Cooling metric offers simple way to compare batteries

Dr Greg Offer of the Electrochemical Science and Engineering Group

Researchers at Imperial College London have developed a new measure for the rate of heat removal from battery packs.

The development of lithium-ion batteries, used to power everything from mobile phones to electric cars, has to date been driven largely by the desire to increase the amount of energy a single cell can store, with cell manufacturers competing on energy density specifications.

Despite their propensity to overheat, far less attention has been paid by industry to the question of keeping lithium-ion batteries cool, with no standard way of quantifying and comparing the thermal performance of different cells.

This is a problem when it comes to designing the kind of large battery packs required for electric vehicles and smart grids, according to a team of researchers from the Electrochemical Science and Engineering Group at Imperial.

In a comment piece published today in Nature, the researchers call on the battery industry to routinely measure and report the thermal performance of cells and propose a new metric to do so, the cell cooling coefficient.

The coefficient, which is straightforward to measure in the lab, will help designers to evaluate trade-offs between thermal management and energy density and optimise the performance of battery packs.

Thermal management

Individual lithium-ion cells are made up of layers of different materials sandwiched together or curled into a ‘jelly roll’. When they operate, the cells generate heat which can be removed either from the surface or through tabs.

Cells are most efficient and stable when each layer is at the same temperature. A temperature gradient between the layers results in batteries that supply less useful energy and degrade faster.

“Calculating the temperature gradient inside individual cells is key to designing thermal management systems for battery packs that contain large numbers of cells”, says Dr Greg Offer, reader in mechanical engineering and principal investigator of the Faraday Institution Multi-Scale Modelling Project.

Unlike existing measures of thermal performance, the cell cooling coefficient can be used to describe the temperature gradient across a cell in operation, allowing a designer to easily compare cells that use different cooling methods.

Competitive edge

The electrification of transport and the urgent need to reduce greenhouse gas emissions from the energy sector are driving a growing demand for lithium-ion batteries and competition in the industry is fierce.

The researchers are calling on manufacturers to publish the cell cooling coefficient of their products alongside other common metrics such as energy capacity and discharge rate and Dr Offer believes that doing so could help cell makers stand out from the crowd:

“Some manufacturers might be reluctant to adopt a metric like ours if their products fare poorly in comparison with the competition, but those that embrace it could gain a competitive advantage”.

'Cool metric for lithium-ion batteries could spur progress' by Greg Offer, Yatish Patel, Alastair Hales, Laura Bravo Diaz and Mohamed Marzook is published in Nature.

The work was supported by the Faraday Institution (faraday.ac.uk; EP/S003053/1), grant number FIRG003.