Abstract
Lithium ion batteries are increasingly important in large scale applications where thermal management is critical for safety and lifetime. Yet, the effect of different thermal boundary conditions on the performance and lifetime is still not fully understood. In this work, a two-dimensional electro-thermal model is developed to simulate cell performance and internal states under complex thermal boundary conditions. Attention was paid to model, not only the electrode stack but also the non-core components (e.g. tab weld points) and thermal boundaries, but also the experiments required to parameterize the thermal model, and the reversible heat generation. The model is comprehensively validated and the performance of tab and surface cooling strategies was evaluated across a wide range of operating conditions. Surface cooling was shown to keep the cell at a lower average temperature, but with a large thermal gradient for high C rates. Tab cooling provided much smaller thermal gradients but higher average temperatures caused by lower heat removing ability. The thermal resistance between the current collectors and tabs was found to be the most significant heat transfer bottleneck and efforts to improve this could have significant positive impacts on the performance of li-ion batteries considering the other advantages of tab cooling.
Highlights
Temperature is one of the key limiting factors for battery pack performance and lifetime, where large temperature deviation from ambient could lead to de-rating, accelerated degradation and in extreme cases, thermal runaway.[2,3,4,5,6,7,8] Thermal management systems (TMS) are employed in majority of vehicles to counter these challenges.[9]
Due to heat transfer limitations, thermal gradients will develop between cells in a battery pack as well as within each cell under aggressive usage,[15,16] which has been shown to have an adverse effect on Li-ion cell performance
We have previously studied the effect of TMS choice on the Li-ion cell performance and degradation, where it was shown that surface cooling can cause a higher rate of useable capacity loss compare to tab cooling.[17]
Summary
Overview.—The model presented was developed in MATLAB R2017a using Simulink (v8.8) with the Simscape toolbox (v4.1). A thermal ECN model containing the anode, cathode, separator and current collectors are used. For this approach to work, it was assumed that the cell material and construction are homogenous, it can be modelled with an arbitrary number of identical unit cells.[39]. The cell components (anode, cathode, current collectors and separator) are modelled individually with respective thermal conductivities and specific heat capacities. It was assumed that the heat is only transferred via the metallic current collector due to its large thermal conductivity in comparison with the electrodes and separator. Total equivalent thermal resistance at the surface and the tab of the unit cell is given by: Rtotal,sur f = Rc∗onvection + Ri∗nsulation + Rinter f ace + Rcasing [6]. The measurements were repeated for a range of temperatures (10◦C, 20◦C,30◦C and 40◦C)
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