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.