Nickel—cadmium system

Because the nickel—iron cell system has a low cell voltage and high cost compared to those of the lead—acid battery, lead—acid became the dorninant automotive and industrial battery system except for heavy-duty appHcations. Renewed interest in the nickel—iron and nickel—cadmium systems, for electric vehicles started in the mid-1980s using other cell geometries. 

Figure 8. Ideal discharge characteristic, and discharge characteristic of a nickel/cadmium system.

The nominal capacity of every system is defined by a specific value of C for example, for the nickel-cadmium system, it is Cj20. By discharging with a higher current, the final capacity obtainable becomes lower. 

Figure 9. Dependence of the cell voltage on the charge capacity for three different currents in the nickel/cadmium system.

Although one of the most common storage batteries is called the nickel/cadmium system ( NiCad ), correctly written (-)Cd/KOH/NiO(OH)(+), cadmium is not usually applied as a metal to form a battery anode. The same can be said with regard to the silver/cadmium [(-) Cd / KOH / AgO (+)] and the MerCad battery [(-)Cd/KOH/HgO(+)]. The metallic negative in these cases may be formed starting with cadmium hydroxide, incorporated in the pore system of a sintered nickel plate or pressed upon a nickel-plated steel current collector (pocket plates), which is subsequently converted to cadmium metal by electrochemical reduction inside the cell (type AB2C2). This operation is done by the customers when they start the application of these (storage)... 

The thermodynamic properties of magnesium make it a natural choice for use as an anode material in rechargeable batteries, as it may provide a considerably higher energy density than the commonly used lead-acid and nickel-cadmium systems, while in contrast to Pb and Cd, magnesium is inexpensive, environmentally friendly, and safe to handle. However, the development of Mg-ion batteries has so far been limited by the kinetics of Mg " " diffusion and the lack of suitable electrolytes. Actually, in spite of an expected general similarity between the processes of Li and Mg ion insertion into inorganic host materials, most of the compounds that exhibit fast and reversible Li ion insertion perform very poorly in Mg " ions. Hence, there... 

The iron-nickel oxide alkaline battery system has many features in common with the nickel-cadmium system discussed above. It was first developed by Edison in the USA at the turn of the century and was patented in the same year as Jungner s first nickel-cadmium US patent, 1901. Iron can be regarded as a favourable active battery material because of its low cost, high theoretical specific capacity (twice that of cadmium) and non-toxic, pollution-free characteristics. However, because its reduction potential is below that of hydrogen, and since hydrogen overvoltage is low on iron, charge retention is poor and efficiency is low. 

Batteries based on the iron-nickel oxide system are now being developed for electric vehicle applications. These use fibre-plaque electrodes, as described above for the nickel-cadmium system, and incorporate electrolyte circulation systems to permit removal of gases evolved during charge... 

Replacing zinc with cadmium reduces the OCV by approximately 0.4 V, but increases the cycle life of the system considerably. This cell is very similar to the nickel-cadmium system, but has an energy density higher by about a third. The cost of the system has restricted its application to small button cells. 

Figure 6.11 Theoretical energy density of various Li/intercalation couples in comparison with the lead-acid and nickel-cadmium systems.

Today the most important electrochemical storage systems for stationary applications are the lead-acid and the nickel/cadmium systems. Both of them have advantages and disadvantages which carefully have to be considered for best selection. 

Nevertheless nickel/cadmium systems are economic under certain conditions, e.g. at very low temperatures. 

Battery development is ongoing. The lead-acid and nickel/cadmium systems will hold their dominant position in the field of stationary batteries. In the field of small portable power the nickel/metal hydride system has advantages because of its higher energy density compared to nickel/cadmium. 

The basic electrochemistry is the same for the vented pocket plate, sintered plate, fiber and plastic-bonded plate types as well as for other variations of the nickel-cadmium system. The reactions of charge and discharge can be illustrated by the following simplified equation ... 

During charging, the secondary cell receives the same amount of electric energy as that previously released, and this is stored in the form of chemical energy (see Figure 1.11 for nickel-cadmium system). Terminal voltage, charging time, number... 

Where a rechargeable power source is required for portable-in-use apparatus, the nickel-cadmium system is generally specified. Nickel-cadmium batteries and cells are available in a wide range of cylindrical and button sizes, and they are ideally suited to high-rate applieations. Both sintered electrode cylindrical batteries and mass plate electrode button cells constructions are available. The following discussion refers largely to the former type, since these oceupy the major part of the market. 

For many everyday batteries, the choice is still between the lead—acid and the nickel-cadmium systems, rather than the more recently developed systems discussed below. 

For many everyday battery applications, the choice is still between the lead-acid and the nickel-cadmium systems, rather than the more recently developed systems. Each of these two main types of sealed battery has its own advantages and the choice between nickel-cadmium and lead-acid batteries depends very much on the particular application and on the performance characteristics required. 

Most lithium systems operate over temperature ranges similar to those of nickel—cadmium systems (—40 to +60°C). However, lithium systems, capable of operating throughout a temperature range of —55 to -1-125°C, are also available. 

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