Destructive Physical Analysis Research Articles

Ultrasonic assessment of aging in lithium metal pouch cells

Lithium metal anodes are prone to non-uniform lithium plating that can lead to dendrite formation, internal short circuits, and catastrophic ignition. This type of structural defect is not captured by battery management systems, prompting the need to advance nondestructive evaluation (NDE) techniques that are suitable for periodic monitoring of the internal structure. An ultrasonic inspection technique is evaluated here as a means of identifying flaws and irregular lithium plating that can be a precursor to dendrite formation and, ultimately, battery failure.Lithium metal pouch cell batteries were aged through electrochemical cycling at a range of moderate constant-current charging and discharging rates. They were inspected intermittently using a tailored local ultrasonic resonance spectroscopy (LURS) approach that measures the local through-thickness resonances of the battery as an indicator of the underlying structure. LURS results showed clear structural resonances in cells in the range of 3–7 MHz, which shifted significantly to lower frequencies as batteries aged. In addition to point measurements, scanning LURS measurements were implemented to map resonance frequencies over the electrode surface. These resonance maps revealed spatial variations in structure that developed during aging and showed better contrast to structural variation than achieved using conventional amplitude-based c-scans of the specimens.NDE results from pouch cell measurements were compared to a destructive physical analysis of the electrode structures using scanning electron microscopy (SEM). The SEM results showed differences in the end-of-life lithium anode structure at different charge rates. The results of this study show the capability of LURS as a means for identifying changes in battery structures during aging and establish resonance trends that may be applied in future work for prognostic modeling of batteries in service.

Evaluation of Copper and Gold Wire Bonds in PEM Devices

Abstract The shift from gold to copper wire bonds used in plastic encapsulated microcircuits (PEMs) has been gradual for the last few years. Many agencies and critics question the long-term robustness and reliability of copper wire bonds; however, PEM manufacturers have been thoroughly evaluating and optimizing the copper wire bond process over the last few years to produce highly reliable products. When the manufacturing process is optimized and closely controlled and monitored, copper wire bonds can be a reliable and stronger alternate to gold wire bonds. This paper details a copper wire bond study that specifically compares copper wire against gold wire bonds following combined environmental life stress test conditions. In this study, bond pull testing indicates good performance of both copper and gold wire bonds. The methods used in this analysis (destructive physical analysis, wire pull testing, and environmental testing) were effective techniques for evaluation of bond wire integrity, detection of manufacturing defects, and overall assessment of copper and gold wire bond processes. Based on wire bond testing in this analysis, the reliability and integrity of optimally developed copper wire bonds appear to be superior to or even stronger than gold wires in the same package type. Equivalent reliability was demonstrated for both copper and gold wire bonds in this test.

Effect of temperature on vibration durability of lead-free solder joints

Safety critical electronics in automotive, avionic or military applications have to work reliably in a range of harsh conditions, including simultaneous mechanical and thermal loads (Gromala, 2021; Qi et al., 2009 [1, 2]). The durability of electronics, more specifically their board level interconnects, under vibration and thermal loads must be investigated and understood for interconnects assembled with lead-free solder alloys. In this study, isothermal vibration fatigue tests have been carried out at −40 °C, 25 °C and 125 °C. A novel test specimen, specifically designed for combined vibration and temperature cycling experiments, was used for this study. The test PCB contains leadless chip resistor (LCR) components, soldered using the SAC105 solder alloy (Sn98.5Ag1.0Cu0.5). Failed specimens at each temperature were subjected to destructive physical analysis (DPA) for root cause analysis (RCA) of the observed failures. Dynamic finite element analysis (FEA) was carried out and combined with the experimental results in order to generate fatigue strain-life (SN) curves. Two opposing effects of increasing temperature - viz. decrease of fatigue strength for high-cycle fatigue (HCF) and increase of fatigue ductility for low-cycle fatigue (LCF) - are hypothesized to be responsible for this non-monotonic behavior.

