Abstract:

Ternary lithium-ion cells, recognized for their superior electrochemical properties, are extensively utilized in the energy storage sector. This research uses commercial 18650 lithium nickel-cobalt-manganese (NCM)/graphite cells as experimental samples to explore the effects of float-charging at 4.2, 4.4, and 4.6 V. We conducted a series of analyses including capacity tests, incremental capacity analysis, and impedance tests, complemented by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) of electrode materials from disassembled cells. The influence of float-charging on thermal safety of NCM cells was studied through the accelerating rate calorimeter (ARC) test. Our findings indicate that higher float-charging voltages accelerate cell aging, with an average capacity loss rate of 1.166%/day at 4.6 V. Float-charging at elevated voltages intensifies the interface reactions between the electrolyte and electrode, resulting in a thicker solid electrolyte interface (SEI) film and a substantial increase in impedance. Furthermore, it induces corrosion of the positive collector, leading to aluminum precipitation and deposition at the negative electrode, which further accelerates the capacity attenuation. The onset temperature of self-heating (T_onset) of the cells is reduced after float-charging, indicating a decrease in thermal safety. Through float charging process analysis, failure material characterization, and thermal runaway experiments, this study provides theoretical insights and technical guidance on the impacts of float-charging on the electronic and thermal safety performance of lithium-ion cells.

Key words: lithium ion cell, float-charging, thermal runaway, failure mechanism, thermal safety