Gas Evolution during Unwanted Lithium Plating in Li-Ion Cells with EC-Based or EC-Free Electrolytes

Lithium plating can be induced in any Li-ion cell that has a graphite negative electrode by increasing the charge rate sufficiently at fixed temperature or by lowering the temperature at a fixed charge rate. Recently, ethylene carbonate (EC)-free electrolytes, such as 1 M LiPF6 in 98% ethyl methyl carbonate (EMC): 2% vinylene carbonate (VC), have been shown to passivate lithiated graphite effectively and allow Li[Ni0.42Mn0.42Co0.16]O2/graphite (NMC442/graphite) Li-ion cells to operate effectively for hundreds of charge discharge cycles. High charge rates and low temperatures were applied to Li[Ni1/3Mn1/3Co1/3]O2/graphite (NMC111/graphite) pouch cells containing EC-free EMC-based electrolytes with additives to study unwanted lithium plating. In such electrolytes, the plated lithium metal is not well passivated and reacts to create gas while in the same cells with 1 M LiPF6 EC:EMC (3:7) electrolyte no gas is observed when Li plating occurs because EC passivates metallic Li well. The volume of the pouch cells with EC-free electrolytes increased sharply when Li plating occurred as measured using in-situ methods. In cells having both EC-based and EC-free electrolytes, lithium plating was the cause of rapid capacity loss at high charge rates at both 10°C and 22°C.