Understanding Thermal Runaway in Lithium-Ion Batteries

Lithium-ion battery cutaway illustrating thermal runaway progression from internal damage and heat buildup to fire, gas release, and cell-to-cell propagation.

Lithium-ion batteries power electric vehicles, battery energy storage systems, portable equipment, and an expanding range of applications. When a lithium-ion battery fails in a specific way, the result can be a self-sustaining thermal and chemical event — thermal runaway — that presents hazards distinct from conventional fires and conventional electrical failures.

This article explains what thermal runaway is, how it begins and propagates, what hazards it creates, why it may persist or recur, and why understanding the underlying process matters for first responders, electrical professionals, facility personnel, and others who may encounter lithium-ion battery systems.

This is an educational and hazard-awareness resource. It does not provide universal firefighting procedures, battery service instructions, or suppression tactics. Tactical decisions at specific incidents depend on battery chemistry, system design, manufacturer guidance, department SOPs, and incident conditions. For operational response guidance, see EV Battery Fires and Thermal Runaway, BESS Safety for First Responders, and EV Battery Emergency Response.

What Thermal Runaway Is

Thermal runaway is a self-sustaining chain reaction within a battery cell in which heat generation exceeds the cell's ability to dissipate that heat. As the cell temperature rises, internal chemical reactions accelerate and generate additional heat. That additional heat drives further reactions — which generate more heat still. The cycle continues, accelerating, until the cell can no longer contain it.

The result can include:

  • Intense, sustained heat release
  • Flammable and toxic gas generation
  • Venting of cell contents
  • Pressure buildup and rupture
  • Fire
  • Propagation to adjacent cells and modules

Thermal runaway is not primarily an electrical event in the conventional sense. It is a thermally-driven chemical process that can produce electrical hazards, fire hazards, and toxic hazard simultaneously — and that can continue even if the battery is electrically disconnected. This distinction matters operationally: electrical isolation does not stop an internal thermal runaway process that is already underway.

Basic Cell Construction: What You Need to Know

Understanding why thermal runaway occurs begins with understanding what is inside a lithium-ion cell.

A lithium-ion cell contains:

  • Anode — typically graphite; stores lithium ions during charging
  • Cathode — typically a lithium metal oxide compound; the positive electrode; chemistry varies significantly by battery type
  • Electrolyte — a flammable liquid or gel that allows lithium ions to move between anode and cathode; the electrolyte is flammable and begins to break down at elevated temperatures
  • Separator — a thin membrane that keeps the anode and cathode physically apart; the separator prevents internal short circuits; if the separator fails from heat, damage, or manufacturing defect, the anode and cathode can contact each other directly

This internal structure explains why lithium-ion cells contain the conditions for thermal runaway within their design: flammable electrolyte, heat-sensitive separator, and electrochemical energy stored in close proximity. External factors that disrupt thermal stability or physical integrity can initiate a chain of reactions that the cell cannot stop on its own.

How Thermal Runaway Begins: Initiating Conditions

Thermal runaway can be initiated by several types of events. The common thread is that something raises cell temperature, causes internal short circuit, or drives the cell outside its designed operating conditions.

Physical Damage — Crush, Puncture, or Deformation

Physical compression, puncture, or crushing of a cell can directly damage the separator, allowing the anode and cathode to contact each other. This internal short circuit generates heat rapidly. Physical damage from vehicle collisions, dropped equipment, or structural failure can initiate thermal runaway. The damage may not produce immediate visible signs — delayed onset is documented.

Internal Short Circuit

An internal short circuit can result from separator failure due to manufacturing defect, degradation from age or charge cycling, contamination during manufacturing (metallic particles bridging the separator), or physical damage. An internal short circuit creates a direct conductive path inside the cell, generating heat that can initiate the thermal runaway cascade.

Electrical Abuse — Overcharge

Charging a cell beyond its designed voltage limit (overcharging) forces excess lithium into the anode, where it can deposit as metallic lithium rather than intercalating into the graphite structure. Metallic lithium deposits can form dendrites — needle-like structures that can penetrate the separator and cause internal short circuit. Overcharging also generates heat and can cause electrolyte decomposition.