Anomalous Behavior During Low Rate Discharge of Li/CFX Cells

Lithium-carbon monofluoride (Li/CFx) D-sized battery cells discharged at very low rates (C/1800) were found to deliver inconsistent capacities. These effects were found to be absent as the discharge rate was increased to C/600. Destructive physical analysis of the cells discharged at C/1800 confirmed the effects of corrosion, manifesting as preferential utilization of the Li anode around the copper current collector, discoloration of the remaining excess Li, and more crucially loss of adhesion to the copper current collector, resulting in a premature completion of discharge. This anomalous behavior may be attributed to the competing corrosion reaction at the Li anode during discharge, whereby Li+ is lost to the electrolyte at a rate that is similar to the competing current-producing charge transfer process. At higher rates, this corrosion process is masked by the more significant discharge current generated by the full cell, which occurs at a much higher rate. The inconsistent capacity delivery may relate to the uneven breakdown of the surface film at these very low rates, where the current density is very low. Experimental data from both D-sized and three-electrode cells describing this effect are presented, as well as possible remedies for cells where very low discharge rates are desired.

Cell Supply Chain Management with Approval and Surveillance Program

Although many battery safety standards and certifications are in place to minimize hazards or prevent safety-related incidents in the field, many publicized field incidents have increased public concerns about LiB safety. Most battery field incidents reported that the cell internal short circuit initiated the causes, and many experts blamed battery industries do not have the test system that can predict the field incident nor reflect how the incidents occur. The certification is required when a cell model is developed and just before that goes into the market.Some intended or unintended modifications are in the cell model after the model is certified, and some failure analysis reports blame unknown and unproven changes on the cell model were the root causes of the incidents.To eliminate and prevent the root causes of battery field incidents by unknown and unproven changes on the cell model, we developed a verification process named cell supply chain management with approval and surveillance program. The verification process makes a technical database of all the analysis and test results of each step of a cell model as its fingerprint. The database enables the process can identify and prevent any unknown and unproven changes in the cell model through its life span.The verification process has: 1) Cell electrical properties, 2) X-ray investigation and Destructive Physical Analysis with -electrode alignment check with overhang gap, - electrode assembly (Jelly roll) function and dimension measurement, - insulation mechanism check and measurement, 3) Electrode dimension measurement and composition and materials analysis, and 4) Electrolyte composition analysis.This paper explains and proposes the concept and analysis method of each verification process and validates the cell model verification process by comparing the analysis results of each step of a cell model manufacturing. Figure 1

Development of Cell Selection and Screening Method and Its Validation Process for the Multi-Combination Battery System

