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Author : Carlo Saling & Alexander Kemmling, REMBE

(Click here to view article in digital edition)

Alongside the many performance advantages of lithium-ion batteries, safety-related investigations show that battery cells can pose a risk if temperature limits are exceeded. Above corresponding temperatures, strong exothermic chemical reactions can be initiated in the material at a cell level, which can spread to the entire module or even to the entire battery system. This can result in temperatures of up to 700°C within a short time. These chemical chain reactions at a cell level cannot be interrupted, which is why they also are referred to as thermal runaway. As the thermal decomposition of the battery cell produces ignitable gas mixtures, there is a significant risk of fire and explosion.1

The temperature above which there is a risk of thermal runaway depends significantly on, for instance, the cell type. According to the literature, lithium-ion cells are not usually designed for operating and storage temperatures above 60°C.2 In addition to temperature, there can also be other causes of thermal runaway: for example, internal or external short circuits (e.g. due to quality defects, deformation, external fire load, damaged cells) or to high currents during charging or discharging.

To reduce these risks, high demands are placed on the safety and reliability of battery cells. Manufacturers have to successfully pass through many tests in the run-up to a market launch, including tests that go far beyond normal use (e.g. nail penetration test). Protective measures are also implemented in battery systems in order to avoid a thermal runaway scenario during operation or to identify it at an early stage. The measures include:

- Safety systems inside the cell, such as safety valves or explosion vents

- Battery management systems that monitor, among other things, electricity, voltage and temperatures

- Venting of the battery module housing

- Circuit breaker/galvanic isolation

Figure 4 (above) – Representative gas composition for thermal runaway.

Thanks to their high storage density, lithium-ion batteries are increasingly being used as BESS in order to secure the energy supply or balance out fluctuations from renewable energy sources in the power supply. These battery systems – also referred to as battery storage power stations – play an important role in the global expansion of the renewable energy supply. The battery modules in the power stations are mostly installed in large numbers in man-high racks inside 20- or 40-foot containers.

Despite the high requirements on safety and reliability of battery cells, which the manufacturers ensure with intensive tests and by implementing extensive protective measures, accidents have taken place in various power storage systems in recent years.

Figure 5 (above) – Explosion parameters of hydrogen and carbon monoxide 8,9

Detailed assessment of the potential risk and possible safety concepts

The residual risk of the thermal runaway scenario – e.g. due to damaged cells – brings with it a high risk of fire and explosion. The chemical chain reactions that lead to these high temperatures can ignite the electrodes (lithium compounds, graphite) and lead to dangerous metal fires. The electrolyte liquid between the electrodes consists of organic solvents, which vaporise at temperatures above 80°C.3,4 This volume expansion by a factor of 1000, which occurs with a phase transition from a liquid to a gaseous aggregate condition, leads to high pressures within the cells. In order to prevent the battery cells from bursting during thermal runaway, explosion vents or, where applicable, safety valves are present, which “release” the gas into the environment and protect the cells from bursting.

For example, in a scenario where a battery module with 24 conventional 18650 battery cells (3.7 volt, 3000 mAh) is experiencing thermal runaway, the fire load is transferred to neighbouring cells within seconds. A chain reaction occurs, which is accompanied by cyclical explosive flames from the cells. The explosive flames occur due to the discharge of evaporating, combustible electrolytes. There is also a risk that glowing metal parts and other burning parts of the battery will be expelled.5

Particularly batteries with high power densities, such as those used in vehicles and battery energy storage systems, can release several thousand litres of gas into the environment within seconds – depending, among other things, on the cell type, storage capacity and state of charge.6 These gas mixtures contain flammable components such as hydrogen, hydrocarbon and carbon monoxide,7 as well as toxic flue gases (see Figure 4).

Figure 6 (left) – Maximum explosion pressure for combustion in a 20-litre vessel as part of thermal runaway for an 8.7-Wh cell with a state of charge SOC of 100% 10

Hydrogen and carbon monoxide are gases that can form explosive mixtures with oxygen from the air over wide concentration ranges (see Figure 5). Both gases have low minimum ignition energies, meaning that even small electrostatic charges or hot surfaces are sufficient to ignite this mixture. There is a risk of gas explosion when unburned gas accumulates in the environment.

The use of GSME gas detectors is a proven method of identifying thermal runaway. The simultaneous monitoring of numerous pyrolysis gas concentrations, such as carbon monoxide, hydrogen and hydrocarbons, in the parts per million range offers a detailed insight into the process status. If previously set limit values for the pyrolysis gas concentrations are exceeded, the GSME gas detector emits an electrical alarm signal, which can be used as a warning and to initiate further measures.

A major challenge in practice is that thermal runaway cannot be stopped or extinguished using classic fire protection measures. It must, therefore, be expected that the explosive gases can ignite outside of the battery cell/housing and lead to an explosion.

If hydrogen-air mixtures ignite in closed systems, explosion pressures of up to 8 bar can occur (see Figure 6). These pressures exceed the strengths of the containers and battery rooms in which the battery energy storage systems are located. In particular, doors have low pressure shock resistance and can represent dangerous weak points.

To prevent bursting or flying debris with explosive flames, explosion vents have proven of use as predetermined breaking points, which vent the explosion pressure into the environment in a controlled manner. Extensive explosion vents that are ready for series production have been developed that also open safely and completely at low burst pressures. This solution offers the advantage that the strength of the housing and doors can be much lower, which brings with it huge cost savings. Explosion vents are recommended as protective measures in many standards and are explicitly required by law in certain regions. The combination of both products forms the basis for an optimally designed protective system, consisting of preventative and constructive fire protection and explosion safety.

1 Lithium – Batterien – Brandgefahren und Sicherheitsrisiken, Risk Experts, Dr. Buser

2 Brandschutz-Forschung, IMK Bericht 175, KIT, Herr Kunkelmann – 2017

3 DGUV – Hinweise zum betrieblichen Brandschutz bei der Lagerung und Verwendung von Lithium-Ionen-Akkus – 2020

4 Brandschutz-Forschung, KIT, Forschungsbericht 175, Jürgen Kunkelmann – 2016

5 DGUV – Hinweise zur Brandbekämpfung von Lithium-Ionen- Akkus bei Fahrzeugbränden – 2020

6 Exponent Inc. – Thermal Runaway and Safety of Large Lithium-Ion Battery Systems – 2015

7 ISHPMIE - Explosibility Properties of Gases from Lithium-Ion Energy Storage Battery Thermal Runaways, Adam Barowy – Braunschweig 2020

8 BG RCI Magazin – Ex-Zonen für Wasserstoff-Elektrolyseanlagen – 2014

10 Exponent Inc. – Thermal Runaway and Safety of Large Lithium-Ion Battery Systems – 2015

Carlo Saling , Sales Executive Explosion Safety, Key Accounts D-A-CH, joined REMBE in 2017 and has 15 years of explosion safety experience. His primary tasks are to work out safety concepts of equipment and to carry out safety scans onsite to support operators.

Alexander Kemmling , Sales Executive Explosion Prevention, Key Accounts D-A-CH, joined REMBE in 2014. In the beginning, Alexander focused on industrial measurements and automatic sampling systems for bulk materials at REMBE Kersting GmbH. He later switched to the REMBE Explosion Safety team where he is responsible for the sales of fire and explosion prevention equipment.

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