Promising battery technology packs more performance.
Cai, K, MK Song, E Cairns and Y Zhang. 2012. Nanostructured Li2S−C composites as cathode material for high-energy lithium/sulfur batteries. Nanoletters http://dx.doi.org/10.1021/nl303965a.
A new type of lithium battery brings researchers a step closer to a more powerful, stable and longer lasting battery that could power the cars and personal electronics of the future. The new lithium-sulfide version can hold up to three times more energy than current lithium ion batteries, yet it is made with safer and efficient processes that use very little energy, demand no solvents and produce almost no waste. A new technique mixes lithium, sulfur and carbon nanotubes to make a new conductive electrode. While the battery is promising, more refinements are needed before it can be marketed to consumers.
Batteries power everything from cell phones to automobiles. Today, devices that demand the most power rely on lithium-ion batteries. These rechargeable batteries can store more energy with less material than most other types of batteries.
Energy is generated in the battery's electrodes. Batteries generate electricity when tiny charged atoms move from a negatively charged electrode to a positively charged one.
In disposable alkaline batteries, the reactions that make energy are irreversible. The batteries cannot be recharged after they are depleted.
In rechargeable batteries, adding electricity reverses the energy reactions. When the batteries are recharged, the atoms move in the opposite direction – from the positive to negative electrode. With each cycle of charging and discharging, the battery’s electrodes degrade and its capacity to store energy drops.
Lithium-ion and other rechargeable batteries can be recharged and reused multiple times. However, the energy capacity of rechargeable batteries should remain stable over hundreds or thousands of uses.
Researchers are trying to replace lithium ion technology with a more efficient lithium-based battery. One of the most promising is based on lithium-sulfur technology. A battery made out of lithium and sulfur could potentially produce large amounts of energy using only modest amounts of materials.
Lithium-sulfur batteries may pack three times as much energy into the same amount of space as a typical lithium-ion battery. An increase in electrical capacity of this magnitude would significantly increase the range of electric cars and the use times for personal electronics.
So far, lithium-sulfur batteries are not adequate. They quit working after a couple of uses, making them worthless for most consumer products.
Researchers at Lawrence Berkeley National Labs applied a technique called ball milling to form a composite material of lithium sulfide and carbon. The material was used to make electrodes for a rechargeable battery.
In ball milling, chemicals are put into a stainless steel container with hard ceramic balls and shaken very fast. The high-energy molecular collisions create a fine, well mixed powder of the new compound.
Unlike many chemical synthesis techniques, ball milling doesn't require any solvent and does not produce waste products. The majority of waste from conventional chemical reactions comes from potentially harmful solvents, and this process works best when no solvent is added. These attributes make the process both safe and environmentally friendly.
By creating a better electrode material using ball milling, the scientists hoped that they would be able to create a longer-lasting battery that can store and deliver more energy.
The electrode material made by ball milling binds together in a way that makes it more stable and more conductive than previously-discovered materials.
The material has high energy output and is stable over many more charge/discharge cycles then previous lithium-sulfur batteries.
At the optimum processing conditions and electrode charging, these lithium-sulfur batteries can store and deliver 610 watt-hours per kilogram. This is enough energy to power an average laptop for 30 hours. It is also more than three times the capacity of typical lithium-ion batteries, which deliver 120-200 watt-hours per kilogram.
This is the highest measured capacity for a lithium-sulfur battery ever reported.
A new way to mix chemicals makes a longer-lived battery that stores more energy and remains stable over time and many uses.
These new lithium-sulfide batteries use electrodes formed using a special ball-milling technique. This way of making the electrodes increases the batteries' safety and efficiency while using no solvents and creating very little waste.
The authors acknowledge more work is needed to improve the long-term stability of the batteries. After a single charge/discharge cycle, the batteries lose almost half their capacity. Luckily the batteries only lose a little more energy during the next 50 cycles. By the end of 50 cycles, the energy capacity has dropped by about two-thirds of the original capacity. Even after 50 cycles the energy capacity is about equal to a brand new lithium-ion battery.
Lithium-ion batteries degrade much slower. The typical lithium-ion battery used in personal electronics only loses 10 - 15 percent of its capacity over 50 cycles. It retains about 75 percent capacity after 250 cycles.
Batteries for electric vehicles need to withstand thousands of charge cycles, so significant progress is needed before the new lithium-sulfide batteries make their way into consumer products.
As a first step, the researchers made additional improvements to the lithium-sulfide battery stability by adding a small amount of carbon nanotube material to the electrode. This improved the cycling stability by 15 - 20 percent, showing that there may be potential for further improvements with more advanced materials.
Even with these hurdles, ball milling is an efficient way to make next-generation batteries. More adjustments are needed to improve the battery's long-term stability before they will be found in consumer products.
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Ji, L, M Rao, H Zheng, L Zhang, Y Li, W Duan, J Guo, EJ Cairns and Y Zhang. 2012. Graphene oxide as a sulfur Immobilizer in high performance lithium/sulfur cells. Journal of the American Chemical Society 133(46):18522-18525.
Tarascon, JM, and M Armand. 2001. Issues and challenges facing rechargeable lithium batteries. Nature 414:359-367.
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