A commercial-off-the-shelf (COTS) cylindrical 18650 Li-Ion cells have been proven to be extremely reliable power sources from many electronic applications for long periods and adopted to the EV applications. The cylindrical cells have been considered a reliable power source for the aerospace battery system, thanks to their high energy and power density. However, some electronic applications' field failure histories and some test programs have indicated that batteries built with cylindrical 18650 cells in multi-cell configurations (series and parallel) have experienced safety issues under various test conditions. Another requirement of the aerospace battery is a long-life cycle under harsh operating conditions. A tiny potential risky outlier cell, which cannot be identified and detected during the cell manufacturing process, can severely affect the multi-combination battery system's safety and reliability, like the EV, ESS, and Aerospace battery system. This paper studied a cell selection and screening method and developed a validation process to evaluate the efficiency and reliability of the cell selection and screening process. A cell selection method includes visual inspection, mass and dimension check, some electrical tests (OCV, CCV, AC Impedance), capacity measurement, and cell level safety test according to widely accepted cell safety standards and battery industrial best practices of the lithium-ion battery. The cell screening method and procedure followed the process and criteria on the NASA technical document (Specification for Acceptance Testing Commercial Lithium-Ion Cells). This program adopted the 6-sigma concept on the data analysis of the cell selection and screening process, which rejects the outlier cells of the 3-sigma level of each test item. The validation process was designed with two test conditions: 1) cycle test on the regular operation of the Human-spaceflight's mission operating conditions and 2) accelerate test condition of the maximum (most severe) cycle condition of the cell makers specification document. The regular operation cycle test was designed to evaluate the screening process's efficiency, whether the 3-sigma level screening is acceptable to the performance of the battery system using all passed cell samples from the screening. The regular cycle test selects four types of 4S string battery groups from different combinations of the cell samples. And it was performed according to the Human-spaceflight's mission operating conditions to validate the 4S can meet the required cycle condition. On the other hand, the accelerated cycle test was designed to identify the difference of cell degradation and capacity decay between the two types of cell combinations by the cycle test. One group use cells randomly selected samples from the non-screen cell, and the other group used selected cells from passed group cell as a control sample. Both 4S strings were cycled at the accelerate test condition, the maximum (most severe) cycle condition of the cell makers specification document. The battery capacity decay and the cell's internal resistance changes by DC pulse technics were checked regularly every 100 cycles during the cycle test.The performance of both 4S was almost similar until 200 cycles. However, the non-screen 4S string's capacity decay speed was much faster and reach death after the 500 cycles, while the screened 4S runs without any severe capacity decline until 800 cycles. EIS (Electrochemical Impedance Spectroscopy) technique was applied to each cell of the 4S string. The DPA (destructive Physical Analysis) and electron microscope analysis were used to identify the electrode's degradation degree and study the cell failure mechanism during the accelerated cycle tests. The DPA and electron microscope analysis (SEM-EDS) identified that gradual damage of the cathode electrode material of the non-screened 4S string during the accelerated cycle test was the leading cause of the cell capacity decay. This research discusses that non-screened 4S string may have different characteristic cell among the four cells in the 4 S string compare to the screened 4S string. That can initiate the stress to the cell and then increase the cell internal resistance, following enhanced cell heat generation and eventually getting the cell imbalance. The cell imbalance and heat generation get the cell getting worse after 200 cycles and eventually kill the cell and 4S sting. This research concludes that the used cell screening process is effective and beneficial as a cell screening process for the multi-cell battery system, especially for the long-term cycle-life battery system, like EV, ESS, and aerospace applications. Figure 1

Performance of Commercial Li-Ion Cells for Future NASA Missions and Aerospace Applications

Li-ion batteries are being widely used in planetary missions, based on their impressive performance characteristics, including high specific energy, long life, and the ability to operate over a wide temperature range. Custom large-format Li-ion cells were used in the early planetary missions, e.g., Mars Rovers, Mars Landers, and Mars and Jupiter Orbiters. At the same time, several other robotic missions have utilized and are planning to utilize commercial 18650 lithium-ion cells, e.g., Mars Express, Lunar Reconnaissance Orbiter, Europa Clipper, as well as human-rated astronaut Extra-Vehicular Activities. Interest in commercial 18650 lithium-ion cells for NASA missions has increased in recent years due to distinct advantages over custom large-format cells, including higher specific energy and energy density, enhanced safety and reliability, modularity and redundancy in the battery design, and simpler charge methodology. Multiple commercial 18650 cells are available which can provide a specific energy of ∼250 Wh∙kg−1 (20%–50% over custom cells) and a power density of ∼2 kW∙kg−1. Selected cell types with different chemistries and from multiple high-quality manufacturers, including the MJ1, M36, HG2 models from LG Chem; the 35E, 36 G and 30Q models from Samsung; the BJ and GA models from Sanyo/Panasonic; and the VC7 from Sony/Murata, were evaluated for their applicability in space applications. The performance tests included cycle life at different temperatures and depths of discharge, and high-power characterization at different temperatures; to supplement typical electrical measurements of voltage, capacity, and energy, Electrochemical Impedance Spectroscopy (EIS) was used to characterize the health of some cells on an ongoing basis. In addition, destructive physical analysis was carried out on these cells to analyze their electrode materials and correlate them with their performance characteristics. The demonstrated performance of several cells indicated their viability for further extensive space-qualification prior to their implementation in space missions.

