Page 58 - Handbook of Battery Materials

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24 1 Thermodynamics and Mechanistics

is the decomposition of the electrolyte solution (for example, water to hydrogen
at the negative electrode or to oxygen at the positive electrode). In some battery
systems these evolved gases react back with formation of educts. For example,
in the nickel–cadmium cell oxygen is formed at the positive electrode and reacts
back at the negative electrode, warming up the cell [8].
To avoid this problem, computer-controlled charging systems in modern battery
stacks regulate the voltage for each individual cell.

1.4.6
Coulometric Efﬁciency and Energy Efﬁciency
The efﬁciency during an energy conversion is deﬁned as the ratio of the energy
converted to the energy consumed. This parameter is only decisive for secondary
systems.The charge (Q charge )necessary to load asecondarycell,is always higherthan
the charge (Q discharge ) released during discharge. This is caused by an incomplete
conversion of the charging current into utilizable reaction products. Useless
side reactions with heat production may occur. Here, numerous parameters are
important such as the current density, the temperature, the thickness, the porosity
of the separator, and the age of the cell.
There are two possible ways to describe the efﬁciency of batteries – the coulo-
metric efﬁciency and the energy efﬁciency.
• Coulometric efﬁciency:
Q discharge
q Ah = (1.34)
Q charge
1
The reciprocal value f = of the coulometric efﬁciency is called the charging
q Ah
factor. The coulometric efﬁciency for electrochemical energy conversion is about
70–90% for nickel–cadmium and nearly 100% for lithium-ion batteries [14].
• Energy efﬁciency:
U discharge
q Wh = q Ah · (1.35)
U charge
Here, U discharge and U charge are the average terminal voltages during charge
and discharge. The discharge voltage is normally lower than the charge voltage
because of the internal resistance and overpotentials. For this reason the coulo-
metric efﬁciency is always higher than the energy efﬁciency. It is inﬂuenced by
the same terms as the charge efﬁciency but in addition by the discharge current
and the charging procedure.

1.4.7
Cycle Life and Shelf Life
Another important parameter to describe a secondary electrochemical cell is the
achievable number of cycles or the lifetime. For economic and ecological reasons,

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