Thermal Analysis of Lithium-ion Batteries
We established a comprehensive thermal analysis capability that enables us to identify and measure exothermic and endothermic reactions within a lithium-ion battery cell.
WHY THERMAL ANALYSIS OF LITHIUM-ION BATTERIES MATTERS
The use of lithium-ion batteries is undergoing dramatic growth, primarily due to their industry-leading energy density and comparatively long battery life.1 As the leading rechargeable battery for consumer electronics and electric vehicles (EVs), and a rapidly growing source for energy storage, lithium-ion batteries are playing an increasingly critical role in facilitating energy sustainability.2 One critical concern, however, is that a small percentage of lithium-ion batteries experience internal short circuits (ISCs) that result in thermal runaway, the rapid buildup of heat within the battery that leads to the explosive release of energy or fire.3 Because the most significant safety issues for lithium-ion batteries are related to heat generation, thermal analysis — examining the impact of temperature on lithium-ion battery performance and safety — provides a critically important way to pinpoint, understand and mitigate the risks.4
There are three potential causes of heat generation that can make lithium-ion batteries unsafe:
- An improper load (i.e., flow of electric energy) between the positive and negative poles in a battery can generate heat. This can be caused, for example, by an external short circuit (e.g., a DC charger is connected with the polarity reversed) or by high-rate charging/discharging. In addition, more heat can be generated by overcharging or over-discharging the battery. Improper load will usually heat the entire cell uniformly, as the electrochemical reaction occurs along both the electrodes.
- Heat can also be generated from inside the battery cell by an ISC caused by contamination that occurred on the production line (e.g., introducing metal particles into the cell) or by dendrite formation (i.e., lithium metal deposits that occur due to a polarization effect or after overcharge conditioning). An ISC can generate a substantial amount of heat that will accumulate locally inside the cell within seconds.
- Unsafe heat generation in a lithium-ion battery can also occur when the ambient (or surrounding) environment overheats the battery. This external overheating of a lithium-ion battery will sometimes trigger exothermic (i.e., heat-releasing) reactions inside the battery. The generated heat may then induce additional heat-generating reactions, eventually resulting in thermal runaway.5 Even if the external overheating is minor and causes only a very preliminary self-heating in the battery (i.e., the initial self-heating can be triggered between 60°-70°C), there may be safety issues in further use of the battery because the protective layer (i.e., solid electrolyte interface or SEI) between electrode and electrolyte may have been destroyed.
In all three cases, when the heat generation inside the battery exceeds the battery’s dissipation capacity, the result will be thermal runaway, in which an initial heat event in the battery triggers additional exothermic reactions very rapidly in a chain reaction. When this happens, the battery will typically catch fire, rupture or explode.6
WHAT DID UL DO?
We established a comprehensive thermal analysis capability that includes all existing techniques, which enables us to measure exothermic (heat-emitting) and endothermic (heat-absorbing) reactions within a lithium-ion battery on a constituent material level or to measure the reactions in a cell as a whole.7 This allows us to examine and better understand the potential safety risks of different materials and components in a battery, as well as their complex electrochemical and functional interactions and how these are affected by heating conditions.8 We use five core methods to conduct thermal property analysis of lithium-ion batteries:9
Our research has produced several key findings about the types of fundamental thermal reactions caused by the overheating of lithium-ion batteries (with a standard LiCoO2 chemistry).
- Separator Melting — Lithium-ion battery separators have a porous structure and are made of polyethylene (PE) or polypropylene (PP). Under normal conditions, PE will melt at approximately 130°C and PP at around 160°C in endothermic reactions, helping block ion transfer and shut down the inner circuit of an overheating battery.
- Decomposition of Electrolyte — Lithium-ion battery electrolytes are organic solvents that decompose under overheating conditions. The decomposition of the solvent will sometimes produce active products as well as gaseous substances that pressurize the cell. Generally, electrolyte decomposition does not result in significant heat generation by itself, but the side products will sometimes react with the battery electrodes at higher temperatures.
- Reduction Reaction of the Anode (i.e., negative electrode) with the Electrolyte — the SEI, a thin film that has the same chemistry as a liquid electrolyte but is in a different form, is easily formed on the anode-electrolyte interface. Even mild heating of a lithium-ion battery will stimulate SEI formation, but the SEI film will melt if the battery is further heated to 80°-120°C, and it will release a small amount of heat. Under an ARC test, the initial SEI decomposition within a cell can be triggered as early as 60°-70°C under the simulated adiabatic condition.
- The Oxidation Reaction of the Cathode (i.e., positive electrode) with the Electrolyte — When the temperature of a lithium-ion battery goes beyond 180°C, the cathode and the electrolyte will interact in a way that releases significant heat. If this heat cannot be dissipated effectively, it will finally lead to material decomposition of the cathode, releasing more heat and causing thermal runaway.
- Decomposition of Electrode Materials — At higher temperatures, typically 150°-300°C, the electrode material will decompose. Once this reaction is initiated, a substantial amount of heat is generally released, causing the battery to explode or catch fire. In addition, the decomposition of the cathode will produce oxygen gas, which further increases the potential for thermal runaway.10
Safeguarding lithium-ion batteries is made more difficult by the complex interactions of more than 10 types of materials and components. With a comprehensive set of thermal analysis tools and methods, UL is able to holistically examine and better understand how these batteries react to heat conditions, which is important because all lithium-ion battery safety issues involve heat generation. Our thermal analysis is especially critical today, as the industry pushes to advance lithium-ion battery performance through the use of new materials and battery designs. With thermal analysis, we are continuing to help pinpoint safety risks, identify safe performance envelopes and develop effective risk-mitigation approaches.11