How is the latest progress in international lithium-sulfur battery research and development?

　　Lithium-ion batteries contain two electrodes-a cathode and an anode. The two are separated by a liquid dielectric that moves lithium ions back and forth during the charging cycle. At the anode, lithium atoms are sandwiched between the highly conductive carbon-graphite layer. When the battery discharges, lithium atoms give up electrons and generate electricity.

　　This is not a super factory, at least not for now. But Oxis chief technology officer David Ainsworth said the company is eyeing a bigger "cake": the $100 billion electric car market. "The next few years will be very critical." Ainsworth said, he and others regard lithium-sulfur batteries as the "successors" of lithium-ion batteries and will become the dominant battery technology.

　　They were encouraged by a series of latest reports. According to the report, many performance and durability challenges faced by this technology can be overcome. "You will see progress in many areas." said Brett Helms, a chemist at Lawrence Berkeley National Laboratory in the United States. However, people such as Linda Nazar, a chemist at the University of Waterloo in Canada and a pioneer in lithium-sulfur batteries, are cautious. She believes that creating a lithium-sulfur battery with high capacity, low cost, light weight, compactness, and safety is "really an arduous task." Improving one factor usually comes at the expense of other factors. "You can't optimize all factors at the same time." Nazar said.

　　Lithium-ion batteries contain two electrodes-a cathode and an anode. The two are separated by a liquid dielectric that moves lithium ions back and forth during the charging cycle. At the anode, lithium atoms are sandwiched between the highly conductive carbon-graphite layer. When the battery discharges, lithium atoms give up electrons and generate electricity.

　　"The positively charged lithium ions thus obtained move into the electrolyte. After powering many devices from mobile phones to Tesla cars, the electrons eventually return to the cathode, which is usually a mixture of different metal oxides. There, the positive lithium ions in the electrolyte "snuggle" near the metal atoms that have absorbed the traveling electrons. Charging reverses this molecular pattern because the applied voltage will push lithium ions out of their metal host and return to the anode.

　　Metal oxide cathodes are reliable, but these metals, which are usually a combination of cobalt, nickel, and manganese, are expensive. At the same time, these cathodes are heavy because it takes two metal atoms to "hand in hand" to fix a single electron. This limits the performance of the battery to approximately 200 watt-hours/kg (Wh/kg). Sulfur is much cheaper, and each sulfur atom can hold two electrons. In theory, a battery with a sulfur cathode can store 500Wh/kg or more.

　　However, sulfur is not an ideal material for electrodes. First, it is insulating: it cannot pass electrons to lithium ions that travel through the anode.

　　In 2009, an event that affected the overall situation happened: A research team led by Nazar discovered that sulfur can be embedded in a cathode made of conductive carbon like the anode. Although this method works, it brings other problems. Carbon forms like graphite have high porosity. This increases the overall size of the battery, but the storage performance is not enhanced.

　　This means that more expensive liquid dielectrics are needed to fill these pores. More seriously, when lithium ions combine with sulfur atoms at the cathode, they will react to form soluble molecules called polysulfides. These molecules will float away, degrading the cathode and limiting the number of charging cycles. Polysulfide can also migrate to the anode. There, they will cause further damage.