A pair of breakthroughs in lithium-ion battery technology could have long-range effects for consumers and industry. At the US Department of Energy’s Oak Ridge National Laboratory (ORNL), scientists have designed and tested an energy-dense, all solid lithium-sulfur battery that has four times the energy density of conventional lithium-ion batteries. According to Chengdu Liang of ORNL, the lithium-sulfur battery has the potential “to reduce cost, increase energy density and improve safety compared with existing lithium-ion technologies.”
Rather than using a liquid electrolyte as has been the norm in batteries for about two centuries, the Oak Ridges group uses a new sulfur compound in the battery’s cathode instead. To do so, the team had to synthesize a “never-before-seen” class of sulfur materials that conduct ions as well as the lithium metal oxides used in conventional batteries.
The shift from liquid to solid is being called “game changing” by the researchers at ORNL as it “eliminates the problem of sulfur dissolution.” Liquid electrolytes have been used in previous attempts to create lithium-sulfur batteries because of their ability to conduct ions. It was the dissolution of the lithium polysulfide compounds used in these batteries that allowed the conduction of ions. However, the dissolution also led to the batteries’ rapid break down.
With the ionically-conductive cathode, the ORNL battery maintained a capacity of 1200 milliamp-hours (mAh) per gram after 300 charge-discharge cycles at 60 degrees Celsius. A traditional lithium-ion battery cathode has an average capacity of 140–170 mAh/g. Accounting for the fact that lithium-sulfur batteries deliver half the voltage of lithium-ion versions, the eight-fold increase in capacity translates into four times the energy density of lithium-ion technologies.
The all-solid design also increases battery safety by eliminating flammable liquid electrolytes that can react with lithium metal. The ORNL battery’s use of elemental sulfur, a plentiful industrial byproduct of petroleum processing, is another significant advantage.
“Sulfur is practically free,” Liang said. “Not only does sulfur store much more energy than the transition metal compounds used in lithium-ion battery cathodes, but a lithium-sulfur device could help recycle a waste product into a useful technology.”
A patent on the ORNL sulfur battery design is pending.
And in Germany, scientists at the Centre for Solar Energy and Hydrogen Research Baden-Wurttemburg (ZSW) have developed super-efficient lithium-ion batteries with great potential for energy storage systems. In an electric vehicle they’re expected to retain 85 per cent of their capacity after being charged every day for “about 27.4 years.”
A press release from ZSW says that after 10,000 complete charging and discharging cycles, with a complete charge and discharge cycle per hour, the lithium-ion batteries retain more than 85 per cent of the initial capacity. This means that an electric car with those batteries could be fully charged every day for about 27.4 years and still be going strong. The power density of these batteries, which measures the available power per unit of weight, is also very high at 1,100 watts per kilogram. For an electric vehicle, this means short charging times and a superior acceleration capability.