Electrical Abuse — Overdischarge and High Discharge Rates

Deeply discharging a cell below its minimum voltage, or drawing current at rates beyond cell design, can cause structural damage to electrode materials that increases internal resistance and heat generation.

External Heating and Fire Exposure

External heat — from fire exposure, proximity to hot surfaces, or other burning battery cells — can raise a cell's temperature above the threshold at which internal exothermic reactions begin. This is one of the primary mechanisms of cell-to-cell propagation: heat from a failing cell raises adjacent cells to their own thermal runaway threshold.

Manufacturing Defects

Contamination introduced during manufacturing, non-uniform electrode coating, and other production variations can create latent conditions for thermal runaway that may not manifest until the cell has been in service.

State of Charge

Higher state of charge means more stored energy available to drive exothermic reactions. Fully or nearly fully charged cells generally present greater thermal runaway potential than deeply discharged cells, though even partially charged cells can enter thermal runaway under sufficient thermal or mechanical stress.

The Thermal Runaway Cascade: What Happens Inside

Once initiated, thermal runaway in a cell typically progresses through a sequence of internal events, though the specific thresholds and timing vary substantially by chemistry, cell format, state of charge, and conditions.

Onset of Elevated Temperature and Exothermic Reactions

As temperature rises beyond the cell's stable operating range, materials inside the cell begin to react exothermically. The electrolyte begins to decompose. Solid electrolyte interphase (SEI) layers on the electrodes, which are normally stable, can break down and react. These initial reactions generate heat that further elevates temperature.

Separator Failure

If temperature continues to rise, the separator — often a polymer film — can soften, shrink, or melt, allowing direct contact between anode and cathode. This dramatically increases the rate of internal short circuit and heat generation.

Electrolyte Decomposition and Gas Generation

Elevated temperature causes the electrolyte to decompose, generating flammable and toxic gases including hydrogen, carbon monoxide, carbon dioxide, and in many lithium-ion chemistries, hydrogen fluoride (HF). Gas generation increases cell internal pressure.

Venting

Cells are typically designed with pressure-relief vents that activate when internal pressure reaches a threshold. Venting releases hot gases and, in some cases, fine particulate matter. Venting may produce audible sounds — hissing, popping, crackling — and visible vapor or smoke. Vented gases may be flammable and toxic. The vent jet can be a direct ignition source or can ignite adjacent materials.

Thermal Runaway and Full Cell Involvement

If the exothermic reactions cannot be stopped, the cell enters full thermal runaway. Cathode material can decompose and release oxygen internally, sustaining combustion even in a reduced-oxygen external environment. Temperature can rise very rapidly. Cell rupture and fire may follow.

This process is not identical across all lithium-ion chemistries or cell formats. The stages are presented here as a framework for understanding — not as a universally precise sequence.

Why Chemistry Matters: Not All Lithium-Ion Is the Same

"Lithium-ion" describes a family of battery technologies, not a single chemistry. The cathode material — the component that stores and releases lithium ions at the positive electrode — varies among commercial battery types and significantly affects thermal behavior.

NMC (Nickel Manganese Cobalt Oxide)

NMC cathodes are widely used in EV traction batteries and some BESS applications. NMC chemistry offers high energy density. At elevated temperatures, NMC cathodes can release oxygen that supports internal combustion. NMC cells can enter thermal runaway at lower temperatures than some other chemistries.

LFP (Lithium Iron Phosphate)

LFP cathodes are used in many BESS installations and increasingly in EVs. LFP chemistry offers lower energy density than NMC but greater thermal stability — LFP cathodes are less prone to oxygen release at elevated temperatures, and the temperature threshold for thermal runaway is generally higher than for NMC. However, LFP cells are not immune to thermal runaway. Under sufficient mechanical damage, external heat exposure, or electrical abuse, LFP cells can enter thermal runaway and propagate.

Other Chemistries

Lithium cobalt oxide (LCO) is used in consumer electronics. Lithium manganese oxide (LMO) and other variants are used in various applications. Each has different thermal behavior.

The practical implication: the chemistry of the battery involved in an incident is often unknown to first responders. Assuming that any lithium-ion installation is immune to thermal runaway based on presumed chemistry is not a reliable or safe operational assumption.

