Batteries: Heat and temperature
By Apoorv Shaligram
These days when we speak of batteries powering vehicles, the question of how thermally stable they are often comes up. Temperature and heating have a big impact on battery life and safety. Thus, it stands to reason that thermal behavior and stability is a very important parameter when it comes to selection of battery cells and design of battery packs.
To start with, heat is the cause of death for batteries. Just as for us humans, there may be multiple reasons for death, but it is eventually either the heart or the brain stopping its functions that becomes the cause of death, there are many ‘natural’ or ‘external’ reasons for death of batteries, but they all boil down to heat being the cause of ‘natural death’ for batteries or also the cause behind ‘sudden death’ (battery fires and explosions). To understand how to make long lasting batteries, it is important to understand how heat causes eventual battery failure and hence, it is important to understand how heat and temperature affects batteries.
Mechanism for gradual failure of Li-ion batteries:
There are two (interlinked) reasons behind gradual failure of Li-ion batteries. The first is that the electrolyte in the battery keeps getting decomposed ever so slightly in each charging cycle of the battery forming a film on the anode surface. The second is that the film formation also consumes some of the active Lithium in the cell, thus reducing the amount available for energy storage. This film formation results in the battery’s internal resistance to keep increasing over time, eventually causing enough of a barrier to the electrochemical reaction at the battery anode that the Li-ions start plating the surface rather than taking part in the desired reaction and getting stored in the anode. When Lithium gets plated o the anode surface, it leads to the electrolyte getting exposed to Lithium metal which it is not inert to, and hence a large amount of electrolyte degradation occurs. This process repeats in every further cycle and it is all downhill from there.
The role of heat:
So why does heat affect the process of electrolyte decomposition? Like any other chemical reaction, the decomposition of electrolyte requires energy and has a certain rate of reaction kinetics associated with it. Usually, this energy comes from the charger, where a small fraction of the input current goes into decomposing the electrolyte (Yes, we kill the battery by charging it over and over again). The more the energy available, the higher will be the extent of electrolyte decomposition. Thus, most degradation happens when the battery is at the upper end of its voltage window. Higher temperature causes the molecules to be at a higher energy state, and hence further accelerates the kinetics of this reaction. Thus at higher temperature, the same cell will degrade faster even if charged the same number of times at the same current. Heat generated by the cell in charging and discharging directly affects the cell temperature and impacts electrolyte decomposition. On the other hand, lower temperature can also play a role. When it is too low, temperature can slow down the desired reaction of energy storage and hence cause Lithium plating while charging. This can result in not just a large amount of electrolyte decomposition due to the sudden exposure to lithium metal, but also dendritic growth that can internally short circuit the battery*. Thus from a thermal point-of-view, the most important parameters in selection of cells or battery packs is the operational conditions it is rated for.
How to prevent death (or at least, how to stave off the inevitable)?
On the operations side, it is important to keep temperatures in check, especially in the charging cycles. Modulating charging current is an important part, which the BMS (battery management system) can help with. On the battery pack front, it is important to have a good thermal design. The battery pack should be designed for the expected heat output at the prescribed charging rates.
- Heat dissipation and cooling mechanisms remain in focus. All this becomes even more critical for fast charging where the rate of heating increases exponentially with increase in charging current.
- If heat cannot be dissipated, managing the temperature through thermal mass becomes another method (Use of PCM or phase change materials to stop temperatures from rising beyond a point).
- Cell selection becomes the other big decision. Cells need to be selected based on how much heat they generate, how efficiently they can be cooled and most importantly, how resilient they are to capacity degradation due to heating.
If we track the evolution of Tesla’s battery pack technology, we will see some pointers along these lines. They first went with a very large number of small format 18650 cells that allowed efficient cooling with an active liquid cooling mechanism, then started working their way towards larger formats (21700 and now the move towards 4680) that are more resilient to heating, but also towards reducing heating itself.
While cooling and thermal management remains the focus as of today, the future will be about cells that do not heat. Going for smaller cells may help solve the cooling problem, but it adds a whole lot to the cost of battery pack operations! If we are to make electric vehicles mainstream, we need affordable battery packs that can give great ‘refueling’ experience while lasting the test of time. The answer lies in efficiently manufacturing large format cells that do not heat. It may need a more fundamental change than just tweaks in the size of cells or its chemistry…
*I had once written a nice essay on Lithium plating and dendritic growth in cells a long time back. I will find those old notebooks some day and post that blog online as well.
