MATERIALS TECHNOLOGY EV battery thermal


Alysha Liebscher and Gary Gayman of Morgan Advanced Materials discuss the phenomenon of thermal runaway in electric vehicles and the options available to manufacturers


E


lectric and Hybrid vehicle production is going through high levels of growth as consumers become more aware of global sustainability issues. People are exploring alternative choices in transport and the cars


they buy. Despite this underlying willingness to move


towards electric transport there is still anxiety and consumers are expecting greater charge capacity, faster charging rates and longer range. To this end, automotive battery manufacturers are


pouring huge effort into developing lithium-ion battery packs that can carry cars further and further. The current state-of-the-art is reflected by a Japanese car manufacturer that is likely to produce extended range versions of its vehicles this year with active thermal management. Thermal management is key and while


range is important for use, thermal management is vital to the actual safety of the battery, vehicle and its occupants. This is due to the phenomenon of thermal runaway, a dangerous reaction that can occur in lithium-ion batteries.


WHAT IS THERMAL RUNAWAY? Increasing the range of an electric vehicle can be done in multiple ways. This includes having larger battery packs with more modules and cells, through to putting in higher energy density cells with higher capacity. However, all systems are susceptible to thermal runaway, some more so than others. Each cell in a lithium-ion battery contains


electrolyte, a flammable liquid. If the cell short- circuits, the electrolyte can combust and the pressure within the cell will rapidly increase until the cell vents the flammable electrolyte. Temperatures at the ruptured cell can quickly increase to above 1,000°C. This rapid and extreme rise in temperature is termed thermal runaway and when it initiates the same reaction in adjacent cells, it is known as thermal runaway propagation. When thermal runaway happens, it can produce


smoke, fire and even explosions. Occupants need to have as much time as possible to escape the vehicle


26 /// Testing & Test Houses /// March 2019


if it does occur. Although thermal


runaway is life-threatening, to date there is yet to be global regulation in place. Whereas China has implemented the GB/T 31485 standard, the UN has only proposed legislation. This leaves automotive manufacturers with the choice of whether or not to design their battery packs with systems able to deal with thermal runaway incidents. It’s up to their own risk assessment programmes to determine how likely thermal runaway is to occur. Yet putting in place any protection is likely to


hinder the range capacity of the vehicle – available space is a premium and including more protective materials means there is less room for the cells.


BEYOND THE MIDDLE GROUND Seemingly, there is no middle ground between range and thermal stability. However, it does not need to be the case that battery manufacturers compromise safety for range, or vice versa.


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