Italian researchers have developed a lithium ion battery with graphene as anode, which shows better performance than the commercial lithium-based battery (the energy density is about 25 % higher). The key to these results was the use of graphene in form of spreadable ink – a suspension of graphene nanoflakes smaller than 100 nm.
What Are Lithium-Ion Batteries?
Lithium-ion batteries (LIBs) are probably the most used batteries for portable electronic devices, such as mobile phones or laptops.
In these batteries, the electric current is created by the movement of ions of lithium (Li+) between the two electrodes (cathode and anode). In such devices the cathode is generally made of a compound which contains lithium; lithium cobalt oxide (LiCoO2) and lithium iron phosphate (LiFePO4) are the most common ones. The anode is made of graphite.
Lithium-Ion Batteries: Limits
Despite their extensive use, LIBs present some limits, such as temperature sensitivity and possible safety concerns.
Another problem is that their energy density is relatively low; because of this, at present they cannot be used for other applications, such as electric vehicles and plants for renewable energy (i.e. wind, solar).
One way to increase the energy density would be the use of an anode with a higher specific capacity than graphite. The characteristics of the anode, however, should be appropriate for the electrode to work with lithium ions.
Using Graphene For Batteries
Graphene is a carbon-based material made of just one layer of atoms, bonded to each other with a 120 o angle. Due to its high electric conductivity and surface area, it is an ideal material for electrode fabrication.
The use of graphene in LIBs, however, presents some problems. For instance, repulsion forces between the Li+ ions and the graphene sheets reduce the efficiency of the system.
Several studies were performed to modify graphene structure, to reduce the repulsion of the lithium ions.
Solving the Problems of Graphene
Recently Italian researchers obtained interesting results in this field, as reported in an article published in Nano Letters. The study was performed in the University Sapienza of Rome, in cooperation with the Istituto Italiano di Tecnologia (Genova), Scuola Normale Superiore (Pisa) and Istituto Processi Chimico-Fisici (Messina).
The key element of this study was the fabrication of the anode using graphene in the form of nanoflake ink.
Graphene Nanoflake Ink
Graphene nanoflake ink was prepared following a multistep process.
First, a thin film of graphene, prepared by Chemical Vapor Deposition, was dispersed in an appropriate organic solvent (N-methyl-2-pyrrolidone).
Then, the researchers treated the dispersion with ultrasound (ultrasonication). With this combination of actions, the team formed graphene flakes of various dimensions.
After the ultrasonication, the researchers ultracentrifuged the dispersion at 25000 rpm. They performed the ultracentrifugation to separate the larger/thicker flakes from the smaller/thinner ones. Larger flakes sedimented at the bottom of the solution, while smaller and lighter graphene flakes remained suspended in the solution.
At the end of the ultracentrifugation, the researchers separated the supernatant – the liquid remaining above the residue at the bottom – by pipetting. This liquid contained graphene flakes with dimensions in the range between 30 and 100 nm.
The researchers carefully placed drops of the suspension, which was now, essentially, spreadable graphene ink, on the surface of a copper substrate. The metal was then heated at 400 oC, to completely remove the solvent, leaving behind the graphene. The researchers used this material as a lithium-ion battery anode, and LiFePO4 as cathode.
When tested, this lithium-ion battery showed excellent performance, as its energy density was 190 W h kg-1.
This value is about 25 % higher than that of LIBs currently used.
Professor Vittorio Pellegrini, one of the leading scientists involved in the study, commented to Decoded Science on the importance of these results.
“These results are important for two main reasons.
The first is that the anode fabrication is a relatively simple process; in fact, with the use of the spreadable graphene ink, the process can be easily scaled up at reasonable costs.
The second is that the performance of this LIB battery is better than the current commercial ones. This opens to the door to many different applications. To achieve these performances, it was very important to have graphene in the form of nanoflakes; in fact this reduces remarkably the lithium ion repulsion, and allows a greater charge storage.”
New Lithium-Ion Batteries?
This new type of battery could make huge improvements in various types of technology. Longer-lasting batteries at a reasonable cost – all thanks to graphene and materials science.
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