What you will learn:

• How did polymer current collectors prevent thermal runaway in 18650 Li-ion cells?
• The “Operando” (live) high-speed x-ray radiography was used to assess experiences of nail penetration abuse.
• The post-mortem 3D CT scan presented a visual analysis of the cells after abuse.
• Explanation of the short circuit isolation mechanism in polymer current collectors.

The quest for safer lithium-ion batteries continues to drive advanced research, particularly with concerns about the prospect of thermal runaway, with most cases of thermal runaway being attributed to an internal short circuit. Such a short circuit generates an increasing amount of heat which can trigger a failure in adjacent batteries and start fires. It has been shown that the temperature inside the battery can reach 800°C and more in just seconds.

To better understand the probe, then design and validate a solution, researchers from the National Renewable Energy Laboratory (NREL) worked with a global team including NASA, University College London, The Faraday Institution in Oxford, National Physical Laboratory in London and The European Synchrotron in France.

Their approach involved material substitution for the current collector in these batteries that conducts electricity to and from the negative and positive terminals. This structural and electrical element is usually made of aluminum or copper. Instead, the researchers used a polymer current collector (PCC), which is a collector with an inner layer of plastic between the metal. (Fig. 1). The PCC was manufactured by Soteria Battery Innovation Group and custom 18650 batteries (18 mm in diameter and 65 mm in height, a widely used, industry standard size) were manufactured by Coulometry.

% {[ data-embed-type=”image” data-embed-id=”60c264df2a3ece35428b47c2″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”1. Nail penetration and failure mechanism. (A) Nail penetration of a standard commercial metal CC cell. (B) Failure mitigation mechanism of aluminum (Al) PCC and copper (Cu) PCC cell during nail penetration; scanning electron microscopy (SEM) insets of the PCC cross-sections illustrate the ∼8-μm polymer substrate “core” and ∼0.5-μm metal-film coating.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/06/ED_interest_safer_battery_Fig1.60c264df12351.png?auto=format&fit=max&w=1440″ data-embed-caption=”1. Nail penetration and failure mechanism. (A) Nail penetration of a standard commercial metal CC cell. (B) Failure mitigation mechanism of aluminum (Al) PCC and copper (Cu) PCC cell during nail penetration; scanning electron microscopy (SEM) insets of the PCC cross-sections illustrate the ∼8-μm polymer substrate “core” and ∼0.5-μm metal-film coating.” ]}%

The researchers initiated a controlled, instrumented failure by driving a nail into the cells to trigger a short circuit (these aren’t hardware nails: each had a built-in K-type thermocouple). The nail penetrated to a depth of 9mm, an event that took between 1.1 and 1.6 seconds (Fig. 2).

% {[ data-embed-type=”image” data-embed-id=”60c26502f3a390b7418b47d1″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”2. “Post-mortem” analysis of cells, where the numbered arrows represent the same feature observed across multiple locations and aren’t limited to the examples marked, in which “1” indicates the negative Cu PCC, “2” highlights the positive Al PCC, and “3” shows the separator. (A) Reconstructed tomogram of the entire nail-penetrated cell (Al PCC + Cu PCC), showing evidence of the shear forces exerted on the electrode assembly. (B) Cylindrical cross-section of the nail penetration path shown in (A) of a cell (Al PCC + Cu PCC), providing visualization of the structure and indicators of the phenomena that occurred during mechanical abuse. (C) Cylindrical cross-section orthogonal slice of the nail-penetration path in the penetrated direction for a cell (Al PCC + Cu PCC). (D) Post-nail-penetration test OCV measurement showed 4.077 V for the cell; note the nail hole in the side.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/06/ED_interest_safer_battery_Fig2_web.60c26501a5e05.png?auto=format&fit=max&w=1440″ data-embed-caption=”2. “Post-mortem” analysis of cells, where the numbered arrows represent the same feature observed across multiple locations and aren’t limited to the examples marked, in which “1” indicates the negative Cu PCC, “2” highlights the positive Al PCC, and “3” shows the separator. (A) Reconstructed tomogram of the entire nail-penetrated cell (Al PCC + Cu PCC), showing evidence of the shear forces exerted on the electrode assembly. (B) Cylindrical cross-section of the nail penetration path shown in (A) of a cell (Al PCC + Cu PCC), providing visualization of the structure and indicators of the phenomena that occurred during mechanical abuse. (C) Cylindrical cross-section orthogonal slice of the nail-penetration path in the penetrated direction for a cell (Al PCC + Cu PCC). (D) Post-nail-penetration test OCV measurement showed 4.077 V for the cell; note the nail hole in the side.” ]}%

