Battery Claims Greater Capacity than Lithium-ion

Advanced Battery Technology, Sep 2005

Oxis Energy in Oxfordshire, U.K., is developing a rechargeable battery with more capacity than lithium-ion.

"It has a high gravimetric density, twice the energy density per unit weight of Li-ion, and not just on the first charge-discharge cycle," said director Dr. Matthew Frohn. "Cycle life is certainly in excess of 300 cycles," he added.

The chemistry inside is a form of lithium-sulphur. "There are a number of interesting approaches in Li-S chemistry," said Frohn. "However, there are some intrinsic problems."

These include a high level of self-discharge and low cycle life through dendrite growth. Through undisclosed modifications to Li-S cell chemistry and structure, Oxis has invented what it dubs a lithium-sulphide cell.

"It is a significant improvement over lithium sulphur," said chief technologist Dr. Gleb Ivanov.

The cell has a metallic lithium anode, something which has caused nervousness in the past following spontaneous combustion with now defunct rechargeable lithium cells over a decade ago. Li-ion and Li-polymer cells have no metallic lithium and only form it when they are abused.

"Our cell electro-chemistry is a combination of lithium and poly-sulphur and is extremely safe. It has an intrinsically safe mechanism; the lithium metal is effectively passivated," said Ivanov.

"We have done nail penetration, over-charge and short-circuit tests and all tests were positive," he said.

Ivanov claims another advantage: "You can draw 2C or 4C," he said, "and we rate our capacity at C/2. Li-ion cells are rated at C/4 discharge."

The cells can deliver higher power levels than most Li-ion cells, and their capacities are rated conservatively.