4.2.3. Parameters (*Ea*, *A*0)

Energy of activation *Ea* and pre-exponential factor *A*<sup>0</sup> were calculated thanks to Equation (13) and the calculated parameters (*l*, *m*) (Figure 9).

**Figure 9.** *Ea* and pre-exponential factor *A*<sup>0</sup> for different ageing conditions.

Group *G*1 has the smallest energy of activation. The energy required to start the reaction is smaller than the energy of activation of the other cells. No extra SEI has been formed in this group and it is well known than SEI plays a protector role in the thermal stability. However we can notice that the activation energy values within this group are not identical (Figure 9).

Cells aged at 0 ◦C have the highest energy of activation in group *G*1. A high energy of activation means that the reaction is slow. This can be explained by the fact that these cells have a different morphology (as shown by parameter *m*) and so require more energy to trigger the reaction. Post-mortem analyses confirm that these cells have been transformed. GD-OES exhibits that lithium is available at the surface of the electrode. EIS has shown that the interface capacitance has grown, revealing a more porous morphology. The porosity may create an imbalance in the concentration distribution of Li. SEI can be formed irregularly on the pores and so creates zones with Li and zones without Li [17].

In group *G*2, cells have quite the same energy of activation but the pre-exponential factor is different. The pre-exponential factor of the cell aged at CV is lower than for one the other cells of group *G*2. The pre-exponential factor includes many constants describing the initial state of the sample, such as three-dimensional shape factors of initial particles, molecular mass, density, stoichiometric factors of chemical reaction, active surface, number of lattice imperfections, etc. EIS shows that the structure of the interface is closer to the structure of cells aged at 45 ◦C and the cells aged at 25 ◦C. Extra SEI formed in these conditions is formed by solvent degradation like for the cells aged at the OCV. The EIS and the GD-OES analyses show that the quantity of SEI is more important in the case of the CV ageing process. This can explain the differences in the value *A*0.
