Lithium-ion rechargeable batteries, the mainstay of battery technology for many of today's consumer and enterprise devices and applications, currently use carbon-based materials as anodes, with capacities up to 370 mAH/g. Researchers have developed a new anode material which promises to almost double that capacity.
The joint research team from the Nanostructured Semiconducting Materials Group at the International Centre for Materials Nanoarchitectonics, NIMS, and the Georgia Institute of Technology developed the anode material by forming nanoparticles made of silicon-metal composites on metal substrates.
The resulting anode material had high capacity - almost twice as high as conventional materials - and a long cycle life.
In terms of anodes for lithium-ion batteries, theoretically, the use of pure silicon could yield a maximum capacity of 4,200 mAh/g. However, this is not practical, because pure silicon is highly expandable, with its volume growing by three to four times when lithium ions are incorporated into it. During repeated charge-discharge cycles, the stress caused by this volume growth can cause pure silicon anode materials to crack, resulting in very short battery life.
In order to achieve some of the capacity-boosting capabilities of silicon, while avoiding its physical vulnerabilities, the researchers are developing composites for use as anodes. The joint research groups formed one-dimensional germanium nanowires on metal substrates and then created nanostructured silicon-metal composites using the nanowires as a base material layer.
The formed nanostructured material has numerous cavities existing inside aggregated nanoparticles of about several tens of nanometers to a hundred nanometers. There also are larger cavities present between the silicon-metal composites and the germanium nanostructures. These cavities act as buffer space, absorbing the stress generated by the expansion of the pure silicon.
The material also regulates the composition of silicon and metal elements in the silicon-based nanostructure, because the material consists of not only pure silicon but also metal atoms that are spontaneously provided from the substrate via the underlying germanium nanostructures and incorporated into the growing silicon material, forming silicon-metal composites.
The researchers confirmed that the capacity of the new anode material was about double the capacity of current anode materials, and its cycle life was also extended compared to conventional materials.