Many researchers are interested in lithium-sulfur batteries, because they can offer up to four times the energy density of lithium-ion chemistries. However, current lithium-sulfur batteries have a much shorter lifespan.
In a paper published this week in Nature Communications, a team from the Department of Energy’s Pacific Northwest National Laboratory (PNNL) described a “hybrid” anode that could quadruple the life of lithium-sulfur batteries.
“Lithium-sulfur batteries could one day help us take electric cars on longer drives and store renewable energy more cheaply, but some technical challenges have to be overcome first,” said Jun Liu, the paper’s corresponding author. “PNNL’s new anode design is helping bringing us closer to that day.”
The problems with lithium-sulfur batteries have to do with unwanted side reactions that start on the sulfur-containing cathode, which slowly disintegrates and forms molecules called polysulfides that dissolve into the battery’s electrolyte liquid. The dissolved sulfur eventually develops into a thin film called the solid-state electrolyte interface layer. The film forms on the surface of the lithium-containing anode, growing until the battery is inoperable.
Most lithium-sulfur battery research to date has centered on stopping sulfur leakage from the cathode. But the PNNL team focused on the battery’s other side by adding a protective graphite shield to the anode. The shield moves the sulfur side reactions away from the anode’s lithium surface, preventing it from growing the debilitating interference layer. Combining graphite from lithium-ion batteries with lithium from conventional lithium-sulfur batteries, the researchers dubbed their new anode a hybrid of the two.
When equipped with a conventional anode, the team’s battery stopped working after about 100 charge/discharge cycles. With PNNL’s hybrid anode, the system worked well past 400 cycles.
“Sulfur is still dissolved in a lithium-sulfur battery that uses our hybrid anode, but that doesn’t really matter,” Liu said. “Tests showed a battery with a hybrid anode can successfully be charged repeatedly at a high rate for more 400 cycles, and with just an 11-percent decrease in the battery’s energy storage capacity.”
Image courtesy of Huang et al, Nature Communications 2014