Search

Cooling electrical tabs of the cell instead of the lithium ion cell surfaces has shown to provide better thermal uniformity within the cell, but its ability to remove heat is limited by the heat transfer bottleneck between tab and electrode stack. A two-dimensional electro-thermal model was validated with custom made cells with different tab sizes and position and used to study how heat transfer for tab cooling could be increased. We show for the first time that the heat transfer bottleneck can be opened up with a single modification, increasing the thickness of the tabs, without affecting the electrode stack. A virtual large-capacity automotive cell (based upon the LG Chem E63 cell) was modelled to demonstrate that optimised tab cooling can be as effective in removing heat as surface cooling, while maintaining the benefit of better thermal, current and state-of-charge homogeneity. These findings will enable cell manufacturers to optimise cell design to allow wider introduction of tab cooling. This would enable the benefits of tab cooling, including higher useable capacity, higher power, and a longer lifetime to be possible in a wider range of applications.

Lithium-ion battery research has historically been driven by power and energy density targets. However, the performance of a lithium-ion cell is strongly influenced by its heat generation and rejection capabilities which have received less attention. The development of adequate thermal metrics able to capture the anisotropic thermal conductivity and uneven internal heat generation rates characteristic of lithium-ion cells is therefore paramount. The Cell Cooling Coefficient (CCC), in W.K−1, has been introduced as a suitable metric to quantify the rate of heat rejection of a given cell and thermal management method. However, there is no standardised methodology defining how to measure the heat generation capabilities of a cell. In this study, we applied the CCC empirical methodology to evaluate the rates of irreversible heat generation at various operation conditions, providing maps which give a complete insight into cell thermal performance. The maps derived show how the most important operational variables (frequency, C-rate, SOC and temperature) influence the cell thermal performance. These maps can be used along with the CCC by pack engineers to optimise the design of thermal management systems and to down select cells according to their thermal performance.

Lithium-ion battery development is conventionally driven by energy and power density targets, yet the performance of a lithium-ion battery pack is often restricted by its heat rejection capabilities. It is therefore common to observe elevated cell temperatures and large internal thermal gradients which, given that impedance is a function of temperature, induce large current inhomogeneities and accelerate cell-level degradation. Battery thermal performance must be better quantified to resolve this limitation, but anisotropic thermal conductivity and uneven internal heat generation rates render conventional heat rejection measures, such as the Biot number, unsuitable. The Cell Cooling Coefficient (CCC) is introduced as a new metric which quantifies the rate of heat rejection. The CCC (units W.K−1) is constant for a given cell and thermal management method and is therefore ideal for comparing the thermal performance of different cell designs and form factors. By enhancing knowledge of pack-wide heat rejection, uptake of the CCC will also reduce the risk of thermal runaway. The CCC is presented as an essential tool to inform the cell down-selection process in the initial design phases, based solely on their thermal bottlenecks. This simple methodology has the potential to revolutionise the lithium-ion battery industry.

There is no universal and quantifiable standard to compare a given cell model’s capability to reject heat. The consequence of this is suboptimal cell designs because cell manufacturers do not have a metric to optimise. The Cell Cooling Coefficient for pouch cell tab cooling (CCC tabs ) defines a cell’s capability to reject heat from its tabs. However, surface cooling remains the thermal management approach of choice for automotive and other high-power applications. This study introduces a surface Cell Cooling Coefficient, CCC surf which is shown to be a fundamental property of a lithium-ion cell. CCC surf is found to be considerably larger than CCC tabs , and this is a trend anticipated for every pouch cell currently commercially available. However, surface cooling induces layer-to-layer nonuniformity which is strongly linked to reduced cell performance and reduced cell lifetime. Thus, the Cell Cooling Coefficient enables quantitative comparison of each cooling method. Further, a method is presented for using the Cell Cooling Coefficients to inform the optimal design of a battery pack thermal management system. In this manner, implementation of the Cell Cooling Coefficient can transform the industry, by minimising the requirement for computationally expensive modelling or time consuming experiments in the early stages of battery-pack design.

Lithium ion batteries (LIBs) are increasingly important in ensuring sustainable mobility and reliable energy supply, storing and managing energy from renewable sources [1]. Temperature is a critical factor in LIBs performance optimisation where large temperature deviations within the cell could lead to accelerated degradation and in extreme cases, thermal runaway. Thermal management has therefore become the focus of intensive research in an attempt to improve battery performance and lifespan [2-5]. Despite the growing research interest in this area, cell heat generation and heat dissipation pathways are not usually considered when designing a cell. This typically leads to cells with thermal bottlenecks prone to internal thermal gradients. With the goal of improving performance and lifetime, a two-dimensional electro-thermal model has been developed to simulate cell performance and internal states under complex thermal boundary conditions [6]. This model can be used to assess different cooling strategies and parameters such us tab position and dimensions can be optimised from the thermal performance perspective for a particular cell chemistry and geometry. In this study, a novel experimental procedure is employed to evaluate cell heat generation and dissipation for various operation conditions. The two-dimensional electro-thermal model was employed to assess the internal temperature distribution during the measurements and to verify the heat dissipation patterns observed during the experiments. As a result, a new metric, the Cell Cooling Coefficient (CCC) is proposed to evaluate the thermal pathways of a cell cooled via its tabs. International Energy Agency. Tracking Clean Energy Progress 2017. 1–82 (2017). doi:10.1787/energy_tech-2014-en Bandhauer, T. M., Garimella, S. & Fuller, T. F. A Critical Review of Thermal Issues in Lithium-Ion Batteries. J. Electrochem. Soc. 158, R1–R25 (2011). Troxler, Y., Wu, B., Marinescu, M., Yufit, V., Patel, Y., Marquis, J.A., Brandon, N.P. & Offer, G. J. The effect of thermal gradients on the performance of lithium-ion batteries. J. Power Sources 247, 1018–1025 (2014). Hunt, I. A., Zhao, Y., Patel, Y. & Offer, G. J. Surface Cooling Causes Accelerated Degradation Compared to Tab Cooling for Lithium-Ion Pouch Cells. J. Electrochem. Soc. 163, A1846–A1852 (2016). Khan, M. R., Swierczynski, M. J. & Kær, S. K. Towards an Ultimate Battery Thermal Management System : A Review. (2017). Batteries 3, 9 (2017) Zhao, Y., Patel, Y., Zhang, T. & Offer, G. J. Modeling the Effects of Thermal Gradients Induced by Tab and Surface Cooling on Lithium Ion Cell Performance. J. Electrochem. Soc. 165, A3169–A3178 (2018).