New Energy Vehicles

09 Jul 2019

UK Researchers Propose Cell Cooling Coefficient to Quantify Heat rejection from Li-ion Batteries

09 Jul 2019  by Green Car Congress   

08 July 2019 in an open-access paper in the Journal of the Electrochemical Society, researchers from Imperial College London and the Faraday Institution propose a new metric—the Cell Cooling Coefficient (CCC)—to quantify the rate of heat rejection from Li-ion batteries.

The CCC (units W.K−1) is constant for a given cell and thermal management method and is therefore suited for comparing the thermal performance of different cell designs and form factors, the researchers said. By enhancing knowledge of pack-wide heat rejection, uptake of the CCC will also reduce the risk of thermal runaway, they proposed.

Thermal management is especially important for Li-ion batteries in demanding applications such as in hybrid and electric vehicles in which cells generate a considerable amount of heat in operation. Failure to remove the heat efficiently can increase cell temperature, leading to accelerated degradation.

However, removing the heat creates thermal gradients within cells due to the finite and anisotropic thermal conductivity. The impedance of a cell is a strong function of temperature, and therefore thermal gradients cause different regions to have different impedances which results in current inhomogeneities. The consequence is accelerated and varying rates of degradation, observed between layers within a cell and between cells in a pack. Counterintuitively, the contribution of these thermal gradients to degradation can sometimes be larger than the effect of higher average absolute temperatures.

Considerable improvements in battery lifetime, through the design of better thermal management systems, is essential for innovation in the field. However, the impact of internal thermal gradients is rarely considered in cell design. Cell heat generation and heat rejection pathways are often overlooked, power and energy density are optimised instead. However, a badly designed cell from a thermal management perspective could lead to reduced power, less usable capacity and reduced energy density at pack level. Currently it is impossible without extensive modelling or testing for systems engineers to understand which cells have been designed well for thermal management from information contained on a specification sheet. There is therefore a need for a simple metric which if introduced would allow cell designers and systems engineers to evaluate cells against each other in terms of their ability to reject heat. Including this metric on cell specification sheets would have the potential to revolutionise the whole industry by making optimising cells for thermal management just as important as optimising for power and/or energy.

In this study, a new metric, the Cell Cooling Coefficient (CCC) with units of W.K−1, and a standardised method of measuring it, is introduced to evaluate the thermal pathways of a cell based on its physical design. This singular metric quantifies the rate of heat rejection, through different thermal pathways within the cell geometry, as a result of internal thermal gradients. As it is independent of cell design, form factor, or internal materials, this allows comparison between different cell formats, chemistries and geometries, unachievable with present industry standard measures. A cell with a higher CCC would enable higher continuous powers to be used, with smaller thermal gradients within the cell and therefore higher usable capacity. This translates to a lower average cell temperature during operation which combined with smaller thermal gradients would lead to a longer life. This new metric should enable not only end-users, system and design engineers but also cell designers, manufacturers and developers to compete on designing cells that can be effectively thermally managed, offering significant improvements in performance, lifetime and cost at the system level.

—Hales et al.

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