Lithium-Sulfur Battery Lifetime Increased By Stanford Research Team
26 February 2013
Currently widely used in portable electronic devices, lithium-ion (Li-ion) batteries are very expensive and their low energy density imposes limitations on their use in the automotive industry.
Lithium-sulfur (Li-S) batteries are viewed as one of the alternatives due to their better gravimetric energy density and lower cost, thus increasing the driving range of electric vehicles at an affordable price. However, one of the biggest drawbacks of Li-S batteries is their short lifecycle. A significant portion of their initial capacity is lost during each discharge/charge cycle. The typical Li-S lifetime is a few tens of cycles.
However, a team of researchers from Stanford University, led by Yi Cui, a professor of materials science and engineering, has developed a modified Li-S battery that can retain more than 80% of its 1180 mAh/g design capacity after 300 discharge/charge cycles. The team claims there is the potential for similar capacity retention over thousands of cycles.
The improvement in Li-S batteries' lifetime was achieved by identifying a mechanism that causes capacity decay after cycling. While recharging, lithium sulfide binding to the hollow carbon nanofiber cathode surface is of key importance for creating good conductivity and, as a result, better charge flow. During Li-S battery discharge, lithium sulfide partly detaches from the carbon, which causes conductivity loss and prevents full recharging capability.
The breakthrough came with adding hydrophilic and lipophilic polymers to the carbon nanofiber surface. These polymers' properties allow for better lithium sulfide binding to the carbon cathode and better electrical contact. This cathode structure of Li-S battery reduces the decay to about 3% over the first 100 cycles, and less than 20% decay for more than 300 cycles.
Nevertheless, these numbers are still far from the best Li-ion lifespan parameters, which can come close to 10,000 cycles. The results of Yi Cui's team in nanomaterials design currently allow for a lifespan of about 1,000 cycles for Li-S batteries and further improvements are expected to be made.