<i>Backscatter</i><br> Focus on Lithium-ion Batteries

Backscatter
Focus on Lithium-ion Batteries

BY Donald Christiansen Posted: 24 Feb 2017

Nearly 3 million Galaxy Note 7 smartphones were recalled by Samsung because of fires triggered by short-circuited batteries. In seeking the cause, Samsung first, and logically, focused on the batteries themselves.

The lithium-ion batteries were, in theory, ideal for the thin design and long-life expectancy of the Galaxy 7 smartphone. Lithium ions, emanating from the negative electrode, provide a high energy density. Earlier lithium batteries were prone to dendrite growth, which could extend to the positive electrode and result in short circuits. Subsequent modifications of the anode structure enabled the formation of a solid electrolyte interface (SEI) on both anode and cathode to prevent lithium ions from aggressive reaction with solvent (internal short circuit and thermal runaway). However, damage to the SEI can be caused by excessive heat, mechanical damage to the cell structure, and aging issues.

Suspects

The initial Samsung failures had been caused by a deformation of the battery casing. The electrodes were thus pressed together, shorting them and igniting the entire smartphone.

The company then switched to a second battery supplier. But poor welding of the cathode tab in these cells created sharp burrs that were forced into the anode due to its swelling. A short circuit between the copper of the anode and the cathode tab resulted, leading to thermal runaway.

Ultimately production ceased and all the Galaxy Note 7s were recalled. Samsung then announced the formation of a battery advisory group of academic and research center experts, and reported that it would initiate stronger quality assurance processes with random inspections of suppliers’ batteries.

What appears to have been missing in the design and development of the Samsung Galaxy was a suitable program of stress testing of the operating device prior to its large-scale rollout. Many observers subsequently suggested that the blame for its failure cannot be attributed solely to the batteries used, but rather to the overall design of the smartphone case and the location of the battery within it. It was also noted that, if unusual heating were to occur, the built-in battery could not be removed and replaced.

Next?

The unfortunate Samsung experience does not signal the demise of the lithium-ion battery. The cells are also specified for large-scale inclusion in energy storage associated with national power-grid facilities in the United States, as well as for use in electric cars. Some 8000 individual cells are reportedly used in a Tesla car. These are slightly larger (22 mm dia by 70 mm long) than the standard (18 mm dia by 65 mm long) cell.

In neither application is the concern for safety and reliability absent. In the case of the power grid, vast quantities of batteries are installed in fixed locations, offering prime targets for sabotage or terrorist attacks. And when based in vehicles, will the batteries be able to withstand a crushing force of 15000 pounds in a head-on collision?

The Samsung experience may prove a valuable incentive to electric-vehicle designers, who will be challenged to develop test procedures that simulate real-world operational stresses, including those that arise from long-term vehicle use and operation in varying climate conditions.

A 2012 proposal (see Resources, Stephens et al.) called for a comprehensive study of the hazards of lithium-ion batteries in hybrid-electric, plug-in hybrid-electric, and battery-electric vehicles, including identifying failure modes, risk levels, and mitigation strategies. By 2014, the National Highway Traffic Safety Administration (NHTSA) had begun reviewing incidents involving lithium-ion based batteries in vehicles. In a study of Tesla Model S incidents it identified several safety-related concerns, including multi-cell crushing, thermal propagation and containment, fault detection, and passenger compartment isolation. In two Model S vehicles that struck objects on the road, the battery enclosure was penetrated. Two Chevrolet Volt vehicles caught fire while parked, and first-responder actions and external fire exposure protection were seen as issues.

Delayed Failure

Electrochemical damage, including dendrite growth and SEI breakdown can result from hundreds of charge/discharge cycles in normal service, or may be exacerbated by excursions outside the normal duty cycles. Thermal runaway and short circuits may result. Experts at NHTSA note that key parameters related to damage growth (and its control) are inferred through computer models, but not actually measured. These observations may have greater import to electric-vehicle makers than to smartphone designers. Normal use for electric vehicles is predicated on up to 15 years of service and 150,000 or more operating miles. Samsung and its competitors deal with anticipated product life cycles that are a fraction of that.

Lithium-ion batteries get high marks because they can charge, discharge, and store high levels of energy. Yet the experts note that high levels of storage equate to high levels of energy discharge in case of failure. They also warn that while the safety of lithium-ion batteries can be managed, if their operating range is extended as a result of current R&D, they may in the future be operated closer to the limits where damage is initiated and failures occur.

Other factors must be considered when lithium-ion batteries are used in support of the national grid. I’ll leave that discussion for another time, but meanwhile welcome your observations.

The lithium-ion battery application challenge looms large. The cell failures are not fail-safe but are catastrophic and a potential danger to humans. Beta testing procedures do not appear applicable as they are more useful in soliciting feedback from users concerning new product features and ease of use. The necessary reliability test and maintainability procedures appear to be incumbent on the particular product or system manufacturer, and not solely on the battery suppliers.

Resources

  • Stephens, D., P. Gorney, and B. Hennessey, “Failure Modes & Effects Criticality Analysis of Lithium-Ion Battery Electric and Plug-in Hybrid Vehicles Project Overview,” Jan. 26, 2012, SAE Government/Industry Meeting.
  • Smith, B., B. Park, and W. Godfrey, “Lithium-ion based Rechargeable Energy Storage System Real-World Incident Review, GTR Status Update,” NHTSA, May 2014.
  • Zachary, G. P., “What Frankenstein Can Teach Engineers,” IEEE Spectrum. Feb. 2017.
  • Consumer Reports on Samsung Galaxy Note 7   http://www.consumerreports.org/smartphones/samsung-investigation-new-details-note7-battery-failures/
  • Christiansen, D., “When Designers Should Say ‘No,’” Today’s Engineer, June 2010.
  • Christiansen, D., “Accidents Waiting to Happen,” Today’s Engineer, October 2003.
  • Cardwell, D., "Moving beyond cars,Tesla uses batteries to bolster power grid, " Deccan Herald, 13 February 2017

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Donald Christiansen is the former editor and publisher of IEEE Spectrum and an independent publishing consultant. He is a Fellow of the IEEE. He can be reached at donchristiansen@ieee.org.

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