88
and impose less caustic conditions on the electrodes than liquids. Furthermore, the
leakage and flammability issues are thwarted in many cases. This chapter is intended
to discuss the main types of technologies that utilize such composite solid electrolytes, and to compare and contrast the needs that may be met with them, as well as
how they might be tailored for specific applications. In particular, organic polymers
and composites thereof, along with metal organic coordination polymers, have
shown considerable promise in this area.
Among types of ionic conductors, “mixed” conductors are able to transport both
electrons and ions. Those solids with preferential ionic conductivity are often
referred to as solid electrolytes or superionic conductors (which implies a high conductivity value, see specific definition below), allowing for the rapid movement of
ions through the network. When only one ion moves in response to a field, the material is known as a single-ion conductor, a feature that is often achieved via incorporation of anchored counter-ions in the solid matrix. A detailed description can be
found in “Solid Electrolytes: Materials, Properties and Applications.” [2]. To
behave solely as an electrolyte in applications, the material must be electrically
insulating in order to prevent short-circuiting (i.e. be a dielectric). However, electrodes must possess both ion and electron conducting properties. The focus here is
on the electrolyte, but the interested reader is directed to consider other work on
polymeric nanocomposites for electrode materials, a rich field of study [3].
Ionic conductivity can be formulated similarly to electronic conductivity, by considering the size, the concentration, and mobility of the charge. In Eq. (1) below, the
product of these terms describes the ionic conductivity (σ i ), where mobility (μ) is
defined as the speed of charge movement in the presence of an electric field, Z is the
magnitude of the charge of the ion, and n is the concentration of the carriers.
σ
µ
i
n Z e
= · · ·
(1)
Ionic conductivity is typically not as high as electronic conductivity, so it can be a
limiting factor in device performance. Traditionally in solid electrolytes, hopping of
either cations or anions between lattice sites occurs when a potential is introduced.
The process is activated, with an associated Arrhenius activation energy (E a ). The
defects required for mobility can be intrinsic (Frenkel or Schottky) or extrinsic
(introduced by doping). Carrier concentration will vary with temperature for thermally induced intrinsic defects. For example, the α phase of AgI occurs above
147 °C, whereby Ag
+
ions exhibit liquid-like movement due to temperature-induced
cationic disorder. Ideally, one looks for ion specificity, low activation energies to
initiate transport, and high ion conductivities. The possible mechanisms are dependent upon the system and are a continual area of study as new, diverse materials for
good electrolyte behavior are developed that may require adjustment to the classic
models. Low associated E a often indicates that a hopping mechanism, such as in the
well-known Grotthuss mechanism for hydrated protons (See Fig. 2), is likely vs. a
vehicle-type mechanism, whereby charges move along with assistance from another
species. These concepts are also useful for describing polymeric electrolytes. In
terms of porous coordination polymer frameworks, intrinsic usually refers to
C. A. Bauer
and impose less caustic conditions on the electrodes than liquids. Furthermore, the
leakage and flammability issues are thwarted in many cases. This chapter is intended
to discuss the main types of technologies that utilize such composite solid electrolytes, and to compare and contrast the needs that may be met with them, as well as
how they might be tailored for specific applications. In particular, organic polymers
and composites thereof, along with metal organic coordination polymers, have
shown considerable promise in this area.
Among types of ionic conductors, “mixed” conductors are able to transport both
electrons and ions. Those solids with preferential ionic conductivity are often
referred to as solid electrolytes or superionic conductors (which implies a high conductivity value, see specific definition below), allowing for the rapid movement of
ions through the network. When only one ion moves in response to a field, the material is known as a single-ion conductor, a feature that is often achieved via incorporation of anchored counter-ions in the solid matrix. A detailed description can be
found in “Solid Electrolytes: Materials, Properties and Applications.” [2]. To
behave solely as an electrolyte in applications, the material must be electrically
insulating in order to prevent short-circuiting (i.e. be a dielectric). However, electrodes must possess both ion and electron conducting properties. The focus here is
on the electrolyte, but the interested reader is directed to consider other work on
polymeric nanocomposites for electrode materials, a rich field of study [3].
Ionic conductivity can be formulated similarly to electronic conductivity, by considering the size, the concentration, and mobility of the charge. In Eq. (1) below, the
product of these terms describes the ionic conductivity (σ i ), where mobility (μ) is
defined as the speed of charge movement in the presence of an electric field, Z is the
magnitude of the charge of the ion, and n is the concentration of the carriers.
σ
µ
i
n Z e
= · · ·
(1)
Ionic conductivity is typically not as high as electronic conductivity, so it can be a
limiting factor in device performance. Traditionally in solid electrolytes, hopping of
either cations or anions between lattice sites occurs when a potential is introduced.
The process is activated, with an associated Arrhenius activation energy (E a ). The
defects required for mobility can be intrinsic (Frenkel or Schottky) or extrinsic
(introduced by doping). Carrier concentration will vary with temperature for thermally induced intrinsic defects. For example, the α phase of AgI occurs above
147 °C, whereby Ag
+
ions exhibit liquid-like movement due to temperature-induced
cationic disorder. Ideally, one looks for ion specificity, low activation energies to
initiate transport, and high ion conductivities. The possible mechanisms are dependent upon the system and are a continual area of study as new, diverse materials for
good electrolyte behavior are developed that may require adjustment to the classic
models. Low associated E a often indicates that a hopping mechanism, such as in the
well-known Grotthuss mechanism for hydrated protons (See Fig. 2), is likely vs. a
vehicle-type mechanism, whereby charges move along with assistance from another
species. These concepts are also useful for describing polymeric electrolytes. In
terms of porous coordination polymer frameworks, intrinsic usually refers to
C. A. Bauer