Using the concepts of homogeneity in additional tests of electronic components for space applications

In modern conditions in the Russian Federation the equipment of spacecraft with a highly reliable electronic components is possible only through testing technical centers which act as a link between manufacturers of electronic components and their consumers. Given the lack of specialized production of electronic components of the «Space» category in our country, this is the only alternative way. Testing technical centers carry out the formation of batches of electronics for space applications by conducting additional tests of electronic components for general industrial use allowing the rejection of elements containing defects that can manifest themselves during long-term operation in outer space. A modern spacecraft contains about 100–200 thousand electronic components. Taking into account the fact that the on-board equipment of the spacecraft during its operation cannot be repaired, it is obvious that extremely high reliability requirements are imposed on the electronic components of space applications. In this regard the improvement of the methodology for increasing the reliability of the electronic components of space applications is of paramount importance for the further development of the space industry. This article outlines an approach to improving the technology of additional tests in testing technical centers, based on the use of the concepts of homogeneity of the tested batch of electronic components which makes it possible to meaningfully generate samples for destructive physical analysis and radiation resistance tests, as well as to additionally identify emission elements that are potentially unreliable elements.

Can automotive MEMS be reliably used in space applications? An assessment method under sequential bi-parameter testing

Commercial Off-The-Shelf (COTS) automotive inertial measurement units were subjected to severe accumulative stress conditions: thermal shocks (high temperature gradients), temperature cycling (low gradients) and mechanical vibration are combined in an A-then-B sequential testing procedure, with the aim to promote failure acceleration and improve lifetime prediction. The maximum stresses, applied individually, did not cause failure on the selected components. On the other hand, accumulative bi-parameter testing conditions resulted in die attach delamination. Three batches of devices tested with different preconditioning (A-then-B or B-then-A) display different reliability figures. Failure mode and effects analysis (FMEA) is established. A Finite Element Analysis (FEA) is done based on a destructive physical analysis of the devices to confirm the correlation between the stresses applied and the physics of the failure in order to understand the thermomechanical behavior of the devices, linking it to observed failures.

Determination of Possible Reversible Lithium Plating through Electrochemical Analysis at 30 °C in Lithium Ion Cells

Fast charging of lithium ion cells is an important hurdle to overcome for the increased penetration of electric vehicles into the transportation market. During the fast charging of lithium ion cells with today’s current negative electrode active materials, namely graphite, lithium metal plating has a high likelihood of occurring. This occurs as most of the lithiated graphite potential is within 100 mV of Li/Li+ potential where a small polarization can result in lithium metal plating on the negative electrode. This plating leads to safety issues, and capacity fade, among other things. As the lithium metal is plated on the graphite negative electrode it can either become isolated and inactive, resulting in a loss of active lithium and capacity fade in the cell, or the lithium remains active during cell cycling where it may either migrate into the negative electrode over time or travel to the cells positive electrode during discharge.To date most lithium metal plating identification techniques, involve destructive physical analysis. The current study presents an electrochemical approach and technique which may identify reversible lithium metal plating on graphite negative electrodes during fast charging of lithium ion cells.

Lithium-ion cell response to mechanical abuse: Three-point bend

Abstract Previous studies have investigated the conditions required to immediately initiate thermal run-away, however there is little information available about the development of internal cell defects in abused cells that lead to thermal run-away or failure as a result of continued cycling. In this study cells are subjected a three-point bending abuse at levels that physically damage the cells but do not cause immediate thermal run-away. The abused cells are then cycled at multiple temperatures for up to 1000 cycles. Samples were examined at 100, 200, 500 and 1000 cycles using techniques including electrical analysis, computed tomography X-ray imaging, destructive physical analysis, and cross-section analysis. The electrical performance (capacity, impedance) of the abused cells was not significantly different than that of the control group. The damage imposed during the abuse protocol did not lead to thermal runaway of the cells used in this study; instead, failures presented themselves as high resistance short circuits that were sometimes not apparent during cell cycling. It is hypothesized that these short circuits formed during cycling as a result of the natural expansion and contraction of the electrodes.

