proposition of a transient method for non-destructive thermal characterization of lithium-ion pouch cells;
analysis of the dependence of the battery specific heat capacity and thermal conductivity on the SoC;
suggestion for an enhanced passive battery cooling system with heat pipes, which decreases the non-uniform temperature distribution on the pouch cell surface;
analysis of the optimal placement of heat pipes in a passive battery cooling system;
proposition of a steady-state method for the thermal characterization and generation of the thermal model of a complex battery system.
The author has published research results related to the topic of the doctoral dissertation in the following publications:
1. Murashko, K., Pyrhönen, J. and Laurila, L. (2013), 3D thermal model of a lithium ion battery for hybrid mobile working machines: determination of the model parameters in a pouch cell, IEEE Transactions on Energy Conversion, vol. 2 (1), pp. 335–343.
In this paper, the author described the detailed methodology for establishing the thermal model of a single pouch cell, where the temperature dependence of the cell parameters was taken into account.
2. Murashko, K., Pyrhönen, J. and Laurila. L. (2013), Optimization of the lithium-ion battery passive thermal control system with the heat pipes embedded into aluminium plate, in Proceedings of the EPE 13 ECCE Europe, Lille, France, pp. 1–10.
In this paper, the author proposed a methodology for the optimization of the passive cooling system of the lithium-ion pouch cells, which decreases the temperature non-uniform distribution on the pouch cell surface.
3. Minav, T. A., Murashko, K., Pyrhönen, J. and Laurila, L. (2013), Forklift with lithium titanate battery during lifting/lowering cycle: analysis of the recuperation capability, Automation in Construction, vol. 35, pp. 275–284.
In this paper, the author was responsible for generating a lithium-ion battery pack model and a model of the electrical drives that were used in the modelling of the forklift operation and the analysis of the recuperation capability. The model of the hydraulics of the forklift, the experimental stand and the forklift operation control principles were provided by T. Minav.
4. Murashko, K., Mityakov, A., Pyrhönen, J., Mityakov, V. Y. and Sapozhnikov, S. S.
(2014), Determination of the thermal parameters of the pouch lithium ion battery, Journal of Power Sources, vol. 271, pp. 48–54.
In this paper, the author proposed a transient method for the non-destructive thermal characterization of the lithium-ion pouch cell and analysed the dependence of the thermal parameters on the SoC and the measurement point.
5. Murashko, K., Huapeng Wu and Pyrhönen, J. (2014), Comparison of the control principles for the battery cooling system in the hybrid bus, in Proceedings of the EPE 14 ECCE Europe, Lappeenranta, Finland, pp. 1–10.
In this paper, the author proposed a thermal protection system for a lithium-ion battery pack and analysed different control principles in order to operate the battery pack at an optimal operating temperature.
6. Minav, T., Murashko, K., Åman, R., Pyrhönen, J. and Pietola, M. (2015), Towards better energy efficiency through systems approach in an industrial forklift, Journal of Automobile Engineering, vol. 229 (3), pp. 273–282.
In this paper, the author was responsible for analysing the efficiency of the lithium-ion battery pack in different operating conditions of the forklift. The efficiency of the other parts of the forklift was analysed by T. Minav and R. Åman.
2 Losses in the lithium-ion pouch cell
This chapter presents a method for measuring the heat losses in the Altairnano LTO/
NMC pouch cell, which is from here onwards referred to as the LTO pouch cell. The dependences of the heat losses on the operating cycle, SoC, the cell geometry and the cell operating temperature are analysed. The focus is on the short charging/discharging cycles, which are dominating in the application of the battery pack in HEVs.
2.1
Structure of the lithium-ion pouch cellThe structure of the Altairnano lithium-ion pouch cell under study is shown in Fig. 2.1.
Fig. 2.1: Structure of the Li-ion pouch cell under study (Murashko et al., 2013).
This structure can be divided into N units. Each unit consists of two current collectors, a negative electrode, a positive electrode and a separator. Both sides of the current collectors are used, which decreases the material demand and reduces the cell internal losses.
The positive electrode of the LTO pouch cell is made of lithium nickel manganese cobalt oxide (NMC), and the negative electrode is made of lithium titanium oxide (Li4/3Ti5/3O4) called titanate. The separator is usually made of a suitable polymer fabric, and an organic carbonate solution electrolyte (LiFP6) is used as the electrolyte in this cell. The current collectors for the positive and negative electrodes are made of aluminium in the LTO pouch cells. In other lithium-ion cells, graphite is usually used as the material for the negative electrode. However, it has a small ionization potential (Beguin and Frackowiak, 2010), and it is not possible to use an aluminium current collector because of the intercalation of lithium ions in aluminium instead of graphite. Intercalation in copper does not take place in these conditions, and therefore, copper is used as the current collector for the graphite negative electrode. The lithium titanium oxide has higher ionization potential than aluminium, which provides an opportunity to use aluminium as the material of the negative current collector in the LTO pouch cells. Further, unlike batteries with a graphite electrode, LTO batteries are considered free from solid electrolyte interface
(SEI) film (He et al., 2013), which plays a significant role in the battery performance and affects the battery operating characteristics such as the maximum charging speed and the lifetime of the battery. Thus, LTO batteries can be charged with the same maximum C-rate current that is used for discharging. This property is not too common in different battery types, and this was one of the most important reasons for selecting the LTO pouch cells as a potential battery for heavy machinery.
The LTO pouch cell used in the tests and for the verification of the proposed methodology has the following dimensions: 256 mm 259 mm 12.7 mm (width height thickness), which was given in the data sheet. The thicknesses and numbers of constituent elements in the cell were assumed the same as in a similar Altairnano 50 Ah LTO pouch cell. The required parameters were measured by disassembling of the Altairnano 50 Ah LTO pouch cell. The thickness of the pouch was taken the same as in (Ye et al., 2012, Taheri et al., 2014). The values are given in Table 2.1.
Table 2.1: Thicknesses and numbers of constituent elements in the LTO pouch cell under study, which were measured by disassembling of the Altairnano 50 Ah LTO pouch cell.
Parts of the cell positive electrode
negative electrode
separator current collector
pouch
Thickness, μm 120 100 20 25 100
Number of parts in the cell
40 39 80 79 2