode surface. More uniform and has a higher Young's modulus. Subsequently, an in-situ heating transmission electron microscope was used to observe that CEI, which is mainly composed of organic matter, has poor thermal stability and will continue to expand at high temperatures. It cannot protect the cathode well, and the loss of oxygen in the cathode material causes cracks and holes. The inorganic-based CEI can well maintain the mechanical stability of the material at high temperatures.
Figure 2. Differences in physical and chemical properties of different components of CEI. Image source: Angew. Chem. Int. Ed.
Further research results show that spinel coating and inorganic-based CEI can well maintain the cycle stability of layered cathode materials under high voltage and improve the rate performance. . This result was recently published in Angewandte Chemie International Edition.
Tuning Interphase Chemistry to Stabilize High-Voltage LiCoO2 Cathode Material via Spinel Coating
Junxiang Liu, Jiaqi Wang, Youxuan Ni, Jiuding Liu, Yudong Zhang, Yong Lu, Zhenhua Yan, Kai Zhang, Qing Zhao, Fangyi Cheng, Jun Chen
Angew. Chem. Int. Ed., 2022, DOI: 10.1002/anie.202207000
Instructor introduction
Cheng Fangyi
https://www.x-mol.com/university/faculty/11824