A Cell Selection Method and Validation Process for the Aerospace Battery

This paper studied a cell selection method and its validation process for the aerospace battery. A cell selection method includes visual inspection, mass and dimension check, electrical tests (OCV, CCV, AC Impedance), capacity measurement and soft-short test. The 6-sigma concept was adopted on data analysis of the cell selection. All the cells outside of the 3-sigma level were rejected, and accepted cells were selected for the validation testing. The validation process executes cycle testing of the four types of 4S string battery group according to the mission operating conditions of the Human-spaceflight followed by analysis of the battery capacity during the cycle and by analysis of non-destructive and DPA (destructive physical analysis) of the cells. The effectiveness of each item on the cell selection method was identified and discussed by non-destructive and DPA analysis with some electron microscope analysis. This paper recommends this proven selection method and validation process for the future aerospace battery.

Commercial 18650 Lithium-Ion Cells for High-Energy, High-Power, and Radiation Applications

Interest in commercial off-the-shelf (COTS) cylindrical lithium-ion cells for NASA missions has increased in recent years due to their increasing performance and demonstrated reliability. There is particular interest in cells in the 18650 format, due to their wide availability from well-established vendors. Multiple commercial products are available, offering greater than 250 Wh∙kg-1 and/or 3 kW∙kg-1 at the cell level. High specific energy has made small cells competitive with heritage large-format prismatic lithium-ion cells for spacecraft power systems; high power capability could offer an alternative to thermal batteries in some applications. The descent stage of the Europa Lander mission concept was baselined with a combination of a high specific energy lithium primary battery and thermal batteries to provide the required energy content and high-power pulse capability during deorbiting and landing. High specific energy and power lithium-ion cells could fulfill the functions of both of these, while also allowing for cell pre-screening and testing, unlike primary or thermal batteries. In support of this concept and future NASA missions, we have performed extensive evaluation of the components and performance characteristics of a number of COTS 18650 Li-ion cells, including destructive physical analysis, cycle life, rate capability, and ionizing radiation tolerance. ACKNOWLEDGEMENT The work described herein was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Support from the Europa Lander Mission concept and the NASA Engineering Safety Commission is gratefully acknowledged. Pre-Decisional Information – For Planning and Discussion Purposes Only.

Mechanical collapse as primary degradation mode in mandrel-free 18650 Li-ion cells operated at 0 °C

Low temperature charging of Li-ion batteries threatens undesired deposition of lithium dendrites which are often blamed for catastrophic, thermal runaway failures. This work identifies three distinct degradation modes experienced by 0 °C operated 18650 Li-ion batteries, revealing safety and performance risks beyond lithium plating. Two equivalently rated (2.6Ah) commercial cells from different vendors, one possessing a mandrel and one lacking, were cycled at 0 °C. Through a combination of techniques, it is identified that the first degradation mode is dominated by cell polarization at low temperature, causing lower coulombic efficiency and undesired side reactions. Next, mechanical deformation or jellyroll collapse occurs, followed by accelerated metal plating. However, in the mandrel-containing cell, physical constraint of the electrodes prevents both the deformation and metal plating modes. These degradation modes, difficult to isolate with electrochemistry alone, were diagnosed with non-destructive micro X-ray computed tomography and destructive physical analysis and confirmed with differential capacity assessment. The implication of the physical conditions induced by 0 °C operation on cell safety was assessed with accelerated rate calorimetry. Our observations emphasize the presence of thermo-mechano-electrochemical interplay in commercial cells operated in extreme conditions and assesses the safety implications of their coupling.

Transient failure mode of (Ba,Ca)(Ti,Zr)O3 pulsed power capacitor based on electromechanical coupling mechanism

A (Ba,Ca)(Ti,Zr)O3 pulsed power capacitor (BCZTPPC) is used as the power source to provide an instantaneous energy shock in spacecraft systems. In this paper, a transient failure mode about BCZTPPC is discussed and illustrated by an electromechanical coupling mechanism after analyzing the consequences of ring down tests, images of destructive physical analysis, finite-element analysis and numerical analysis. Results obtained point out that if the discharge frequency of BCZTPPC is equal to natural frequency of dielectric material, a mechanical resonance would occur based on reverse piezoelectric effect. Such an electromechanical coupling progress can fracture the inner ceramic layers and form internal cracks across the inner electrodes. As a consequence, the high-voltage immediately destroys the capacitor by dumping all the energy on the these fractured areas the next time a discharge occurs since the cracks across inner electrodes can be seems as short circuit under a high applied voltage. It is concluded that such an instantaneous slightly overloaded mechanical energy shock caused by electromechanical coupling progress is fatal to a BCZTPPC.

