Chapter 1
General Aspects

1.1 Definition

Batteries are a variety of galvanic cells, that is, devices containing two (identical or different) electron-conducting electrodes, which contact an ion-conducting electrolyte. Batteries are destined to convert the energy of a chemical reaction between solid electrode components into electrical energy providing an electric current (when the circuit is closed) between two not-identical electrodes having different values of the electrode potential (positive and negative terminals). A battery comprises one or several single galvanic cells. In each such cell a comparatively low voltage is generated, typically 0.5-4 V for different classes of cells. Where higher voltages are required, the necessary number of cells is connected in series to form a galvanic battery. Colloquially, the term "battery" is often used to denote single galvanic cells acting as electrochemical power sources as well as groups of single cells. This is retained in this book. Some battery types retain the term "cell" even for groups of single cells (e.g., fuel cell, not fuel battery). The term "cell" is also used when it is necessary to compare different aspects of single-cell and multicell batteries.

1.2 Current-Producing Chemical Reaction

Reactions in batteries are chemical reactions between an oxidizer and a reducer. In reactions of this type, the reducer being oxidized releases electrons while the oxidizer being reduced accepts electrons. An example of such a redox reaction is the reaction between silver oxide (the oxidizer) and metallic zinc (the reducer):

in which electrons are transferred from zinc atoms of metallic zinc to silver ions in the crystal lattice of silver oxide. When reaction (1.1) is allowed to proceed in a jar in which silver oxide is thoroughly mixed with fine zinc powder, no electrical energy is produced in spite of all the electron transfers at grain boundaries. This is because these transfers occur randomly in space and the reaction energy is liberated as heat that can raise the temperature of the reaction mixture to dangerous levels. The same reaction does occur in batteries, but in an ordered manner in two partial reactions separated in space and accompanied by electric current flow (Fig. 1.1).

Figure 1.1 Schematic of a silver-zinc battery.

In the simple case a battery (cell) consists of two electrodes made of different materials immersed in an electrolyte. The electrodes are conducting metal plates or grids covered by reactants (active mass); the oxidizer is present on one electrode, the reducer on the other. In silver-zinc cells the electrodes are metal grids, one covered with silver oxide and the other with zinc. An aqueous solution of KOH serves as electrolyte. Schematically, this system can be written as

When these electrodes are placed into the common electrolyte enabling electrolytic contact between them, an open circuit voltage (OCV) e develops between them (here e = 1.6 V), zinc being the negative electrode. When they are additionally connected by an electronically conducting external circuit, the OCV causes electrons to flow through it from the negative to the positive electrode. This is equivalent to an electric current I in the opposite direction. This current is the result of reactions occurring at the surfaces of the electrodes immersed into the electrolyte: zinc being oxidized at the negative electrode (anode)1

and silver oxide being reduced at the positive electrode (cathode)

These electrode reactions sustain a continuous flow of electrons in the external circuit. The OH- ions produced by reaction (1.4) in the vicinity of the positive electrode are transported through the electrolyte toward the negative electrode to replace OH- ions consumed in reaction (1.3). Thus, the electric circuit as a whole is closed. Apart from the OCV, the current depends on the cell's internal resistance and the ohmic resistance present in the external circuit. Current flow will stop as soon as at least one of the reactants is consumed.

In contrast to what occurred in the jar, in the batteries, the overall chemical reaction occurs in the form of two spatially separated partial electrochemical reactions. Electric current is generated because the random transfer of electrons is replaced by a spatially ordered overall process (current-producing reaction).

1.3 Classification

By their principles of functioning, batteries can be classified as follows:

1. Primary (single-discharge) batteries. A primary battery contains a finite quantity of the reactants participating in the reaction; once this quantity is consumed (on completion of discharge), a primary battery cannot be used again ("throw-away batteries").
2. Storage (multiple-cycle) batteries (also called secondary or rechargeable batteries). On the completion of discharge, a storage battery can be recharged by forcing an electric current through it in the opposite direction; this will regenerate the original reactants from the reaction (or discharge) products. Therefore, electric energy supplied by an external power source (such as the grid) is stored in the battery in the form of chemical energy. During the discharge phase this energy is delivered to a consumer independent of the grid. During the charging phase the electrode reactions and the overall current-producing reaction occur in the direction opposite to that during discharge. Thus, these reactions must be chemically reversible (the notion of chemical reversibility must not be confused with that of thermodynamic reversibility). Good rechargeable batteries will sustain a large number of such charge-discharge cycles (hundreds or even thousands). The classification into primary and storage batteries is not rigorous because under certain conditions some primary battery may be recharged and storage batteries after a single use are sometimes discarded.

   The silver-zinc battery is a storage battery: after discharge, it can be recharged by forcing through it an electric current in the reverse direction. In this process the two electrode reactions (1.3) and (1.4) as well as the overall reaction (1.2) go from right to left.

3. cells. In the fuel-cell mode of operation, reactants are continuously fed into the cell (or battery) while reaction products are continuously removed. Hence, fuel cells (the more appropriate term of fuel battery is not commonly used) can deliver current continuously for a considerable length of time, which largely depends on external reactant storage.

   Batteries are also classified according to their chemistry (their system), that is, the chemical nature of reactants. The above-mentioned battery with silver oxide as an oxidant at the positive electrode and metallic zinc as negative electrode is called "silver-zinc battery."

   Sometimes other methods of classification are also used, for example, on the basis of the application (stationary or mobile batteries), shape (cylindrical, prismatic, disk-shape batteries), size (miniature, small-sized, medium-sized, or large-sized batteries), electrolyte type (alkaline, acidic, or neutral electrolyte, with liquid or solid (solidified), or molten salt electrolyte), voltage (low voltage or high voltage batteries), electric power generation (low power or high power batteries), and so on.

1.4 Thermodynamic Aspects

Each electrode j of a battery brought into contact with the electrolyte develops a certain electrode potential Ej. The concept of "potential" is an experimental, undefined parameter, that is, it has a real physical meaning and reflects a real physical phenomenon, but cannot be determined from experimental data (even from thought experiments). Only potential differences between the given electrode and another electrode (reference electrode) are measurable. (Similarly, the height of a certain geographic point is defined and can be measured only when referred to the height of another point, e.g., sea level). Values of electrode potentials are commonly referred to as the potential of the standard hydrogen electrode (SHE). Potentials of different electrodes can be either negative (i.e., more negative than the potential of the SHE) or positive. The OCV of a battery U is the potential difference between the positive electrode and the negative electrode:

According to this definition, the OCV is always positive (provided the potentials of both electrodes are referred to the same reference electrode).

Thermodynamically, electrode reactions can be either reversible or irreversible. In case of a reversible reaction, the electrode potential is called reversible (thermodynamic electrode potential). The corresponding OCV is traditionally called "electromotive force" (EMF) and is denoted as e.

The EMF of a battery with reversible electrodes can be defined by the thermodynamic relation

where ?G is the difference of the Gibbs energy G during the current-producing reaction-the difference of the Gibbs energies of all reactants and...