The advance, described in a paper published in Nature Nanotechnology, could lead to smaller, cheaper and more efficient rechargeable batteries.
‘Of all the materials that one might use in an anode, lithium has the greatest potential,’ 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.’
We believe we can realise a practical and stable lithium metal anode that could power the next generation of rechargeable batteries
In addition to Zheng, the research team includes Guangyuan Zheng, a doctoral candidate in Cui’s lab and first author of the paper, and Steven Chu, the former US Secretary of Energy and Nobel Laureate who recently resumed his professorship at Stanford.
‘In practical terms, if we can improve the capacity of batteries to, say, four times today’s, that would be exciting. You might be able to have cell phone with double or triple the battery life or an electric car with a range of 300 miles that cost only $25,000 - competitive with an internal combustion engine getting 40mpg,’ Chu said in a statement.
Most lithium ion batteries work similarly with the key components including an anode, the negative pole from which electrons flow out and into a device, and the cathode, where the electrons re-enter the battery once they have travelled through the circuit. Separating them is an electrolyte, a solid or liquid loaded with positively charged lithium ions that travel between the anode and cathode.
During charging, the positively charged lithium ions in the electrolyte are attracted to the negatively charged anode and the lithium accumulates on the anode. Today, the anode in a lithium ion battery is actually made of graphite or silicon.
Engineers would like to use lithium for the anode, but so far they have been unable to do so because the lithium ions expand as they gather on the anode during charging.
All anode materials, including graphite and silicon, expand during charging, but not like lithium. Researchers say that lithium’s expansion during charging is virtually infinite relative to the other materials. Its expansion is also uneven; causing pits and cracks to form in the outer surface.
The resulting fissures on the surface of the anode allow the lithium ions to escape, forming hair-like or mossy growths, called dendrites that can short circuit the battery.
Preventing this build-up is the first challenge of using lithium for the battery’s anode. The second engineering challenge is that a lithium anode is highly chemically reactive with the electrolyte, using up the electrolyte and reducing battery life. An additional problem is that the anode and electrolyte produce heat when they come into contact.
To solve these problems the Stanford researchers built a protective layer of interconnected carbon domes on top of their lithium anode, described by the team describe as a nanosphere.
The Stanford team’s nanosphere layer resembles a honeycomb: it creates a flexible, uniform and non-reactive film that protects the unstable lithium from the drawbacks that have made it such a challenge. The carbon nanosphere wall is 20nm thick.
‘The ideal protective layer for a lithium metal anode needs to be chemically stable to protect against the chemical reactions with the electrolyte and mechanically strong to withstand the expansion of the lithium during charge,’ Cui said.
The Stanford nanosphere is made of amorphous carbon, which is chemically stable, yet strong and flexible so as to move freely up and down with the lithium as it expands and contracts during the battery’s normal charge-discharge cycle.
The nanospheres improve the coulombic efficiency of the battery. Generally, to be commercially viable, a battery must have a coulombic efficiency of 99.9 per cent or more over as many cycles as possible. Previous anodes of unprotected lithium metal achieved approximately 96 per cent efficiency, which dropped to less than 50 per cent in just 100 cycles. The Stanford team’s new lithium metal anode is said to achieve 99 per cent efficiency even at 150 cycles.
‘With some additional engineering and new electrolytes, we believe we can realise a practical and stable lithium metal anode that could power the next generation of rechargeable batteries,’ Cui said.