A Study on Effects of Copper Wrap Specifications on Printed Circuit Board Reliability.

During the manufacturing of printed circuit boards (PCBs) for a Flight Project, it was found that a European manufacturer was building its boards to a European standard that had no requirement for copper wrap on the vias. The amount of copper wrap that was measured on coupons from the panel containing the boards of interest was less than the amount specified in IPC-6012 Rev B, Class 3. To help determine the reliability and usability of the boards, three sets of tests and a simulation were run. The test results, along with results of simulation and destructive physical analysis, are presented in this paper. The first experiment involved subjecting coupons from the panels supplied by the European manufacturer to thermal cycling. After 17 000 cycles, the test was stopped with no failures. A second set of accelerated tests involved comparing the thermal fatigue life of test samples made from FR4 and polyimide with varying amounts of copper wrap. Again, the testing did not reveal any failures. The third test involved using interconnect stress test coupons with through-hole vias and blind vias that were subjected to elevated temperatures to accelerate fatigue failures. While there were failures, as expected, the failures were at barrel cracks. In addition to the experiments, this paper also discusses the results of finite-element analysis using simulation software that was used to model plated-through holes under thermal stress using a steady-state analysis, also showing the main failure mode was barrel cracking. The tests show that although copper wrap was sought as a better alternative to butt joints between barrel plating and copper foil layers, manufacturability remains challenging and attempts to meet the requirements often result in features that reduce the reliability of the boards. Experimental and simulation work discussed in this paper indicate that the standard requirements for copper wrap are not contributing to the overall board reliability, although it should be added that a design with a butt joint is going to be a higher risk than a reduced copper wrap design. The study further shows that procurement requirements for wrap plating thickness from Class 3 to Class 2 would pose little risk to reliability (minimum 5 μm/0.197 mil for all via types).Experimental results corroborated by modeling indicate that the stress maxima are internal to the barrels rather than at the wrap location. In fact, the existence of Cu wrap was determined to have no appreciable effect on reliability.

(Invited) Effect of Aging on Mechanical Properties of Lithium Ion Cell Components

The mechanical properties of aged and fresh lithium ion cell components are evaluated in this paper. Cells components were obtained from destructive physical analysis of 40Ah NMC/Graphite-based pouch cells before and after cycling and were subjected to mechanical testing. The aging tests comprised of cycling the cell across a voltage window of 4.1V to 3.0V at room temperature (25℃). Using a 2C charging rate and 1C discharging rate, the cells were subjected to over 5600 cycles before a 80% drop in the name-plate capacity was observed. Mechanical tests, including compression test, tensile test and indentation test, were conducted on the cell components to investigate differences in the mechanical performance. Comparison of the fresh and aged cells components shows that cycling the cells has different degrees of impact on the different cell components. Anodes suffered the most serious deterioration in mechanical properties while separators remained intact under the test condition investigated.

