Kinetic Study of Parasitic Reactions in Lithium-Ion Batteries: A Case Study on LiNi0.6Mn0.2Co0.2O2
Authors: Zeng Xiaoqiao , Xu Guiliang , Li Yan , Amine Khalil , Chen Zonghai 
Published: February 2020

The global R&D effort has been driven by the growing application of high energy-density lithium-ion batteries to electrify the transportation system and achieve significant reduction on non-renewable fuels consumption and greenhouse gas emission. However, high initial energy-density still generally be obtained with the price of a reduction on battery safety and life, which are fundamentally connected to the parasitic reactions in the cell at various temperatures. The safety issue is chemically related to violent reactions occurring at elevated temperatures and “extreme” potentials while the battery lifetime is more directly influenced by the extremely slow reactions continuously running under “normal” operating conditions, leading to continuous consumption of lithium reservoir and the buildup of side products inside the cell. Hence, practical high energy-density lithium-ion chemistry needs to be carefully balanced between energy-density and safety/lifetime of the chosen chemistry. The electrochemical validation of the effectiveness of those approaches for improving battery life is practically trivial, but extremely time-consuming, especially for those chemistries that have already been demonstrated capable of being charged/discharged for more than thousand cycles. Therefore, the fundamental understanding/measurement of parasitic reactions not only helps to select a proper life improving strategy, but also substantially shortens the lengthy electrochemical validating process.
In this work, a home-built high-precision leakage current measuring system was deployed to investigate the parasitic reaction kinetics between lithium transition metal oxide cathode (LiNi0.6Mn0.2Co0.2O2) and conventional carbonate electrolyte (1.0 M LiPF6 EC/EMC 3:7). The working electrode was held at a specific potential using the source meter, presuming that the state of the charge or the lithium concentration in the working electrode will reach an equilibrium state after the constant-voltage charge/discharge. During this process, the electron obtained from the environment by oxidizing the solvent, will be electrochemically monitored by the external circuit. The measured leakage current is practically proportional to the reaction rate of parasitic reactions between the working electrode and the electrolyte. Hence, the static leakage current can be used as a quantitative indicator for the reaction rate of the side reactions. The kinetic study revealed a strong dependence of parasitic reactions on the practical upper cutoff potential in terms of both the reaction mechanism and the reaction rate. The kinetic data also indicated a significant change of reaction mode at ~4.5 V vs. Li+/Li. The study implies that a different strategy might be needed for effective mitigation of the parasitic reactions for materials targeted for operation at a potential higher than 4.5 V vs. Li+/Li. It demonstrated that the kinetic study could be crucial in developing advanced electrode materials with a fundamental balance between energy density and battery life.

Figure 1