4
Components and Materials for
Electrochemical Supercapacitors
4.1 Introduction
4.1.1 Traditional Capacitors
As discussed in Chapter 1, capacitors are known for their extremely high
power density and cycle lifetimes, lending to their significance in the world
of analog and digital electronics. An ideal capacitor is a passive device best
visualized as two separated metal plates composed of conductive films such
as stainless steel, nickel, or aluminum. The two plates allow a static charge to
build up and be released quickly, resulting in high power density. Between
the plates is a dielectric, a thin insulating material that has the potential to
polarize in an electric field.
Charges in the dielectric move to generate an internal electric field and
form equilibrium with the field present on the two plates. By using higher
permittivity dielectrics, it is possible to store higher levels of charge in
the internal field. Alternatively, capacitance is increased by reducing the
separation between the plates. Further, a capacitor’s performance can be
altered by the breakdown potential of the dielectric. When breakdown
occurs, all the stored energy between the plates is lost. To avoid dielectric
breakdown, a minimum separation distance limits capacitance at higher
operating voltages.
Dielectric materials vary by cost and the capacitance needed for a specific
application. Glass, ceramic, and mica papers are high quality, low capacitance
dielectrics with extremely high breakdown resistance. Conversely, metalized polymer foils such as polystyrene, polyethylene terephthalate (PET),
and Teflon (PTFE) are single-piece dielectric films that offer better capacitor
performance. In recent years, polymer foils have come to dominate the static
capacitor market because they have better stability at high temperatures, can
be manufactured at lower cost, and age better than dielectric papers.
135
Components and Materials for
Electrochemical Supercapacitors
4.1 Introduction
4.1.1 Traditional Capacitors
As discussed in Chapter 1, capacitors are known for their extremely high
power density and cycle lifetimes, lending to their significance in the world
of analog and digital electronics. An ideal capacitor is a passive device best
visualized as two separated metal plates composed of conductive films such
as stainless steel, nickel, or aluminum. The two plates allow a static charge to
build up and be released quickly, resulting in high power density. Between
the plates is a dielectric, a thin insulating material that has the potential to
polarize in an electric field.
Charges in the dielectric move to generate an internal electric field and
form equilibrium with the field present on the two plates. By using higher
permittivity dielectrics, it is possible to store higher levels of charge in
the internal field. Alternatively, capacitance is increased by reducing the
separation between the plates. Further, a capacitor’s performance can be
altered by the breakdown potential of the dielectric. When breakdown
occurs, all the stored energy between the plates is lost. To avoid dielectric
breakdown, a minimum separation distance limits capacitance at higher
operating voltages.
Dielectric materials vary by cost and the capacitance needed for a specific
application. Glass, ceramic, and mica papers are high quality, low capacitance
dielectrics with extremely high breakdown resistance. Conversely, metalized polymer foils such as polystyrene, polyethylene terephthalate (PET),
and Teflon (PTFE) are single-piece dielectric films that offer better capacitor
performance. In recent years, polymer foils have come to dominate the static
capacitor market because they have better stability at high temperatures, can
be manufactured at lower cost, and age better than dielectric papers.
135