Storage of electrical energy is a serious problem. The requirement to provide power on demand in short bursts, directly by generation, forces us to size the generation capability larger then the greatest projected demand. Chemical generation requires the same sizing problem along with recharging or renewing the device. Super capacitors and ionic batteries may provide storage of electricity and supply it directly without efficiency robbing conversion in and out of chemical solutions.
Paper like electrical energy storage in super capacitor – batteries
University researchers have developed a paper-like material for lithium-ion batteries that has the potential to boost by several times the specific energy—or amount of energy—that can be delivered by the battery. The new material produced by researchers at the University of California-Riverside’s Bourns College of Engineering is composed of sponge-like silicon nanofibers. The research team used a technique known as electrospinning, in which 20,000 to 40,000 volts are applied between a rotating drum and a nozzle to emit a chemical compound. The compound is composed mainly of tetraethyl orthosilicate (TEOS), frequently used in the semiconductor industry. The nanofibers are then exposed to magnesium vapor to produce the sponge-like silicon fiber structure.
The material could be used in batteries for electric vehicles (EVs) and personal electronics, according to the researchers.
Conventionally produced lithium-ion battery anodes are made using copper foil coated with a mixture of graphite, a conductive additive and a polymer binder. However, the performance of graphite has been nearly tapped out, according to the UC Riverside researchers, so they are experimenting with silicon, which has a specific capacity that is nearly 10 times higher than graphite.
Silicon has a downside: it suffers from significant volume expansion, which can quickly degrade the battery. The silicon nanofiber structure created in the lab of Mihri Ozkan, a professor of electrical and computer engineering at UC Riverside, circumvents this issue and allows the battery to be cycled hundreds of times without significant degradation, according to the researchers. The researchers contend that eliminating the need for metal current collectors and inactive polymer binders while switching to an energy dense material such as silicon will significantly boost the range capabilities of electric vehicles.
Free-standing materials grown using chemical vapor deposition, such as carbon nanotubes or silicon nanowires, can only be produced in very small quantities (micrograms). By contrast, the UC Riverside researchers were able to produce several grams of silicon nanofibers at a time even at the lab scale.The researchers’ next step is to implement the silicon nanofibers into a pouch cell format lithium-ion battery, which is still a larger scale battery format that can be used in EVs and portable electronics.
The UC Riverside Office of Technology Commercialization has filed patents for inventions reported in the research paper. The paper, “Towards Scalable Binderless Electrodes: Carbon Coated Silicon Nanofiber Paper via Mg Reduction of Electrospun SiO2 Nanofibers,” has been published in the journal Nature Scientific Reports. The research was supported by Temiz Energy Technologies.
The paper describing the research was authored by Mihri Ozkan along with Cengiz S. Ozkan, a professor of mechanical engineering, and six of their graduate students: Zach Favors, Hamed Hosseini Bay, Zafer Mutlu, Kazi Ahmed, Robert Ionescu and Rachel Ye.
1 Farad is a serious hand full
Researchers at LinköpingUniversity’s Laboratory of Organic Electronics, Sweden, have developed what they call “power paper” – a new material that consists of nanocellulose and conductive polymer, capable of storing energy.
One sheet of power paper is 15 cm in diameter, a few tenths of a millimeter thick, and capable of storing as much as 1 farad, similar to that of supercapacitors currently on the market. The team’s material takes a few seconds to re-charge and can be re-charged up to hundreds of times. The material the researchers used to create the power paper, looks and feels like a strong sheet of plastic paper.
This piece of power paper can store 1 farad. (Source: Thor Balkhed)
“Thin films that function as capacitors have existed for some time. What we have done is to produce the material in three dimensions. We can produce thick sheets,” says Xavier Crispin, professor of organic electronics and co-author of the team’s article.
The paper is based on nanocellulose, cellulose fibers that are broken down into fibers about 20 nm in diameter once in contact with high-pressure water. With the cellulose fibers in a solution of water, an electrically charged polymer that is also in a water solution is added to the mix which is when the polymer forms a thin coating around the fibers.
“The covered fibers are in tangles, where the liquid in the spaces between them functions as an electrolyte,” says Jesper Edberg, a doctoral student who conducted the experiments.
The new cellulose-polymer material has set a new world record in simultaneous conductivity for ions and electrons, part of the reason why the team feels it is contains a high capacity for energy storage.
Unlike batteries and capacitors on the market today, power paper is constructed with simple and readily available materials, is waterproof, and does not require the use of harmful chemicals.
The team will face its next challenge in the development process – figuring out how to develop an industrial-scale process for this. Linköping University has just received funding to work on a paper machine that will produce power paper.
Another fast-charging battery promises longer life.
The automotive industry is eager to see an electric vehicle (EV) battery that can be fully charged within five minutes, about the time it takes to fill a conventional vehicle’s gas tank. Actual charging time for an EV is over four hours.
Scientists at the Nanyang Technological University (NTU) in Singapore have developed such a battery. In tests at the university labs, the researchers demonstrated that the new battery could be charged up to 70% of its total charge in just two minutes, while enduring 20 times more charging cycles than today’s batteries. The team of researchers, led by Professor Chen Xiaodong from the School of Material Science and Engineering at NTU, developed this battery by replacing the traditional lithium-graphite anode in lithium-ion batteries with a new gel material made from titanium dioxide. This compound is very abundant, cheap and safe for human handling. It is commonly used as a food additive or in sunscreens to absorb UV rays.
