Tech + Engineering

01
Aug

Triple the Lifetime of Lithium Car Batteries Using Carbon Nanospheres

One of the biggest barriers to wider adoption of electric cars is that they constantly need to be recharged. The range of most electric cars is stuck below 100 miles (unless you’re in a position to drop $70k on a Tesla), which isn’t very practical for anyone with a long commute or a penchant for road trips.

These vehicles would be more useful if their batteries could store more energy. The “holy grail” of energy-dense batteries would be one with pure lithium at one end, but lithium is a temperamental material. A Stanford team led by Professor Yi Cui has shown that a layer of hollow carbon nanospheres might be able to make lithium more manageable.

lithium-metal
Pure lithium holds a lot of energy but can be tricky to work with.

First: a battery primer. Batteries work by making electrons flow through a circuit. The two ends of the battery are made of two different materials: one that electrons want to leave—the anode—and one where they’d love to be—the cathode. A battery is set up so that electrons can’t go through the battery to get from one end to the other, forcing them to travel through the wires outside the battery that make up the rest of the circuit. Along the way, those electrons power a lightbulb or a smartphone screen. Inside the battery, positive ions flow from the anode to the cathode, through a conductive material called an electrolyte. In rechargeable batteries, an external current can make the process run in reverse, giving the anode a fresh supply of electrons.

Most rechargeable batteries use lithium ions as the positively-charged particles that get shuttled back and forth inside the battery. But for decades, scientists have been trying to find a way to use lithium for another part of a battery: the anode. Lithium is both very light and very eager to release its electrons, which makes it a perfect anode material for batteries that need to be both powerful and portable. But so far, there have been big reasons not to use lithium anodes. When lithium ions return to a lithium anode during recharging, they form random, clumpy shapes that decrease the battery’s efficiency.  And the high reactivity that makes lithium such a good anode material also means that it can interact violently—and dangerously—with the materials in the electrolyte.

The carbon nanospheres—which Cui and his colleagues created by coating microscopic  plastic beads with carbon dust—cover a lithium anode like an upside-down egg carton.

When lithium ions migrate over from the cathode during recharging, this carbon shell keeps them organized. It also protects the highly reactive lithium metal from interacting with the charged particles in the electrolyte.

The result is a battery that’s more efficient and less dangerous. Henry Gass, writing for Scientific American:

Previous lithium anodes achieved a 96 percent coulombic efficiency, but that dropped to less than 50 percent after 100 cycles. This new lithium anode achieves a 99 percent efficiency for 150 cycles. Cui said that over the next few years, the team hopes to refine the battery design to both improve on the coulombic efficiency and sustain it for 500 to 1,000 cycles.

This technology isn’t ready for commercial application yet—batteries need to have an efficiency greater than 99.9% to be practical. If they can get there, though, Cui and his colleagues say we’re looking at batteries that could power electric cars for three hundred miles at a time. That’s the start of a pretty respectable road trip.