Ultracapacitors
Electric double-layer capacitors (or "ultracapacitors") are used in some electric vehicles, such as AFS Trinity's concept prototype, to store rapidly available energy with their high power density, in order to keep batteries within safe resistive heating limits and extend battery life.
Since commercially available ultracapacitors have a low energy density no production electric cars use ultracapacitors exclusively.
Read more about this topic: Electric Vehicle Battery