Cell-to-Cell and Module-to-Module Propagation

Thermal runaway in a single cell rarely remains isolated in a battery pack. The process by which a failing cell initiates thermal runaway in adjacent cells is called thermal propagation or cell-to-cell propagation, and it is one of the defining hazards of lithium-ion battery incidents at scale.

How Propagation Occurs

A cell in thermal runaway releases substantial heat. This heat is transferred to adjacent cells by:

  • Conduction — direct heat transfer through physical contact or shared structural components
  • Convection — hot gases and vapor from the venting cell heating adjacent surfaces
  • Radiation — radiant heat from fire or hot surfaces
  • Hot gas jet impingement — the venting gas jet from one cell can directly heat or ignite an adjacent cell's casing or vent

As adjacent cells absorb heat and reach their own thermal runaway thresholds, they in turn release heat, creating a cascade that can progress through a module, and from module to module through a pack.

Design Factors Affecting Propagation

Battery pack designers work to limit propagation through thermal barriers, spacing between cells or modules, thermal management systems, and pack architecture. The effectiveness of these design features varies by manufacturer and system design, and may be compromised by physical damage, fire exposure, or flooding.

Scale Implications

In a residential BESS, thermal runaway propagation may involve a relatively small number of cells. In an EV traction battery pack or a large commercial BESS, propagation can involve thousands of cells and produce sustained, high-intensity fire and heat release. The scale of the installation affects the scale of the incident.

From the responder's perspective: apparent localization of fire or smoke within a battery system does not establish that propagation is complete or contained. Battery fire that appears to involve only part of a system may continue to propagate as adjacent cells reach thermal runaway thresholds.

Hazardous Gas Production

Battery thermal events produce gases that present both flammability and toxicity hazards. The specific gases and quantities depend on battery chemistry, cell format, state of charge, temperature, and degree of involvement.

Gases documented in lithium-ion battery thermal events include:

  • Hydrogen (H₂) — highly flammable; lighter than air; can accumulate in enclosed spaces
  • Carbon monoxide (CO) — toxic; product of incomplete combustion of electrolyte and other organic materials
  • Carbon dioxide (CO₂) — combustion product; displaces oxygen in enclosed spaces
  • Hydrogen fluoride (HF) — documented in NMC and many other lithium-ion chemistries; highly toxic; reacts with moisture including respiratory tissue; presents serious inhalation hazard; NIOSH has documented HF release in lithium-ion thermal events
  • Volatile organic compounds (VOCs) — flammable and toxic; products of electrolyte decomposition
  • Other compounds — specific gas composition varies by chemistry and conditions

Gas production can occur before visible fire develops — during the venting phase. Responders may encounter toxic and flammable gases from a battery that has not yet ignited. Atmospheric monitoring and respiratory protection are relevant considerations that depend on incident conditions, available resources, and department SOPs.

In enclosed or confined spaces — building interiors, parking structures, battery rooms — gas accumulation presents additional hazard. NFPA 855 requirements for ESS installations include provisions for gas detection and ventilation for this reason.

Heat, Fire, and the Oxygen Problem

A unique characteristic of certain lithium-ion cathode materials (particularly NMC and related chemistries) is that at elevated temperatures, the cathode can decompose and release oxygen. This oxygen is generated internally within the cell — meaning the battery can sustain internal combustion even in an environment with limited external oxygen.

Conventional fire suppression approaches work in part by limiting oxygen availability. For a battery fire where oxygen is being generated internally, suppression strategies must account for this characteristic. This does not mean battery fires cannot be managed — it means that management requires awareness of how these fires behave differently.

The specific implications for suppression depend on the battery chemistry, the scale of involvement, and available resources. Fire suppression tactics are established by department SOPs, manufacturer ERGs, and incident command — not by universal rules.

Venting: What It Looks and Sounds Like

Venting is a significant precursor event and, when it occurs, one of the most recognizable indicators of battery thermal involvement.

