The rapid evolution
Guest article by Claus Bünnagel, chief editor of ’busplaner’ magazine
Until now, conventional graphite anodes – the negative terminal on a battery – have always presented the same problem. Using a thin layer of graphite limits a cell’s energy capacity. A thick graphite anode can hold a higher lithium-ion charge, but this makes fast charging more difficult because the ions must travel further in the anode. At this point silicon comes into play. It is cheap, readily available worldwide, and can alter the crystal structure of the anode in such a way as to significantly reduce lithium plating, which can negatively affect performance. This technology is expected to break through in the next three years. When that happens, charging an electric vehicle to 75 to 80 per cent will take five to ten minutes, with no need to dispense with high-capacity cells. At present, an energy density of up to 350 Wh/kg is achievable. Even operating a vehicle at temperatures of -20°C and below is conceivable, as is a minimum of 1,000 charge cycles.
Graphene instead of graphite
However, the solution to future fast-charging may lie completely elsewhere. The atomic structure of graphene is similar to that of graphite, which is currently still used in anodes. But there is one important difference. Unlike graphite and every other material currently available, graphene is two-dimensional. It consists of only a single layer of carbon and is only one atom wide. At the same time, the honeycomb arrangement of the atoms makes graphene incredibly strong. In terms of strength this new material can be compared with steel, but it is also just as pliable as rubber. It is also an excellent heat and electricity conductor, and has a huge surface as well as many other positive attributes.
Using orthosilicic acid, graphene can be formed into microscopic spherical particles. Used as anode material and with a thin coating on the cathode, these particles help shorten charging times to 12 minutes and increase capacity by 45 per cent compared with a conventional lithium-ion battery.
Graphene in supercapacitors
Experiments are also taking place using graphene in supercapacitors. This type of supercapacitor consists of only two layers of graphene separated by an electrolyte, in the form of an easily applied thin coating. This prevents the graphene layers making contact and reacting to form conventional graphite. The advantages are a relatively high energy density as far as supercapacitors go, along with typically fast charging and discharging times.
Solid-state batteries are the future
Besides lithium metal and lithium sulphur batteries, solid-state batteries also offer good prospects for the future. Their weakness to date is the low number of charge cycles, which a solid-state ceramic separator aims to eliminate in the future. Thus the goal is to be able to fast-charge a solid-state battery to 80 per cent capacity in 15 minutes. After 800 charge cycles or several hundred thousand kilometres, battery capacity can still be expected to exceed 80 per cent. Furthermore, a solid-state battery is reportedly extremely fire resistant and functions within a broad temperature range without performance loss, even at temperatures of down to -30°C. The next step will be to convert single-layer to multi-layer cells and prepare them for mass production.