Page 670 - Handbook of Battery Materials

644 18 Polymer Electrolytes

is reflected in the growing commercial interest in polymer electrolyte batteries as
well as more fundamental studies of ternary systems.

18.3.1
Gel Electrolytes

Gel electrolytes are attractive alternatives to the dry electrolytes, particularly with
respect to higher, more practical, ionic conductivities. Two distinct methods can
be used to achieve macroscopic immobilization of the liquid solvent: increase the
viscosity of the liquid electrolyte by adding a soluble polymer, for example, PEO,
poly(methyl methacrylate) (PMMA), polyacrylonitrile (PAN), poly(vinylidene fluo-
ride) (PVdF), and so on. [32, 34]; or load the liquid electrolyte into a microscopous
matrix, for example, porous polyethylene [102, 103].
The solidity of gel electrolytes results from chain entanglements. At high
temperatures they flow like liquids, but on cooling they show a small increase
in the shear modulus at temperatures well above T g . This is the liquid-to-rubber
transition. The values of shear modulus and viscosity for rubbery solids are
considerably lower than those for glass-forming liquids at an equivalent structural
relaxation time. The local or microscopic viscosity relaxation time of the rubbery
material, which is reflected in the T g , obeys a VTF equation with a pre-exponential
factor equivalent to that for small-molecule liquids. Above the liquid-to-rubber
transition, the VTF equation is also obeyed, but the pre-exponential term for
viscosity is much larger than is typical for small-molecule liquids and is dependent
on the polymer molecular weight.
Addition of a plasticizing solvent to a polymer–salt system modifies the electrolyte
by lowering the T g through an isothermal increase in the system’s configurational
entropy, and this consequently increases the mobility of all particles. A suitable
−1
choice of organic solvent can lead to very high conductivities (10 −3 –10 −2 Scm )
while still retaining the rubbery character of the material. The challenge is to find
the right combination of components to give high ionic conductivity, chemical
stability, and a wide voltage window, and to be able to resist steady stresses over a
practical temperature range.
Gel electrolytes based on polystyrene, poly(vinyl chloride), poly(vinyl alcohol),
PAN and PVdF, various salts, and high-dielectric-constant solvents have been
investigated since the early 1980s [34, 104–106]. The polymer imparts mechanical
stability; solvating power is not a requirement. Conductivity is critically affected by
the physical properties of the solvent, such as the viscosity, mobility, and dielectric
constant, and by the concentration of salt in the electrolyte. A high dielectric
constant increases the level of salt dissociation, whereas low viscosities lead to high
ionic mobility. The main role of small molecules is therefore to plasticize the host
polymer, improving flexibility and segmental motion, and to solvate the cation (or
the anion in some instances), which reduces ion–ion interactions.
PAN-based systems have been the focus of much recent interest. Although the
polymer is assumed to be nonsolvating, NMR studies [107] suggest that there may be
some competition (albeit small) for solvation between the polymer and plasticizing