J. D. Loud, Exponent Failure Analysis Associates, Menlo Park, CA; X. Hu, Exponent Failure Analysis Associates, Natick, MA

Summary: The consumer product industry continues to show more and more demand for portable electronic devices that require stored electrical energy. Li-Ion battery technology has become the most popular choice because of its high energy density, high cell voltage, low self-discharge rate and no “memory effect” on its capacity. However, it is the high energy density inherent in Li-Ion batteries that can result in unsafe failures. Failure can occur due to problems within a cell, or because of external factors that impact a cell. Undesired failures range from inconvenient overheating of the cell that causes minor damage to surrounding materials, to energetic venting with flames. While these potential failures have been known and explored for many years, it is only with the proliferation of these devices in society that these undesirable, low probability events are gradually becoming known. Extensive efforts have been devoted to reducing the probability that such events will occur, including external circuit protection to prevent electrically-induced events, and novel cell designs and manufacturing methods to prevent failures. The need is great, however, to conclusively establish the cause of such undesired events so that steps can be taken to prevent future events and also to determine whether a field population has an unacceptably high risk of failure and should be removed from service. Understanding the failure mechanisms of a lithium ion cell and the root causes of the failure is very important in establishing a complete fault tree. Possible causes for a Li-Ion failure include: 1. An internal cell short circuit (manufacturing-related). Possible causes of an internal short circuit include: a. A burr or other metal processing anomaly that stresses the separator and eventually results in an electrode-to-electrode short circuit. b. Contamination in the jellyroll that eventually penetrates the separator, resulting in an electrode-to-electrode short circuit. c. Contamination in the jellyroll that dissolves, plates out on the separator and dendritically grows through the separator, resulting in an electrode-to-electrode short circuit. d. A lead anomaly that stresses the separator and eventually results in an electrode-to-electrode short circuit. e. Misalignment of the winding such that contact between the electrodes (including active materials) occurs. f. A lead to case or lead to jellyroll short circuit. The routing, length and insulation on the leads that bring the cell power out of the cell is important to prevent short circuits. 2. An internal cell short circuit (externally caused). Possible causes for such an event include: a. External mechanical crush of the cell can. b. A pinpoint dent in the cell can. c. Excessive mechanical impact. 3. An external cell short circuit. The routing and insulation on the external leads is important to prevent external short circuits. 4. Overcharge of the cell. Overcharge of the cell may result in internal heating and precipitate a cell internal short circuit. Such internal short circuit events may be quite energetic since the amount of energy stored at the time of the incident is higher than that of a fully charged cell. 5. Overdischarge of the cell. Overdischarge of the cell will result in dissolution of the copper current collector and subsequent plating of the dissolved copper onto the adjacent separator. Repeated overdischarge will result in dendrite growth of the copper through the separator and eventually an electrode-to-electrode short circuit. 6. Electrode degradation. A local anode-to-cathode imbalance can cause localized degradation (oxidation) of the electrodes. The oxidation process will generate heat as well as gasses that will diffuse to the adjacent electrode and cause additional oxidation. This process may result in cell overheating and venting. 7. External heat. Externally heating a cell will cause it to fail such that it will vent to dissipate its energy in an undesirable manner. There is a wide range of damage that occurs in Li-Ion incidents. The goal in a root cause failure analysis is to identify the surviving characteristics present in the remaining evidence to determine which of the above caused each incident. However, because of the nature of a Li-Ion incident, it usually results in charred or burnt material. There is often only limited surviving evidence for the failure investigator to evaluate. Since 1996, when the authors first became involved in evaluating Li-Ion incidents, they have developed and refined a systematic methodology through numerous Li-Ion incident investigations. This methodology, which is tied to the above fault tree, is described as follows: 1. System level visual inspection looking for evidence of: a. External heating/fire attack, b. Mechanical impact or compression, or c. Liquid intrusion, etc.; 2. System level X-ray inspection of the protection circuitry, looking for evidence of circuit or component failure; 3. Protection circuit/charge circuit functionality testing; 4. Cell level X-ray inspection to characterize the damage pattern of the incident cell and prepare for destructive inspection of cell remains; 5. Cell disassembly and microscopic inspection of cell remains: a. Evaluate overall damage pattern of remaining cell electrodes; b. Microscopic inspection of copper electrode, looking for signs of melted copper; 6. Supplemental analysis may include: a. EDX (Energy-dispersive X-ray spectroscopy), looking for potential contaminants; b. XRD (X-ray diffraction) analysis of remaining active material in evaluating potential overcharge conditions; c. Replication testing; d. Survey of exemplar cells; e. Review of manufacturing operations; f. Statistical analysis of failed units. This paper presents detailed explanations of each step in combination with typical photographs from selected representative incidents. This paper can be used as a guideline for investigators in Li-Ion incident failure analyses.