A Stanford team, led by the battery master himself — Yi Cui — has developed a new lithium-ion battery electrode that still works at 85% capacity after 6,000 charge/discharge cycles, compared to current lithium-ion batteries (found in your laptop, iPad, smartphone) that are usually depleted after around 1,000 charges. The new electrode also has the possibility of increasing lithium-ion battery capacity by up to 10 times.

In a lithium-ion battery there is an anode (negative electrode) and cathode (positive) separated by a liquid lithium salt electrolyte, with the capacity (milliamp-hours, mAh) being a direct function of all three materials. Currently, almost every lithium-ion battery uses a graphite (carbon) anode, which has a specific capacity of 400 mAh per gram, which means the anode has to be relatively large to store a decent amount of power. Yi Cui’s team, however, has successfully built a double-walled silicon nanotube anode that has a capacity of around 4,000 mAh per gram. In other words, this anode is an important precursor to lithium-ion batteries with 10 times their current capacity.

Now, it has been known for some time that silicon would make a better anode than carbon — one silicon atom can bind to four lithium ions, while it takes six carbon atoms to bind a single lithium ion, resulting in a very high power density — but until now it has been impossible to build a silicon anode that can survive more than a few charge/discharge cycles. Basically, when charging, silicon can absorb so many lithium ions that the anode swells to four times its original size — and then during discharge, as the lithium ions travel to the cathode, it returns to its original size. After a few these expansion/contraction cycles, the silicon anode is destroyed.

Yi Cui’s double-walled silicon nanotubes, however, are rugged — and they’re also covered in a thin layer of silicon oxide, which is strong enough to keep the nanotubes from expanding. As a result, this new anode can survive 6,000 charge/discharge cycles without being significantly damaged.

The next step is to simplify the production of these nanotubes — it’s currently a four-step process that starts with polymer nanofibers, which is then reduced to carbon, and then coated with silicon (pictured right) — and then to build an actual battery with double the energy density of current lithium-ion batteries. This might seem like a humble goal after saying that silicon anodes can increase power density by 10 times, but as we mentioned, the electrolyte and cathode are also important. That bulging-with-ions silicon anode is useless if the cathode doesn’t have enough space to receive them all. Eventually, though, Yi Cui seems confident that extensive gains in lithium-ion power density can be achieved.

Now you might like to read about IBM’s lithium-air battery, or Yi Cui’s everlasting battery cathode

Read more at SLAC or read the paper [paywalled]