Study of correlation between aging at low temperatures and safety of commercial 18650-type lithium-ion cells

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Abstract

Safety of electric vehicles (EVs and PHEVs) with lithium-ion batteries is currently one of the main obstacles besides the high costs and the low range compared to vehicles with combustion engines. For the commercialization of EVs it is important to overcome those challenges, especially with safe and abuse tolerant lithium-ion batteries. Over the life-time of the battery different aging mechanisms take place inside the cells [1]. These effects can influence the cell behavior during abnormal conditions, like environmental effects or electronic malfunctions. Safety in terms of lithium-ion cells refers to a mutual agreement between cell producer and end user to operate the cells only in the mentioned conditions by the manufacturer. To examine a correlation between aging at low temperatures and safety, two types of commercial 18650-type cells from a well-known manufacturer were used in this study. Cell A (2.2 Ah) with Li(Ni0.5Co0.2Mn0.3)02 as cathode and graphite as anode material was selected to represent a state-the-art cell. Cell B (3.0 Ah) containing a LiCoO2 cathode and a graphite anode with an Al2O3 coating was selected to determine the influence of a porous Al2O3 safety coating on the anode. The cells were aged at 0°C until a state of health (SOH) of 70%. The main aging mechanism that influences the cell safety was identified as lithium metal deposition with caused side reactions. Heat-Wait-Search (HWS) experiments were conducted on aged and uncycled cells in an accelerating rate calorimeter (ARC). An ARC generates a worst-case scenario for abuse tests because of the lack of heat dissipation from the cell to the surrounding environment. Within the study, we examined the influence of the state of charge (SOC) ranging from 0%, 50% and 100% as well as the SOH to determine how the cell cycle life is affected by the applied aging strategy. Post-mortem analyses including SEM and solid state NMR were performed to underline the ARC and electrochemical results. The results are beneficial for future lithium-ion cells. A correlation between aging at low temperatures and the thermal abuse of commercial lithium ion cells could be shown in this study.

Speakers from the University of Münster

Friesen, Alex	Münster Electrochemical Energy Technology Battery Research Center (MEET)
Horsthemke, Fabian	Münster Electrochemical Energy Technology Battery Research Center (MEET)
Mönnighoff, Xaver	Münster Electrochemical Energy Technology Battery Research Center (MEET)
Schappacher, Falko Mark	Münster Electrochemical Energy Technology Battery Research Center (MEET)
Winter, Martin	Münster Electrochemical Energy Technology Battery Research Center (MEET)