The Department of Energy’s Pacific Northwest National Laboratory has created a new variety of lithium-ion battery that can store at least twice the amount of energy found in your conventional smartphone or laptop battery. Unlike some other battery advances that won’t see the light of day for years to come (if at all), this energy storage breakthrough could actually find its way into commercial devices fairly soon. As is fairly normal nowadays, nanotech is the magic ingredient; nanostructured silicon sponges to be exact.

Almost every lithium-ion battery (LIB) in existence today consists of a graphite electrode, an electrolyte (usually a lithium salt), and a metal oxide electrode (usually an oxide containing lithium). When you charge a LIB, the graphite electrode (anode) sucks up lithium ions; when you discharge a LIB, a chemical reaction causes the ions to flow out of the graphite to the metal oxide eletrode (cathode), creating electricity. [Read: How does a lithium-ion battery work, and why are they so popular?] One of the key limitations to a battery’s capacity is how many ions you can cram into the anode — and in the case of graphite, the answer is “not many.”

Enter silicon. While it takes six carbon (graphite) atoms to bind to a single lithium ion, a single silicon atom can bind to four lithium ions. The exact maths are a little bit complex (silicon atoms are larger than carbon atoms), but it ultimately means silicon anodes can theoretically store more than 10 times as much energy as graphite. In practice, because there are other aspects of the battery chemistry to consider, a silicon anode can realistically double or triple a lithium-ion battery’s energy capacity. In short, silicon is the key to smartphone and laptop batteries that last days, and smartwatches and other wearable computers that last long enough to make them actually desirable for everyday use. [Read: DOE calls for a chemical battery with 5x capacity, within 5 years – can it be done?]

The silicon anode's structure during lithiation (being recharged) and delithiation (discharge)

The silicon anode’s structure during lithiation (being recharged) and delithiation (discharge). You can see how the spongy gaps are smaller/filled in by lithiation.

The thing is, we’ve known about silicon’s excellent energy capacity for ages. The problem is, silicon absorbs so many ions that it physically expands to four times its original size. Lithium-ion batteries, which have to be tough and rigid (because they’re explosive), obviously can’t handle a component that regularly expands and contracts by such a huge margin. Now, however, researchers at the DOE’s PNNL have fabricated a silicon electrode that only expands by 30% rather than 400% — and 30% is workable, for commercial LIB designs. [doi:10.1038/ncomms5105 – “Mesoporous silicon sponge as an anti-pulverization structure for high-performance lithium-ion battery anodes”]

PNNL’s secret sauce is the development of a mesoporous silicon sponge — basically a piece of silicon that’s riddled with holes. Instead of expanding outwards by 400%, the silicon instead expands to fill the spongy holes. The porous silicon anode has an energy density of 750 mAh per gram (about twice that of graphite). Furthermore, its structure seems to be incredibly rugged: After 1,000 charge/discharge cycles, the prototype battery still retained 80% of its total energy capacity.

The silicon anode's performance, over hundreds of charge/discharge cycles

The silicon anode’s performance, over hundreds of charge/discharge cycles

Moving forward, the PNNL team now have to create a larger prototype — something that might power a smartphone — and they need to streamline the production process of the electrode, so that it’s commercially viable. There’s reason to believe that they’ll succeed on both counts, though: Amprius, a lithium-ion battery startup, is already shipping batteries with a silicon anode. Their energy density gains have been more modest (~10-50%), but it’s fairly clear by this point that silicon will probably be the next big breakthrough in LIB energy density, driving the mobile computing ever onward.