Page 181 - Lindens Handbook of Batteries

7.6 PRINCIPLES OF OPERATION

can be quite large when very high surface area activated carbons are used and can have a much more
rapid response than the lead electrode. The electrolyte phase is similar to that of the standard lead-
acid battery so the reader is referred to Chaps. 16 and 17. Further information on the use of this type
of battery in microhybrid vehicles is found in Chap. 29.
Sulfuric acid has also been used in the vanadium and other redox types of batteries. Here the
solution also contains vanadium ions of different valence states (uranium ions and other ions have
also been tested) with a vanadium (II–III) couple used on the negative electrode side and a vana-
dium (IV–V) couple used on the positive electrode side. A simple microporous separator can be
used to separate the two solutions when they are in the interelectrode space, although sometimes an
ion-specific separator is used (proton conductive). No particular harm occurs when the solutions
interdiffuse except in lowering the round-trip energy efficiency of the process since both solutions
contain vanadium ions. When the system is not undergoing charge or discharge, the solutions are
pumped into storage containers, and when the system is operating, the solutions are pumped into
the interelectrode space in a flow-through cell. Even in these systems, one must be concerned about
gas evolution since the overpotential of the charging regime can carry the cell into the gassing range
(see Chap. 30 for details).
Occasionally other acids have been employed for acid battery types. For example, a HBF elec-
4
trolyte has been used with lead/lead dioxide electrodes. These types are not in current production,
however, so they will not be discussed.

7.3 NONAqUeOUs eleCTROlyTes

This section is arbitrarily separated into organic solvent-based electrolytes, inorganic solvent-based
electrolytes, ionic liquids, solid polymer electrolytes, and ceramic/glassy electrolytes. The use of
polymeric materials to cause gelation of the electrolyte (used mainly with organic solvents) has
little effect on the basic electrolyte properties such as conductivity and diffusivity, except for a
major effect on viscosity. However, the electrolyte in most modern batteries has very little convec-
tive flow, so viscosity has little effect on battery operation. The greatest development has been with
organic solvent-based electrolytes as they have been used in numerous primary lithium batteries
and many variations of the lithium-ion battery type. Inorganic solvent-based electrolytes have been
used mainly in liquid cathode batteries. The others are still in the development stage, but will be
discussed briefly.

7.3.1 Organic-Solvent Electrolytes
The greatest electrochemical use for organic-solvent electrolytes has been in the fields of primary
lithium batteries and rechargeable lithium-ion batteries. The successful application in primary lithium
batteries predates that of the rechargeable batteries by several decades, even though work was insti-
tuted on both primary and secondary batteries at about the same time in the early 1960s. Techniques
for handling and purifying the organic liquids had to be developed first, with special attention
to the contamination of water and other impurities. More than a decade of work was required to
understand the importance of reducing impurities to the ppm level from salts and solvents. Cathode
materials and other cell components often have much more adsorbed water than was commonly
realized so that methods to remove water from all components had to be developed as well. A good
compilation of the techniques required to study nonaqueous electrochemistry is given in Ref. 9.
In addition, it was necessary to develop an understanding of the stability of purified solvents and
salts with active materials of both electrodes, especially the negative electrode. Studies of the
electrochemical window on materials such as conductive diamond, platinum, or glassy carbon
during cyclic voltammetric sweeps in the negative direction (cathodic scans) required much
interpretation because the film-forming properties of these materials was very different from that of
lithium metal (or in later work on carbons or graphites). Likewise, anodic scans on inert substrates