Page 669 - Handbook of Battery Materials

18.3 Hybrid Electrolytes 643

Provided there is sufficient dissociation and properly designed ligands, then, in
principle, highly conducting polymer electrolytes can be formed for any cation.
Dissociation can be improved by using anions with highly delocalized charge,
but cation mobility is still undesirably low. A new approach to polymer electrolyte
design which could enhance cation mobility is the formation of ionic rubbers. The
aim is to produce a material which combines the advantages of a superionic glass
(decoupled cation motion) and the macroscopic rubbery properties of a polymer
electrolyte [5, 97–100]. The polymer need only be present as a small percentage
of the total electrolyte to impart flexibility to the system. Complexing lithium
chlorosulfonate with aluminum chloride to give a mixed anion, [AlCl 3 − SO 3 Cl] ,
−
produces a material with a room-temperature conductivity better than 10 −3 Scm −1
and a 4.0 V electrochemical window. LiAlC1 4 (T g = −39 C) is also being inves-
◦
tigated as a potential electrolyte component. In combination with complexes of
Li(CF 3 SO 2 ) 2 N and AlCl 3 , ambient conductivities range from 10 −4 to 10 −5 Scm −1
[100]. Simpler imides like dichlorosulfonimide and fluorochlorosulfonimide en-
hance the conductivity further. Again, these systems exhibit a wide electrochemical
stability window ∼5V.

18.3
Hybrid Electrolytes

The polymer electrolytes discussed so far suffer from a number of disadvantages.
Firstly, they exhibit low conductivities in comparison with liquid or solid (crystalline
or glassy) electrolytes at or below room temperature. The best all-amorphous
systems have conductivities less than 10 −4 Scm −1 at room temperature. These
ambient temperature conductivities may be insufficient in some cases for the power
required by a lithium battery. Secondly, the interfacial impedances present at both
the lithium anode (passivation) and composite cathode (passivation, contact) are
in addition to the ohmic losses in the electrolyte. Thirdly, the lowness of cation
transference number, although similar to the values in liquid systems, is a major
issue since the total conductivity is lower and could limit the use of solvent-free
polymer electrolytes except in the form of extremely thin films or above room
temperature.
One way of addressing these issues has been to form polymer hybrid electrolytes
[101]. These generally consist of a polymeric component and either (i) a liquid
phase, (ii) another polymer, conducting or nonconducting, (iii) a solid ceramic
or glassy component, or (iv) a combination of these. Many hybrid systems, par-
ticularly those containing a liquid component, have not been viable for practical
lithium-metal-based electrochemical cells until now. Despite their superior con-
ducting properties, the addition of a liquid component reintroduces the undesirable
physical and electrochemical properties of a liquid-based electrolyte: mechanical
instability, solvent combustibility, severe interfacial reactions, and the tendency for
lithium dendrites to form on cell cycling. Lithium-ion or ‘rocking-chair’ batteries
do not use lithium metal, making gel-type materials much more attractive. This