Page 657 - Handbook of Battery Materials

18.2 Solvent-Free Polymer Electrolytes 631

low (e.g., for PEO, ∼5–10), and ion association will reduce the dissociation effect
in the entropy. Experimentally, there is widespread evidence for ion association in
polymer electrolytes [12]. These ion pairs or higher aggregates may be contact or
solvent-separated species. In general, high salt concentrations are likely to favor
contact ion pairs (or aggregates). In long-chain polyethers, steric factors also need
to be considered. To avoid polymer chain strain, the ion’s coordination sphere may
not be saturated, making it easy for empty sites around the cation to be occupied
by anions. This would lead to the formation of contact ionic clusters, even at low
salt concentrations. Experimentally, however, it can be difficult to make a specific
identification of species present [8, 13, 14].
In solvents lacking hydrogen-bonding ability (low acceptor number), anion
stability depends on charge dispersion. Large anions with delocalized charge require
little solvation. Salts of singly charged polyatomic anions such as in LiCF 3 SO 3
or LiClO 4 will dissolve easily in polyethers. These salts also tend to have low
lattice energies. Salts containing monatomic anions may be soluble in polyethers,
−
−
provided they are large and polarizable, for example, I ,Br . Some theoretically
−
−
−
suitable anions for polymer electrolytes are ClO 4 ,CF 3 SO 3 , (CF 3 SO 2 ) 2 N ,BF 4 ,
−
−
−
−
−
−
BPh ,AsF 6 ,PF 6 ,SCN ,and I . However, coordination anions like AsF 6 are
−
4
sources of Lewis acids, particularly when associated with lithium, and are capable
−
of inducing polymer chain scission. A choice of ClO 4 with associated dangers
restricts its commercial use, while CF 3 SO − complexes have a very unfavorable
3
phase diagram [8], restricting their application to amorphous polymer hosts.
Noncoordination anions with extensive charge delocalization have been very
successful in enhancing the performance of dry polymer electrolytes. Examples of
these are given in Table 18.1.
The major breakthrough has been with bis(trifluoromethanesulfonyl)imido
(TFSI) [15–17]. The size and conformation of the imide anion in the polymer
complex results in the chains being forced apart [18], reducing their ability to pack
into a regular structure, and thus lowering the melting point of the crystalline phase.
This is accompanied by a several-fold increase in conductivity. The charge delocal-
ization concept has been extended to carbon analogs, (CF 3 SO 2 ) 2 CRH [19, 20], but
electrochemical instability and synthetic problems have limited progress to date [21].
Table 18.1 Some imide ions and carbanions used in salts
to enhance polymer electrolyte conductivity and reduce
crystallinity.
Name Acronym Structure References
Bis(trifluoromethanesulfonyl)- TFSI [N(CF 3 SO 2 ) 2 ] – [15]
imido
(Methoxypropyltrifluoro- MPSA [(CF 3 SO 2 )N(CH 2 ) 3 OCH 3 ] – [20]
methanesulfonyl)amino
Bis(trifluoromethanesulfonyl)- TFSM [(CF 3 SO 2 ) 2 CH] [19]
methyl