Computational Modeling of Lithium-ion Batteries
We developed a unique thermal model of the common 18650 lithium-ion battery cell that enhances our ability to mathematically simulate, explore and understand the causes and severity of internal short circuits.
WHY COMPUTATIONAL MODELING OF LITHIUM-ION BATTERIES MATTERS
Lithium-ion batteries are a vital source of sustainable energy and are being used across an expanding array of applications — from portable electronic devices to electric vehicles to stationary energy storage. Importantly, the high energy density that contributes to the performance and popularity of these batteries also makes a small percentage of them vulnerable to internal short circuits (ISCs) that can result in fires or explosions. Computational modeling is a method UL is using to better understand ISCs, along with the mechanisms of heat generation that occur due to the electrochemical reactions within a lithium-ion battery and the primary causes of thermal runaway, which represents the worst-case — and most hazardous — scenario of possible lithium-ion battery failure modes.
The energy density of lithium-ion batteries has tripled since their commercial introduction in 1991.1 Energy density — or a battery’s energy delivery capability relative to its weight or volume — is one of the advantages of lithium-ion batteries, which have the highest energy density and the highest voltage of any commercially available rechargeable battery.2 This was largely accomplished by packing more active material into the cell and making the electrodes and separators thinner.3 Ultimately, this means that lithium-ion batteries are smaller and lighter than other batteries; this is one of the primary reasons for their growing popularity.
The power that a lithium-ion battery packs into its volume is largely achieved by the use of an electrolyte composed of lithium salts in flammable organic solvents such as ethylene carbonate and ethyl methyl carbonate. This differs from the electrolytes in other common types of batteries, which are composed of acid or base aqueous solutions that are nonflammable.4 The flammable electrolyte, coupled with increasingly thin and light separators, increases the risk of failure. Currently, lithium-ion batteries have a failure rate of approximately one in 10 million cells.5 Although this may seem relatively low, in the context of the 4.4 billion lithium-ion battery cells manufactured in 2012,6 the failure rate equates to 440 potential battery failures in one year, a figure that could increase as the lithium-ion battery market doubles over the next four years.7
Lithium-ion batteries have been involved in several highly publicized incidents over the past several years.8 In many cases the battery failures were linked to ISCs that led to thermal runaway, resulting in the explosive release of energy along with fire.9 Thus, it became imperative for UL to gain a better understanding of ISCs to identify the factors that cause the most heat generation within the shortest time, leading to catastrophic battery failure.
WHAT DID UL DO?
We developed a unique computational thermal modeling capability to enable us to mathematically simulate, explore and understand the causes and severity of ISCs. Our thermal model was developed using SC/Tetra computational fluid dynamic (CFD) software and built on a complex geometric representation of the 18650 type or other cylindrical lithium-ion battery, making it especially suitable for these cells. The model provides a robust simulation of lithium-ion battery characteristics because all of the thermal and physical properties of each component are based on information from investigated literature or real experimental data.10 This gives us the ability to investigate all the factors involved with an ISC.
UL’s thermal model takes into account different sources of heat generation — electrical, electrochemical and chemical — and encompasses the factors related to material and construction design that are the dominant elements in determining the capability of a cell to dissipate excess heat. This enables UL scientists and engineers to investigate the components and/or ISC conditions that are the critical factors in determining the temperature rise distribution within the whole cell as well as outcomes involved with an ISC under steady and pseudo-steady state conditions.11 Our key findings include:
KEY CHARACTERISTICS OF AN ISC EVENT
Our model showed that other than in a case caused by mechanical abuse, most ISC events begin on a small scale in a localized area within a single lithium-ion battery cell, which results in localized heating at the very beginning of the event. Further, once the ISC is triggered, all safety designs outside of cells have no effect, and most of the safety devices within the cells will also be bypassed (e.g., the fuse that is designed to open to vent excess voltage or heat).12
Three principal findings relative to ISC behaviors were observed from our Indentation Induced ISC Test (for more information, refer to the article “Indentation Induced ISC Test”). The worst-case scenario of an ISC can occur when the resistance at the point of the ISC is similar to the resistance level of the whole cell. This leads to the maximum generation of Joule heat from the ISC point to the surrounding area in the battery. In addition, our model showed that the location of an ISC is directly related to its severity. An ISC located in the center of the bottom side of a cylindrical lithium-ion battery exhibits the highest rise in temperature, reaching 1,066°F.
Our third key finding about ISC behaviors is counterintuitive. Assuming the same electrochemistry energy and constant resistance at the ISC point and within the whole cell, we found that the smaller the initial ISC, the greater its severity and the more likely it is to lead to thermal runaway (i.e., the rapid buildup of heat inside the battery that can lead to fire or explosion).13
FAILURE MECHANISMS IN AN ISC
Using our thermal model, we were able to assess the impact of materials, product design, battery construction and electrochemistry on ISCs. We identified the heat-generation profiles through three different heating sources. One common heat source in a lithium-ion battery is the result of localized overheating by the Joule effect, which means the current flows through the ISC point in the battery, generating localized overheating due to the impedance at the ISC bridging point. The second heat source is when an ISC triggers an electrochemical reaction in the battery material via electron transfer that leads to “globalized” overheating of the battery. The final heat source we detected involves an ISC that creates a localized overheating condition that consequently causes material decomposition around the point of the ISC.
The profiles of all three heat sources can be simulated using different material properties, which can be flexibly set by the simulation tool. We also developed a geometry model, composed of a detailed spiral “jelly roll” structure that includes all the components within the cell with appropriate thermal property settings. This enables us to create a virtual heat-conducting medium that can simulate the heat dissipation capability of a real battery. This simulation is important because the heat-generation rate and a battery’s ability to dissipate heat are the two critical factors involved with thermal runaway and potentially significant fire and explosion hazards. We observed that the faster the heat builds within the battery, the less likely the battery’s fail-safe devices will be able to effectively vent the excess heat quickly enough, and the more likely the ISC will lead to thermal runaway. 14
Computational modeling is an important tool we use to gain insight into how a product functions, how it malfunctions and the risks involved. For lithium-ion batteries, UL applied this tool to develop a thermal model to help us proactively gain a better understanding of how ISCs work and why some lithium-ion battery cells fail while others do not. With new insights about the characteristics and behaviors of ISCs as well as their failure mechanisms, we are better able to help manufacturers safeguard the batteries they make across the design, materials specification, manufacturing and quality assurance processes.