Audible indicators associated with venting:

  • Hissing — gas escaping through pressure-relief vents
  • Popping or crackling — individual cells venting; small internal ruptures
  • High-pitched whistling — pressurized vent gas escaping through small openings

Visual indicators associated with venting:

  • White, gray, or colored smoke or vapor from enclosures, wheel wells, battery cabinets, or vents
  • Visible misting or steam-like emission from battery equipment
  • Deformation or swelling of cell casings or battery enclosures

Venting may provide a warning before fire develops — but it may also occur rapidly and without extended warning time before ignition. Responders should not rely on venting indicators as a reliable early warning system, or assume that the absence of venting means the battery is stable.

Thermal Runaway vs. Electrical Hazard: A Critical Distinction

Thermal runaway is a thermal/chemical phenomenon. It is distinct from — but concurrent with — electrical hazards in a battery system. These two hazard types interact and overlap, and neither controls the other.

Thermal runaway does not eliminate electrical hazard:

  • A battery in thermal runaway still contains stored electrochemical energy
  • Fire-damaged conductors and terminals may remain energized
  • Physical damage from thermal events can expose energized internal components
  • Suppression activity does not de-energize battery cells

Electrical isolation does not stop thermal runaway:

  • Opening a disconnect switch or disconnecting external power does not stop an internal thermal reaction already underway
  • A battery that has been electrically isolated may continue to undergo thermal runaway internally
  • Electrical isolation is necessary for qualified electrical work but does not establish thermal safety

Fire suppression does not establish electrical safety:

  • Extinguishing visible flames does not eliminate stored battery energy
  • Post-fire battery assemblies may retain substantial charge
  • An electrically safe work condition as defined by NFPA 70E requires isolation and verified de-energization by qualified personnel — it cannot be established by fire suppression

This means an incident involving battery thermal runaway may simultaneously present thermal hazard, fire hazard, toxic gas hazard, and electrical hazard — and that managing one does not manage the others. Incident assessment must account for all hazard types.

For electrical hazard awareness at emergency scenes, see Arc Flash Awareness for First Responders, Electrical Hazards During Emergency Response, and BESS Safety for First Responders.

Re-Ignition and Delayed Thermal Events

Two characteristics of lithium-ion battery thermal events have significant operational implications that set them apart from most conventional fire incidents: re-ignition after apparent suppression, and delayed onset after an initiating event.

Re-Ignition After Apparent Suppression

When visible flames are suppressed, battery cells may still be at elevated temperature and may contain stored energy. Cells that have not yet reached their thermal runaway threshold may continue to absorb heat from adjacent cells that are already involved. The suppression of surface fire does not necessarily cool the interior of a battery pack to a stable state.

NTSB Safety Report SR-20-01 documents EV battery reignition after initial suppression, in some cases after the vehicle had been moved or was in storage. This is not an anomaly — it is a recognized characteristic of lithium-ion battery fires that responders and post-incident handlers should plan for.

Delayed Onset After Physical Damage

Physical damage to a battery — from collision, crush, puncture, or flooding — may not produce immediate visible signs of thermal involvement. Internal cell damage sufficient to initiate thermal runaway may not be externally visible and may not manifest immediately. NHTSA guidance explicitly recognizes that physical damage may cause immediate or delayed toxic and flammable gas release and fire.

The operational implications are consistent across both:

  • Apparent extinguishment is not confirmation of battery stability
  • Apparent absence of damage or symptoms is not confirmation that the battery is safe
  • Continued monitoring, conservative handoff procedures, and communication of battery involvement to post-incident personnel are operationally appropriate regardless of apparent incident status
  • The duration of required monitoring and the specific approach depend on battery type, incident conditions, manufacturer guidance, and department SOPs — there is no universal monitoring period

Where Thermal Runaway May Be Encountered

The phenomenon of thermal runaway is not limited to any single application. Lithium-ion batteries are used across a wide and expanding range of systems:

Electric vehicles — traction battery packs in battery-electric, hybrid, and plug-in hybrid vehicles. The scale of EV traction batteries means that thermal runaway involving a significant portion of the pack can produce sustained high-intensity fire. For detailed EV operational response, see EV Battery Fires and Thermal Runaway and EV Battery Emergency Response.

Battery Energy Storage Systems (BESS) — residential, commercial, industrial, and utility-scale installations increasingly use lithium-ion BESS. These systems can range from a single wall-mounted residential unit to containerized megawatt-scale installations. For BESS-specific hazard awareness, see BESS Safety for First Responders and Battery Energy Storage System (BESS) Electrical Safety.

