1. Marco Teórico
1.3. Accidentes Globales de Mercancías Peligrosas
The measured OCV depends on how it is measured, by charging or discharging; it could be different depending on the method. Therefore, two SoC curves can be obtained for charge and discharge. This variation of OCV is known as OCV hysteresis [124, 125]. An example of OCV hysteresis has been shown in Figure 33. The charge-discharge OCV curves were obtained by the incremental OCV method.
Figure 33: Hysteresis between OCV curves (of NMC cell) obtained by charging the cell in steps from 0% SoC to 100% SoC and discharging the cell in step from 100% to 0% SoC.
Cells with lithium iron phosphate electrodes or nickel hydroxide electrodes are known to have stable hysteresis [114, 121, 126]. Hysteresis in a battery corresponds to the existence of several possible thermodynamic equilibrium potentials at the same SoC of the cell. Positive electrodes with lithium iron phosphate as the active material are known to exhibit a hysteretic phenomenon [123, 127]. Srinivasan and Newman [121] provided an explanation for hysteresis based on the existence of a lithium rich and lithium deficient phase within an active particle. They termed the explanation as thepath dependent shrinking core model, whereby during discharge a shrinking particle core of LiyFePO4and a growing outer crust of Li(1-x)FePO4occurs,
while during charge a shrinking core of Li(1-x)FePO4 and a growing crust of
LiyFePO4 occurs (considering mole fractions x and y are close to zero) as shown in
Figure 34. As a consequence of shrinking core, the juxtaposition of the phases at 50 % SoC depending on immediate history can be clearly seen. The corresponding chemical potential of the particle at this point, and therefore open circuit potential, will be different. With a further charge, the composition of the core can be different
even at the same SoC and same immediate history (in this case it is charge). Therefore, depending on this two-phase particle composition there will be OCV hysteresis.
Figure 34: Illustration of path dependence shrinking core model as proposed by Srinivasan and Newman [121]. SoC of LFP electrode was adjusted to 50 % SoC by (a) charging from 0 % SoC and (b) discharging from 100 % SoC. The change of the core or further growth of new shell is shown due to further charge from 50 % SoC in both cases.
More recent work in explaining hysteresis has extended the single particle two phase transition of LiFePO4. Dreyer et al. [127] argued that if the active material particle
has a non-monotonic chemical potential with regards to its lithium mole fraction and in the presence of many such particles in the positive electrode, the chemical potential of the electrode will be different at the same SoC depending on the path taken to reach the particular SoC. In comparison to the path dependent shrinking core model a notable revision is the interconnectedness of many particles with a non-
monotonic chemical potential function. While in the former explanation a particle is assumed to be stable when it has reached its inhomogeneous two phase state (regions of low and high lithium mole fraction within the particle), in the latter the particle reaches a homogeneous stable state by distributing the lithium ions to neighbouring particles and decreasing its chemical potential during charge; similarly an inhomogeneous particle will admit lithium ions from neighbouring particles during discharge. This interchange of ions occurs when the mole fraction of an inhomogeneous particle reaches its maximum or minimum chemical potential (non- monotonic potential function) leading to different overall chemical potential, and therefore open-circuit potential of the electrode depending if it is charging or discharging. Therefore, while battery being charged or discharged, to estimate OCV by BMS, an accurate hysteresis data is crucial.
Although, OCV hysteresis is important for OCV measurement and thus operation of BMS, none of the current standards include any OCV test procedure. As OCV hysteresis is dependent on the accurate measurement of the OCV, a standard OCV test procedure is also essential.
3.5 Summary
Four characterisation tests have been reviewed in this chapter. Capacity test with constant current is commonly used to assess battery capacity. Existing standards suggest to measure capacity with a matrix of temperatures (e.g. -10°C, 0°C, 25°C, 40°C) and different constant current value (e.g. C/10, C/3, 1C, 3C). Battery capacity was historically measured in Amp-hours (Ah), which continued in capacity test procedures for automotive application. However, measure of battery capacity in
Watt-hours could be more appropriate for automotive application where energy is more important and will be further discussed in Section 4.1.
Internal impedance of the battery is an important parameter, as it indicates physical and electrochemical status within the cell and its power capability. The pulse power test directly measures the power capability of the cell. Although three different types of pulse power test present in existing standards, the HPPC and pulse profile used in IEC standard are widely accepted to measure power capability and DCR (real part of impedance). However, DCR fails to provide insight to the battery’s electrochemical and physical status. For this reasons pulse power test will not be primarily investigated as part of this thesis. On the other hand, EIS directly measures impedance of the battery cell at a wide range of frequencies and electrochemical and physical processes within the cell are separated in the frequency domain. Although, EIS has been widely used by electrochemists, it is a fairly new technique (compared to pulse power and capacity test) to characterise li-ion cells for automotive applications. Therefore, further investigation of EIS test for automotive application will be performed later in this thesis.
Though OCV is a common parameter of ECM; existing standards fail to provide an adequate measurement. The OCV test procedures currently being used employ a very slow rate (e.g. C/25) charge-discharge or discharge the cell in steps. Slow rate charge-discharge is not ideal due to inclusion of kinetic contribution. OCV measured by step discharge exclude kinetic contributions; however, are not well developed yet e.g. little research has been done on requirement of the length of rest period. Further research with current issues of step OCV test will be discussed in Section 4.3.