Investigating the Electrochemical Properties of Lithium-Ion Battery Cathode Materials Controlled by Interfacial Engineering

Abstract

The cathode material is an important part of lithium-ion batteries (LIBs), and various studies have been conducted until now. Among the cathode materials, we researched the control of interfacial surfaces in lithium manganese oxide (LiMn2O4), lithium nickel cobalt manganese oxide LiNi0.8Co0.15Mn0.05O2, and lithium nickel oxide (LiNiO2) through different experimental methods, focusing on enhancing the electrochemical performance and improving the problems of each material. At first, we designed the experiments on the synthesis of lithium excess LiMn2O4 (LMO) by exploiting an intriguing thermal phenomenon, thermally induced grain fining, to sheds light on how it affects the mechanism and kinetics of lithiation, and, furthermore, the electrochemical behavior of LMO. Detailed insights into the lithiation mechanism and kinetics reveal that the use of a finely grained, porous Mn3O4, which possesses an open crystal structure, is a key to the success of incorporating excess Li. Additionally, this in-depth electrochemical investigation verifies a very recent theoretical prediction of faster Li diffusion kinetics enabled by excess Li. Next, we focused on the enhancement of cycling performance for the Ni-rich layered LiNixMnyCozO2 (NCM) materials are used in lithium-ion batteries because of their high cathode capacity. Generally, the high nickel content leads to severe capacity fading and structural collapse after repeated charging and discharging. To mitigate these problems, we heat-treated NCM in an NH3 atmosphere to form Ni2+ pillars between Li layers. The Ni2+ pillars improve the cycling stability of NCM by the alleviating degradation of the Li layer. This is a simple and effective method to improve the stability of Ni-rich NCM Lastly, in the study of LiNiO2 (LNO), we found that there was a difference in cell stability when a method other than the same charge and discharge process commonly used in the driving process of cathode material was tried. As a result of trying various methods, when the asymmetric of fixing the discharging rate and making the charging late slower is obtained, the cycling ability rapidly improved at a specific condition. We found that the cycling ability was improved under the asymmetric slow charge/fast discharge process, and after disassembling each cycled cell, we analyzed the difference on the surface of the electrodes through material characteristic analysis. As a result of the analysis, we concluded that the protective layer formed on the surface during the asymmetric slow charging and fast discharging process inhibited the further reaction between the cathode material and the electrolyte and induced the creation of a uniform layer, thereby maintaining stability even during long-term operation. We revealed that the stability, which is the weak point of conventional Ni-rich cathode materials, can be controlled by simply manipulating the charging and discharging rate.