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SIMULATION OF INTERNAL SHORT CIRCUITS IN LITHIUM-ION CELLS

Simulation of Internal Short Circuits in Lithium-Ion Cells

Abstract Understanding the root causes and mitigating the safety hazards associated with internal short circuits in lithium-ion cells is an active area of research and a needed update for battery safety standards. For battery safety standards, there is a need for a practical, safe and reliable test method for battery safety standards that can assess the safety performance of lithium-ion cells under the conditions of an internal short circuit. In this paper, we provide details of a proposed test method which relies on external, localized indentation of cell casing to induce an internal fault that might simulate some field failures.

Introduction Lithium-ion cells have been powering a wide range of electrical and electronic devices from consumer products and medical equipment to automotive systems and space applications. However, some highly publicized failures of products powered by lithium-ion cells, such as laptops and electronic toys, have been reported.2 In some instances, a large number of products have been recalled. A portion of these failures has been linked to overheating of the battery resulting in fire and explosion. Fires on some cargo planes carrying bulk shipments of lithium-ion cells have regulators (United Nations and Federal Aviation Authority in the US) concerned about safe handling procedures.3 Insurance companies have funded research to support fire protection strategies for bulk storage of lithium-ion batteries.1 Finally, the most recent and publicized fire incidents involving a GM Volt and two Boeing 787 airplanes, have brought this technology under intense governmental scrutiny. 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.4

Though the lithium-ion cell is designed with integrated passive safeguards (and active safeguards in the case of pack designs), the sheer number of lithium-ion cells, the complexity of the cell electrochemistry, and the numerous usage conditions present challenges not only to the design of safe cells, but to the design of tests for battery safety standards. Moreover, lithium-ion cells are not only being used in small numbers to power an individual hand-held device, but many thousands of these commercial, off-the-shelf (COTS) cells are packaged together (with monitoring systems) to power electric vehicles and even spacecraft. In some cases, such as transportation,

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White Paper by

Alvin Wu

Mahmood Tabaddor, PhD

Carl Wang, PhD

Judith Jeevarajan,1 PhD

Corporate Research, UL LLC

1NASA Johnson Space Center


customized lithium-ion cells are being designed and employed where field usage history, which exists for COTS, is not available yet. Considering the effort in understanding and mitigating the failure of a single cell, these challenges are likely to increase manyfold for modules/packs. The ability to access and translate the results of battery safety research and field failure information into a consensus-based safety standard is a challenge that can only be addressed through open cooperation between governmental research organizations, cell manufacturers, safety stakeholders, and standards organizations. In this paper, we describe one promising new internal short circuit simulation method that is being developed for battery safety standards by UL through such a cooperative approach with other battery research organizations.

Lithium-Ion Cells

Basics of Cell Operation In the terminology of batteries, (secondary) lithium-ion batteries are rechargeable while (primary) lithium batteries are not. The main electrochemical components of a lithium-ion cell are the cathode, anode, separator and electrolyte (Figure 1). The electrochemically active materials for the electrodes are a lithium metal oxide (such as LiCoO2) for the cathode and graphitic carbon for the anode. These materials are each bound to a metal foil current collector through a binder. The electrodes are electrically isolated from one another by a micro-porous separator film within a non-aqueous electrolyte. The electrolyte allows only for transfer of ions and can be liquid, gel, polymer, or solid (such as ceramic).

Batteries basically convert chemical energy into electrical energy during discharge through oxidation/reduction (redox) reactions. For lithium-ion cells, the current between the electrodes is generated by the migration of positively charged lithium-ions. As a battery is cycled, these lithium cations are inserted into or extracted from the active electrode materials. Specifically, lithium-ions, travel from cathode to anode during a charging condition and in the reverse direction during a discharging condition. Electrons travel via external circuitry driven by a load to help equilibrate charge difference between the electrodes thereby generating a current.6

Research into the safety of lithium-ion cells has been ongoing for over a decade while battery safety standards have been in existence for even longer. Since lithium-ion cells bring together highly energetic materials in contact with a flammable electrolyte, when exposed to many different abuse conditions (vibration, high-temperature environments, crushing, etc.), the initiation of heat-generating reactions which can lead to fire and explosion are a concern. For that reason, the core tests within consensus-based battery safety standards, such as UL 16427 and IEC 62133,8 subject cells to anticipated abuse conditions. Cells are designed with integrated passive safety features such as a circuit interrupt device, a pressure vent and/or shutdown separator to help mitigate an over-current condition or excessive pressure buildup within the cell.9 However, such features cannot address all potential abuse conditions. In

Figure 1: Components of a cylindrical lithium-ion cell (left), and, discharging mechanism for lithium-ion cell (right)5


addition, cell designers and researchers have been investigating a range of options for improving cell safety, such as cell chemistries, including less reactive materials for anode, cathode, and/or electrolyte, without compromising performance. Nevertheless, field failures have been recorded involving explosive release of energy along with fire. There have been some ideas on the underlying initiators for such failures, known as thermal runaway, in cells. One source describes some causes of internal cell faults leading to field failures: contamination, electrode damage, mechanical damage, and thermal abuse.10

