Page 232 - Handbook of Battery Materials

7.2 Possibilities for Bromine Storage 201

than the characteristic value for elemental Br 2 [38]. While theoretical studies predict
−
that the free Br ion has a linear symmetric structure [39–43], a broadly scattered
3
variety of linear and nonlinear symmetric and asymmetric Br ions were reported
−
3
to exist in a crystalline environment [44, 45] and in solution [35, 36], according to
the size and shape of the corresponding countercations.
Br forms V-shaped planar species [42, 43] with relatively strong terminal and
−
5
weak central bonds. Raman spectra in aqueous and acetonitrile solutions [36]
exhibit terminal stretching modes of 250 and 257 cm −1 which are essentially
−
higher than those in Br . Structural data from experimental studies exist only
3
for crystalline compounds in which the Br − ions adopt linear geometry. In a
5
theoretical study using density functional theory [43], a central bond angle of 114.6 ◦
and a central terminal bond ratio of ∼1.111 were obtained; the latter is somewhat
smaller than the 1.181 estimated from XRD experiments on linear Br units in
−
5
TMA·0.7H 2 O·xHal 5 H [32].
It is evident that the shapes and relative stabilities of the polybromide anions
depend to a large extent on the nature of the countercations. The tendency to form
a nonaqueous phase is determined by the chemical environment, and depends
particularly on the degree of hydration of the ions. Hence, important properties of
bromine-storing phases can be varied over a wide range by adjusting the type and
composition of the quaternary ammonium salts used. Additional parameters, such
−
as pH, concentration of supporting salts (i.e., C1 ) also producing heteroatomic
polyhalide ions [46], and temperature effects, enhance the difficulties of a systematic
investigation of the polybromide structure in practical battery electrolytes.
An overview of the most important quaternary ammonium salts tested for
possible applicability in zinc–bromine batteries is presented in Table 7.2. A rough
classification has been applied according to the substance classes of the substituents
attached to the nitrogen.
The two basic requirements for efficient bromine storage in zinc–bromine bat-
teries, which need to be met in order to ensure low self-discharge are a substantial
reduction of equilibrium vapor pressure of Br 2 of the polybromide phase in associ-
ation with low solubility of active bromine in the aqueous phase. As mentioned by
Schnittke [4] the use of aromatic N-substituents for battery applications is highly
problematic due to their tendency to undergo bromination. Based on Bajpai’s
pioneering work on vapor pressures [56] (see Figure 7.2) and Eustace’s [73] study
of Br 2 distribution between the complex phase and overstanding aqueous solution
in combination with stability considerations N-methyl N-ethylmorpholinium bro-
mide (MEM), and N-chloromethyl N-methylpyrrolidinium bromide (C-MEP) and
analogous aliphatic heterocyclic compounds have confirmed their advantages for
possible application in zinc-flow batteries.
Eustace noticed the correlation between asymmetric N-substitution and low
◦
melting points (at temperatures ≥15 C) of the substances [73] observed in this
study. The importance of sufficiently high densities of the ‘fused salts’ for an
efficient separation of the complex phase and the aqueous solution was emphasized.
Mixtures of various quaternary ammonium bromides were tested subsequently
[48, 49, 66, 75], aiming particularly at the avoidance of crystallization in the