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A research team at Rice University has overcome the problem of lithium metal building dendrites when used in batteries. They designed a novel electrode composed of a graphite-carbon nanotube hybrid with unprecedented lithium storage capacity, which is now being tested in a full battery at pilot scale.
For charging portable electronic devices, we most commonly resort to lithium (Li) ion batteries. These rechargeable devices rely on that Li ions incorporate into the electrode materials, a process that is called intercalation.
The amount of energy that can be stored in the intercalation compounds is limited. “Lithium-ion batteries have changed the world, no doubt”, says chemist James Tour, who led the present study, “but they’re about as good as they’re going to get. Your cell phone’s battery won’t last any longer until new technology comes along.”
One promising new technology, which overcomes this problem, are so-called Li metal batteries with electrodes based on metallic Li. They can theoretically deliver ten times higher capacities than those of Li ion devices.
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However, during battery operation, Li metal is known to form dendrites and other unstable structures. Dendrites are small elongated branched structures that grow from the electrode into the electrolyte of the battery. After some time, they can reach to the counter electrode and create a short circuit, which will cause the battery to fail, and in the worst case to catch fire or explode.
Now researchers from Rice University have found a solution to the dendrite problem: They discovered that Li metal can be stored as a coating on a graphene-carbon nanotube anode.
During battery operation, the coating will be repeatedly applied and stripped of so that no dendrite formation can occur. With this technology, mobile phones could in future last longer and charge faster. The study was recently published in the high-impact journal ACS Nano.
Electrode based on a graphene-carbon nanotube hybrid prevents dendrites
The graphene-carbon nanotubes that the novel electrode is based on is a hybrid of graphene and carbon nanotubes. More precisely, the hollow nanotubes are with one of their open sides covalently bonded on to the thin graphene sheet, looking a little like a nanoforest.
Although the research group at Rice already developed this material five years ago, they only realized its potential one day in 2014 when Adbul-Rahman Raji, a former PhD student in the lab and first author of the current study, started to experiment with lithium metal and the hybrid nanoforest.
“I reasoned that lithium metal must have plated on the electrode while analysing results of experiments carried out to store lithium ions in the anode material combined with a lithium cobalt oxide cathode in a full cell”, says Raji. When they looked at the voltage profile of the cell, they noticed that it was very flat. “At that moment, we knew we had found something special.”
A week later, Raji and co-author Rodrigo Villegas Salvatierra, a Rice postdoctoral researcher, had a closer look at the material under the microscope. To their surprise, they found that Li metal did not form any dendrites when deposited into a standalone anode made of the hybrid nanoforest. “We were stunned to find no dendrites grown, and the rest is history”, Raji says.
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Unprecedented lithium storage capacity makes novel battery last longer
In the hybrid material, the scientists observed a lithium storage capacity of 3,351 milliamp hours per gram, which is close to the theoretical maximum and ten times higher than in Li ion batteries.
These superior properties are thanks to the high surface area and low density of the nanoforest hybrid. This means that there is a lot of space for the Li to coat the nanoforest surface when the battery charges. Tour explains that with this material, the volume is used to a maximum.
The research team then went on to test the anode in a full battery with a sulphur-based cathode. “Many people doing battery research only make the anode, because to do the whole package is much harder”, Tour says. “We had to develop a commensurate cathode technology based upon sulphur to accommodate these ultrahigh-capacity lithium anodes in first-generation systems.”
When testing the full batteries, they found that it retained 80% of its initial capacity after 500 charge-discharge cycles. This corresponds to approximately two years of normal mobile phone use, which is very promising news for all mobile phone users.
If and when the novel battery will be available on the market is unclear but the research team has already taken the next step. Tour: “We’re producing these full batteries, cathode plus anode, on a pilot scale, and they’re being tested.”
Sources:
- (2017) Lithium Batteries with Nearly Maximum Metal Storage, ACS Nano, doi: 10.1021/acsnano.7b02731.
- Phys. Org (2017) Graphene-nanotube hybrid boosts lithium metal batteries, available at: https://phys.org/news/2017-05-graphene-nanotube-hybrid-boosts-lithium-metal.html (assessed on 19/05/2017).
- Image Credit: Shutterstock.com/JevantoProductions
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