Effects of Cell Reversal on Li-Ion Batteries

Cell reversal in lithium ion (Li-ion) batteries is the condition of the anode electrochemical potential rising above that of the cathode, resulting in a negative voltage measured at the cell level. There are two primary reactions that occur at the anode at high potentials which increase cell impedance: oxidation of copper current collector, and oxidization of the carbonate electrolytes to CO2. At the cathode, the reducing potential can lead to the electrodeposition of copper to form dendrites, which pose a shorting risk if they bridge the anode and cathode. Cell reversal can be caused by poorly matched cells, a failure of the battery management electronics, or a defective cell in a pack. Under these conditions, one or several of the cells can go into reversal causing performance decreases or even a dangerous thermal runaway event. This paper examines a pack of commercial 18650 Li-ion cells in simulated geosynchronous orbit (GEO) test under conditions where one or more cells were forced into reversal. Panasonic B cells were matched and assembled into a pack to create a virtual cell. Measurements were collected using matched current shunts, and cells were cycled at a 60% DOD, but with only 90% of removed charge replaced each cycle to simulate a battery management system failure. After failure, cells were non-destructively examined using CT X-ray, and then dissected for failure analysis. The parallel cell pack operated far into reversal, with up to four complete cycles (Fig. 1) completed before all cells shorted. CT X-ray scans of the cells after shorting detectable amounts of copper dendrites in the cell, and demonstrated the first nondestructive test for cell reversal in Li-ion (Fig. 2). Destructive physical analysis of the cells showed extensive copper corrosion at the anode current collector as well as copper dendrites that were found to have fully penetrated the separator in selected areas. These results show that these cells can operate several cycles into deep reversal without going into thermal runaway, despite the observed growth of copper dendrites which pierce the cell separator. This has strong implications on cell safety and battery management. Figure 1

A Combined Aging and Abuse Study of Li-Ion Cells

Lithium-ion batteries have been powering a wide range of electrical and electronic devices, from consumer products and medical equipment to automotive systems and space applications. Some highly publicized failures of products powered by lithium ion cells, such as laptops, cellphones and electronic toys, have been reported recently. A portion of these failures has been linked to overheating of the battery resulting in fire and explosion. Overall, these incidents have catalyzed a great deal of research and design activities aimed at trying to understand the causes of such failures and help guide safer cell designs. In this work, the degradation mechanisms in commercial 18650 lithium ion cells, due to long term cycling under nominal-use charge discharge cycling as well as under electric vehicle drive cycling, are studied. In addition, studies were carried out after subjecting the cells to overcharge and external short circuit testing on fresh as well as cycled cells. During the external short test, the cells were subjected to a low resistance load (50 mΩ). Actual results indicate that for a very short time the cell experiences a very high current causing a rise in the cell temperature (>70 oC). In the overcharge test, the cell is charged up at a constant C-rate until the test is stopped due to either an abnormal event or the cell protection activation. Preliminary results have shown lithium metal deposition, as expected, in the negative electrode once that the intercalation rate has been overcome by the diffusion rate. Also, cell temperature has shown a significant increment during the overcharge process. Finally, the electrodes and electrolyte from the cycled cells were subjected to a similar analysis to understand the degradation mechanisms in long term use as well as to understand their behavior under abuse conditions after long term use. Understanding what causes this temperature rise and its effect on the electrodes could provide some information about how to prevent thermal runaway in cells and batteries used in various applications for long periods. The cells under these tests have been characterized by conducting a Destructive Physical Analysis (DPA). The study is being accomplished through charge/discharge cycling, internal resistance measurement, capacity fading, and impedance spectroscopy. Cell temperatures are monitored in all the experiments. The morphological changes are studied by extracting the electrodes from the abused cells and analyzing them by microscopy and spectroscopy techniques. This analysis will help to obtain the changes in the electrochemical properties of the cell as well as its morphology.

A Micro-/Macroscopic Safety Mechanism Study for Li Ion Battery

In order to understand the battery safety mechanisms including cell degradations on abuse conditions and field safety incidents, we used micro-/macroscopic tool to analyze the Li-ion battery design and manufacturing status which have strong direct relationship with the failure. The micro-/macroscopic analysis tool was applied to both gap measurements for the DPA (Destructive Physical Analysis) and the non-destructive failure/structure analysis of computerized tomography for the damaged samples after abuse testing. Through the combination analysis of the gap measurement by micro-/macroscopic tool and the non-destructive failure/structure analysis by X-Ray with the result of cell abuse verification tests, we can identify the latent root causes of field incident, study the battery safety mechanism and classify the safety risk level of each cell model. It can be suggested that a micro-/macroscopic analysis tool is a good method to study lithium ion battery safety mechanism and the root cause analysis of the failure.