Battery Failure Analysis and Characterization of Failure Types 2021

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All Li-ion batteries are susceptible to this type of failure, but their thermal stability and thermal runaway temperature is tied strongly to the cell’s cathode chemistry. Li-ion batteries are often referred to by their chemistry, which is dictated by the cathode chemistry. Lithium iron phosphate (LFP) and lithium cobalt oxide (LCO), are two examples. The bonding characteristics and chemical structure of the cathode makes the battery more or less chemically and thermally stable, with LFP type batteries being far more stable than LCO type batteries. These different chemistries result in different physical crystal structures that encompass the cathode, which strongly controls cell stability and how fast a particular battery can be charged and discharged safely. These crystal structures also affect Li-ion mobility or how quickly and efficiently they can be inserted during intercalation (charging/discharging). For example, LCO batteries have higher nominal voltages giving them a higher energy density, but the layered structure of the cathode can limit the mobility of the Li-ions making it more dangerous to force higher charge/discharge rates. Conversely, lithium manganese oxide (LMO) batteries have 3-dimensional spinel structures that enhance intercalation, allowing these cells to charge and discharge safely at higher rates. Forcing high charge/discharge rates puts stress on the battery electrodes and can also result in heating, which can lead to thermal runaway. For this reason, consideration of the cell cathode chemistry is an important factor when determining a particular application, as improper operation of the battery can lead to a thermal runaway event.
If a thermal runaway failure occurs, it is often important to determine why the event happened. This could be important to operators to potentially prevent a future event, for insurance and potential litigation, and for reporting to regulatory agencies. A fast response and taking measures to preserve the site and potential evidence or artifacts of interest are essential to ensure an accurate origin and cause investigation can be thoroughly performed. As part of the investigative effort, data review, e.g., SCADA, collecting information, reviewing any available footage, and collecting drone footage using infrared thermography, can all be methods used to aid in heat mapping to identify the origin or probable origins. If an approximate origin is identified, or multiple probable origins are identified, collection of evidence, establishing chain of custody, and further laboratory analysis would be prudent. Using the correct methods and analytical techniques will help to identify the failure mechanisms involved, and combined with other obtained information, a methodical approach using causal mapping can help to identify one or multiple causes or contributing factors to the event, and to establish a timeline and sequence of events.
Examination and analysis of physical evidence obtained from the scene is typically conducted in a forensic laboratory, such as BakerRisk’s Forensic Materials Engineering Laboratory. Methodical photo- documentation of the as-received condition of collected evidence, and documentation of the process of destructive testing activities, are essential activities. The following are useful examination methods for assessing collected evidence:
• Non-destructive examination: aside from visual examination and low magnification optical microscopy, one useful tool would be computed tomography (CT) scanning of modules or cells. Prior to any opening, removal, or sectioning of the evidence, imaging of the interior acquired via non-destructive means can be useful prior to proceeding with destructive activities.
• Microscopic examination: using data previously collected non-destructively can aid in subsequent destructive activities. Opening of a cell using a glove box and sectioning of cells to reveal the interior of a cell, including the jellyroll, is a necessary step to better understand a cell’s construction. Evaluation of cross-sections allows for assessing the quality of spot welds and measuring spacing and distances. Examples of this type of analysis are shown in Figure 2, which was collected by BakerRisk in our materials and testing laboratory for a button cell Li-ion battery
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