Experimental Procedure

The experimental procedure followed was as shown in Figure 4-3. The following sections describe each of its stages.

Figure 4-3: Sequence of experimental procedure stages

4.5.2.1 Pre-testing cell performance check

These tests were conducted on the cells before the external short circuit test regime to ensure that their capacity and internal resistance values were sound.

Capacity and internal resistance check

The procedure in Section 3.5.1 was followed to check the capacity and internal resistance of cells.

Galvanostatic electrochemical impedance spectroscopy (GEIS)

GEIS is a common non-destructive technique used to study the transfer behaviour of systems in a number of applications. For Li-ion cells the electrochemical impedance can be measured at different conditions providing key electrochemical parameters of a sealed battery [105], [106]. The impedance is determined as a function of a voltage response in response to a current input. A range of input frequencies can be used, giving the advantage to separate the individual components contributing to the impedance, such as series resistance, charge transfer resistance and double layer capacitance, which can be used to evaluate the performance of a battery [107], [108]. In this research the galvanostatic impedance measurements were performed in the frequency scan range of 10 mHz to 10 KHz before testing using Bio-logic VMP3 cycler and EC-lab V11.02 software. The value of the excitation current signal was set to 800 mA and the temperature of the thermal chamber was set to 25 °C. The sampling rate was set to 6 points per decade. Section (A) of the appendix shows a screenshot of the EIS software settings.

Pre-testing performance check and characterisation External short circuit testing Long-term cycling or storage Post-testing characterisation

4.5.2.2 The external short circuit event

The EXSC tests were composed of two sets of experiments to investigate two parameters; the short resistance value and the short duration. For the first set, different shunt resistors were used to investigate the effect of the resistance on the behaviour of the cell during short-circuit. The combined resistances of the whole external circuit, including the circuit elements, the cable connections and contact resistances were measured using a micro-ohmmeter to be 0.562, 0.788, 24.0, 236, 501 and 821 mΩ. During the second set the lowest value of short resistance (0.562 mΩ) was used to short-circuit cells for different durations (20, 10, 5, 1, and 0.5 s). The second set was composed of three groups. Cells in Group 1 were only short-circuited for different durations. In Group 2, after the EXSC test, cells of the short durations 5, 1, and 0.5 s were cycled for 155 cycles within 2-3 hours after the EXSC. In Group 3, cells were shorted (for 5, 1, and 0.5 s), then stored for an extended period (31 days, equivalent to the time taken to complete 155 cycles) before being cycled for 155 cycles. The aim of Groups 2 and 3 was to compare the effect of storing sort-circuited cells to immediately cycling them after an EXSC. Each test was repeated 3 times. The shorting time was controlled by remotely operating the contactor using a programmable power supply (ROHDE & SCHWARZ HMP4040). Table 4-2 summarises the experiments. Table 4-2: Summary of EXSC experiments

Test Set Group No. Parameter Investigated

Parameter Values Test Stage(s)

A 0 Short resistance value 0.562, 0.788, 24.0, 236, 501 and 821 mΩ EXSC only B

1 Short duration 20, 10, 5, 1 and 0.5 s, using 0.562 mΩ

EXSC only

2 Short duration 5, 1 and 0.5 s, using 0.562 mΩ

EXSC  cycling

3 Short duration 5, 1 and 0.5 s, using 0.562 mΩ

EXSC  storage

4.5.2.3 Cycling and storage after external short circuit

Full charge-discharge cycles were conducted on the cells to investigate their EXSC cycling behaviour. Cells were cycled between 4.2 and 2.7 V for 155 cycles using Maccor cycler. Non-abused cells were also cycled as control. The cycling method affects the cell performance, reliability and lifetime [109]–[111], hence the same cycling profile as presented in Section 4.5.2.1 was followed for all cells. The internal resistance of the cells was measured every 5 cycles using the DCIR measurement method outlined in Section 4.5.2.1. Figure 4-4 shows the cycling regime of the charge/discharge cycles after the overcharge event. A screenshot of the cycling programme is shown in Section (B) of the appendix.

Figure 4-4: Post external short circuit cycling regime

4.5.2.4 Electrochemical impedance spectroscopy

EIS measurements were done on all cells after all testing stages (EXSC, storage and cycling) to investigate the change in their impedance as a result of the previous testing stage. The same EIS settings as in Section 4.5.2.1 were used.

4.5.2.5 Post-mortem analysis

After testing post-mortem analysis was performed on a number of cells to investigate any changes in their internal components. Three cells were used; cells shorted for 20 and 0.5 s and a non-abused cell used as a reference. The cells were opened in an argon filled glovebox in an abuse chamber. The oxygen and water vapour concentration inside the glovebox were below 0.1 ppm. The cell components (mainly electrodes and separator) were visually inspected. Samples of the cathode and anode

were cut and washed with DMC (di-methyl carbonate) as preparation for SEM. SEM images were obtained using a Carl Zeiss Sigma Field Emission Scanning Electron Microscope (FE-SEM). The acceleration voltage was 5 kV and the lens working distance was approximately 2 mm.

4.6 Results and Discussion