In our lifetimes, we’ve gone from popping batteries into our remote-control toy cars to stepping into fully electric vehicles. This technological leap has been made possible in part by the evolution of traditional lithium ion to lithium metal batteries (LMBs), rechargeable batteries with about 10 times higher specific energy than their predecessors.
Still, today’s LMBs are not quite suited for a greener electricity-powered future. One limiting factor is that over time, lithium builds up on the LMBs’ electrodes and creates ‘dendrites’ that overheat when the battery is in use, ultimately shortening the batteries’ lifespans and posing dangerous safety hazards.
“Much research is directed towards suppressing the dendrites through physical blocking or chemical regulating the deposition process of lithium metal,” said Zhaolin Liu, a Principal Scientist at A*STAR’s Institute of Materials Research and Engineering (IMRE). “However, the thermodynamic factors associated with the formation of dendrites, which can be key to understanding LMB failure, are often neglected.”
Liu teamed up with A*STAR colleague Xiaowei Wang; and researchers in China from Xi’an Jiaotong University, Shanghai Institute of Space Power Sources, Guangdong Jiana Energy Technology Co. and Shenzhen University to think up ways of dialling down the hotspot-driven dendrite formation by first mapping the thermodynamic behaviours in LMBs.
In a series of simulations, the research team confirmed that factors such as an LMB’s surface energy, Li-ion concentration gradient and migration energy, and temperature all influence the electrochemical reaction of lithium while the battery is in use.
“The fast-charging process of LMBs accelerates local heat generation and the formation of local hotspots exacerbating the Li dendrite formation,” said Liu.
Armed with these insights, the researchers designed a graphene-coated separator that shields lithium metal electrodes from forming dendrites. Graphene—carbon atoms arranged in a honeycomb structure—was chosen for its remarkable heat and electrical conductivity.
“The high thermal conductivity of graphene allows it to act as an in-situ thermal dispersion medium to prevent local thermal accumulation,” said Liu, adding that graphene-coated separators acted as a ‘heat-sink’ to cool off hotspots and extend the battery life.
While this is a big step up for LMBs, Liu cautioned that some hurdles still stand in the way before next-generation LMBs featuring graphene-coated separators can be commercialised. For instance, the graphene layer must bind tightly to the battery components and uniformly coat the separator for optimal efficiency, both of which are difficult to achieve on an industrial scale.
Nonetheless, the team plans to take on the challenge and is now working on assembling the prototype of a rechargeable LMB with a graphene-coated separator.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).