Límite de los Gastos Públicos

A total of six cells were provided by an industrial partner for the experimental work. To ensure a small spread of variation between individual specimens and ensure robustness of the testing

122 outcomes, a full characterisation of the cells’ attributes prior to performance and degradation testing was deemed to be required. The characterisation process employed within this research is defined in Table 6-4 and discussed below. A full characterisation refers to the entire table; a partial characterisation refers to those tests marked in bold. Implementation of either a full or partial characterisation test is discussed below.

Unless otherwise stated, all tests were conducted using the “BITRODE MCV 16-100-5” system.

The sampling rate for cell current and voltage measurements during charge and discharge were set to 10 Hz, the fastest sampling rate available with the equipment. During resting periods, where a less dynamic cell response is expected, the sampling rate was set to 1 Hz to reduce the amount of data collected. The results from the initial characterisation tests are presented and discussed within the next chapter in section 7.2.

Assessment Test Description SOC C-Rate

Discharge energy capacity

Standard test to determine the

discharge capacity of every cell 100-0% 1C Slow charge

TABLE 6-4–CHARACTERISATION TESTS.TESTS IN BOLD CONSTITUTE PARTIAL CHARACTERISATION TESTS DURING DUTY CYCLE DEGRADATION STUDY.

6.3.1 Galvanostatic energy capacity

The galvanostatic energy capacity tests are reviewed within section 2.5.1. This test is selected as it is commonly used to identify the energy content that can be extracted from a cell during discharge, or stored within a cell under charge. As the amount of energy which can be stored or

123 extracted is current dependent, the current used is kept the same throughout the duration of testing. Test were conducted for charging and discharging.

For each discharge energy capacity test, cells were charged using a CC-CV schedule with a charging current of 26.5 A (0.5 C) up to the upper voltage limit of 4.2V. Subsequently, the cells were kept at this voltage until the current dropped below 2A, after which the cells were left to equilibrate for 1 hour. This state was defined as fully charged or 100% SOC. Following this rest period, the cells were discharged to the lower voltage limit of 2.7V with a current of 53 A (1 C), followed by another resting period of 1 hour to let the cells equilibrate again. The discharge energy capacity was defined as the recorded dissipated energy capacity during the discharge.

Following the discharge capacity tests, cells were subsequently charged with a smaller current of 5.3 A (0.1C) up to the cells’ upper potential limit of 4.2 V, and again held there until the charging current dropped below 2 A. This lower current was chosen such that the resulting voltage vs.

capacity profile could be used for dQ/dV analysis during degradation testing as described within section 2.5.2. For a true pOCV analysis, to minimise effects of polarisation, some researchers suggest that the currents employed should ideally be at C/25 or lower [38]. However, the internal resistance of the cells employed within [38] was reported to be 0.18 Ω, compared to 0.0013 Ω for the cells employed within this research. With the constrained access to experimental facilities associated with this work, and accounting for the low internal impedance declared by the cell manufacturer, the current of C/10 was deemed acceptable. The dQ/dV curves can be obtained by differentiating the capacity vs voltage data. The charge capacity was then defined as the total energy sunk into each cell during this charging step. The results of this test are presented and discussed within section 7.2.1.

6.3.2 Pulse-Multisine Characterisation

Within this study, the Pulse Multisine Characterisation (PMC) test is chosen over the HPPC test, that is defined within the IEC 62660 standard [34], to characterise cell internal resistance. Unlike the HPPC test, the PMC test does not require a 30-minute equilibration time between each individual current pulse. As such it requires less testing time whilst providing equally reliable characterisation results [142].

Testing was carried out at five different SOCs, at 95%, 80%, 50%, 20% and 10%. Each cell was charged to 100% SOC using the same CC-CV schedule as the one prior to the discharge capacity test. Following this charging step, the cell was left to equilibrate for 1 hour before being discharged at 26.5 A to 95% SOC. SOC intervals were determined via coulomb counting based on

124 the rated coulombic capacity of the cell; i.e. a 5% DOD discharge from 100% SOC to 95% SOC requires the dissipation of 2.65 Ah. Following the SOC adjustment, the cells were left to equilibrate for another hour before they were subject to five consecutive repetitions of the PMC profile during which voltage and current measurements are acquired. Upon completion of this test, the cells were left for 1 hour to equilibrate and for their temperatures to drop back down to 20 °C, followed by the next SOC adjustment period from 95% to 80% SOC. These steps were repeated for all five SOCs at which the tests were conducted. The composition of the PMC profile used is illustrated within Figure 6-3.

125 FIGURE 6-3–PULSE MULTISINE PROFILE COMPOSITION:(A) BASE SIGNAL WITH PULSES OF 357.6A DISCHARGING AND 67.6A CHARGING CURRENT AMPLITUDE;(B) ZERO-MEAN MULTISINE SIGNAL WITH A PEAK AMPLITUDE OF 43.4 A;(C) COMBINED PULSE MULTISINE SIGNAL USED FOR PMC CHARACTERISATION TESTS

The profile consists of a charge-neutral base signal illustrated in Figure 6-3a, which is superimposed with a zero-mean multisine signal (illustrated in Figure 6-3b), resulting in a charge sustaining combined profile as shown in Figure 6-3c. The base signal comprises a 5 second discharging pulse with a magnitude of 357.6 A, followed by a resting period of 20 seconds. This is followed by a charging pulse with an amplitude of 63.6 A followed by another 20 second resting period. To maintain charge neutrality, the duration of the charging pulse is 28.1 seconds. The multisine signal is a random-phase multisine as defined within [141]. It has a flat spectrum with

126 a uniform distribution of random phases between – 𝜋 and 𝜋 radians. This results in a zero mean profile in which the magnitudes of the profile have a normal distribution. The peak amplitude of the multisine is 43.4 A such that the peak discharging and charging currents of the combined signal do not exceed 400 A and 106 A, respectively. The recorded current and voltage signals are subsequently processed to parameterise an NL-ECM, the structure and parameterisation of which are initially described within section 2. The results of this test are described and discussed within section 7.2.2.

6.3.3 Electrochemical Impedance Spectroscopy

As identified within the literature review, the Nyquits plots resulting from EIS testing can be used to model the response of cells to different frequency inputs and map this response to the SEI, charge transfer phenomena, and diffusion effects. Thus this test gives a deeper insight into the processes occurring within the cell.

Electrochemical Impedance Spectroscopy (EIS) testing was carried out at five different SOCs at 95%, 80%, 50%, 20% and 10% during full characterisation tests and at 95%, 50% and 10% SOC during partial characterisation tests. The charging and SOC adjustment procedures prior to each EIS test followed the same protocol as for the PMC tests. Following SOC adjustment, an extended equilibration period of at least 4 hours was employed as recommended by Barai et al. [145], to allow the dynamics of the cell to stabilise before measurements were made. The EIS tests were carried out in galvanostatic mode using a Solartron EnergyLab XM” system with a 2-Ampere booster. Each cell was excited with a sinusoidal AC current with an RMS value of 1.41 A, and cell impedance was measured in a frequency range from 10 mHz to 10 kHz with 10 frequency points per decade as suggested within [136]. To analyse the impedance measurements, the resulting Nyquist plots were subsequently fitted to an ECM using “Scribner® ZView2”. The process of ECM fitting and results of these tests are detailed within section 7.2.3.