Page 671 - Handbook of Battery Materials

18.3 Hybrid Electrolytes 645

solvent in PAN-based gel electrolytes. This makes for a rather complex ion-transport
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mechanism. Probing on a microscopic level, such as through Li NMR [107], shows
that the polymer can impart a strong influence on ionic conductivity, even on
short-range ionic motion, and that impedance of ionic motion can be correlated
with T g . IR spectroscopic studies of ‘kinks’ in the log σ vs 1/T plots observed when
PAN or PVdF is added to a liquid electrolyte with relatively high concentrations
of salt show that they are not related to changes in mechanical properties but are
believed to result from direct ion–polymer interactions or changes in the ability
of the low-molecular-weight solvent (high or low dielectric constant) to solvate
the salt as a result of the presence of the polymer. Increasingly, the influence of
interactions between components of the gel electrolytes is being scrutinized [108,
109]. With regard to safety, the Sony Corporation [110] have investigated PAN gels
and claim that, given the right combination of polymer/salt/solvent, gels can have
−1
remarkably high conductivities (∼3 × 10 −3 Scm ) and electrochemical stability
(>4.5 V), and exhibit superior thermal stability (and hence safety) in comparison
with other gels and liquid solvents. This thermal stability is said to result from
cyclization experienced by PAN at high temperatures.
The role of plasticizers in ion–ion interactions may not be straightforward either,
judging by IR data on plasticized (PEO) 9 LiCF 3 SO 3 [108]. Propylene carbonate (PC)
reduces ion association, but a large percentage by weight is required to achieve
this. The material becomes more amorphous at room temperature as a result of
preferential interaction of PC with pure PEO in the heterogeneous system to form
conducting pathways. Despite PC being a good solvent for the salt, at least 50 wt%
is required before any significant interaction is detected.
The conductivity of gelled electrolytes is determined primarily by the liquid and
salt components. High liquid content, of the order of 40%, is required to attain
conductivities comparable with those of the corresponding liquid electrolyte. At
these liquid loading levels there is often insufficient mechanical strength, and
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although this effect may not be noticeable on 1–2 cm laboratory cells, it is certainly
evident on scale-up [111], Polymer blends such as PEO–MEEP are much more
mechanically stable than MEEP itself and more conductive than PEO, but there is
little overall improvement of the room temperature conductivity, even when they
are complexed with plasticizing salts [112]. A novel approach to mechanically stable,
highly conducting electrolytes is to use a dual-phase electrolyte (DPE) made up of
two different polymers, one a supporting latex, the other a latex containing polar
units which are fused together [113, 114]. An example is a styrene–butadiene rubber
(SBR) fused with an acrylonitrile–butadiene rubber (NBR). When immersed in a
lithium salt solution, only the polar phase takes up solvent to form ion-conducing
pathways while the nonpolar phase imparts mechanical strength. Alternatively, a
core-shell latex can be synthesized by polymerizing a nonpolar monomer (e.g.,
polybutadiene) in a fine dispersion of a polar polymer (e.g., poly(vinylpyrrolidone)).
A polar polymer shell forms around the stabilizing nonpolar core. The latex
particles collapse on removal of the dispersion medium, causing the cores to fuse
partially, and once again the polar phase takes up electrolyte solution to form
ion-conducting pathways [114]. Figure 18.7 shows a schematic of DPE electrolytes.