Non-destructive testing of lithium-ion batteries

Martin Zäpfel, February 2017

Facing a clear trend towards mobile power supply,^[1] non-destructive testing of lithium-ion batteries becomes crucial. Batteries based on the lithium-ion technology are produced on a large-scale^[2] for mobile phones and other handheld or medical device applications,^[3] and will be more and more utilized in the field of automotive and even in aircraft technology.^[4][5] Quality and safety standards must be observed for these components with regard to the field of application by non-destructive testing.

Motivation

Safety is the most important criterion concerning electric-car batteries.^[5][6] In case of failure an amount of thermal energy can be released which is six times as much as the electric energy stored in the lithium-ion battery.^[6] This led to callbacks of millions of batteries,^[7][8] recently by Samsung which recalled the Galaxy Note 7 due to the risk of fire.^[9] To prevent those damages, non-destructive testing methods exists in variable types, for any of the different shapes and material combinations of lithium-ion batteries that influence performance, costs and safety. As an example, figure 1 offers a detailed insight into a common lithium-ion battery by x-ray computer tomography scanning.

Fundamentals of lithium-ion batteries

Advantages of lithium-ion batteries are high specific energy, performance and efficiency as well as low self-discharge.^[10] Different shapes of cells, cylindrical, coin, prismatic or pouch cells^[6][10] as shown in figure 2, consist of the same typical components: cathode (LiFePO4, LiCoO2…) and anode (graphite, Li …) electrodes, an electrolyte (solid or liquid, consisting of organic solvent, lithium salt, additives), a separator (PP, PE…), current collectors and a battery box.^[6] These cells are combined to modules that, together with for example a temperature management system, form the overall battery.^[10] As the cell itself with its chemical components can induce a hazardous reaction, it is also referred to as a battery.

Failure mechanism

Lithium-ion batteries may fail during each stage of life cycle due to manufacturing errors, abuse conditions or degradation.

Failure causes during production:^[6]

• Inclusion of air or water
• Damage of layers or the separator
• Mechanical deformation
• Defect in liquid or gas tightness
• Divergent filling level
• Welded defect
• Particle contamination

Failure causes due to abuse conditions:^[3][6]

• Mechanical: e.g. internal short-circuit due to dendrites or particles
• Electrical: e.g. overcharge which may lead to reactions, formation of gas and temperature increase
• Thermal: e.g. flame exposure

Failure causes due to degradation:^[11]

• Reformation of solid electrolyte interface
• Contaminations/ migration of reaction products
• Lithium plating
• Corrosion
• Gassing

In case of failure, high temperature due to internal reactions can smelt the separator of a lithium-ion battery which may lead to a further increase of the temperature and finally to a thermal runaway.^[6] Non-destructive testing methods can contribute to prevent those damages.

Testing methods

The testing methods can be applied either during the process of production or on the final product to detect failures or to analyse the state of degradation. In fact, the quality of production influences the process of degradation as well.^[10] Common methods are:

• Light optical systems for the inspection of the separators^[12][6]
• NIR spectroscopy for the detection of water^[6]
• IR thermography for the detection of particles^[13][14]
• High-precision weighing for the inspection of tightness and filling level^[6]
• Electrochemical impedance spectroscopy^{[15][16][17][18]} for the inspection of the welding of current collector or the detection of lithium plating^[19][20]
• Cycle tests (charging, discharging) for the determination of state-of-charge/ state-of-health^[6][21]
• X-ray computer tomography^[22][23]
• Neutron radiography^{[24][25][26][27]}
• Combination of previous methods^[28]

The previously listed testing methods are specified in table 1:

Table 1: Description of testing methods

Light optical systems

The articles Close-range photogrammetry as well as Shearography describe the application of light optical systems. Based on electromagnetic waves, light optical systems allow the contactless measurement and analysis of failures, e.g. damages of a separator.

