Lithium-Sulfur – hot air or hope?

In the past week there have been two press releases regarding advancements of Lithium-Sulfur (Li-S) batteries. What do they promise, and what are they quietly sweeping under the run when it comes to how they’d work in electric cars?

First was last week, Sion Power announced they’ve partnered with BASF to commercialize their lithium-sulfur batteries. This is a very important step – they’re working on trying to make actual product, which not too many battery “breakthrough” announcements make it to.

From a 2006 Sion Power presentation, they discuss their 350Wh/kg battery, which is excellent, however they don’t mention power density or cycle life, which are standard problems for Li-S batteries. Graphs in the presentation showed lower energy capacities (250Wh/kg) and low power density ratings – around 300W/kg. A 35kWh battery would weigh 140kg (less than a Volt pack) but only be able to produce about 42kW, thats only 56HP – not enough to propel an electric vehicle. Likewise, another issue is the cycle life of the Li-S batteries – only 50-75 cycles to 80%. Batteries for electric cars would need to be capable of thousands of cycles to satisfy the 10 year/150,000 mile warranty requirement.

The ultimte theoretical limits of Li-S batteries are quite impressive – 2500Wh/kg and 2600W/kg – however there is a long way to go before we can get anywhere near those figures. Outside of the high energy density of the batteries, the Li-S chemistry can provide comparable output to other novel Li-based technoloiges – 150Wh/kg and 1,000-1,500W/kg.