A breakthrough in electrochemistry at Cambridge university could lead the way to rechargeable super-batteries that pack five times more energy into a given space than today’s best batteries, greatly extending the range of electric vehicles and potentially transforming the economics of electricity storage.
Chemistry professor Clare Grey and her team have overcome several technical challenges in the development of lithium-air batteries – the only cells theoretically capable of giving electric cars the range of petrol and diesel vehicles without having to carry excessively bulky and heavy battery packs.
If the technology can be turned from a laborary demonstrator into a commercial product, it will enable a car to drive from London to Edinburgh on a single charge, with batteries that cost and weigh one-fifth of the lithium-ion cells that power today’s electric cars.
“What we’ve achieved is a significant advance for this technology and suggests whole new areas for research,” said Prof Grey. “We haven’t solved all the problems inherent to this chemistry but our results do show routes forward.”
Because lithium-air has such a big theoretical advantage over lithium-ion which dominates rechargeable batteries today – its energy density is potentially 10 times greater – researchers around the world are working on lithium-air.
A research paper published in the journal Science shows that the Cambridge group has overcome some of the practical problems of the technology, particularly chemical instability that led to a rapid fall-off in performance of the lithium-air cells demonstrated previously.
The basic chemistry of lithium-air batteries is simple. The cell generates electricity by combining lithium with oxygen to form lithium peroxide and is then recharged by applying a current to reverse the reaction. Making these reactions take place reliably over many cycles is a formidable challenge.
The Cambridge scientists adjusted the chemistry in several ways to make it more controllable. For example, they converted lithium peroxide to lithium hydroxide (a compound that is easier to work with), they added lithium iodide to the system and they made a very porous “fluffy” electrode from graphene, a new form of carbon discovered 12 years ago at Manchester University.
The system demonstrated in the Cambridge lab is 90 per cent efficient, according to the researchers, and it can be recharged more than 2,000 times. But they say at least another decade of work is likely to be required to turn it into a commercial battery for cars and for grid storage – storing the intermittent output of solar and wind generators for use when needed.
The Cambridge research so far has been funded by the UK Engineering and Physical Sciences Research Council, the US Department of Energy and the EU, with support from Johnson Matthey, the UK advanced materials company.
“We have patented the technology and the intellectual property is owned by Cambridge Enterprise, the university’s commercialisation arm,” said Prof Grey. “We are working with a number of companies to take it forward.” (c) 2015 The Financial Times Ltd.
Researchers have developed a new method for harvesting the energy carried by particles known as ‘dark’ spin-triplet excitons with close to 100% efficiency, clearing the way for hybrid solar cells which could far surpass current efficiency limits.
The team, from the University of Cambridge, have successfully harvested the energy of triplet excitons, an excited electron state whose energy in harvested in solar cells, and transferred it from organic to inorganic semiconductors. To date, this type of energy transfer had only been shown for spin-singlet excitons. The results are published in the journal Nature Materials.
In the natural world, excitons are a key part of photosynthesis: light photons are absorbed by pigments and generate excitons, which then carry the associated energy throughout the plant. The same process is at work in a solar cell.
In conventional semiconductors such as silicon, when one photon is absorbed it leads to the formation of one free electron that can be extracted as current. However, in pentacene, a type of organic semiconductor, the absorption of a photon leads to the formation of two electrons. But these electrons are not free and they are difficult to pin down, as they are bound up within ‘dark’ triplet exciton states.
Excitons come in two ‘flavours’: spin-singlet and spin-triplet. Spin-singlet excitons are ‘bright’ and their energy is relatively straightforward to harvest in solar cells. Triplet-spin excitons, in contrast, are ‘dark’, and the way in which the electrons spin makes it difficult to harvest the energy they carry.
“The key to making a better solar cell is to be able to extract the electrons from these dark triplet excitons,” said Maxim Tabachnyk, a Gates Cambridge Scholar at the University’s Cavendish Laboratory, and the paper’s lead author. “If we can combine materials like pentacene with conventional semiconductors like silicon, it would allow us to break through the fundamental ceiling on the efficiency of solar cells.”
Using state-of-art femtosecond laser spectroscopy techniques, the team discovered that triplet excitons could be transferred directly into inorganic semiconductors, with a transfer efficiency of more than 95%. Once transferred to the inorganic material, the electrons from the triplets can be easily extracted.
“Combining the advantages of organic semiconductors, which are low cost and easily processable, with highly efficient inorganic semiconductors, could enable us to further push the efficiency of inorganic solar cells, like those made of silicon,” said Dr Akshay Rao, who lead the team behind the work.
The team is now investigating how the discovered energy transfer of spin-triplet excitons can be extended to other organic/inorganic systems and are developing a cheap organic coating that could be used to boost the power conversion efficiency of silicon solar cells.
The work at Cambridge forms part of a broader initiative to harness high tech knowledge in the physical sciences to tackle global challenges such as climate change and renewable energy. This initiative is backed by the UK Engineering and Physical Sciences Research Council (EPSRC) and the Winton Programme for the Physics of Sustainability.
Read more on the subject by clicking here.
See more at: http://www.cam.ac.uk/research/news/hybrid-materials-could-smash-the-solar-efficiency-ceiling#sthash.loqUPAFy.dpuf
