Designing a Safer Lithium Battery

In the world of batteries, lithium-ion batteries are like the Golden State Warriors: powerful, trendy, and highly explosive.

Like all batteries, lithium-ion batteries function by the presence of  a standard oxidizing agent at the cathode and a reducing agent at the anode; for lithium-ion batteries, the anode is made of carbon while the cathode uses a cobalt-based metal. The battery also holds an electrolyte-rich mixture, typically a lithium-based salt from which it takes its name. When the battery runs, the positively charged lithium ions flow towards the cathode; to reduce excess charge buildup, electrons simultaneously move towards the cathode, producing a constant current flowing from anode to cathode. Designed in the 1970s by Exxon, lithium-ion batteries have been renowned for their power and efficiency. What’s more is that, unlike many contemporary batteries, they’re readily rechargeable: force the lithium ions to return to the anode, and the battery is as good as new.

Though they have become one of the most commonly used batteries in modern technology,  many lithium-ion batteries suffer from one fatal flaw. When a battery has run for too long, lithium ions begin accumulating on the cathode and start forming solid metal. These metal extensions can eventually grow across the battery from the cathode like miniature tree branches. If they happen to touch the anode of the battery, a short-circuit results and the electrolyte media, which is flammable, can explode.

Though the probability of such an occurrence is relatively low, the prospect of an exploding battery still poses a threat to life and property. That’s the main problem with the hoverboards seen recently: the lithium metal “spikes” can reach the anode while recharging and cause a catastrophic short-circuit. Perhaps a more frightening prospect is that the lithium-ion batteries in our cell phones and computers are similarly in danger of explosions. While rarer and certainly less-publicized than the recent hoverboard fires, cell phone explosions do occur, and almost always, the lithium-ion battery is to blame.

When a battery has run for too long, lithium ions begin accumulating on the cathode and start building up into metal. These metal extensions can eventually grow across the battery from the cathode like miniature tree branches. If they happen to touch the anode of the battery, a short-circuit results; the electrolyte media, which is flammable, can explode.

The lithium-ion battery is wondrous, energy-dense, and powerful. There’s even talk of using packets of lithium-ion batteries for the next electric cars. But before further development and use of these batteries can occur, there must first be a way to make them safer. Picture the hoverboard explosion, a smaller and more localized occurrence, and then picture scaling it up to an electric car containing multitudes of these miniature bombs. Imagine if all these batteries went up in flames: not an implausible prospect, as the explosion of one battery can set off a chain reaction that ignites the rest. What will happen when laptop and cell phone use becomes more common in third-world countries, where manufacturing and safety regulations are often overlooked? Fortunately, Professors Yi Cui and Zhenan Bao of Stanford University have a potential answer.

Professor Cui’s group designed a thin copper nanolayer that was placed between the cathode and anode of the battery. While the nanolayer is thin and porous enough to allow lithium ion diffusion to the cathode with no loss in battery efficiency, it can block the lithium metal extensions that form as lithium builds up on the cathode, preventing a dangerous short-circuit. When these extensions, or dendrites as Cui called them, touch the copper nanolayer, a signal can be sent warning the user that the battery needs to be replaced. Cui’s battery was a breakthrough, but had one major issue: upon the copper nanolayer being penetrated by the lithium dendrites, the nanolayer ceases to become porous, and the battery becomes useless as the lithium aggregates on the cathode. This problem was addressed by Professor Bao’s group, who kept the general premise of Cui’s idea but used another nanomaterial as the detector. Bao turned to another invention of her lab, a similarly-designed nanomaterial developed as a temperature sensor for prosthetic skin.

A thin, impenetrable layer of polyethylene with nickel spikes serves as a substitute for the copper nanolayer. The nickel spikes are covered in graphene, a strong conductor of electricity; in its original form, the nickel spikes touch, and the graphene can conduct electricity through the battery as usual. Upon reaching 70 degrees Celsius, however, the polyethylene begins to rapidly expand, and the nickel spikes are driven apart and no longer able to conduct electricity. Bao’s group found that conductivity dropped by a factor of 100 billion when hot temperatures were applied. Drop the temperature, however, and the polyethylene rapidly contracts to where the nickel spikes can begin conducting electricity again. In this way, the lithium-ion battery has its own self-warning system of when to shut down, while still retaining its efficiency. The battery is also reusable, thereby eliminating the chief design flaw in Cui’s battery.

Using innovative tactics in nanomaterials and materials science, Cui and Bao have made a discovery that could pave the way for continued and extended use of lithium-ion batteries. By taking a crucial step toward promoting safety, they could indeed promote the use of such batteries to improve energy efficiency and industrial productivity.

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