Demand for lithium ion batteries that power devices is on the rise. User industries from electric cars to cell phones are demanding more batteries and more capacity from each battery. To help meet this demand, Environmental Molecular Science Laboratory (EMSL) users and researchers put their energy behind a new idea that, literally, gives batteries a bit of room to grow. Lithium ion batteries generate electricity by shuttling lithium ions through an electrolyte. In a fully charged battery, lithium ions are stored in a cathode, such as lithium cobalt oxide (LiCoO2). When in use, lithium ions flow from the cathode through an electrolyte into the anode, most commonly made of carbon. During recharging, the ions are pushed back to the cathode where they started. Researchers built upon current technology by making a new type of anode that consists of single silicon nanoparticles inside carbon shells, much like yolks inside eggs. In the new design, lithium ions flow from the cathode through the electrolyte, diffuse through the carbon shells, and enter the silicon-which can hold ten times as many lithium ions as carbon alone. By leaving just the right amount of space, the lithiated silicon nanoparticles swell to fill, but not burst, the carbon shell. This results in a lithium ion battery system that compared to commercial batteries holds seven times more energy and can be discharged and recharged five times as many times before it wears out. Critical to its good performance, the new system forms a stable crust, a solid electrolyte interphase, on the anode that is a consequence of electrolyte decomposition. Moreover, the team's manufacturing process is affordable, efficient, and can be readily scaled up.
In another research, a team of scientists from Germany and Japan have presented a new principle for storing energy in lithium ion batteries using a porous polymer framework. This could give these new batteries double the energy storage of conventional lithium ion batteries. The lightest of all metals, lithium batteries have made all manner of electronic devices compact and portable, ushering in the era of miniaturised mobile technology. But their use beyond these applications has been limited as they struggle to match the power output of the combustion engine, for example. Also, the transition metals they commonly use are becoming more scarce and expensive. Ken Sakaushi at the Dresden University of Technology and co-workers aim to solve both of these issues with their demonstration of a novel energy storage principle for a cathode based on a porous organic polymer framework material. In a traditional lithium battery, Sakaushi explains, electrons are transferred from the anode to the cathode by reducing a positive charge (p-doping) or creating a negative one (n-doping) within the cathode, with the corresponding movement of either anions or cations, respectively. 'Our idea is to combine these into one process,' he reveals. 'It uses both anions and cations to transfer electrons during discharge.' The key to realising this idea lies in the use of a triazine-based polymer as the cathode material. Triazine's electrochemical behaviour makes it uniquely suited for this purpose as it can exist in both p-doped (positive) and n-doped (negative) states. 'The most important feature of our cathode is a continuous, linear transition between these states during charge and discharge,' he says, which effectively doubles its capacity compared to traditional cathodes. The use of polymeric frameworks for these purposes also has other advantages: selecting the monomers (in this case p-dicyanobenzene) gives precise control over the pore size and distribution to deliver high surface area and allow rapid transport of the ions into and out of the electrode. They can also be lighter than the transition metal oxides normally used. 'These are very interesting materials,' says Laurence Hardwick from the University of Liverpool, UK. 'People are now trying to use them in applications such as energy storage-this is the first or second paper in this area. It's a very interesting, novel approach for ion storage in this class of solids.'