Environmental concerns lead many researchers to develop alternative fuel sources to overcome problems such as carbon dioxide (CO₂), sulfur dioxide (SO₂), nitrogen oxides (NO_x) and particulate matter emissions. Battery powered electric vehicles (BEV) are one of the strongest alternatives to replace fossil fuel based vehicles for being more environmentally friendly in terms of pollutant and greenhouse gas emissions [1].

Batteries containing olivine cathode materials (e.g. LiFePO₄, aka LFP) are highly promising for electric vehicle (EV) applications due to their properties such as high gravimetric capacity, fast charging, and lower material and maintenance costs.  However, certain properties of LFP batteries are still need to be improved for mass production. High temperature operation is one of the main concerns about BEVs since temperature of batteries increase during both charging and discharging as a result of thermochemical heating, joule heating and ambient temperature. Even though thermally stable materials are used for a battery, there is still a risk of explosion as a result of the localized high temperatures during operation [2].

It can be seen from the literature that most of the modeling studies tend to accept heat generation uniform for all cell components [3,4]. However, joule heating and electrochemical heating affect each layer differently. For instance, heat generation related to electrical heating (joule heating) varies proportional to the internal material resistances while heat generation related to electrochemical reactions (electrochemical heating) primarily affects solid-electrolyte interfaces (SEI) in lithium-ion battery systems [5]. Since instantaneous local temperature rise is the main risk factor for Li-ion battery related explosions, local heat generation behavior should be understood in order to develop a proper battery cooling mechanism for their utilization in battery powered electrical vehicles.

Aim of this study is to create a thermal and electrochemical model for 20Ah commercial LiFePO₄batteries to examine the heat generation in each battery layer as well as to study the time dependent heat distribution in the batteries.

Battery models containing different number of cells are prepared with a finite element simulation software called COMSOL Multiphysics. Each cell having a copper negative current collector, graphite negative electrode, LiPF₆ w/EC:DEC (1:1) electrolyte, LiFePO₄positive electrode and aluminum current collector is modeled exactly the same with the purchased commercial 20Ah batteries. Then, heat generation at certain points and cross-sectional areas where temperature difference is expected to be high are examined. Heat generation behavior of selected points and layers are shown on 1D, 2D and 3D graphics for batteries composed of different number of layers. In order to verify the model results, charge-discharge tests are applied to 20Ah batteries that contain 30 layers and surface temperatures are measured for 6 different (3C, 2C, 1C, 0.7C, 0.5C, 0.2C) current rates. Measurements are compared to results obtained for the model having 30 layers. Each experiment is repeated for different ambient temperatures (-20, -10, 0, 10, 20, 30, 40, 50 °C) in a thermal chamber (POL-EKO Aparatura) to estimate the heat generation characteristics of LFP batteries under different environments.

The project is believed to help improve efficiency of cooling mechanism development studies for Li-ion batteries.     

REFERENCES

1. Notter D. A. et al. Environ. Sci. Technol., 44 (2010), 6550–6556
2. Cho J. et al. Energy Environ. Sci., 4 (2011), 1621
3. Cao B. et al. Journal of Power Sources, 195 (2010), 2393–2398
4. Selman J.R. et al. Journal of Power Sources 83 (1999), 1–8
5. Wang C.Y. et al. Int. J. Energy Res., 34 (2010), 107–115