How batteries work Chapter 17 Safety

Chapter 17

Safety: thermal runaway, protection strategies and the operating window

Can a battery catch fire, and how do we keep it safe?

Where we are. In Lab 1 we learned to measure a cell and to find out what has gone wrong inside it. Two questions remain, and they matter most to the person holding the cell. What happens when it is abused, so that its stored energy comes out all at once? And what becomes of its materials when it is finished?

Figure 17.1 A loop of three steps, heat released, temperature rises, reactions speed up, with a cooling arrow leaving it; beside it a schematic temperature trace that either levels off or runs away depending on how strong the cooling is.
The runaway loop: heat that makes more heat. Try this: slide the cooling down slowly and find where the trace stops levelling off.
SourceThermal runaway as a self-sustaining, autocatalytic reaction of an electrode with the electrolyte, and heat released at the electrode surfaces, after Winter and Brodd 2004, sections 1.2 and 1.3. The temperature trace is a drawing device with two outcomes, computed from a toy rule, not a model of any cell; it carries no numbers.

Let us begin with the energy itself. A charged cell holds its energy in two materials kept apart. If they meet through a short circuit, the whole stored energy comes out as heat.1 The worst case is thermal runaway: the reaction of an electrode with the electrolyte becomes self-sustaining and enters an autocatalytic mode, the heat it releases speeding it up. Because this heat is released inside the cell, at the electrode surfaces, cells meant for high currents must be built to shed it.1 The SEI we met in chapter 10 plays a part here too: its decomposition promotes thermal runaway.2

The history of the rechargeable lithium cell is written in fires. Exxon’s rechargeable lithium-metal cell of the 1970s grew dendrites across the electrolyte to the positive electrode; the internal short ignited the flammable electrolyte and the programme was abandoned.3 In 1989 the first lithium-metal rechargeable cell sold for consumer use, Moli Energy’s Li/MoS2, was recalled after cells vented with flame in the field: the porous, high-surface lithium deposited by cycling reacts with the electrolyte strongly enough to do that, and cycling makes cells less tolerant of abuse.4

A cell is safe as long as its heat can leave faster than its reactions make it.

Figure 17.2 Two views on a temperature axis from 80 to 420 degrees. First, bars for the temperature ranges of eight exothermic reactions in a lithium-ion cell, from the decomposition of the SEI at 100 to 130 degrees to the reaction of lithiated graphite with the binder at 220 to 400 degrees, with a dashed line at 130 degrees for a standard heating test. Second, for four positive electrode materials, the ranges where each releases heat and oxygen, lowest for lithium cobalt oxide and highest for lithium iron phosphate.
negative electrodepositive electrodeelectrolyteoxygen released
The order in which a hot cell starts to burn. Try this: find the reaction that releases the most heat per gram, and the material that holds its oxygen longest.
SourceTemperature ranges and enthalpies of the exothermic processes from Faruque et al. 2026, Table 23, which compiles separate studies; the positive electrodes from their Table 4 and the order of onset from their section 9.1.3; the 130 °C heating test of GB/T 31485 from their section 9.1.6.

The chain of heat

Which reactions feed the loop of figure 17.1, and in what order? A 2026 review gathers measured temperature ranges and heats of reaction for the main exothermic processes in a lithium-ion cell. The SEI decomposes first, from about 100 to 130 °C, releasing 186 to 257 J/g. Lithiated graphite reacts with the electrolyte between about 110 and 290 °C, with 1460 to 1714 J/g, the largest heat in the table, and with the PVDF binder between 220 and 400 °C. On the positive side, charged LixCoO2 reacts with the electrolyte from about 167 °C and decomposes from about 178 °C, and the electrolyte itself decomposes from about 225 °C.5 The ranges come from different studies, so read them as a ladder, not as the history of one cell.

The positive electrode matters as much. In the review’s comparison, LiCoO2 releases heat from about 180 to 220 °C and oxygen from about 200 to 250 °C; layered nickel-manganese-cobalt oxides release oxygen at a high state of charge from about 180 to 220 °C; LiMn2O4 decomposes at about 220 to 250 °C; and LiFePO4 releases heat only at about 270 to 320 °C and holds its oxygen to about 400 °C, thanks to its strong P–O bonds. The temperature at which heat release starts rises in the order LiCoO2 < NCA < NCM < LiFePO4.5 This is the thermal side of the trade-offs in chapter 13.

Cooling is the other side of the loop. Battery packs shed heat with air blown by fans, with liquid cooling by water or glycol, with phase-change materials or with heat pipes; liquids are preferred where much heat must go, because they conduct and absorb heat better than air.5

Figure 17.3 Six layers of protection from the outside in: electronics that watch every cell, a pressure switch in the cap, a shutdown separator, a ceramic-coated separator, additives in the electrolyte and the chemistry itself. Choosing a kind of abuse marks which layers act, which help less and which cannot help.
Layers of protection, and which one answers which kind of abuse. Try this: choose Short inside and see why prevention matters more than any switch.
SourceElectronic control of each cell’s voltage, current limiting, the pressure-operated current interrupter, the shutdown separator and its limits, and the copper dissolution on overdischarge after Brandt 1994; electronic protection of LiCoO2 cells after Winter and Brodd 2004, section 2.6; the Al2O3-coated separator after Goodenough and Park 2013; overcharge additives after Abe 2014, pp. 186 to 190. Which layers answer which abuse is the page’s reading of those statements.

