The lithium-ion battery is one of the most important technologies for electric vehicles, portable devices, and energy storage. However, safety has always been the sword of Damocles hanging over its head. Kai Liu, an assistant professor at Tsinghua University, researches high-safety lithium-ion batteries. At the material level, he proposed a series of new methods to improve the safety of lithium-ion batteries, which may avoid the trade-off between high energy density and high safety.
He systematically investigated the thermal runaway mechanism of lithium-ion batteries and categorized it into three stages. In Stage 1 (the early stage), Kai introduced a "smart feedback" coating layer on lithium metal electrodes. When the lithium dendrites grow and pierce the coating, the latter will harden itself, "press" the dendrites, and effectively suppress their growth.
In Stage 2 (the middle stage), he proposed a novel “chemical reaction-etching” mechanism. Some chemicals were added to the separator of the battery that can "eat up" the highly active lithium metal that enters the separator. This method of using chemical quenching to slow down the growth rate of lithium dendrites has prolonged the lifespan of lithium metal anodes by 5 times.
In Stage 3 (the late stage), he invented a thermal triggered "molecular fire extinguisher." He encapsulated flame retardants in a polymer shell made of electrochemically inert, low-melting-point materials. During normal charging and discharging, the shell can prevent dissolution of the retardant into the electrolyte and mitigate the the negative effects on battery performance. When the temperature increases during the thermal runaway, the shell will melt and release the flame retardant into the highly flammable electrolyte and effectively suppress the combustion. Without affecting the performance of the battery, he shortened the self-extinguishing time of the electrolyte by nearly 30 times.