Applying Fault Tree Analysis Methodology
A unique methodology applied by UL to showcase how potential defects can create unsafe operations for a lithium-ion cell.
WHY FAULT TREE ANALYSIS MATTERS
Several highly publicized fire, explosion and product recall incidents have raised concerns about the overall safety of lithium-ion batteries. There is an urgent need to understand the root causes of these incidents and to promote open cooperation between governmental research organizations, cell manufacturers, safety stakeholders and standards organizations to develop consensus-based updates to the safety standards. UL’s innovative application of the Fault Tree Analysis methodology enhances our ability to identify and catalog the root causes of battery failures and to engage multiple organizations in dialogue to help improve battery safety.
Lithium-ion batteries are popular because they have several advantages relative to competing technologies: they generally have much higher energy density — the amount of energy they can store per kilogram of battery.1 These batteries hold their charge, losing about 5 percent of their charge per month compared with a 20 percent loss per month for NiMH batteries,2 and have no memory effect as other batteries do. This means that lithium-ion batteries do not have to be completely discharged before recharging.3 These batteries can also last through hundreds of charge/discharge cycles.4
Lithium-ion batteries, however, are not flawless. There have been a number of failure incidents that have brought these batteries under intense governmental scrutiny.5 These developments underscore the urgent need to understand the root causes and safety hazards associated with ISCs in lithium-ion cells and to update safety standards.
WHAT DID UL DO?
Given the importance of lithium-ion batteries to so many applications — from consumer electronics to transportation to stationary energy storage for energy utilities — UL has been conducting a broad range of research on different kinds of lithium-ion battery chemistries and formats. Specifically, we have been overseeing a variety of nondestructive analyses of lithium-ion batteries to understand structural elements and impedance, abuse tests to examine battery performance under “worst conditions,” and material analyses to better understand how the different components and materials in lithium-ion batteries respond under various conditions.6
UL is also actively engaged in reviewing publicly available lithium-ion battery research, which shows a strong focus on understanding and mitigating cell failure modes involving internal short circuits (ISCs).7 Although only brief accounts of field failures are available, a number have noted the presence of manufacturing defects that led to ISCs within the cell.8 UL applied the Fault Tree Analysis methodology to the results of its battery safety research and field failure information to translate them into an accessible, logical format that identifies both the immediate and root causes of lithium-ion battery failures.
Fault Tree Analysis is a symbolic logic analytical technique in which an undesired event is defined — in this case, a lithium-ion battery failure incident. The event is resolved through research into its immediate causes. The resolution of events continues until the root causes are identified at the appropriate level. A logical diagram, called a fault tree, is constructed that shows the logical event relationships.9
Fault Tree Analysis is a disciplined approach that provides a framework for the rigorous examination of a fault event (e.g., a battery failure incident). By employing this methodology, UL explicitly shows all the different relationships that are necessary to result in battery failure and gains an in-depth understanding of the logic and root causes of the incident.10
A Fault Tree Analysis has been developed to understand the possible causes of lithium-ion cell failures with a focus on incidents involving fire and explosion. The analysis presented here is for demonstration purposes only. Because of this, it captures the main points and is not developed in great detail. We conduct much more detailed analyses depending on the specific product and failure conditions.
Fault Tree Analysis of a lithium-ion cell that has become unsafe11
This specific fault tree depicts the following causal event relationships and logic:
- The principal event is the unsafe operation of the lithium-ion cell.
- A few possible examples are an electrolyte leakage, which could release toxic gases, or the deflagration of vented volatiles, which could lead to fire/explosion or an inability to operate the safety-critical device that is powered by a lithium-ion cell.
- Looking specifically at the hazard of deflagration of vented volatiles, three basic events are listed, which must ALL happen for a detonation to occur:
- Ignition source: contact of the volatiles with a hot surface (or possibly the volatiles are already at a high temperature)
- Fuel source: vented volatiles from the cell
- Ambient air: oxygen needed to facilitate combustion (within the flammability limits)
- Next, when the vented volatiles from the cell event are examined, four causal events are identified, each of which must occur for the volatiles to vent from the cell:
- A sufficient state of charge of the cell (stored energy)
- Exothermic reactions
- An ISC to provide a pathway for charge flow that leads to a localized heat source
- Inadequate cooling to provide sufficient heat dissipation
When an ISC exists with a sufficient state of charge, then the charge flow results in localized heat generation. This will heat up the cell locally and possibly activate exothermic reactions among the active materials within the cell. If there is insufficient heat dissipation, then the heat generated by the exothermic reactions within the cell will feed back into the remaining materials that have not reacted, continuing the buildup of heat.
- There are four different types of ISCs, and this fault tree focuses on the most energetic — anode to aluminum (AI) film (UL applies Fault Tree Analysis to the other ISC types as well) — and identifies two potential root causes of this type of ISC:
- A breach of the separator by a particle, or foreign object damage, caused by a manufacturing defect
- A damaged separator due to external forces
Our Fault Tree Analysis combines the results of several publicly available research studies and graphically depicts the causes and relationships between events that lead logically from a manufacturing defect or damage from an external force to the unsafe operation of a lithium-ion cell.
UL is constantly seeking to improve the safety of lithium-ion batteries. This requires a systematic approach, an in-depth understanding of lithium-ion battery field incidents, and a focused effort in research and standards development to address the root causes of these incidents. The innovative application of Fault Tree Analysis to lithium-ion battery failures is the New Science that adds an extra dimension of rigor to UL’s approach. It provides a transparent and detailed record of the analysis into the causes of battery failures, which makes Fault Tree Analysis an effective tool to communicate and build consensus, both within UL and with our various research partners and safety stakeholders. Fault Tree Analysis also helps us identify what new research is indicated — to explore and validate new potential causes of battery failures, as suggested by other research findings or field incidents. Fault Tree Analysis is central to how UL helps ensure lithium-ion battery safety.