9. Lithium-Ion Batteries, Safety

Safety of lithium-ion batteries is a critical topic that has not received adequate attention in the past, largely due to the fact that data regarding safety failures have been severely restricted. As a result, there are numerous misunderstandings in a field that has not received the same degree of scientific and technical rigor as other areas of lithium-ion battery technology development. However, safety of lithium-ion batteries will become even more important as lithium-ion technology enters transportation markets. Under suitable triggers, Li-ion cells can experience thermal runaway, i.e., the rapid increase in cell temperature accompanied by venting, vent-with-flame, ejection of cell parts, fire, and explosion. Safety failures of lithium-ion cells can result from a variety of triggers including overcharging, overheating, crushing, mechanical impact, and external short circuits. Safety tests have been devised for all these abuses, with varying degrees of fidelity. However, most safety incidents that have taken place with lithium-ion batteries occur due to the slow and rare development in cells of internal short circuits that mature to the point that they result in thermal runaway. Most safety tests carried out in the laboratory or factory do not replicate the conditions by which safety incidents actually occur in the field. These issues are characterized in detail, and an improved overall framework for considering lithium-ion battery safety is suggested.