The rapid development of lightweight, high-capacity battery technology throughout the 21st century has unlocked the potential for portable power. Today, everything from our wireless earbuds to the cars we drive now depend on lithium ion (Li-ion) battery cell technology.
To keep up with this demand for Li-ion cells, manufacturing has had to drastically improve its processes and procedures to maximise productivity. In 2022 the total battery consumption for Electric Vehicles (EV) registered in each country increased by 72% to 517.9 GWh, with market leaders such as CATL almost doubling its capacity to 191GWh .
A single 18650 cell has a capacity of around 10Wh, which means that millions of cells need to be produced each year. As these numbers continue to grow, naturally the risk of failure during service also increases, despite the fact that failure rates are extremely low at an estimated 1 in 10 million cells .
Some failures simply result in a reduction in capacity, whereas others lead to thermal runaway and the catastrophic failure of the battery pack as well as the product it powers.
What is thermal runaway?
Thermal runaway is the nomenclature for an event in which a single cell experiences self-heating caused by an exothermic chemical reaction between the materials within. The exothermic reaction ultimately leads to the electrical and mechanical failure of a cell.
A Li-ion cell is designed to contain a high amount of energy in a small volume and when a major failure occurs this energy is released quickly. In isolation, this is a dramatic event, but if it occurs in the tightly packed environment of a high energy dense EV battery, one cell failure can cause neighbouring cells to fail, often resulting in a fire and the catastrophic destruction of the entire battery system. This event is called thermal propagation.
Li-ion cells are prone to thermal runaway due to the breakdown of the internal components and chemistry which emits gases such as hydrogen and other hydrocarbons which then ignite, contributing to the exothermic reactions . The battery housing and wiring may also ignite, creating more heat and therefore more cells to fail in a thermal propagation event.
Causes of thermal runaway
The destructive nature of a catastrophic battery failure makes tracing the cause relatively difficult. Consequently, battery and cell manufacturers conduct detailed test programmes to fully understand the cause and behaviour of the failure modes of their products. Typically, there are four main mechanisms that can trigger the thermal runaway process:
- Overcharging: during slow and fast charging
- Short circuit events: coolant leakage, mechanical failure
- Excessive cell temperature: cooling system failure, unexpected conditions
- Physical damage: road traffic accidents
‘Normally Battery Management Systems (BMS) cut the current to the battery when it is fully charged, but if that system fails it will keep charging until there's too much energy in the cells,’ explains Oscar Crespo, Product Manager at Bold. ‘These systems are usually reliable, but they can fail occasionally. To gain a better understanding of thermal runaway we need to analyse the battery under these conditions which involves triggering a thermal runaway event. A common technique is using nail penetration tests, but it is very unusual for this to occur during service. We have determined that thermal runaway is more likely to arise due to overheating, so that is what we focus on in the design, development and testing of our batteries.’
Mitigating thermal runaway
Reducing the risk of thermal runaway starts with preventing it from occurring in the first place and then focusing on containing it to avoid spreading to other cells, minimising the overall damage to the pack. ‘At Bold, the number one priority is to design batteries that prevent thermal runaway first and then secondly, but no less importantly, contain it,’ highlights Crespo.
Cooling systems are an important tactic to controlling battery temperatures in EVs when driving and charging, particularly during fast charging. These systems can involve flooding the pack with dielectric coolant or using metallic heat sinks (also known as cold plates) containing liquid are positioned above and below the cells. By keeping the cells within their optimum temperature range, they can maintain efficiency. This not only minimises the risk of hot spots that can lead to instant cell failure, but also reduces cell ageing which can result in higher operating temperatures later in a battery’s life.
If prevention has not been possible, containment of the resulting release of energy from thermal runaway comes down to material choice. ‘There is no flame that replicates the violence of the cell going into thermal runaway,’ says Crespo. ‘The energy release is almost near the level of an explosion but the pressure cannot build so you get deflagration and near molten metal particles ejected with the flames directly into the materials within the battery.
‘We have set ourselves the target of withstanding 2,000 degC [3,632 degF], but there is no material that can do that indefinitely. So we are working with a range of materials that can cope over a period of time, at least for the duration of the event. Thermoset composite materials are currently the most suitable. They offer resistance to high temperatures and are also extremely lightweight which is why Bold utilises these materials throughout its batteries designs.’
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