Dependence of the LTO pouch cell losses on the cell geometry 64

The geometry and dimensions of the negative and positive electrodes of the Altairnano 60 Ah pouch cell are shown in Fig. 2.24 (a) and (b), respectively.

Fig. 2.24: Dimensions of the negative (a) and positive (b) electrodes of the Altairnano pouch cell.

The potential distributions on the positive and negative electrodes when the cell operates with a simplified operating cycle, shown in Fig. 2.22, were calculated by using Eqs. (2.5) and (2.6). The calculation results for the potential distribution on the negative and positive electrodes at the end of the last discharge cycle are shown in Fig. 2.25 (a) and (b), respectively. The arrows in Fig. 2.25 show the gradient of the potential distribution in the logarithmic scale.

Fig. 2.25: Potential distribution on the negative (a) and positive (b) electrodes at 5 C-rate discharging in the LTO pouch cell under study.

The calculated current density between the current collectors and the heat losses in the LTO pouch cell unit at the end of the last discharge cycle are shown in Fig. 2.26 (a) and (b), respectively. The LTO pouch cell unit consists of half of the positive current collector, positive electrode material, a separator with an electrolyte, negative electrode material and half of the negative current collector.

Fig. 2.26: Current density between the current collectors (a) and the heat losses (b) in one unit of the LTO pouch cell under study at a 5 C-rate current.

As it can be seen in Figs. 2.25 and 2.26, the non-uniform voltage distribution, which is caused by the geometry of the electrode, leads to a non-uniform current density in the cell. This, in turn, leads to non-uniform heat losses in the cell. In addition, the current, which flows in the electrode, produces non-uniform Joule losses. The highest heat losses were found close to the positive and negative terminals of the cell, and the lowest heat losses were generated on the opposite side of the cell. The average value of heat losses at the time instant under consideration is 0.259 W, and the standard deviation (SD) of the heat losses in the unit on the cell surface, which is parallel to the current collectors, is 0.0482 W.

The non-uniform distribution of heat losses can be decreased by modifying the cell geometry so that the positive and negative terminals are placed on opposite sides of the pouch cell (Sun et al., 2012). This arrangement was verified by calculating the heat generation in a similar fictitious cell, but only the positions of the positive and negative terminals were changed. These terminals were placed in the middle of the opposite sides of the LTO pouch cell. The potential distributions on the negative and positive electrodes in this case are shown in Fig. 2.27 (a) and (b), respectively. The arrows show the gradient of the potential distribution in Fig. 2.27. The calculated current densities between the current collectors and the heat losses in the unit of the LTO pouch cell are shown in Fig.

2.28 (a) and (b), respectively.

Fig. 2.27: Potential distribution on the negative and positive electrodes at 5 C-rate discharging in a fictitious structure, where the terminals were placed in the middle of the opposite sides of the cell electrodes.

An analysis of heat losses in the pouch cell where the positive and negative terminals are placed on opposite sides of the cell showed that the SD of the heat losses on the cell surface parallel to the current collectors decrease by 62.6 % compared with the previous cell geometry and is equal to 0.0180 W. The average value of losses for the fictitious cell geometry, where the terminals are placed on opposite sides of the pouch cell, is 0.267 W.

This shows the advantage of such an LTO pouch cell geometry.

Fig. 2.28: Current densities between the current collectors (a) and the heat losses (b) in the unit of the fictitious LTO pouch cell, where terminals were placed in the middle of the opposite sides of the cell electrodes at a 5 C-rate current.

2.4

In this chapter, a LTO pouch cell model was used to analyse the heat generation in the LTO pouch cell. A modified Thevenin’s equivalent circuit was used to estimate the LTO pouch cell terminal voltage during operation of the cell. In this circuit, a large number of parallel-connected RC circuits were replaced by one circuit, where the direct current resistance Rdrc and the constant phase element CPE were connected in parallel. The proposed equivalent electrical circuit was verified by experiments, and it showed a high agreement between the measured and calculated data.

In addition, the dependences of the losses on the SoC, temperature and LTO pouch cell design were considered. In principle, the heat generated in the cell during its operation is calculated as a sum of irreversible and reversible heat losses. The dependences of the LTO pouch cell ohmic and direct current resistances on SoC, operating current and temperature were analysed by experiments. The analysis showed an increase in the values of the ohmic and direct current resistances, when the SoC was decreased. The ohmic resistance Rohm does not depend on the operating current. However, the direct current resistance Rdrc decreases when the operating current increases in the range from 0 to 180 A, and it almost does not change with a further increase in the operating current. In addition, the cell operating temperature has a significant influence on the values of the ohmic and direct current resistances and may limit the operation of the LTO pouch cell at temperatures below 0 °C.

The influence of cell geometry on the heat losses was analysed by applying the presented 3D model of the pouch cell. The analysis showed that the placement of the positive and negative terminals on the same side of the cell leads to a considerable non-uniformity of heat loss generation in the cell. Further, it was shown that the placement of the positive and negative terminals on the opposite sides of the cell could significantly decreases the non-uniformity of the loss generation in the LTO pouch cell.

3 Determination of the cell thermal parameters

It was shown in Chapter 2 that in the determination of the LTO pouch cell thermal parameters, it is not possible to approach the issue of internal heat caused by the cell operation similarly as it was done in (Fleckenstein et al., 2013), because the heat generation is non-uniform in the LTO pouch cell. Therefore, an external heat source should be considered, and a modified heat-flow meter method can be used to determine the LTO pouch cell specific heat capacity and the along- and through-plane thermal conductivities. The modified methodology allows simultaneous estimation of the parameters under study during the transient process. The method is based on an assumption that the LTO pouch cell can be regarded as an infinite plate with anisotropic thermal properties, because the thickness of a pouch cell is usually much smaller than the flat surface dimensions.

3.1