*3.4. Influence of Salt, Concentration and Temperature*

Motivated by the enhanced cycle life at lower temperatures of the cells containing LiFSI with a concentration of *c* = 2 M in the DME electrolyte, the influence of the salt concentration and the used salt on the degradation was investigated. Therefore, the concentration was reduced from *c* = 2 M to *c* = 1 M and the used salt was changed from LiFSI to LiTFSI at a concentration of *c* = 1 M. In Figure 6 the impact of the electrolyte composition is visualized. The influence of the concentration is presented in Figure 6a,c,e. The graphs differ for the considered cell temperatures. In Figure 6b,d,f, the effects of the used salt are also given at different temperatures.

Independent of the operating temperature, decreasing the LiFSI concentration from *c* = 2 M to *c* = 1 M in the electrolyte drastically impaired the cell's performance. This is clearly evident in the reduced cycle life of the cell with lower concentrations. The limiting value of *<sup>η</sup>*CE > 0.95 is reached after 50 cycles for the experiments with *<sup>c</sup>* = 1 M at *T*Cell = 25 ◦C. This is three times less than the lifetime of an equivalent cell at a concentration of *c* = 2 M. Similar results were found at a cell temperature of *T*Cell = 40 ◦C. The maximum number of cycles increased from *n*max = 40 to *n*max = 130 as a consequence of the increased concentration. The cells performing at *T*Cell = 60 ◦C showed, in general, the worst stability. The cell with 1 M LiFSI reached a cycle number of *n*max = 20. The maximum number of cycles was minimally increased to *n*max = 60 by increasing the concentration to *c* = 2 M.

**Figure 6.** Cycle performance of cells with Cu/Li structure. All cells ran with the C-rate of *I*Cell = 1 C. (**a**,**c**,**e**) Both cells use LiFSI Li salt in electrolyte with the different concentrations of *c* = {1, 2} M represented by orange and blue colors, respectively. (**b**,**d**,**f**) Both cells use a salt concentration of *c* = 1 M. Blue points represent the cell using LiFSI salt, and black points show the results of a cell using LiTFSI. (**a**,**b**), cell temperature is set to *T*Cell = 25 ◦C. (**c**,**d**), cell temperature is set to *T*Cell = 40 ◦C. (**e**,**f**), cell temperature is set to *T*Cell = 60 ◦C.

Replacing LiFSI at a concentration of *c* = 1 M with LiTFSI at the same concentration affects the cells' performances drastically. Similar to the effects seen by reducing the salt concentration, the use of LiTFSI negatively influences the cell degradation. At none of the investigated temperatures did the experiments using LiTFSI show stable behavior, not even

for a small number of cycles. Independent of the applied temperature, the cells show low and random values for the CE and no trend in the degradation behavior is evident. The cells with LiTFSI also frequently show values above the theoretical maximum value of *<sup>η</sup>*CE > 1, which is a sign of the inhomogeneous, poor and weak SEI formation potential of LiTFSI salt [24]. This might be because of the lower LiF content formed during the degradation of LiTFSI, which plays a major role in stabilizing the cell performance, resulting in a longer cycle life [25]. A general trend of CE development observed in Figures 4–6 is that the random behavior (noise-like) is a sign of instability. The longer cells run smoothly, the better the cycling performance and lifetime get. A CE value of higher than one could be a sign of micro Li plating, while a CE value of lower than one could be caused by the loss of deposited lithium in the form of SEI or dead lithium. A common behavior in all cells is that the cell cyclability reduces significantly as soon as noises start.
