Page 27 - Lindens Handbook of Batteries

1.4 PRINCIPLES OF OPERATION

The most advantageous combinations of anode and cathode materials are those that will be light-
est and give a high cell voltage and capacity (see Sec. 1.4). Such combinations may not always be
practical, however, due to reactivity with other cell components, polarization, difficulty in handling,
high cost, and other deficiencies.
In a practical system, the anode is selected with the following properties in mind: efficiency as a
reducing agent, high coulombic output (Ah/g), good conductivity, stability, ease of fabrication, and
low cost. Hydrogen is attractive as an anode material, but obviously, must be contained by some
means, which effectively reduces its electrochemical equivalence. Hydrogen is the active material in
metal-hydride anodes (see Chap. 22). Practically, metals are mainly used as the anode material. Zinc
has been a predominant anode because it has these favorable properties. Lithium, the lightest metal,
with a high value of electrochemical equivalence, has become a very attractive anode as suitable and
compatible electrolytes and cell designs have been developed to control its activity. With the devel-
opment of intercalation electrodes, lithiated carbons are finding wide use in lithium-ion technology.
Lithium alloys are also being explored for use as anodes in lithium-ion batteries.
The cathode must be an efficient oxidizing agent, be stable when in contact with the electrolyte, and
have a useful working voltage. Oxygen can be used directly from ambient air being drawn into the cell, as
in the zinc/air battery. However, most of the common cathode materials are metallic oxides. Other cathode
materials, such as the halogens and the oxyhalides, sulfur and its oxides, are also used for special battery
systems.
The electrolyte must have good ionic conductivity but not be electronically conductive, as this
would cause internal short-circuiting. Other important characteristics are nonreactivity with the elec-
trode materials, little change in properties with change in temperature, safety in handling, and low
cost. Most electrolytes are aqueous solutions, but there are important exceptions as, for example, in
thermal and lithium anode batteries, where molten salt and nonaqueous electrolytes are used to avoid
the reaction of the anode with the electrolyte.
Physically, the anode and cathode electrodes are electronically isolated in the cell to prevent inter-
nal short-circuiting, but they are surrounded by the electrolyte. In practical cell designs, a separator
material is used to separate the anode and cathode electrodes mechanically. The separator, however,
is permeable to the electrolyte in order to maintain the desired ionic conductivity. In some cases, the
electrolyte is immobilized for a nonspill design. Electrically conducting grid structures or materials
may also be added to the electrodes to reduce internal resistance.
The cell itself can be built in many shapes and configurations—cylindrical, button, flat, and
prismatic—and the cell components are designed to accommodate the particular cell shape. The cells
are sealed in a variety of ways to prevent leakage and dry-out. Some cells are provided with venting
devices or other means to allow accumulated gases to escape. Suitable cases or containers, means for
terminal connection, and labeling are added to complete the fabrication of the cell and battery.

1.2 CLASSIFICATION OF CELLS AND BATTERIES

Electrochemical cells and batteries are identified as primary (nonrechargeable) or secondary (recharge-
able), depending on their capability of being electrically recharged. Within this classification, other
classifications are used to identify particular structures or designs. The classification used in this
handbook for the different types of electrochemical cells and batteries is described in this section.

1.2.1 Primary Cells or Batteries

These batteries are not capable of being easily or effectively recharged electrically and, hence, are
discharged once and discarded. Primary cells in which the electrolyte is contained by an absorbent
or separator material (there is no free or liquid electrolyte) are termed “dry cells.”
The primary battery is a convenient, usually inexpensive, lightweight source of packaged energy
for portable electronic and electric devices, lighting, digital cameras, toys, memory backup, Global