We've reported before on work by scientists to improve lithium-ion chemistries and find alternatives to lithium-ion batteries. This work is driven in part by the belief that the current lithium-ion designs have maxed out their potential (not to mention the problems with fires).
Scientists at the Department of Energy's Oak Ridge National Laboratory have designed and tested one alternative. They say their all-solid lithium-sulfur battery offers four times the energy density of conventional lithium-ion technologies.
An all-solid lithium-sulfur battery developed by an Oak Ridge National Laboratory team led by Chengdu
Liang could reduce costs, increase performance, and improve safety
over designs that primarily use lithium-ion chemistries.
(Source: Oak Ridge National Laboratory)
Chengdu Liang, lead researcher on the project, told us in an email that the design, which replaces lithium with abundant and low-cost elemental sulfur, is still in its nascent stages but shows real promise.
"The all-solid-state format is a completely different design of the battery," he said. "There is lot of room to optimize the design of the battery. This research points out the right scientific direction for high-energy batteries."
The researchers used conversion chemistry. At the discharge step, the sulfur-sulfur single bond breaks and releases energy; at the charge step, it forms and stores energy. "Because sulfur is a lighter element compared to the transition metal compounds, it accommodates two electrons per atom. So it improves the energy by accommodating more electrons with light materials."
The completely solid design also makes the battery safer by eliminating flammable liquid electrolytes that can react with lithium metal. And sulfur is a plentiful industrial byproduct of petroleum processing, making it "practically free."
Scientists have been trying to come up with a viable lithium-sulfur battery for decades, but they were stuck on designs that used liquid electrolytes. The liquid helped conduct ions through the battery by allowing lithium polysulfide compounds to dissolve, but this process caused the battery to break down prematurely, making it inefficient for commercial use. Liang and his team overcame this barrier after six years of identifying the problems associated with the liquid-electrolyte structure and taking a solid-structure approach. "The latest development of high conduction solid electrolytes and emerging of new materials formed the knowledge base of such a new design."
Specifically, the team synthesized a new class of sulfur-rich materials that conduct ions, as well as the lithium metal oxides conventionally used in the battery's cathode. They combined the new sulfur-rich cathode and a lithium anode with a solid electrolyte material (also developed at the lab) to create an energy-dense, all-solid battery. This battery can maintain a capacity of 1,200 milliamp-hours (mAh) per gram after 300 charge-discharge cycles at 60C. A typical lithium-ion battery cathode has a capacity of 140-170mAh/g.
However, lithium-sulfur batteries deliver about half the voltage of lithium-ion versions, so the eightfold capacity increase the Oak Ridge battery demonstrated gives it roughly four times the gravimetric energy density of lithium-ion batteries, Liang said.
This doesn't take into consideration supporting materials in the battery; optimizing those is the next step for the team. Also, new manufacturing methods must be devised before the battery could be made commercially available. It has "a long way to go" before it is ready for prime time.