NIR spectroscopy

Near-infrared (NIR) spectroscopy is a method based on electromagnetic waves near the infrared spectrum, ranging from 800 nm to 2,5 μm. With specific frequencies, atomic bonds of a molecule are stimulated to vibration. Although bands in the NIR region are weaker in intensity than the corresponding fundamental bands, NIR spectroscopy is dominated by overtones and combinations of O-H connections and therefore particularly suitable for the detection of water. In-line measurement, ease of sample handling and rapid analysis even under field conditions are further advantages of NIR spectroscopy.^[29]

IR Thermography

Thermographic inspection detects the infrared radiation emitted from an object. Thus, any defect will appear as a temperature variation identifying damages of the separator or foreign particles.^[6] Analysing a thermal runaway, figure 3 shows the mean surface temperature profiles of three regions on a cell during a thermal abuse test. The detailed articles Infrared thermography of CFRP sandwich structures and Infrared thermography describe the different variations of thermography.

High-precision weighing

By analysing the mass of a lithium-ion battery, high-precision weighing can detect small deviations between the actual mass of a sample battery and the desired value and therefore differences in the filling level. Furthermore, a deviation in mass can also indicate losing electrolyte and therefore leakage.^[6]

Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) as an electrochemical technique, like galvanostatic cycling at different rates or hybrid pulse power characterization, offers an assessment of capacity loss due to increasing resistance.^[30] Hence, the approach of EIS is to apply an electrical pulse to the electrodes. The response to this stimulus, the resulting current or voltage, is observed and analysed.^[31][3]

Cycle tests

Cycle tests, the controlled charging and discharging of lithium-ion batteries, indicate the state-of-charge (SOC) and the state-of-health (SOH) of a battery. Similar to EIS, the resulting current and voltage are analysed over time. Thus, cycle tests can detect changes, but cannot allocate these changes to a specific failure cause, e.g. the SOH is the result of a combination of previous failure causes. In case of noticeable changes, further analysis need to be conducted.^[6]

X-ray computed tomography

X-ray computed tomography (CT) provides high resolution two or three-dimensional images as shown in figure 4 for example to detect internal short-circuits or even to analyse a thermal runaway.^[22] X-ray tomography describes the radiographic testing using x-rays. Thus, x-ray CT can be used to analyse microstructural properties of electrode materials for diagnosing battery failure mechanisms post-mortem as well as in-situ to quantify microstructural processes or degradation.^[22]

Neutron radiography

Whereas X-rays interact with the electrons around the nuclei, neutrons interact directly with the nuclei itself. Thus, neutron radiography can detect light elements such as lithium easily even in a material composition together with middle and heavy elements as iron or lead.^[24] Moreover, neutron radiography allows studying the behaviour of lithium-ion batteries during electrochemical cycling in situ,^[25] e.g. the change of the lithium distribution in the battery,^[24] which can lead to a better understanding of lithium storage mechanisms. However, the use of neutron radiography is limited due to the requirement of a neutron source, such as the research neutron source Heinz Maier-Leibnitz (FRM II), which uses the nuclear fission of uranium.

The combination of previous methods^[28][32] creates an additional benefit by providing an overview of all the different parameters of lithium-ion batteries. Moreover, linking together for example thermal imaging, x-ray micro-tomography and electrochemical impedance allows to analyse specific dependencies and to draw conclusions for further optimization.^[33]

Conclusion

During the last years, efforts have been made to analyse lithium-ion batteries. However, there still exists a huge demand of further research in this field. Especially with increasing demand for safe and efficient batteries, new and optimized test methods adapted to short cycle times will be required.^[6][10]

Linked articles

Nahbereichsphotogrammetrie, Shearography, Infrarot-Thermographie_an_Kohlefaserverbundwerkstoffen, Infrared_Thermography, X-ray_tomography

Further reading

Brodd, R.J. (Hrsg.): Batteries for Sustainability - Selected Entries from the Encyclopedia of Sustainability Science and Technology. Springer Science+Business Media, New York (2013), p. 425 - 450/456.

Links to external videos:

Understanding how Lithium-ion batteries fail, UCL Chemical Engineering

Real-time thermal imaging video of a cell during thermal abuse.^[22]

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