Electronics and separators

If a cell is safe only while its heat leaves faster than its reactions make it, the first task is to stop the faults that start those reactions. Let us go through the safeguards from the outside in. Lithium cells have no built-in chemical way of shedding overcharge, unlike nickel-cadmium, and in a battery of cells in series small differences in capacity and self-discharge push individual cells out of balance. So the end of charge and of discharge is controlled by watching the voltage, cell by cell; Sony’s first lithium-ion batteries carried a circuit that did exactly that and also limited the current.1,4

Inside the cell, the next safeguard is the separator. A separator that loses its porosity when it gets hot stops the ions in a fault that heats the cell evenly, such as a short outside it, but helps much less against a local internal short.4 A ceramic Al2O3 coating on the separator helps keep dendrites from getting through.6

For this chemistry protection is not optional. A textbook list of the drawbacks of lithium-ion cells reads like a list of reasons for these layers: a protective circuit for every cell, a cover against short circuits and, for high-power uses, an advanced battery management system that raises the cost.7

The electrolyte as a safeguard

The electrolyte can be a safeguard too. To see how, consider what overcharge does to the cell. An overcharged cell loses too much lithium from its positive electrode, whose crystal starts to release oxygen and becomes unstable, while lithium metal deposits on the negative electrode; the electrodes then decompose the solvents, and a sudden exothermic reaction follows. Additives in small quantities answer this in three main ways: redox shuttles consume the excess current through a redox process and keep the voltage from rising; aromatic compounds such as biphenyl are believed to polymerize above the maximum operating voltage into an insulating layer on the positive electrode, raising the cell’s resistance; and compounds such as cyclohexylbenzene decompose and give off hydrogen, so that a pressure-operated current interrupter cuts the charging current. None is free: additives always have side effects, and with biphenyl, cycle life at 40 °C up to 4.3 V got worse the more of it there was.8

The electrolyte itself could also be made safer. A nonflammable liquid is a further aim: phosphate esters such as triethyl phosphate have very high flash points, but trimethyl phosphate is unstable against graphite, which led some to judge such electrolytes impractical.8

Safety built into the chemistry

The innermost safeguard is the chemistry itself. The lithium-metal polymer cells that Bolloré has used in electric cars and buses since 2011 are an example: solid polymer electrolytes, more than 3000 cycles without dendrites.9

Figure 17.4 Four families of abuse tests as tiles, electrochemical, mechanical, thermal and the surroundings, each opening its list of tests; beside them the temperature programme of a standard heating test, a ramp at 5 degrees per minute to 130 degrees held for at least 30 minutes; below, the standards that set such tests.
How cells are tested before they are sold. Try this: open each family, then find which test the 130 °C line of figure 17.2 belongs to.
SourceThe four families of tests, the standards and the two GB/T 31485 protocols after Faruque et al. 2026, sections 9.1.4 to 9.1.7 and Tables 25 and 26; the battery management system and insulation-resistance testing after their sections 9.1 and 9.1.4.

How cells are tested

Before a cell is sold, standards check what it does under abuse. A 2026 review groups the tests in four families: electrochemical (overcharge and overdischarge, discharge at high and low temperature, external short circuit, forced discharge), mechanical (drop, heavy impact, nail penetration, shaking and squeezing, acceleration, crush), thermal (heating, thermal shock, spark) and the surroundings (low pressure at high altitude, soaking). Standards that set such tests include the United Nations transport tests UN 38.3, IEC 62133, UL 2580, the Chinese GB/T 31485 for the traction batteries of electric vehicles, SAE J2464 and ISO 16750-2.5

Two GB/T 31485 tests show what such a protocol looks like. In the heating test the cell is heated to 130 ± 2 °C at 5 °C per minute and held there for at least 30 minutes; it passes if it neither burns nor leaks. In the external short-circuit test a fully charged cell, kept at 25 ± 2 °C for 30 minutes, is shorted through 5 mΩ while its voltage and temperature are recorded; it passes if it neither bursts nor catches fire.5 Set 130 °C against figure 17.2: it is the top of the range in which the SEI decomposes.

A battery adds its own safeguards to those of the cell. The battery management system measures voltage, current and temperature with its sensors, keeps the cells within their safe limits, regulates the charge, balances the cells, manages the heat and reports the state of the battery. It estimates the state of charge by counting coulombs, from the open-circuit voltage or from the impedance, and increasingly with model-based filters and machine learning.5 For the high-voltage batteries of electric vehicles, insulation-resistance testing detects insulation degraded by shock and vibration, moisture or heat before it can cause an electric shock or thermal runaway; IEC 62660, ISO 6469, SAE J2464 and GB/T 18384.1 cover it.5

Figure 17.5 A temperature axis from minus 60 to 100 degrees with three bands: consumer cells in use from 0 to 40 degrees, consumer cells in storage from minus 20 to 85, military and automotive cells from minus 50 to 85; notes that cold fast charging risks lithium plating and warmth speeds self-discharge.
The comfort zone. Try this: find the narrowest band, and think where your phone spends its summer.
SourceRanges after Winter and Brodd 2004, section 2.6 (a 2004 snapshot); self-discharge rising with temperature, theirs too; lithium plating favoured by high current density and low temperature after Fichtner et al. 2022; faster SEI growth at high state of charge and high charging rate after von Kolzenberg, Latz and Horstmann 2020.