To enhance the speed of the charging process, Professor Chen developed a method to convert the titanium dioxide particles into nanotubes. These nanotubes help to speed up the charging process by triggering a chemical reaction in the anode of the battery. “Manufacturing this new nanotube gel is very easy. Titanium dioxide and sodium hydroxide are mixed together and stirred under a certain temperature. Battery manufacturers will find it easy to integrate our new gel into their current production processes,” Professor Chen said.
Professor Rachid Yazami, the scientist who 34 years ago invented the graphite anode used in lithium-ion batteries today, said that Chen’s invention is the next big leap the scientific community was waiting for. “While the cost of lithium-ion batteries has been significantly reduced and their performance improved since Sony commercialized it in 1991, the market is expanding towards new applications in electric mobility and energy storage,” said Professor Yazami. “There is still room for improvement and one such key area is the power density – how much power can be stored in a certain amount of space – which directly relates to the fast charge ability. Ideally, the charge time for batteries in electric vehicles should be less than 15 minutes, which Prof Chen’s nanostructured anode has proven to do,” he adds.
A paper published in the journal Advanced Materials describe the invention. The technology invented by Chen is being licensed to a company. It is expected that in two years’ time a new generation of fast-charging lithium-ion batteries will be available. “With our nanotechnology, electric cars would be able to increase their range dramatically with just five minutes of charging, which is on par with the time needed to pump petrol for current cars,” added Prof Chen. “Equally important, we can now drastically cut down the waste generated by disposed batteries, since our batteries last ten times longer than the current generation of lithium-ion batteries,” he adds.
In search of Functional 90% More Efficient, ‘Ultimate’ Battery
Researchers from the University of Cambridge have demonstrated how many of the obstacles standing in the way of developing the “ultimate” battery can be overcome.
What is the “ultimate” battery?
Lithium-oxygen, or lithium-air batteries have been referred to as the “ultimate” battery due to their theoretical energy density, which is 10 times that of a lithium-ion (Li-ion) battery. Such a high-energy density would be comparable to that of gasoline—and would enable an electric car with a battery that is a fifth the cost and a fifth the weight of those currently on the market to drive about 400 miles on a single charge.
As with many other next-generation batteries, there are several challenges that need to be addressed before lithium-air batteries become a viable alternative to gasoline.
Previous attempts at working demonstrators have had low efficiency, poor rate performance, unwanted chemical reactions and can only be cycled in pure oxygen.
Creating the “ultimate” battery
What the researchers created is a working laboratory demonstrator of a lithium-oxygen battery that has very high energy density, is more than 90% efficient and can even be recharged more than 2000 times, showing how several of the problems holding back the development of these devices could be solved.
The demonstrator has a higher capacity, increased energy efficiency and improved stability over previous attempts.
It relies on a highly porous, ‘fluffy’ carbon electrode made from graphene and additives that alter the chemical reactions at work in the battery, making it more stable and more efficient.
‘Fluffy’ Carbon Electrode Made from Graphene and Additives. Image Credit: University of Cambridge“What we’ve achieved is a significant advance for this technology and suggests whole new areas for research—we haven’t solved all the problems inherent to this chemistry, but our results do show routes forward towards a practical device,” says Professor Clare Grey of Cambridge’s Department of Chemistry.
There is a constant effort to achieve a smaller, more efficient battery among researchers. Apart from the possibility of a smartphone, which lasts for days without needing to be charged, the challenges associated with making a better battery could be damaging the take-off of electric cars and grid-scale solar power.
“In their simplest form, batteries are made of three components: a positive electrode, a negative electrode and an electrolyte,’’ says Dr Tao Liu, also from the Department of Chemistry, and the paper’s first author.
In the traditional Li-ion batteries we use in our laptops and electronics, the negative electrode is composed of graphite, the positive electrode is made of a metal oxide, such as lithium cobalt oxide, while the electrolyte is a lithium salt dissolved in an organic solvent. The action of the battery depends on the movement of lithium ions between the electrodes. Li-ion batteries are light, but their capacity deteriorates with age, and their relatively low-energy densities mean that they need to be recharged frequently.
The team’s method uses a very different chemistry than the earlier attempts at a non-aqueous lithium-air battery. It relies on lithium hydroxide instead of lithium peroxide. With the addition of water and the use of lithium iodide as a ‘mediator’, the battery showed less of the chemical reactions, which can cause cells to die, making it more stable after multiple charge and discharge cycles.
The researchers’ method reduced the ‘voltage gap’ between charge and discharge to 0.2 V. A small voltage gap means a more efficient battery.
Other issues that still have to be addressed include finding a way to protect the metal electrode so that it does not form spindly lithium metal fibers, as this can cause batteries to explode if they grow too much and short-circuit the battery.
Additionally, the demonstrator can only be cycled in pure oxygen, while the air around us also contains carbon dioxide, nitrogen and moisture, all of which are generally harmful to the metal electrode.
“There’s still a lot of work to do,” says Liu. “But what we’ve seen here suggests that there are ways to solve these problems—maybe we’ve just got to look at things a little differently.”
“While there are still plenty of fundamental studies that remain to be done, to iron out some of the mechanistic details, the current results are extremely exciting—we are still very much at the development stage, but we’ve shown that there are solutions to some of the tough problems associated with this technology,” says Grey.
The results were reported in the journal Science, and although they are making strides in the right direction, the researchers caution that a practical lithium-air battery still remains at least a decade away.
For more information, visit the University of Cambridge website.