Internal Short Circuits (ISC) A review of the publicly available lithium-ion battery research shows a strong focus on understanding and mitigating cell failure modes, specifically those due to internal short circuits.18

Though only brief accounts of field failures are available, in some cases, the presence of manufacturing defects has been noted to lead to internal short circuits within the cell. A particle, depending upon its size and morphology, may pierce the separator allowing for direct contact between electrodes (internal short circuit). There is also the possibility that the over-charge or over-voltage subjection of lithium-ion cells causes the formation of lithium dendrites -- small, thin, crystal,needle-like structures -- which may eventually puncture the separator leading to an ISC.11 However, once an ISC is established, a localized heat source is generated within the cell. This is termed joule heating and the extent of the heat produced locally depends upon the internal resistance of the cell and state of charge (current flow) along with other factors as shown in the figure below.

When portions of the cell affected by joule heating reach certain temperatures (generally above 120°C), exothermic reactions between the active electrode materials and the electrolyte are initiated. Depending upon the ability of the cell to dissipate the heat, the heat generated may continue to sustain these exothermic reactions with a rapid increase in the temperature of and pressure within the cell (thermal runaway).12 In cases, where the pressure buildup is relieved via the safety vent, then expulsion of the contents is still a chemical hazard.

The combined high pressure and lower modulus of the casing due to high temperatures can compromise the integrity of the casing leading to explosive release of volatile gases, which may further ignite. The high temperature of the cells could certainly lead to ignition of nearby flammable objects. In the case of battery packs (or



bundles of lithium-ion cells), the highly dense configuration of the cells creates a condition where one cell failure could spread to adjacent cells without proper external battery thermal management.

Battery Safety Standards Battery safety standards contain a menu of abuse tests. Table 1 shows the extent of harmonization of some major safety standards for batteries. Though it should be noted that for similarly named test procedures in the various standards, the test details may not be strictly identical

Table 1: Survey of international battery safety standard abuse tests

manner. For example, the number of samples and the state of sample charge prior to testing may vary. However, a review of international safety standards related to lithium-ion cells reveals that, despite harmonization across a large number of abuse tests, only a few are considering the inclusion of specially designed internal short circuit tests.13

As product recalls undermine public confidence, it is imperative to promote the safe commercialization of lithium-ion cells by ensuring that consensus-based

battery safety standards such as UL 1642 and IEC 62133 effectively capture the fast-changing pace of lithium-ion cell safety and design knowledge. Therefore, the key activity for the safety standards for lithium-ion cells is the development of an internal short circuit test suitable for such standards.

Quite often research-level testing cannot meet the best-practices requirements for acceptable safety tests for standards. In addition, the challenges associated with the myriad possible root causes for


an ISC and the various configurations of lithium-ion cells (cylindrical, prismatic, and pouch) and multi-cell (module/pack) configurations may preclude the possibility of a single ISC-specific safety test. A review of publicly available ISC tests (Figure 2) shows that most require either access to the cell during manufacturing to insert a particle (FISC test),14 compromise integrity of casing (ITRI test) or rely upon disassembly of a production cell to insert a particle (SNL test).19

Figure 2: List of ISC tests for lithium-ion cells

Currently, there are only two new simulated internal short circuits (ISC) tests, which are either under consideration or part of some consensus-based standards. One is the Forced Internal Short Circuit (FISC) test and the other is based on an indentation-type approach. The FISC requires the disassembly of the cell with testing being conducted on the jelly-roll (cell internals without the casing) making it useful mainly for research studies.

Generally, the disassembly of a cell is not considered a best practice for tests in safety standards, especially, as this operation for a lithium-ion cell involves a hazardous condition. Instead, we have chosen an approach that will be called an Indentation Induced ISC test. This test is based on a history of indentation type testing by UL in collaboration with NASA 15, 16 and more recently with Oak Ridge National Laboratory (ORNL),20 with the intention of bringing the best ideas into a single ISC test method.


Indentation Induced ISC Test Indentation testing evolved from previous methods that depended upon an object either penetrating (nail penetration test) or crushing (rod circumference crush test) the cell.17 In the penetration approach, there is little deformation of the cell, however, the nail acts as a bridge generating an ISC that is not localized. The breach of the casing also dissipates any hazardous pressure buildup. For the rod circumference crush test, the deformations of the cell are large and generally the safety mechanisms relieve the pressure. Of course, both these may be necessary if they match abuse conditions that the cell can be subjected to in the field. However, neither generates the localized ISC within a closed cell that is considered to simulate the field failures noted previously.

