Cell Temperature Gradient

The maximum temperature differential in a cell is normally specified as ~2°C to minimise the degradation in capacity of the cell. This requirement will drive the cell selection versus application along with the cooling system design.

  • High temperature and the SEI layer on the anode grows faster. If the SEI layer grows fast it tends to be more porous and unstable.
  • At low temperatures we see slower diffusion and intercalation with the possibility of lithium plating. Lithium plating removes lithium from the active cell, reducing cell capacity. Also, lithium plating can subsequently into lithium dendrites that can cause electrical shorts.

In electric vehicles you are likely to see 10°C deltas during extreme discharge events. However, you need to bring these deltas down quite fast. The trouble is that decreasing the coolant temperature to remove the excess heat can increase the delta. Also, these deltas can be reinforced by lower resistance at higher temperature, irreversible heating is proportional to current squared and lower resistance leads to preferential current delivery.

You need to measure the cells you are proposing to use and create detailed models of the cells and cooling system. Also, test the design as early as possible.

Nigel

Normal Cell Operation

Thermal gradients naturally develop in a battery cell based on a number of factors:

  • 3 dimensional form
  • +ve and -ve tab locations
  • heat generation in tab to electrode joints
  • higher heat generation in cathode as it uses an aluminium electrode compared to copper used in anode
  • non-uniform active layers

Anna Tomaszewska et al [3] show simulated (a and c) versus measured (b and d) temperature gradients in a pouch cell during a 5C discharge.

Surface temperature evolution of a pouch cell during 5C constant current discharge obtained by a) simulation and b) measurement at t ¼ 250 s; c) simulation and d) measurement at the end of discharge/t ¼ 667 s
cell resistance vs temperature

A hot area of a cell will have a lower resistance, this means it will provide more current. Heat generation is a function of the current squared and so the hot area will heat up even more, reducing the resistance further and contributing at times to a positive feedback mechanism [2].

Y. Yu et al. [4] instrumented an NMC 21700 production cell with a Distributed Fibre Optic Sensor, this allowed them to measure the temperature inside the core of the cell along the axis and externally on the case of the cell.

1C CC discharge and 1C CC-CV charge over time revolution in 25°C ambient temperature with cell distributed internal and external temperature measured by DFOS; a) current and voltage; b) internal temperature distribution evolution (D1); c) external temperature distribution evolution (D2); d) external temperature distribution evolution (D3); e) illustration of instrumented cell.

The peak temperature difference between the core and surface of the cell, along the cell length can be as high as 9.7°C for a 1C discharge. Conversely as the cell is fully charged, the hottest location within the cell structure moves from the -ve end towards the +ve end, starting 2.6 cm from the negative tab and ending 4.9 cm from the negative tab.

From the extensive Smart Battery Development by WMG they have also observed: Temperature differential between core and surface diverged as battery state of health decreases.

Cooling Systems

In the design of the battery cooling system it is important not to exacerbate the thermal gradient in the cell. This is difficult as removing heat from the cell will mean the cooling system has to be at a lower temperature than the cell. Heating the cell using the cooling system will also impose a temperature gradient on the cell.

heat always moves from hotter objects to colder objects (or “downhill”), unless energy is supplied to reverse the direction of heat flow

Second Law of Thermodynamics
thermal conduction

Thermal Conduction in a Cell

Whatever way we cool a battery cell we will create temperature gradients in the cell.

It is not possible to apply cooling to all of the active area of the electrodes, this would be nice, but would significantly reduce the energy density of the overall battery pack. So we have to apply cooling to the outside surface of the cell.

This means that the design of the cooling system, including coolant flow rates and temperature need to be carefully considered with the electrochemistry of the cell. This also means the sensing and control of the thermal system need to map the electrochemical-thermal properties of the cell.

Mercedes-Benz S-Class S 400 hybrid battery was ground-breaking back in 2009 as it used Lithium-Ion cells rather than Nickel Metal Hydride cells. It also used direct refrigerant cooling with the cooling module surrounding the base of the cells.

Mercedes S400 Hybrid Battery Pack (2009)

At first this appears to be a well engineered design, significantly enhanced by a compact cooling system. However, this system had a number of pack failures due to the significant temperature gradient imposed on the cells by the refrigerant cooling. Hence should be regarded as a poorly executed design and hard to see how the rapid cell failures were not picked up in the engineering phase.

Summary

The knowledge from the fundamental research and the experience of pack design shows us that when cooling and heating battery cells it is easy to impose a temperature gradient. Therefore, consider the following points when designing the cooling system:

  • what is the maximum temperature gradient the cell can withstand? (note that this might be different for a transient event vs continuous operation)
  • what is the best way to apply the heating and cooling to minimise the temperature gradient that will result?
  • how do you avoid sudden changes in coolant temperature?
  • can the coolant system be used in a way to reduce temperature gradients in a cell?

References

  1. Fleckenstein, M., Bohlen, O., Bäker, B., Aging Effect of Temperature Gradients in Li-ion Cells Experimental and Simulative Investigations and the Consequences on Thermal Battery ManagementWorld Electr. Veh. J. 20125, 322-333
  2. Ian A. Hunt, Yan Zhao, Yatish Patel and G. J. Offer, Surface Cooling Causes Accelerated Degradation Compared to Tab Cooling for Lithium-Ion Pouch Cells, Journal of The Electrochemical Society, Volume 163, Number 9
  3. Anna Tomaszewska, Zhengyu Chu, Xuning Feng, Simon O’Kane, Xinhua Liu, Jingyi Chen, Chenzhen Ji, Elizabeth Endler, Ruihe Li, Lishuo Liu, Yalun Li, Siqi Zheng, Sebastian Vetterlein, Ming Gao, Jiuyu Du, Michael Parkes, Minggao Ouyang, Monica Marinescu, Gregory Offer, Billy Wu, Lithium-ion battery fast charging: A review, eTransportation, Volume 1, 2019
  4. Yifei Yu, Timothy Vincent, Jonathan Sansom, David Greenwood, James Marco, Distributed internal thermal monitoring of lithium ion batteries with fibre sensors, Journal of Energy Storage 50 (2022)104291

cooling options

Battery Cooling Options

There are many different options for battery cooling (and heating). These range in capability and complexity from Passive through to Fully Immersed Dielectric.

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