Electrolyte degradation – Additives and their impacts on cell performance

Summary:

NMC622/Graphite-Pouch-Cells with a capacity of 5 Ah and different electrolytes are investigated under cyclic and calendaric ageing. In addition the analysis of gasses formed during formation, reveals different formation reactions for the different additives. While additve-free electrolyte mainly produces ethylene during formation, the main contribution shifts to carbonmonoxide when additives are used. Carbondioxide is only found in cells containing additives.
The main difference during ageing is the evolution of the Pulse-Resistance. Cells containing VC generally show a higher pulse-resistance compared to FEC-containing cells. Under both cyclic and calendaric ageing a drop of the pulse resistance is visible between the first checkup (conducted at 20°C) and later checkups (conducted at 25°C) for VC-containing cells. This drop is not evident for FEC-containing cells. After this initial pulse-resistance increases for cells stored at 100% SOC. At 80% SOC a rise in the same order of magnitude than for 100% SOC is visible for the pulse-resistance of cells containing VC. Cells containing FEC exhibit a more moderate increase of pulse resistance at 80% SOC.
Under cyclic ageing the pulse-resistance increases faster for cells containing FEC, starting from lower intial values but reaching almost the same range as cells containing VC after 300 cycles. The capacity retention of FEC-containing cells is slightly higher under cyclic conditions compared to VC-containing cells.
In summary, the analysis of cells differing only in their electrolyte composition reveals that VC-additive increases pulse-resistance. In addition under calendaric ageing Pulse-resistance of VC-containing cells increases substantially faster at 80% SOC when compared to FEC-containing cells.