The current Indentation Induced ISC test setup for cylindrical lithium-ion cells is shown in Figure 3. The cell is placed in a holder that prevents rotation or translation of the cell. An indenter with a smooth profile presses from above against the cell casing at a constant speed (0.01 – 0.1 mm/s). Test measurements include temperature of the casing surface at a point near the indentation site, distance traveled by indenter (amount of cell casing deflection), applied force through indenter, and open circuit voltage. The cells can be at different states of charge (SOC) or stages of aging. This entire setup is placed within a chamber that allows for control of ambient temperature.

As the indenter presses against the casing, layers of separator, anode

Figure 3: Pictures of Indentation Induced ISC test for cylindrical lithium-ion cells

and cathode immediately below the indentation region are deformed due to localized high curvature (Figure 4). The resulting high stress/strain will lead to a mechanical failure of the separator allowing for direct contact between electrodes at a distance only a few layers below the casing surface (Figure 5). Though this mechanical event cannot be observed and documented in real time, the effect of the separator failure is a sudden drop in the open circuit voltage (Figure 6). For some cells, seconds after a measured drop in the open circuit

Figure 4: CT scan images of cylindrical lithium-ion cell prior to testing (left) and single CT scan image of cell after indentation (right)

voltage (100-500 mV), there is a rapid increase in cell surface temperature (as high as 700°C) with an outcome involving explosive release of gases and flames (Figure 7).

Now the typical risk measure consists of severity of failure multiplied by probability of failure. In forcing a failure, the Indentation Type ISC test is basically measuring the severity of cell failure. This approach is one that has been adopted by NASA in screening of COTS rechargeable batteries for space applications. Cells that


Figure 5: CT scan image of cell showing breakdown of layers directly below indentation region

Figure 6: Measurements taken during the indentation test for a cell undergoing thermal runaway

Figure 7: Picture of cells experiencing thermal runaway (left) and one example of explosive failure of a lithium-ion cell during indentation test (right)


do not perform well under this type of test would then be subjected to a more stringent secondary testing schedule that might help establish the probability of ISC cell failure.

Though the method does not rely on a particle-induced defect of the separator, the key question is whether the damage to the separator is similar to that might be expected in field failures involving particles. To determine what is happening inside, the test was run on a cell at 0% and 50% SOC to help reduce damage to the cell that would make measurements impossible. Figure 8 shows the layers within the cell where a puncture is

Figure 8: Cell windings subjected to indentation under 0% SOC (top pictures) and 50% SOC (bottom pictures)

apparent. This hole has been measured to have a radius of 1-2 mm. For the 50% SOC, some localized heating is apparent. It was also determined that the puncture penetrated several layers, again a feature of the internal short circuit expected for particle-related field failures. These observations strongly suggest that the test method is suitable for simulating ISC conditions that might be representative of field failures for lithium-ion cells.

Aside from simulating an internal short circuit, the test should be sensitive to the relevant design changes that might affect the resulting ISC performance. To date, an analysis of results from many

cells subjected to this indentation type ISC test shows a correlation between test performance (observed severity of failure) to energy density, thermal stability of active materials, material changes (such as STOBA in Figure 9), and chemistry of the cell.

Other Form Factors The UL indentation test for ISC simulation has been modified to test prismatic and pouch cells. The main challenge for pouch cells is the expected inevitability of puncturing the soft casing, especially when it inflates due to outgassing from exothermic reactions set off by the ISC. However, the trends in the data are very similar to those for cylindrical cells.

Figure 9: Data from 18650 type NMC (1950 mAh) cell without STOBA (top) and with STOBA (bottom)


Summary Lithium-ion cells are expected to be a dominant portable energy/power source for electrical and electronic devices for the near future. However, the energetic nature of the active materials within a lithium-ion cell presents safety challenges. One of these challenges is insight into thermal runaway within a cell believed to be responsible for field failures over the last few years. As this research is ongoing, it presents a further challenge to new test development for battery safety standards. Considering the effort in understanding and mitigating the failure of a single cell, these challenges are likely to be increased manyfold for battery modules and packs. For example, one commercially available electric vehicle contains over 6,000 cylindrical (18650) lithium-ion cells.

As this is a very challenging area, an open and cooperative dialogue to share information on the failure modes of lithium-ion cells and to help develop ISC test(s) for consensus-based safety standards is ever-more important.

With the expected growth in using COTS for a variety of applications, it is important the regardless of how the OEM may choose to subject the cells to further in-house testing incoming cells be certified according to a suite of comprehensive abuse tests. This approach is taken by NASA, whereby they select certified COTS cells and still subject the cells to their space application-specific testing.

Even if the cell is customized for an application, subjecting the cell to tests prescribed by safety standards (possibly with some modifications) could help filter and rank different designs and provide greater confidence in safe field performance over the expected life of the battery system.

Acknowledgments The authors would like to acknowledge the tremendous support given by Dr. Hsin Wang (ORNL).


UL and the UL logo are trademarks of UL LLC © 2013. No part of this document may be copied or distributed without the prior written consent of UL LLC 2013.

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