Usually, Li-Ion batteries have carbon (graphite) electrodes, and the performance that we already know, which is not good enough for our cars at this moment. Georgia Institute of Technology researchers developed a new, high-performance anode based on a silicon-carbon nanocomposite material that could well improve the usability of Li-Ion batteries.
They designed their new anode by using a self-assembly technique, which should solve many of the issues of silicon-based battery anodes. The cost is low, simple, and could be easily scaled-up and made compatible with already-established technologies.
In a battery, lithium ions travel between the two electrodes. The more efficiently they enter the two electrodes, the longer the life and the larger the battery’s capacity will be. Graphite anodes use particles ranging in size from 15 to 20 microns. If silicon particles of that size are simply substituted for the graphite, expansion and contraction as the lithium ions enter and leave the silicon creates cracks that quickly cause the anode to fail.
The new nanocomposite material solves that degradation problem, potentially allowing battery designers to tap the capacity advantages of silicon. That could facilitate higher power output from a given battery size – or allow a smaller battery to produce a required amount of power.
“At the nanoscale, we can tune materials properties with much better precision than we can at traditional size scales,” said Yushin. “This is an example of where having nanoscale fabrication techniques leads to better materials.” Electrical measurements of the new composite anodes in small coin cells showed they had a capacity more than five times greater than the theoretical capacity of graphite.
Fabrication of the composite anode begins with formation of highly conductive branching structures – similar to the branches of a tree – made from carbon black nanoparticles annealed in a high-temperature tube furnace. Silicon nanospheres with diameters of less than 30 nanometers are then formed within the carbon structures using a chemical vapor deposition process. The silicon-carbon composite structures resemble “apples hanging on a tree.”
Using graphitic carbon as an electrically-conductive binder, the silicon-carbon composites are then self-assembled into rigid spheres that have open, interconnected internal pore channels. The spheres, formed in sizes ranging from 10 to 30 microns, are used to form battery anodes. The relatively large composite powder size – a thousand times larger than individual silicon nanoparticles – allows easy powder processing for anode fabrication.
The internal channels in the silicon-carbon spheres have two purposes: they admit liquid electrolyte to allow rapid entry of lithium ions for quick battery charging, and they provide space to accommodate expansion and contraction of the silicon without cracking the anode. The internal channels and nanometer-scale particles also provide short lithium diffusion paths into the anode, boosting battery power characteristics.
During their testing procedures, the researchers have charged and discharged the batteries for more than 100 cycles, with no signs of sensible degradation. This would make the new anode fabrication technology suitable for electric cars, solar cells, and many others.
