Tecnologia De Las Baterias

1
Electrochemical Energy Storage
D. BERNDT

1.1

INTRODUCTION

Electrical energy plays an important role in our daily life. It can universally be
applied and easily be converted into light, heat or mechanical energy. A general
problem, however, is that electrical energy can hardly be stored. Capacitors allow its
direct storage, but the quantities are small, compared to the demand ofmost
applications. In general, the storage of electrical energy requires its conversion into
another form of energy. In batteries the energy of chemical compounds acts as
storage medium, and during discharge, a chemical process occurs that generates
energy which can be drawn from the battery in form of an electric current at a certain
voltage.
For a number of battery systems this process canbe reversed and the battery
recharged, i.e. the intake of electric energy can restore the chemical composition that
contains higher energy and can closely reestablish the original structures within the
battery.
As a consequence, two different battery systems exist:
.
.

Primary batteries that are designed to convert their chemical energy into
electrical energy only once.
Secondarybatteries that are reversible energy converters and designed for
repeated discharges and charges. They are genuine electrochemical storage
systems.

Copyright © 2003 by Expert Verlag. All Rights Reserved.

There is no clear border between them, and some primary battery systems permit
charging under certain conditions. Usually, however, their rechargeability is limited.
The first part of this book(Chapters 2 to 14) concerns batteries of larger
capacities that are employed as standby batteries in stationary applications, provide
energy in vehicles like forklift trucks, or stabilize an electrical network like the starter
battery in motor cars. Rechargeable batteries usually are the choice in such
applications, since primary batteries would be too expensive for the required rather
highcapacity. The second part (Chapters 15 to 19) regards batteries mainly in
portable applications and concerns smaller capacities. In this field primary as well as
secondary batteries are employed.

1.2

THE ELECTROCHEMICAL CELL AND THE CELL REACTION

The cell reaction is a chemical reaction that characterizes the battery. When the
battery is discharged, chemical compounds of higher energycontent are converted
by this reaction into compounds of lower energy content. Usually the released energy
would be observed as heat. But in a battery, the cell reaction is divided into two
electrode reactions, one that releases electrons and the other one that absorbs
electrons, and this flow of electrons forms the current that can be drawn from the
battery. Thus the generation or consumptionof energy that is connected to the cell
reaction is directly converted into an electric current. This is achieved in the
electrochemical cell, sketched in Fig. 1.1.
A positive and a negative electrode are immersed in the electrolyte and the
reacting substances (the active material) usually are stored within the electrodes,
sometimes also in the electrolyte, if it participates in the overallreaction. During
discharge, as shown in Fig. 1.1, the negative electrode contains the substance that is
oxidized (i.e. releases electrons), while the positive electrode contains the oxidizing
substance that is reduced (i.e. accepts electrons).
Thus at the negative electrode oxidation of S(N)red occurs according to
SðNÞred ) SðNÞox þ n ? eÀ

ð1aÞ

while S(P)ox is reduced at the positiveelectrode
SðPÞox þ n ? eÀ ) SðPÞred

ð1bÞ

Both together form the cell reaction
SðNÞred þ SðPÞox ) SðNÞox þ SðPÞred þ energy

ð 1Þ

When the battery belongs to the secondary type and is charged, this reaction is
reversed and a corresponding amount of energy has to be supplied to the cell.
The difference of the bonding energy between the composition at the starting
point of the cell...