An advance that could make batteries last a lot longer


THE digital devices that rule modern life may be advancing at breakneck speed, but the rechargeable batteries that power them are making slower progress. The lithium-ion (Li-ion) batteries found in everything from smartphones and laptops to Teslas and Boeing 787s have been on sale since 1991, their performance gradually improving. But to make the kinds of gains seen in the devices they energise, they need more of the one thing they have surprisingly little of: lithium.

Like other batteries, Li-ions consist of two electrodes (an anode and a cathode) separated by an electrolyte. In a typical Li-ion cell, the anode is made of graphite, the cathode is lithium cobalt oxide, and the electrolyte is a solution of lithium salts and organic solvents. Charging the battery drives positively charged lithium ions in the electrolyte to the negatively charged anode, where they accumulate. When the battery is in use, electrons flow from the anode into a device’s circuit and re-enter the battery via the cathode.

The process also creates heat, which for the most part is easily dissipated. But a battery that is charged for too long can form spindly lithium dendrites, or crystals, on the anode, which can cause a short-circuit. Short-circuits also arise from faults in the materials batteries are made from. They can lead to “thermal runaway” and occasionally fires. Such fires are why Boeing 787s were grounded in early 2013 and why millions of laptop and smart phone batteries have been recalled over the years.

Despite this risk, the ultimate goal for many scientists is to build a commercially viable battery containing yet more lithium, as the anode itself. Lithium-metal anodes have the highest energy-storing capacity of any known material and, because lithium is the least dense metallic element, a big power-to-weight ratio too. Yi Cui of Stanford University estimates that a battery with a lithium anode and sulphur cathode (sulphur also has a very high energy capacity) would be able to hold about five times as much energy as today’s Li-ion batteries, weight-for-weight.

The snag is lithium’s high chemical reactivity: it is part of the alkali metal group that includes sodium and potassium, stars of many a chemistry-class calamity. Stick a lithium anode into a battery, start charging, and the lithium will expand dramatically within the space available as the ions gather on it. (Other anode materials expand, but by nothing like as much.) A lithium anode also swiftly forms dendrites and mossy deposits. At best, these reduce its efficiency. At worst, they create an even greater fire hazard.

Dr Cui and his team, which includes Steven Chu, a Nobel physics laureate and former US energy secretary, believe they have ways to reduce that risk. One is to enclose the lithium anode in a thin film—5,000 times thinner than a human hair—of carbon “nanospheres”. The carbon film will not react with either the lithium or the electrolyte. It is also immensely strong and flexible, enabling it to expand and contract along with the lithium. And although lithium ions can flow through it to the anode, enabling the battery to operate as if the film were not there, it prevents dendrites (which are thousands of times larger than the ions) from forming.

One atom at a time

The researchers’ latest solution is to use an advanced two-dimensional (2D) material—in other words, comprising a single layer of atoms—to sheathe the lithium anode. The materials they chose are layered 2D hexagonal boron nitride or graphene. But the 2D material is also flawed, in a good way: when it is grown, tiny atomic-level cracks form across its surface. These enable lithium ions to flow through, but are small enough to stop the dreaded dendrites.

Both approaches, along with others the Stanford team will not yet reveal, have a few years of development ahead. For a rechargeable battery to be commercially viable, it must have a charging efficiency (the percentage of electrical energy stored during charging that is recoverable during discharge) of greater than 99.9% over as many discharge/recharge cycles as possible. Today, Dr Cui’s nanosphere-based batteries achieve 99.6%, while his 2D batteries manage around 97%, both over 100 or so cycles. Gaining those final few points may take time. But Dr Cui is optimistic, and says he can see the way to mass production.

Those who scoff at such claims should consider Dr Cui’s record. He has already co-founded one startup, Amprius, that works on silicon-anode batteries and has secured $60m in funding from several big venture-capital firms and from Google’s chairman Eric Schmidt (who sits on the board of The Economist’s parent company). If Dr Cui’s research pans out as he expects, it may be folded into Amprius. Or, he muses, “We may just do another startup.”

From the print edition: Science and technology