185
Components and Materials for Electrochemical Supercapacitors
between increased stability ionic liquids and lowered conductivity can be
seen in Table 4.9 [113].
IL electrolytes have high thermal stability that creates an opportunity for
operation in high temperature environments. At high temperatures, the low
conductivity that limits IL performance is overcome by increased ion mobility (kinetic energy), resulting in higher conductivity, greater device power,
and better response time [113]. However, high heat reduces the potential
window for ion stability and this negatively impacts the power and energy
density. Another way to overcome the low conductivity of ILs is to balance
the high potential windows of ionic liquids with the increased conductivity
and power of organic electrolytes such as propylene carbonate and acetonitrile to optimize conductivity [112]. Utilizing such combinations can prevent
safety issues, reduce toxicity, and result in a device with high energy density
that maintains sufficient power performance [112].
4.3.5 Solid State Polymer Electrolytes
Gel and solid polymer electrolytes aim to combine the function of the electrolyte and separator into a single component to reduce the number of parts
in an ES and increase the potential window through the higher stability
offered by a polymer matrix. A gel electrolyte incorporates a liquid electrolyte into a microporous polymer matrix that holds in the liquid electrolyte
through capillary forces, creating a solid polymer film. The chosen separator
must be insoluble in the desired electrolyte and provide adequate ionic conductivity. Non-polar rigid polymers such as PTFE, PVA, PVdF, and cellulose
acetate offer good ion conductivity when used as gel electrolytes [114]. Based
on the data in Table 4.9, the ionic conductivity of EtMeIm + BF 4 is 14 mS.cm –1 .
Ionic conductivity of the same imidazolium salt used as a gel electrolyte in a
PVdF matrix retains 5 mS.cm –1 [115].
Modern electrolytes need increased stability and ion mobility to operate
at high potential windows. Gel electrolytes allow incorporation of aqueous,
organic, and ionic liquids, depending on the requirements of the ES. Separators
are used in conjunction with the electrolyte to help provide structured channels
and prevent short circuits between the electrodes. The presence of solid electrolyte layers results in a reduced need for robust encapsulation technologies.
In order to combine the two structures, electrolyte is trapped within the
polymer matrix during polymerization. The result is a solid, thin, flexible
electrolyte. Gel electrolytes have clear manufacturing and assembly advantages because of their simplified forms and dual functionalities. However,
the performance is a large factor in the ability of such ideas to proliferate.
Gel polymers offer slightly lower conductance than liquid electrolytes,
but they provide structural improvement that improves the efficiency of ion
transport mechanisms and cycle life [116,117]. Polyvinyl acetate (PVA) has
been shown to offer good results in trapping aqueous electrolytes [116,118,119].
Components and Materials for Electrochemical Supercapacitors
between increased stability ionic liquids and lowered conductivity can be
seen in Table 4.9 [113].
IL electrolytes have high thermal stability that creates an opportunity for
operation in high temperature environments. At high temperatures, the low
conductivity that limits IL performance is overcome by increased ion mobility (kinetic energy), resulting in higher conductivity, greater device power,
and better response time [113]. However, high heat reduces the potential
window for ion stability and this negatively impacts the power and energy
density. Another way to overcome the low conductivity of ILs is to balance
the high potential windows of ionic liquids with the increased conductivity
and power of organic electrolytes such as propylene carbonate and acetonitrile to optimize conductivity [112]. Utilizing such combinations can prevent
safety issues, reduce toxicity, and result in a device with high energy density
that maintains sufficient power performance [112].
4.3.5 Solid State Polymer Electrolytes
Gel and solid polymer electrolytes aim to combine the function of the electrolyte and separator into a single component to reduce the number of parts
in an ES and increase the potential window through the higher stability
offered by a polymer matrix. A gel electrolyte incorporates a liquid electrolyte into a microporous polymer matrix that holds in the liquid electrolyte
through capillary forces, creating a solid polymer film. The chosen separator
must be insoluble in the desired electrolyte and provide adequate ionic conductivity. Non-polar rigid polymers such as PTFE, PVA, PVdF, and cellulose
acetate offer good ion conductivity when used as gel electrolytes [114]. Based
on the data in Table 4.9, the ionic conductivity of EtMeIm + BF 4 is 14 mS.cm –1 .
Ionic conductivity of the same imidazolium salt used as a gel electrolyte in a
PVdF matrix retains 5 mS.cm –1 [115].
Modern electrolytes need increased stability and ion mobility to operate
at high potential windows. Gel electrolytes allow incorporation of aqueous,
organic, and ionic liquids, depending on the requirements of the ES. Separators
are used in conjunction with the electrolyte to help provide structured channels
and prevent short circuits between the electrodes. The presence of solid electrolyte layers results in a reduced need for robust encapsulation technologies.
In order to combine the two structures, electrolyte is trapped within the
polymer matrix during polymerization. The result is a solid, thin, flexible
electrolyte. Gel electrolytes have clear manufacturing and assembly advantages because of their simplified forms and dual functionalities. However,
the performance is a large factor in the ability of such ideas to proliferate.
Gel polymers offer slightly lower conductance than liquid electrolytes,
but they provide structural improvement that improves the efficiency of ion
transport mechanisms and cycle life [116,117]. Polyvinyl acetate (PVA) has
been shown to offer good results in trapping aqueous electrolytes [116,118,119].