Glossary
Short definitions of thermal runaway, thermal propagation, cell-to-cell barriers, aerogel, mica, intumescent materials, propagation tests and related terms.
- BESS / ESS
- Battery energy storage system / energy storage system. Stationary installations, from home batteries to grid-scale containers.
- Cell-to-pack (CTP)
- A pack design that places cells directly in the pack without intermediate modules. It raises energy density but removes module walls that used to slow propagation.
- Ceramic fibre paper
- A flexible paper of alumina-silica or similar refractory fibres with a small amount of binder. Used for high-temperature insulation.
- Ejecta
- Hot solid and liquid particles expelled with vent gas. They can erode barriers and create conductive paths.
- Endothermic material
- A material that absorbs heat by decomposing or releasing water when heated, delaying the temperature rise of the protected surface.
- Intumescent material
- A material that swells into an insulating char when heated, increasing its thickness and insulating effect during a fire.
- LFP / NMC
- Lithium iron phosphate and lithium nickel manganese cobalt oxide cathode chemistries. NMC cells usually reach higher peak temperatures in thermal runaway; LFP cells still vent large volumes of flammable gas.
- Lower flammability limit (LFL)
- The lowest concentration of a flammable gas in air that can ignite. It is used to size ventilation and explosion protection for battery rooms and enclosures.
- Mica
- A family of layered silicate minerals (mainly muscovite and phlogopite) made into paper, sheet or rigid board with a binder. Valued for dielectric strength and flame and particle resistance.
- Phase-change material (PCM)
- A material that absorbs heat as it melts. It is used to buffer temperature peaks; on its own it is not a high-temperature fire barrier.
- Propagation test
- A test in which one cell is deliberately forced into thermal runaway and the response of the module, pack or system is observed. Pass criteria vary by standard.
- SEI (solid electrolyte interphase)
- A thin passivating film on the anode surface. Its decomposition is usually the first exothermic step in thermal runaway.
- Silica aerogel
- A highly porous solid of silica nanoparticles. It has very low thermal conductivity. For battery use it is usually reinforced with fibres to form a blanket, felt or paper.
- Thermal barrier (cell-to-cell barrier)
- A layer placed between cells to slow heat transfer from a failing cell to its neighbours. Often also has to absorb cell swelling and provide electrical insulation.
- Thermal interface material (TIM)
- A material designed to conduct heat, for example from cells to a cooling plate. It does the opposite job from a thermal barrier, and the two are often confused.
- Thermal propagation
- The spread of thermal runaway from the first failing cell to neighbouring cells, modules or racks. Barriers, spacing and venting design aim to stop it.
- Thermal runaway
- Self-accelerating failure of a single cell: internal heat generation exceeds heat loss, the temperature rises, and the reactions speed up further until the cell vents and may burn.
- Trigger method
- How the initiating cell is forced into thermal runaway in a test, for example a heater, nail penetration or overcharge.
- Venting
- Release of gas from a cell when internal pressure exceeds the strength of the vent or casing. Vent gas from lithium-ion cells is hot, flammable and toxic.