Solar-plus-storage systems — residential and commercial solar PV systems increasingly incorporate paired battery storage. A solar incident may involve PV electrical hazards, battery thermal runaway hazards, and utility electrical hazards simultaneously. See Solar Electrical Safety for First Responders.

Portable power equipment — large-format lithium-ion batteries in portable power stations, construction equipment, power tools, and similar applications. Smaller scale but the same chemistry.

E-bikes, e-scooters, and personal mobility devices — lithium-ion batteries in personal mobility devices have been involved in documented residential and commercial fire incidents, including fires in building interiors where battery charging occurs.

Telecommunications and critical infrastructure — backup battery systems at cell towers, data centers, and telecommunications facilities increasingly use lithium-ion technology.

Indicators That May Be Associated With Battery Thermal Involvement

Responders should understand warning indicators associated with battery thermal events, while also understanding their limitations.

Indicators that may be present:

  • Unusual or unexpected heat from battery equipment, enclosures, vehicle undercarriage, or floor areas
  • Smoke or vapor from battery enclosures, cabinets, wheel wells, or equipment vents
  • Hissing, popping, crackling, or high-pitched sounds from battery equipment
  • Chemical, sweet, solvent-like, or irritating odors — though odor descriptions vary and are not universally reliable identifiers
  • Visible swelling or deformation of battery cells, modules, or enclosures
  • Evidence of leakage or discoloration around battery equipment
  • Fire or visible flames from battery areas

Critical limitations:

These indicators are not universally present. Thermal runaway can develop without extended external warning. Physical damage may initiate an internal process that produces no immediate external signs. A battery system that presents none of these indicators may still be undergoing internal changes that will produce thermal runaway later. Indicators are useful prompts for heightened awareness — not a checklist that, when unchecked, confirms safety.

After the Incident: Persistent Hazards

Battery incidents do not necessarily end when visible fire or other acute signs are controlled. Responders, investigators, contractors, and recovery personnel should understand that lithium-ion battery hazards may persist:

  • Remaining stored electrochemical energy in cells that survived or partially survived the thermal event
  • Residual heat in battery modules, retained by thermal mass, that may not dissipate quickly
  • The potential for continued or renewed thermal runaway in cells not yet fully involved
  • Re-ignition potential, particularly if the battery pack has not been stabilized by qualified personnel
  • Electrically energized components in a physically damaged, open, or exposed condition
  • Toxic residues on surfaces, particularly where HF-producing chemistry was involved

Decisions about re-entry, overhaul, recovery, and handoff should account for these persistent hazards. Those decisions depend on incident-specific conditions, battery type, monitoring data, qualified assessment, and department SOPs.

What This Understanding Means Operationally

The purpose of this article is to provide a foundation for understanding why lithium-ion battery incidents behave as they do — so that responders and safety personnel can make better-informed decisions when they encounter them.

The key operational takeaways from the thermal runaway mechanism:

  • A battery that appears controlled may not be stable — heat transfer within the pack may be continuing
  • A battery that appears undamaged may have sustained internal damage sufficient to initiate thermal runaway
  • Electrical disconnection does not stop an internal thermal process already underway
  • Fire suppression does not eliminate electrical hazard
  • The hazards are not over when the visible incident appears controlled
  • Specific tactical decisions require specific information: battery chemistry, system design, manufacturer guidance, and incident conditions — not universal rules applied to all lithium-ion batteries

Related Resources:

Recommended Products:

These products are appropriate for qualified electrical workers and trained safety personnel under proper procedures. They do not stop thermal runaway, make damaged batteries safe, or substitute for incident command decisions, suppression operations, or qualified electrical isolation and verification.

This resource is intended for general educational and hazard-awareness purposes. It does not replace department standard operating procedures, incident-command protocols, applicable codes and regulations, equipment manufacturer emergency response documentation, formal fire-service or electrical-safety training, or the judgment of qualified electrical or safety professionals. Tactical decisions at specific incidents are the responsibility of incident command and must reflect training, SOPs, AHJ guidance, manufacturer information, and conditions present at the scene.