191
Components and Materials for Electrochemical Supercapacitors
choice are aluminum, iron, copper, and steel thin films. In the case of a soft
electrode, the material can be pressed into a metal wire mesh. The sheet resistance of the active material prevents effective conduction over long distances
such as along the length of an electrode film. As a result, current conduction
is applied through the thickness of the active material and then passes to a
larger external end-plate collector through a low resistance solder joint. The
metal collector film backs the active electrode material to minimize electron
transport distance to the highly conductive collector.
Over prolonged cycling time, the connection between the electrode and
the metal collector can be degraded due to shrinkage and loss of electrode
material to the electrolyte. These losses translate to lowered capacitance and
cycle life of the device and offer further reason to use other techniques to
improve connections and reduce resistances. Another important concern on
a system-wide level is the efficient transport of heat from the material. Using
thermally conductive metals helps transport heat to the end plate, allowing
better integration with heat sinks for passive cooling. Aluminum foils are
common choices due to thermal transmission, low cost, high workability,
and good conductivity.
In many cases, electrode materials or pastes are grown or deposited (e.g.,
spray coat, drop coat, spin coat, application of electrode paste) directly onto
collector material to provide a good molecular contact created by drying and
annealing of the electrode [125]. To further minimize resistance, a highly
conductive coating that exhibits improved interfacial contact between components can be applied. Without low resistance connections, the electrode
performance of a good material can otherwise be lost.
One common technique for generating a good contact is the use of silver
paste that adapts to the surface roughness of the active material. Gold and
lead thin films have been shown to significantly enhance interface integration of graphene (electrically and mechanically), resulting in higher device
performance. Strong current collection allows efficient movement of charge
and allows current density as high as 30 A.g
–1 (maintaining 70% of low current capacitance) and performance of 40 Wh.kg –1 and 40 kW.kg –1 [126].
Another technique used by Portet et al. to minimize contact resistance
employs roughening the collector to bring ESR down from 50 to 5 Ω.cm2.
Etching or surface roughening is an effective way to improve the contact
area and increase the number of conductive sites. To further improve contact
resistance, a conductive carbon gel was dried onto the collector surface. The
carbon-coated collector exhibited a minimal interfacial boundary with the
active carbon material and managed to reduce contact resistance to as low
as 1 Ω.cm 2 [127].
For free-standing electrodes, integration with collectors becomes even
more important to avoid resistances because there is no intimate surface
contact. This highlights the importance of high pressure lamination to
reduce contact resistance between collector and active material when an inti
Components and Materials for Electrochemical Supercapacitors
choice are aluminum, iron, copper, and steel thin films. In the case of a soft
electrode, the material can be pressed into a metal wire mesh. The sheet resistance of the active material prevents effective conduction over long distances
such as along the length of an electrode film. As a result, current conduction
is applied through the thickness of the active material and then passes to a
larger external end-plate collector through a low resistance solder joint. The
metal collector film backs the active electrode material to minimize electron
transport distance to the highly conductive collector.
Over prolonged cycling time, the connection between the electrode and
the metal collector can be degraded due to shrinkage and loss of electrode
material to the electrolyte. These losses translate to lowered capacitance and
cycle life of the device and offer further reason to use other techniques to
improve connections and reduce resistances. Another important concern on
a system-wide level is the efficient transport of heat from the material. Using
thermally conductive metals helps transport heat to the end plate, allowing
better integration with heat sinks for passive cooling. Aluminum foils are
common choices due to thermal transmission, low cost, high workability,
and good conductivity.
In many cases, electrode materials or pastes are grown or deposited (e.g.,
spray coat, drop coat, spin coat, application of electrode paste) directly onto
collector material to provide a good molecular contact created by drying and
annealing of the electrode [125]. To further minimize resistance, a highly
conductive coating that exhibits improved interfacial contact between components can be applied. Without low resistance connections, the electrode
performance of a good material can otherwise be lost.
One common technique for generating a good contact is the use of silver
paste that adapts to the surface roughness of the active material. Gold and
lead thin films have been shown to significantly enhance interface integration of graphene (electrically and mechanically), resulting in higher device
performance. Strong current collection allows efficient movement of charge
and allows current density as high as 30 A.g
–1 (maintaining 70% of low current capacitance) and performance of 40 Wh.kg –1 and 40 kW.kg –1 [126].
Another technique used by Portet et al. to minimize contact resistance
employs roughening the collector to bring ESR down from 50 to 5 Ω.cm2.
Etching or surface roughening is an effective way to improve the contact
area and increase the number of conductive sites. To further improve contact
resistance, a conductive carbon gel was dried onto the collector surface. The
carbon-coated collector exhibited a minimal interfacial boundary with the
active carbon material and managed to reduce contact resistance to as low
as 1 Ω.cm 2 [127].
For free-standing electrodes, integration with collectors becomes even
more important to avoid resistances because there is no intimate surface
contact. This highlights the importance of high pressure lamination to
reduce contact resistance between collector and active material when an inti