The application of lithium-ion batteries has greatly improved people’s life style. However, with the rapid development of modern society, people have higher and higher requirements for charging speed, so the research on fast charging of lithium-ion batteries is extremely important. This high-energy-density lithium-ion battery fast charging technology will have broad application prospects in mobile electronic devices, high-power power tools, and electric vehicles. However, the current research on fast charging has been hindered by many obstacles, such as lithium precipitation on the negative side. In order to achieve the improvement of fast charging performance of Li-ion batteries, we must fully understand the changes of electrode materials during the positive and negative electrode processes, but the failure behavior of positive electrodes under fast charging conditions has not been well understood in previous studies.
Recently, Dr. Tanvir R. Tanim (corresponding author) of Idaho National Laboratory published a research paper entitled “Extended Cycle Life Implications of Fast Charging for Lithium-Ion Battery Cathode” in the journal Energy Storage Materials. This article combines electrochemical analysis, failure model and post-test characterization to study the effect of fast charging (XFC) on cathode materials at multiple scales. The experimental samples include 41 G/NMC (the anode is graphite and the cathode is NCM ternary material) pouch cells, cycled up to 1000 times at different fast charge rates (1–9 C) and states of charge. It was found that during early cycling, the problem with the positive electrode was minimal, but later in the life of the battery, the positive electrode showed significant cracks and was accompanied by a fatigue mechanism, and the positive electrode failure began to accelerate. During cycling, the main structure of the cathode remains intact, but the surface particles can be observed to be significantly restructured.
From the analysis of the experimental results, it can be seen that between 1C and 9C, in the cut-off voltage range above 4.1 V, the cathode failure has a significant nonlinear change with cycling. Within 225 cycles, the problem with the positive electrode was less, but it started to worsen after that. The separation (or cracking) between the original particles is the main reason for the deterioration of cathode performance. Electrochemical data, failure models, and SEM characterizations all verify that the intensification of cathode cracks at the end of cycling is due to a mechanical fatigue mechanism. In the constant current charging mode, the fatigue mechanism is more sensitive to the charging depth than the charging rate. During long-term cycling, even under the harshest cycling conditions (9 C, 4.1 V), the bulk crystal structure of the cathode particles remains a layered structure at the end of the cycle, however, high rates lead to severe surface problems , such as thicker CEI layers, more structural changes, oxygen loss, and manganese ion dissolution. In addition, controlling the cut-off voltage of charging can effectively delay the failure of the cathode material even at the most severe 9 C rate, and even avoid the fatigue mechanism.