Manufacturer of Li-ion batteries specify the maximum application temperature of their products to around 50 – 60 °C. If operated at higher temperatures the cells tend to generate gas and age much faster. When the temperature of a battery reaches a critical limit it could even result in a thermal runaway and the explosion of a cell. However, some industries require a thermal stability of their devices for up to 80 °C and higher.

To evaluate the cycling behaviour under these conditions, commercial pouch cells (1.2 Ah) without electrolyte have been obtained from Li-Fun Technology Co. Ltd. Their cell chemistry is based on NMC (111) and artificial graphite. The cells have been filled with electrolyte mixtures containing different solvents and additives. After the formation and degassing step the cells were transferred into an environmental test chamber, set to 80 °C. They have then been stored at different OCV`s or cycled at C/3 (400 mA - CCCV) while measuring the cell impedance, using PEIS. Various post mortem analyses like XPS, XRD and symmetrical coin cells have been used to obtain additional information about the degradation of individual components.

Cells filled with electrolytes containing none or inappropriate additives showed a premature cell swelling and capacity failing during the first 25 cycles (Figure 1 - Electrolyte A and C). By adding 2 % of VC (vinylene carbonate) to Electrolyte A, no visual gas generation could be observed and the capacity loss was much lower (Figure 1 - Electrolyte B). Similar results have been obtained for cells filled and formatted by the industrial supplier (Figure 1 - Electrolyte D). However, cells filled with this electrolyte composition showed a downfall in capacity after around 200 cycles. This behaviour occurred together with an increase in series resistance which is related to the degradation of the electrolyte itself. After opening these kind of cells in a glove box, the separator and electrodes have been found to be completely dry.

During the first 200 cycles, cells with electrolyte B and D do not just show a similar decrease in capacity, but also an increase in charge transfer resistance. Researchers who performed studies with similar electrode materials and experimental parameters concluded that this resistance increase might be caused by the growing of the anodic SEI layer [L. Bodenes, Journal of The Electrochemical Society, 159 (10), A1739-A1746, 2012]. But PEIS measurements done on symmetrical coin cells showed that the cathode resistance is up to ten times larger compared to the anode. This was also confirmed by half-cell results of the aged electrodes, when cycling these against fresh lithium.

Figure 1:        Cycle stability of 1.2 Ah NCM/Graphite pouch cells, filled with different electrolytes, charged and discharged at C/3 – 400 mA (2.5 V – 4.2 V / CC-CV) at 80 °C

Electrolyte A: – EC:EMC (3:7) 1M LiPF₆
Electrolyte B: – EC:EMC (3:7) 1M LiPF₆ + 2% VC
Electrolyte C: – EC:EMC (1:3) 1.2M LiPF₆ + 3% VC + 15% FEC
Electrolyte D: – EC:PC:DEC (35:30:45) 1M LiPF₆ + 1% VC + 2% PES + other