Batteries are inherently inefficient things, and that inefficiency is what leads scientists to make constant attempts to improve them: A battery will never be perfect, but it can always weigh less, last longer and be more efficient. While today’s “lithium ion” batteries contain lithium in the electrolyte only, researchers at Stanford have created what some call the “holy grail” of batteries: By using carbon nanospheres to overcome some of lithium’s limitations, they’ve created a battery with a full lithium anode.

In the future, your smartphone (and even your car) might be much lighter and last a lot longer.

“Of all the materials that one might use in an anode, lithium has the greatest potential. Some call it the Holy Grail,” said Yi Cui, a professor of Material Science and Engineering and leader of the research team. “It is very lightweight and it has the highest energy density. You get more power per volume and weight, leading to lighter, smaller batteries with more power.”

Making it work, though, has obviously proved problematic up to this point.

“Lithium has major challenges that have made its use in anodes difficult. Many engineers had given up the search, but we found a way to protect the lithium from the problems that have plagued it for so long,” said Guangyuan Zheng, a doctoral candidate in Cui’s lab and first author of the paper.

Lithium’s main problem is that the mechanics of a battery lead to its expansion – in a batter with a lithium anode, the positively charged lithium ions in the electrolyte are attracted to the negatively charged anode and the lithium accumulates on the anode. With a nearly limitless expansion capacity, a lithium anode will inevitably crack the batteries outer surface, allowing lithium ions to escape and form moss-like dendrites, which short circuit the batter. Lithium is also highly reactive with the electrolyte itself, using it up and depleting battery life.

To solve this, the engineers created a layer of carbon domes over their lithium anode, in what they carbon nanospheres. Incredibly thin and flexible, the carbon is the perfect foil to the problems plaguing lithium: It’s chemically stable and therefore non-reactive, and it’s also flexible enough to move with the lithium as it expands and contracts.

The batteries are incredibly efficient, but not yet commercially viable. They managed to to get the carbon-coated lithium anodes up to a coulombic efficiency of 99% over 150 cycles, but commercial batteries typically must be 99.9%.

“The difference between 99 percent and 96 percent, in battery terms, is huge. So, 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,” Cui said. “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.”