Lithium-ion batteries are a ubiquitous power source, found in devices ranging from smartphones to electric vehicles. Storing a large amount of energy in a small space, this technology introduces a safety concern known as “off-gassing.” Off-gassing is the release of volatile compounds from inside the battery cell, signaling an internal chemical reaction that indicates a failure state. This venting of gases is a precursor to severe events like fire or explosion, making the phenomenon a serious safety hazard.
The Mechanism of Off Gassing
Gas generation begins when the internal temperature or pressure within the cell rises above safe operational limits. Lithium-ion batteries contain a liquid electrolyte, which is typically a mixture of organic carbonates. When the cell experiences stress, this organic electrolyte begins to decompose through exothermic chemical reactions.
This decomposition generates gaseous products, including carbon monoxide (\(\text{CO}\)) and hydrogen (\(\text{H}_2\)), rapidly increasing the internal pressure. To prevent a catastrophic rupture, most batteries are equipped with a safety vent or pressure relief mechanism. Off-gassing is the expulsion of these gases through the vent or a breach in the casing.
Carbon dioxide (\(\text{CO}_2\)) production often results from secondary reactions. The initial decomposition releases \(\text{CO}\), which can react with atomic lattice oxygen released from the cathode material at elevated temperatures. This chain of reactions accelerates self-heating, pushing the cell toward thermal runaway.
Common Triggers of Battery Failure
Off-gassing and potential failure stem from several forms of abuse. One common operational trigger is overcharging, which occurs when the battery voltage is pushed past its designated safe limit. Excessive voltage breaks down the electrolyte and can lead to the plating of metallic lithium on the anode, which is a precursor to internal short circuits.
Physical damage is another external trigger that compromises the cell’s integrity. Crushing, puncturing, or heavy impact can tear the thin separator layer that keeps the positive and negative electrodes apart. This mechanical abuse causes an internal short circuit, leading to an uncontrolled flow of current and rapid heating.
The third cause is thermal abuse, or exposure to excessive heat, which initiates the decomposition process directly. When a cell is exposed to high ambient temperatures, the internal chemical reactions accelerate, and the heat generation surpasses the cell’s ability to dissipate it. This leads to a self-sustaining cycle of rising temperature and further decomposition, known as thermal runaway.
Composition and Toxicity of Released Gases
The gases expelled during an off-gassing event are flammable and highly toxic, posing a severe risk to human health. Primary gases include Carbon Dioxide (\(\text{CO}_2\)), Carbon Monoxide (\(\text{CO}\)), and Hydrogen Fluoride (\(\text{HF}\)). The composition and concentration of this gas mixture depend on the battery’s specific chemistry, its state of charge, and the failure mode.
Hydrogen Fluoride is formed when the electrolyte salt, commonly lithium hexafluorophosphate (\(\text{LiPF}_6\)), reacts with trace moisture inside the battery or surrounding air. This reaction yields corrosive \(\text{HF}\) gas, which can be generated in large amounts. Exposure to \(\text{HF}\) gas, which may be odorless, causes severe irritation to the eyes, nose, and respiratory tract, and high concentrations can be fatal due to fluid accumulation in the lungs.
The danger of \(\text{HF}\) is amplified because it readily forms hydrofluoric acid upon contact with moisture, including the moisture in the respiratory tract and on the skin. This acid quickly penetrates tissue, leading to systemic poisoning by depleting calcium and disrupting cell function. Carbon Monoxide (\(\text{CO}\)) is also released, presenting an asphyxiation hazard, as it interferes with the blood’s ability to carry oxygen.
Immediate Safety Response and Proper Handling
If a lithium-ion battery begins to off-gas—indicated by signs like swelling, hissing sounds, or an unusual smell—immediate action is necessary to ensure safety. Evacuate the area and ensure the space is well-ventilated by opening windows or doors, as the released gases are toxic and potentially flammable. The battery should be disconnected from any power source and moved to an isolated location, preferably outdoors on a non-combustible surface, if it is safe to do so.
For long-term incident prevention, proper handling practices are essential.
Safe Charging
Always use the manufacturer-approved charger designed specifically for the battery. Never leave charging batteries unattended, especially overnight. Avoid charging devices or batteries on or near soft furnishings like beds or carpets, which are easily combustible.
Safe Storage and Disposal
Safe storage involves keeping batteries in a temperature-controlled environment and away from direct heat sources. Batteries should be stored in a non-flammable container. Any battery showing signs of damage or heat should be placed in a metal container and stored ten meters away from other combustible materials. For a compromised or failed battery, contact a specialized recycler or your local waste management facility for proper disposal, as these items cannot be simply thrown into the regular trash.