updated 05:41 pm EST, Tue December 3, 2013
New cells also remove conflict minerals from construction process
A team of Berkeley Lab battery researchers led by Elton Cairns has invented an improved lithium/sulfur (Li/S) cell that, for the first time with the sulfur-based technology, offers both long cycle life and a high discharge rate in addition to the inherently low cost and light weight of Li/S batteries. Tests show that the Berkeley Lab Li/S cell had an estimated specific energy of nearly 500 Wh/kg, more than double that of current Li-ion cells which run around 200 Wh/kg.
The researchers developed a sulfur-graphene oxide (S-GO) nanocomposite cathode that vastly improves the cycle life of Li/S cells by reducing charge-affecting chemical reactions. It works by immobilizing sulfur and inhibiting polysulfides during operation. The new Li/S cell maintained a high capacity through 1,500 charge/discharge cycles, and also maintained capacity during high discharge and charge rates, such as those required of batteries serving cordless power tools.
In earlier Li/S cells, capacity faded rapidly due to sulfur loss from the cathode by formation of soluble polysulfides in the electrolyte during the charge/discharge cycle. The Berkeley Lab cell avoids this effect through a process that deposits sulfur on ultra-thin cathode surfaces made of nanoporous graphene oxide (GO). Modifying the S-GO composite with a surfactant, CTAB (cetyltrimethyl ammonium bromide) adds a layer of protection for the sulfur attached to the GO, preventing premature maximum charge loss.
In addition, a flexible binder holds the cathode materials in place despite the electromechanical expansion and contraction that occurs during the charge/discharge cycle. The new Berkeley Lab cell employs an improved ionic liquid electrolyte that further reduces polysulfide formation and blocks trace amounts from binding to the lithium metal anodes of the Li/S cells. The combination of features in the Berkeley Lab cell provides an significantly higher level of performance and reliability over older Li/S-based cells.
Lawrence Berkeley claims that the Li metal anode boasts 90 percent less weight than carbon anodes. As an additional bonus, the materials for the battery are abundant, with no "conflict materials" used in production.