The Surface Cell Cooling Coefficient: A Standard to Define Heat Rejection from Lithium Ion Battery Pouch Cells

Abstract

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.

A review of thermal physics and management inside lithium-ion batteries for high energy density and fast charging

“…Note that a rapid change of the battery temperature by a large external heating or cooling power leads to a large temperature gradient. That indicates the optimal battery temperature depends on a balance between the temperature rise and the temperature uniformity [9,[93][94][95][96][97].…”

Cool metric for lithium-ion batteries could spur progress

“…We have developed a metric called the cell cooling coefficient 3,4 . It can be used to describe the temperature gradient across a cell in operation in watts per kelvin (W K -1 ).…”

“…The first step is for the battery industry to report routinely on thermal management. We have developed a standardized performance metric for this purpose 3,4 . It compares different electrochemical cells and can be measured using equipment that is readily available in battery laboratories.…”

The role of cell geometry when selecting tab or surface cooling to minimise cell degradation

“…These are limited to enabling just qualitative comparison because determination of a cell's effective thermal conductivity or characteristic length requires unjustifiable assumptions. In response to this the Cell Cooling Coefficient (CCC) has been developed to provide a universal thermal performance metric, specific to the lithium-ion battery industry, for tab cooling [23] and for surface cooling [24]. The CCC is a constant for each cell (manufacturer and model number), and thermal management method.…”

“…The capability of the two cells to reject heat through either their surface or tabs may be quantified by evaluating the cells' CCCs. The method for empirically deriving the CCC has been published in previous work by Hales et al [23,24],…”