## 2 Experimental
### Long-Term Cycling  
For the ageing study presented in this research work, accelerated ageing tests have been performed by cycling of a high energy lithium-ion battery cell at 35 \({}^{\circ}\)C. The cycling conditions are defined by the following specifications:  
* The State-of-Charge (SoC) (in %) which is defined by the ratio between the battery cell actual capacity and its nominal capacity, as expressed in Equation (1): \[SoC=C_{\mathrm{actual}}/C_{\mathrm{nominal}}\] (1)
* The Depth-of-Discharge (DoD) (in %) which reflects the percentage of battery capacity that has been discharged expressed as a percentage of its actual maximum discharge capacity and is linked to the SoC in Equation (2): \[DoD=100\%-SoC\] (2)
* The middle State-of-Charge (mid-SoC) which is the SoC around which the battery cell is cycled. \begin{table}
\begin{tabular}{l c c} \hline \hline  & **Physical and Mechanical Characteristics** & \\ \hline  & **Thickness** & 7.1 \(\pm\) 0.2 mm \\ \hline
**Length** & without terminals & 217 mm \(\pm\) 1 mm \\
**Width** & without terminals & 130 mm \(\pm\) 1 mm \\
**Weight** & & 428 g \\ \hline \hline  & \multicolumn{2}{c}{**Chemical Characteristics**} \\ \hline
**Positive electrode** & Nickel Manganese Cobalt \\
**Negative electrode** & Graphite \\ \hline  & \multicolumn{2}{c}{**Electrical Characteristics**} \\ \hline
**Minimum discharge voltage (\({}^{1}\))** & 3 V \\
**Maximum charging voltage** & 4.15 V \\
**Nominal voltage** & 3.65 V \\
**Charging current (\({}^{1}\))** & 10 A (0.5 C) \\
**Nominal capacity** & 20 Ah \\
**Operating Temperature** & [-30 \({}^{\circ}\)C,+5 \({}^{\circ}\)C ] \\
**AC impedance at 1 kHz** & \textless{}3 m\(\Omega\) \\
**Specific energy** & 174 Wh/kg \\
**Energy density** & 370 Wh/L \\ \hline \hline \end{tabular} \({}^{(1)}\) recommended by the EIG battery manufacturer.  
\end{table} 
Table 1: Main properties of the high energy lithium polymer EIG C020 battery cells [31]. * Nominal Capacity which is the capacity expressed in Ampere-hours (Ah), specified by the manufacturer at specific conditions. In this research work, the nominal capacity of the studied battery cells is equal to 20 Ah. * The Charge/Discharge current rates expressed in terms of C-rate where the C-rate defines the rate at which a battery is discharged or charged relatively to its nominal capacity. * The Full Equivalent Cycles (FEC) (in case the cycling is performed with a DoD inferior to 100%), defines the amount of cycles needed to equal the same amount of Charge/Discharge capacity observed in case the cell would have been cycled with 100% DoD. * The Capacity Fade (\(\Delta C\)) (in %) indicates the loss of capacity of the battery observed in time by comparison of its actual capacity with its measured initial capacity, as expressed in Equation (3): \[\Delta C=100\%\times(1-\frac{C_{actual}}{C_{initial}})\] (3)
* The State of Health (SoH) (in %) translates the ageing of the battery cell and is defined with the Equation (4): \[SoH=100\%-\Delta C\] (4)  
In this research work, the cycling conditions were defined by 80% Depth of Discharge (DoD), which is defined as 50% mid-State of Charge (SoC) and 1C/1C (i.e., 20 A/20 A) current for the constant charge and discharge currents. The reference amount of cycles is equal to 100 FEC where each cycle is defined by a repetition of one charge and one discharge sequences. The research work presented in this paper is based on one uncycled (or fresh) cell (i.e., cell 1) and one cycled (or aged) cell (i.e., cell 2) that was cycled up to 900 FEC. Table 2 summarizes the testing of the studied cells and their purpose. Cells 1 and 2 were aimed for thermal characterisations of fresh and aged battery cells, respectively. Capacity tests have been performed on both battery cells at the Begin Of Life (BoL) and as part of the Check-Ups (CUs) every 100 FEC on the cycled cell to monitor its SoH evolution and its capacity fade due to the accelerated ageing. This test is defined by a 3 times repetition of charge and discharge sequences at C/3 (6.666 A) current rate.