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Improving lithium battery safety

New experiments characterize battery thermodynamics and highlight ubiquitous design problems that increase the risk of failure.
10 May 2013, SPIE Newsroom. DOI: 10.1117/2.1201304.004823

Lithium-ion (Li-ion) batteries have the highest energy density of all existing battery technologies. However, widespread reports of lithium battery fires aboard Boeing’s Dreamliner airplanes and in other consumer-facing applications have raised questions concerning their use. The recent Li-ion problems are due to a fundamental misunderstanding among the engineering community of the governing principles of battery thermodynamics. Existing solutions for battery safety focus on mitigating symptoms rather than treating the root cause.

The sensational nature of battery fires means that the public’s perceived risk of failure outpaces actual failure rates. As such, brand identities are directly threatened, and companies are being forced to choose less efficient energy storage solutions. For example, a 2008 incident with the US Navy’s Advanced Seal Delivery System attributed to Li-ion failure prompted the Navy to revert to older, less dense battery technologies like zinc-silver and nickel-cadmium (NiCd) for many of their field operations. More recently, Airbus dropped Li-ion technologies in favor of less energy dense NiCd batteries. We have focused on characterizing the thermodynamic basis of heat generation in lithium batteries in an effort to strengthen the foundational knowledge needed to make the operation of Li-ion technologies safe and resliable.

Lithium batteries heat up during charging and discharging. Although, the precise sources of the heat generation are not understood, it is known that they are dependent on factors that include the ambient temperature and the state of charge. The standard explanation for battery heating focuses on internal resistance, Ri, which includes electrolyte resistance, Rs, anode resistance, Ra, and cathode resistance, Rc (and where Ri is often equated with Rs). In actuality, there are at least five different internal components in lithium batteries that generate heat. These include Rs, Ra, Rc, entropy change at the anode, ΔSa, and entropy change at the cathode, ΔSc.1–3 Our work shows that internal components other than Rs depend strongly on temperature (see Figure 1).4 In this analysis, contributions of each component to heating were measured in a 4.4Ah Li-ion cell with a 1C discharge rate at different environment temperatures, Tenv. C is the ampere-hour (Ah) capacity of the cell. Thus, a 4.4Ah cell discharging at 4.4A rate is said to discharge at 1C rate. Our results confirm that Rs is not the dominant source of heating at any temperature.
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