Of course, the problem of “in operando” battery tests (live observation) is twofold: you have to initiate the failure in a controlled manner and then capture the very high speed events induced by the failure. The NREL project used high-speed x-ray imaging at 2,000 frames per second (fps) as well as post-failure analysis including additional CT scans and scanning electron microscope (SEM) imaging.

They concluded that their use of the CCP was successful. Repeated experiments on several batteries demonstrated that adding PCC to the positive electrode was sufficient in all cases to prevent thermal runaway. The specific runaway prevention mechanism and associated squelching was that as the temperature increased, the PCC would retract to isolate the nail from the negative terminal and thus close the short circuit.

As this is an NREL project, the test set-up is sophisticated and advanced. The increase in temperature and time were captured via their newly developed calorimeter and nail penetrating cell chamber, formally referred to as the Fractional Thermal Runaway Calorimeter (FTRC) (Figure 3 and references 1 and 2).

% {[ data-embed-type=”image” data-embed-id=”60c2652ef3a390bc418b4801″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”3. Calorimeter and nail-penetration cell chamber. (A) Rendering of the fully assembled calorimeter inside a thermally insulating calorimeter casing. (B) A magnified view of the FTRC internal structure. (C) Rendering of the Al nail-penetration cell chamber using a steel nail with an internal thermocouple. Additional features include spring-pressurized cell skin thermocouples, gas sealing O-rings, and thermally isolating ceramic.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/06/ED_interest_safer_battery_Fig3_web.60c2652de9bd6.png?auto=format&fit=max&w=1440″ data-embed-caption=”3. Calorimeter and nail-penetration cell chamber. (A) Rendering of the fully assembled calorimeter inside a thermally insulating calorimeter casing. (B) A magnified view of the FTRC internal structure. (C) Rendering of the Al nail-penetration cell chamber using a steel nail with an internal thermocouple. Additional features include spring-pressurized cell skin thermocouples, gas sealing O-rings, and thermally isolating ceramic.” ]}%

In addition, the associated documentation is comprehensive, starting with their article “Prevention of thermal runaway of lithium-ion batteries using current collectors with polymer substrate” published in the journal Physical Sciences cell reports. Other available resources including their high speed videos are also cited in the references below.

Also note: In addition to this project and its documentation, NREL just released its Battery failure database, a compilation of data generated by hundreds of abuse tests (nail penetration, thermal abuse, internal short circuit) conducted on lithium-ion batteries. This comprehensive database provides information on the amount of heat generated by a battery during thermal runaway, as well as hundreds of high-speed x-ray videos of internal failure mechanisms, which you would prefer someone else to. else do rather than try to investigate on your own.

The references

1. Electronic design, “Calorimeter deliberately drives Li-Ion cells into thermal runaway and explosion“

2. NASA, “Improve Battery Safety with Runaway Fractional Thermal Calorimetry”(Excellent PowerPoint presentation); then click on the button “Display the document”

3. Cellular reports Physical sciences, “Lithium-ion battery thermal runaway prevention using polymer substrate current collectors“

4. Cellular Relations Physical Sciences, “Additional information”

5. NREL, “Video S1: High speed (2000 fps) x-ray image of a cell undergoing nail penetration where thermal runaway is observed“

6. NREL, “Video S2: High speed x-ray (2000 fps) x-ray of the cell underlying the penetrating nail where thermal runaway has been prevented“

7. NREL, Battery failure database