A Study on Impedance Change of Lithium-Ion Batteries Depending on Charging Current Profile at Low Temperature

Abstract

Lithium-ion batteries(LIBs) are used in various portable electronic devices due to their advantages such as high energy density, power, and long lifespan. Therefore, various studies are being conducted to improve the capacity and power of LIBs and to increase their stability. As a result, LIBs has become a key component of electric vehicles(EVs) and hybrid vehicles(HEVs), which are rapidly increasing in recent years. However, even in these LIBs, the electrochemical reaction and the diffusion rate of lithium ions rapidly decrease at low temperatures, resulting in serious performance degradation. Therefore, low-temperature charging causes lithium plating and dendrite formation to reduce cell life and in severe cases cause cell rupture. In particulary, in the case of EVs and HEVs that are charged outdoors, low-temperature charging can cause serious problems such as battery rupture as well as shortened lifespan. Therefore, in this study, we analyze the effect of various charging techniques such as CC-CV, multi-stage CC, and pulse on the low-temperature impedance change of LIB and propose an optimal charging technique for stable charging.AT first, test cells for the experiment were fabricated and activated, and then capacity and impedance were measured at room temperature. After that, the charge/discharge cycle experiment of each test cell was performed by applying the CC-CV, multi-step CC, and pulse charging methods at low temperature. After the cycle experiment, the capacity and impedance change of each test cell were measured at room temperature, and the correlation between the capacitance change and the impedance change was analyzed. As a result, the greatest change occurred in the resistance and capacitance components of the SEI (Solid Electrolyte Interface) layer, which is common to all charging methods. However, among the three charging methods, the pulse charging method had the smallest change in capacity and impedance. This is because the repetitive rest period of pulse charging gives relaxation time for diffusion of Li-ions. Next, morphological changes of the LIB anode were confirmed using a scanning electron microscope (SEM). As a result, it was confirmed that the lithium plating and dendrite generated during low-temperature charging additionally reacted with the electrolyte to form an additional SEI layer, thereby changing the thickness and shape of the SEI layer. Therefore, it was confirmed that the change of the SEI layer caused the LIB impedance change. Finally, we confirmed that a low current density or charging current with sufficient break time such as pulse charging is most effective in maintaining the endurance of LIBs at low temperatures.