Wednesday, July 30, 2014

Stanford creates ‘Holy Grail’ lithium battery, could triple smartphone and EV battery life

Lithium anode, feature cropepd 
They’ve done it again: The battery barons of Stanford, led by Yi Cui, have created what those in the industry call the “Holy Grail” of lithium-ion battery design. In specific, they’ve finally worked out how to create a rugged lithium electrode that can increase the capacity of a lithium-ion battery by three to four times — as in, this lithium electrode, on its own, could increase the battery life of your smartphone by three times, or significantly reduce the size and cost of an electric car’s battery pack.

A lithium-ion battery’s capacity (i.e. the amount of work it can do before it runs out of juice) is mostly dictated by how many lithium ions can be sucked up into the anode during charging. (For a more details on lithium-ion battery chemistry, read our featured story about how they work.) In almost all modern LIBs, the anode is made of graphite. Graphite is cheap and long-lasting (it keeps its capacity over hundreds of charge/discharge cycles), but its useful capacity is actually quite low (about 350 mAh/g). Lithium is by far the best anode material with a specific capacity that’s more than ten times that of graphite (3,860 mAh/g), but it degrades very quickly — and, perhaps more importantly, it has a tendency to violently explode when brought into contact with the electrolyte. If these niggling issues could be rectified, a LIB with much higher capacity could be built (not quite 10 times higher though; there are lots of other factors at play that prevent theoretical limits from being hit).

Now, a team at Stanford university, led by Yi Cui — the mastermind behind a huge number of battery breakthroughs that we’ve written about on ExtremeTech — have found a way of creating lithium anodes that keep their capacity over 150 charge/discharge cycles… and don’t explode. [doi:10.1038/nnano.2014.152 - "Interconnected hollow carbon nanospheres for stable lithium metal anodes"]
Lithium anodes crack and form dendrites -- but not with Stanford's new carbon nanosphere coating!
Lithium anodes crack and form dendrites — but not with Stanford’s new carbon nanosphere coating!
Similar to other recent battery breakthroughs, nanotech is the key to Stanford’s new lithium electrode. One of the main problems with lithium is that it expands dramatically when it absorbs ions during charging, creating cracks in the metal. Lithium ions then ooze out of these cracks, forming “mossy” metal deposits known as dendrites. These dendrites very quickly lower the battery’s efficiency, so that it’s fairly useless after just a handful of cycles. To prevent these cracks and dendrites from forming, Stanford deposits a layer of carbon nanospheres on the surface of the lithium anode. As you can see in the photos, these nanospheres create an interconnected series of domes that are strong enough to maintain the lithium’s structural integrity, while still allowing the electricity-carrying ions to pass back and forth. This protective layer, which is about 20nm thick, also prevents the lithium from reacting explosively with the electrolyte.

Some cool microscopic imagery of Stanford's carbon nanospheres
Some cool microscopic imagery of Stanford’s carbon nanospheres

All told, in technical terms, this new lithium anode has a coulombic (Faraday) efficiency of 99% after 150 cycles. Cui says this is short of the 99.9% required for a commercially viable design, but “while we’re not quite to that 99.9 percent threshold, where we need to be, we’re close and this is a significant improvement over any previous design. With some additional engineering and new electrolytes, we believe we can realize a practical and stable lithium metal anode that could power the next generation of rechargeable batteries.”

In real-world terms, this new lithium anode could triple or quadruple the battery life of your smartphone or electric vehicle — or, alternatively, make it so you can get away with a much smaller battery. For EVs, where the cost of the batteries is a major barrier to mass-market pricing and adoption, this could be a very serious breakthrough.

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