So far we have looked at abuse. Ordinary use matters too: temperature and state of charge decide how a cell ages, and we take temperature first. Consumer cells are designed to work between about 0 and 40 °C and to be stored between −20 and 85 °C; military and automotive cells must work from −50 to 85 °C.1 Within those limits, three trends matter. Self-discharge rises with temperature.1 The SEI grows fastest when the cell sits at a high state of charge and is charged fast.10 And at high current and low temperature, lithium can plate on the graphite instead of entering it.11

What that means in your pocket. Read together, those statements favour keeping a cell cool, not leaving it full for long periods, and not fast-charging it in the cold. This is our own reading of the three statements above, not a rule stated in any of the works cited.

A 2026 review gives a narrower band in which performance and ageing stay balanced: 15 to 35 °C.5

References for this chapter

Numbers in the text refer to this list. All references of the course, with the chapters that use each one.

  1. 1 M. Winter, R. J. Brodd, "What Are Batteries, Fuel Cells, and Supercapacitors?", Chemical Reviews 104, 2004, 4245-4269. doi:10.1021/cr020730k
  2. 2 B. Horstmann, F. Single, A. Latz, "Review on multi-scale models of solid-electrolyte interphase formation", Current Opinion in Electrochemistry 13, 2019, 61-69. doi:10.1016/j.coelec.2018.10.013
  3. 3 J. B. Goodenough, "How we made the Li-ion rechargeable battery", Nature Electronics 1, 2018, 204. doi:10.1038/s41928-018-0048-6
  4. 4 K. Brandt, "Historical development of secondary lithium batteries", Solid State Ionics 69, 1994, 173-183. doi:10.1016/0167-2738(94)90408-1
  5. 5 O. Faruque, M. J. Isalm, O. Faruq, M. M. Alam, "A comprehensive review of past and present developments of Li-ion batteries", Discover Sustainability 7, 2026, 881. doi:10.1007/s43621-026-02920-8
  6. 6 J. B. Goodenough, K.-S. Park, "The Li-Ion Rechargeable Battery: A Perspective", Journal of the American Chemical Society 135, 2013, 1167-1176. doi:10.1021/ja3091438
  7. 7 S. A. Arote, "Fundamentals and perspectives of lithium-ion batteries", chapter 1 of Lithium-ion and Lithium–Sulfur Batteries: Fundamentals to Performance, IOP Publishing, 2022, pp. 1-1 to 1-26. doi:10.1088/978-0-7503-4881-2ch1
  8. 8 K. Abe, "Nonaqueous Electrolytes and Advances in Additives", in T. R. Jow, K. Xu, O. Borodin, M. Ue (eds.), Electrolytes for Lithium and Lithium-Ion Batteries, Modern Aspects of Electrochemistry 58, Springer, New York, 2014, 167-207. doi:10.1007/978-1-4939-0302-3_3
  9. 9 H. Zhang, C. Li, G. G. Eshetu, S. Laruelle, S. Grugeon, K. Zaghib, C. Julien, A. Mauger, D. Guyomard, T. Rojo, N. Gisbert-Trejo, S. Passerini, X. Huang, Z. Zhou, P. Johansson, M. Forsyth, "From Solid-Solution Electrodes and the Rocking-Chair Concept to Today's Batteries", Angewandte Chemie International Edition 59, 2020, 534-538. doi:10.1002/anie.201913923
  10. 10 L. von Kolzenberg, A. Latz, B. Horstmann, "Solid-Electrolyte Interphase During Battery Cycling: Theory of Growth Regimes", ChemSusChem 13, 2020, 3901-3910. doi:10.1002/cssc.202000867
  11. 11 M. Fichtner, K. Edström, E. Ayerbe, M. Berecibar, A. Bhowmik, I. E. Castelli, S. Clark, R. Dominko, M. Erakca, A. A. Franco, A. Grimaud, B. Horstmann, A. Latz, H. Lorrmann, M. Meeus, R. Narayan, F. Pammer, J. Ruhland, H. Stein, T. Vegge, M. Weil, "Rechargeable Batteries of the Future—The State of the Art from a BATTERY 2030+ Perspective", Advanced Energy Materials 12, 2022, 2102904. doi:10.1002/aenm.202102904

Cite this page

Using this page in teaching, a thesis or an article? Cite it as follows; the date is the last update of the course.

Shyma Sajeevan, A. (2026). Safety: thermal runaway, protection strategies and the operating window. How batteries work. https://asajeevan.github.io/batteries/safety/

↑