Page 59 - Handbook of Battery Materials

1.4 Criteria for the Judgment of Batteries 25

systems with a high cycle life are preferred. The number of cycles illustrates how
often a secondary battery can be charged and discharged repeatedly before a lower
limit (defined as a failure) of the capacity is reached. This value is often set at
80% of the nominal capacity. To compare different battery systems, the depth of
discharge has to be quoted as well as the number of cycles.
Additionally, batteries deteriorate as a result of chemical side reactions that
proceed during charging and discharging, but also during storage. Cell design,
temperature, the electrochemical system, and the charge state affect the shelf life
and the charge retention of the battery.

1.4.8
Specific Energy and Energy Density

With respect to the specific energy (the electric energy per unit mass) of today’s
battery systems, there is a major difference between the performance of aqueous
systems and that of nonaqueous systems [15]. Apart from batteries for some special
applications, there are

• Aqueous batteries with about 140 Wh kg −1 for primary and about 80 Wh kg −1
for secondary systems
• Nonaqueous batteries withabout400 Wh kg −1 forprimaryand about180 Wh kg −1
for secondary systems
• For comparison: the utilizable electric or mechanic energy of a gasoline engine
is 3000 Wh per 1 kg gasoline.
The zinc–carbon and alkaline manganese cells are primary battery systems, while
lead, nickel–cadmium, and nickel–metal hydride batteries are secondary batteries
with aqueous electrolyte solutions. The aqueous battery systems generally show
only a limited performance at low temperatures. Because of the decomposition
of the water, the voltage of a single cell is limited. For this reason lithium-ion
batteries are of great interest when using organic or polymer electrolytes, allowing
cell potentials of up to 4.5 V to be achieved.

1.4.9
Safety

Batteries are sources of energy and deliver their energy in a safe way when
they are properly used. Therefore it is of crucial importance to choose the right
electrochemical system in combination with the correct charge, discharge, and
storage conditions to assure optimum, reliable, and safe operation.
There are instances when a battery may vent, rupture, or even explode if it is
abused. To avoid this, a cell and/or a battery should include protective devices to
avoid
• application of too high charge or discharge rates
• improper charge or discharge voltage or voltage reversal