CHAPTER 1
Energy Storage in Batteries
1.1 BATTERIES – THEIR HISTORY AND DEVELOPMENT
A battery is a chemical device for the storage of electricity. Since electricity cannot be stored directly (except in electrolytic capacitors or superconducting coils, both of which have major technical and economic limitations) it is necessary to utilize an indirect form of storage. Possibilities include the conversion of electrical energy into potential energy (pumped-hydro schemes), kinetic energy (flywheels), thermal energy (night storage heaters), or chemical energy. One form of chemical energy is hydrogen, generated by electrolysis, which may be stored and subsequently converted back into electricity in a fuel cell. Electrolysers and fuel cells are, like batteries, electrochemical energy-conversion devices. Electrolysers play an important role in the chemical industry (for instance, in the production of chlorine and caustic soda and of metals such as aluminium and copper), while fuel cells are assuming ever greater importance for localized electricity generation, combined heat and power schemes, and as power sources for electric and hybrid electric vehicles. Electrolysers and fuel cells, however, lie outside the scope of the present book.
The convenience of batteries lies in the wide range of sizes in which they may be manufactured or assembled into packs, their ability to supply electrical power instantly, their portability (for smaller sizes), and the option of single-use or multiple-use units. The last-mentioned feature provides a useful means for classifying the many different battery systems into two broad categories: (i) 'primary batteries', which utilize the chemicals once only, in a single discharge, and then are thrown away; (ii) 'secondary batteries', which may be recharged and used again. With the latter batteries, the charging process involves the uptake of electricity and the conversion of the chemicals back into their original forms, so that they are available for a further discharge. Thus, battery charging is a special form of electrolytic process. The discharge-charge cycle may be repeated until the secondary battery deteriorates and its capacity to store charge falls below a practical level. Secondary batteries used to be known as 'accumulators' but the term has largely dropped out of use, at least in the English language. It is quite common (and acceptable) for secondary batteries to be referred to as 'rechargeable batteries'.
It is just 200 years since the invention of the first battery. This has been ascribed to Alessandro Volta (1745–1827), Professor of Natural Philosophy (physics) at the University of Pavia in Italy. His name is commemorated for all time by the unit of electrical potential, the volt. Volta's famous experiment, described in a letter to the Royal Society of London in 1800, consisted of assembling a pile of alternate silver (or brass or copper) and zinc (or tin) discs, with each pair of dissimilar metals separated from the next by a piece of cloth which was saturated with brine. One end of the pile terminated in a silver disc and the other in a zinc disc, and a continuous current of electricity was produced as soon as the two were connected by a wire conductor. This was the first galvanic, or primary, battery and became known as 'Volta's pile'. Batteries have come a long way in 200 years! It is interesting to note that the French word for battery ('la pile') stems directly from Volta's device.
The next significant step in the development of batteries was the invention of the Daniell cell by John Daniell (1790–1845), Professor of Chemistry at King's College, London. In 1836, Daniell took a copper vessel filled with copper sulfate solution and in it immersed a gullet of an ox. This unusual, and somewhat repugnant, receptacle contained a solution of sulfuric acid and a vertical zinc rod. Discharge of the resulting cell caused the zinc electrode to dissolve and copper to be deposited at the positive electrode. The cell gave a voltage of 1.1 V. This was possibly the first practical galvanic cell to give a continuous current of useful magnitude. Further modifications (Figure 1.1) included the use of porous ceramic pots ('separators') instead of animal membranes, substitution of sulfuric acid by zinc or magnesium sulfate, and the development of multi-cell batteries. Daniell cells were adopted by commercial telegraphic systems following a rapid expansion of such services in the early 1850s.
A subsequent major advance was made by the French chemist Georges Leclanche (1839–1882) who, in 1866, invented the primary cell which bears his name (see Section 5.1, Chapter 5). This consists of a zinc rod as the negative electrode and a carbon rod as the positive electrode, both immersed in a solution of ammonium chloride contained in a glass jar. The positive electrode was housed in an inner porous ceramic pot and packed around with a mixture of powdered manganese dioxide and carbon. The cell, which has been extensively developed ever since, gives a voltage of 1.5 V. A major advance took place in the late 19th century when the idea of using a zinc can as both container and electrode was patented and came into general use.
Before the invention of these galvanic cells, the only electricity known and available was static electricity, as produced by friction between dissimilar materials or in thunderstorms. The first recorded electrostatic machine was made in 1663 by the German physicist Otto von Guericke (1602–1686). In 1797, George Pearson (1751–1828) reported that in order to electrolyse water the electrostatic machine had to be discharged 14 600 times, and then produced only about 5.5 ml of a gaseous mixture of hydrogen and oxygen. Not surprisingly, therefore, the availability of a continuous current from a galvanic cell caused a revolution in technology. It formed the basis of telegraphy and of the electric door bell, and later of radio reception. In the chemical field, the supply of electricity allowed the development of electroplating, electroforming and, for the first time, the electrolytic extraction of metals such as sodium, potassium, and calcium. Copper, for use in electrical equipment, could be purified by electrorefining.
The first effective demonstration of a secondary (rechargeable) cell was given in 1859 by the French chemist Gaston Plante (1834–1889). This cell consisted of two concentric spirals of lead sheet, separated by porous cloth and immersed in dilute sulfuric acid within a cylindrical glass vessel...
