ABSTRACT:
John
Kerr and co-workers at Berkeley Lab have developed single-ion
cross-linked comb-branched polymer electrolytes with high
conductivity for use as membranes in lithium batteries, fuel
cells, and electrochromic windows. Solid polymer electrolyte
separators are used in lithium batteries instead of common
organic solvents because (1) they are non-volatile, (2) they
inhibit the growth of dendrites, the tiny metallic snowflake
structures in lithium metal electrodes that lead to battery
failure, and (3) they can be used in very thin films thereby
improving the power performance of the battery and increasing
the energy density.
Solid
polymer electrolytes have been improved by the creation of
single-ion polymer conductors. Single ion conductors, transference
number of one, avoid the development of concentration gradients
that result in low voltage upon discharge and irreparable
damage on charge because the anion is immobilized by covalently
connecting it to the polymer comb. Until now, lithium single
ion polymer conductors have been plagued with low conductivity,
reactivity to lithium, poor cathode compatibility, and mechanical
stiffness that leads to poor processing properties. Kerrs
new cross-linked polymer electrolytes based on trifluoromethylsulfonylmethide,
sulfonate, and fluoroalkylsulfonate and imide anions overcome
these limitations.
The
controllable method of preparation results in a material that
has uniformly excellent mechanical and ion transport properties
that appear to be unaffected by the cross-linking density.
This allows density to be varied to suit the application.
The cross-linked materials achieve much higher lithium ion
conductivities than other cross-linked polymers (10-5 S/cm
at ambient temperatures) and yet also inhibit dendrite growth
due to the mechanical properties. The side chains of the comb-branched
structures are long enough to allow for maximum segmental
motion so that the polymer can effectively penetrate between
the electrode particles and adhere to electrode surfaces while
maintaining the amorphous nature that facilitates high ion
mobility. This overcomes many of the problems involved in
the preparation of good composite electrode structures.
The
capabilities, materials, and principles used for developing
these polymer electrolytes for lithium batteries can be adapted
to develop polymer films for fuel cells and electrochromic
windows. Kerrs group is investigating the use of new
proton solvating functions on comb branch polyether polyelectrolyte
materials to provide water-free membranes that can operate
at high temperatures for fuel cells.
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