Calendar Aging of Lithium-Ion Batteries: Investigating the Influence of Electrolyte Additives in Open-Circuit and Floating State-of-Charge Conditions

Abstract

Battery electrolytes play an important role in battery performance and safety. To improve the formation of a solid-electrolyte interface (SEI) and overall performance, additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are added to the electrolyte. A thorough understanding of the aging process is critical to the safe operation of batteries. The reactions of the liquid electrolyte, its additives and the associated gas formation play a decisive role in cell aging and cell safety. Thus they are in the focus of the BatgasMod project (Modelling of battery gassing) funded by the German Federal Ministry of Education and Research (BMBF) in the framework of the competence cluster Battery usage concepts (BattUse). The aim of the BatgasMod project is the modeling of the gassing behavior of batteries over their lifespan and the combination of electrolyte aging models with battery models for an early prediction of cell behavior in the usage phase.In order to support the modeling with aging data, three different cell variants were produced, differing in terms of their electrolyte composition. The cells were assembled at the MEET by combining commercial LiNi0.6Mn0.2Co0.2O2 (NMC622) cathode and graphite anode material together with a ceramic coated separator. The electrolyte for all three cell types is a mixture of the organic solvents EC and EMC in a ratio of 3:7 with 1M LiPF6. This composition without additives represents the reference cell. To investigate the influence of the additives, cells were prepared with either5 wt.-% vinylene carbonate (VC) or 5 wt.-% fluoroethylene carbonate (FEC) as an additive.First, both at IAM-AWP and ISEA cell formation was performed to study the entire life of the cells, including the critical step of SEI formation in the first cycles. After degassing, the cells were subjected to 10 additional cycles to obtain a stable capacity for the start of the aging tests. The cells were stored in temperature chambers at 25°C with SOCs of 100%, 80%, 30% and 5%. At IAM-AWP measurements were performed under open circuit conditions to allow for self-discharge. For comparison, at ISEA the cells were aged under floating SOC conditions, i.e. the SOCs were kept constant throughout the entire measurement. At predetermined time intervals, the cells were checked for remaining capacity and pulse resistance. In both measurements the influence of the SOC is clearly visible and the high SOC leads to an increased aging. The electrolyte additives showed different results for the different aging tests. The cells without additives outperform the cells with additives under open circuit conditions. This unusual behavior was further investigated and was attributed to the high self-discharge of the cells without additives. As a result, the voltage drop leads to a lower storage SOC, which was beneficial for the aging of these cells. The electrolyte additives showed deviating results for the different aging tests. The floating SOC measurements revealed a superior performance of the cells with additives. In addition, it was found that cells containing VC have lower capacity loss but higher resistance increase than cells containing FEC. Differential voltage analysis and incremental capacity analysis were performed to better understand the dominant aging mechanisms. These methods suggest that a combination of loss of lithium inventory and impedance rise causes cell degradation.These results provide guidance for improving calendar aging by selecting the most appropriate electrolyte additive. Future work will focus on comparing the influence of additives on the safety of fresh and calendar aged cells.