Electrodes pocket plate

Cell construction is mainly confined to two types, using either pocket plate electrodes (vented cells) or sintered , bonded or fibre plate electrodes (vented and sealed cells). In the former, the active materials are retained within pockets of finely perforated nickel-plated sheet steel which are interlocked to form a plate. Positive and negative plates are then interleaved with insulating spacers placed between them. In sintered plate electrodes, a porous sintered nickel mass is formed and the active materials are distributed within the pores. In sintered plate vented cells, cellulose or other membrane materials are used in combination with a woven nylon separator. In sealed or recombining cells, special nylon separators are used which permit rapid oxygen diffusion through the electrolyte layer. 

Iron-nickel oxide cells are always vented. Tubular/pocket plate electrodes are constructed as described above and are generally housed in nickel-plated steel cases. Cells with sintered plate electrodes have smaller inter-electrode spacings. They use synthetic fibre fabrics as separators, and plastic containers. 

Unlike the pocket-plate electrodes, the sintered-plate electrodes do not need the addition of any conductive material (such as graphite) to create enough electronic conductivity of the electrode, because plaque itself provides this function. On the other hand, the sintered-plate electrodes do not need the addition of the binder material to the active material to create the structural integrity of the electrode, because the plaque itself provides this integrity in the first place. Having these structural advantages, the sintered-plate... 

Figure 10.12 Procedure for the manufacture of pocket plate electrodes for a Ni/Cd battery.

Pocket Plate Electrode. This is the same type of electrode used in pocket plate nickel-cadmium and nickel-iron batteries. Electrodes are prepared by loading nickel hydroxide hydrate active material and a conductive additive (graphite and/or nickel flake) into tubular flat pockets which are then assembled into electrodes. Little interest currently exists in using this type of electrode in nickel-zinc cells since modern cells attempt to utilize lightweight electrode construction. 

However, it can undergo self-reductive dissolution (loss of active material) accompanied by oxygen evolution [349]. The active material of the positive electrode (in pocket plate cells) consists of nickel hydroxide mixed with small additions of cobalt and barium hydroxides to improve the capacity and charging/discharging performance and graphite to improve conductivity [348]. 

However, it is recognized that slightly soluble intermediates such as CdO(OH) and Cd(OH)3 are involved. Cadmium does not corrode since its equilibrium potential is more positive than that of hydrogen in the same solution. The active material in pocket plate cells consists of metallic cadmium, with up to 25% of iron and small quantities of nickel and graphite to prevent agglomeration. Two methods of preparation are used. One involves the electrochemical co-reduction of a solution of cadmium and iron sulphate in the other, dry mixtures of cadmium oxide or hydroxide and Fe304 or iron powder are used. In some methods of pocket plate manufacture, the electrode material is pressed into pellets or briquettes before being inserted into the pockets, and various waxes or oils may be used to facilitate this process. 

The active material used in pocket plate cells consists of Ni(OH)2 together with up to 5% of Co(OH)2, Ba(OH)2, etc. to improve cell capacity and cycle life, and 20% of graphite in various forms to increase the electronic conductivity. The nickel hydroxide is precipitated from nickel sulphate in a controlled manner to produce fine particles of large surface area. As in the case of the cadmium electrode, the nickel hydroxide powder may be formed into pellets before insertion into the pockets. 

In pocket plate cells, the active materials are a mixture of finely powdered metallic iron and Fe304. The preparation of this mixture varies from manufacturer to manufacturer, but generally involves a final process in which controlled air oxidation of iron powder or reduction of Fe304 with hydrogen is used to form the appropriate composition. Additives such as cadmium, cadmium oxide or graphite are commonly included to improve the capacity retention and electronic conductance. The performance of the electrode is improved by the addition of up to 0.5% of FeS the mechanism of the sulphide involvement is not well understood. If sulphide is lost by oxidation after prolonged use, small amounts of soluble sulphide may be added to the electrolyte,... 

Pocket plate design is not suitable for the positive electrode because of the infiltration of soluble zincate and the consequent decrease in positive electrode capacity. Porous matrix positives do not suffer so badly from... 

In the USA, BIG RIVER ZINC developed a process designed to produce cadmium oxide directly from pocket-plate negative electrodes. It discontinued this process when the price of cadmium fell very sharply in 1991/1992. 

Industrial Ni-Cd batteries are rugged, long-life, cheap batteries capable of operating at high rates. The so-called pocket-plate battery can stand overcharge, polarity reversal and short-circuits. To better utilize the electrode materials, two other structures have been developed the fiber plate and the plastic-bonded plate. The latter has afforded improved performance characteristics (e.g. an energy density of 110 Wh/1). 

A h, cylindrical cells 0.1—10 A h and button cells 0.04-1.75 A h. The energy density of sintered plate cells is considerably better than their pocket plate counterparts because the electrodes are closer together, and with the high porosity of the plates there is also more active material per unit volume with a 100 A h cell... 

The anode current collector is an annealed, perforated, nickel-plated steel pocket plate assembly. The tubes for the positive electrode are produced from perforated nickel-plated strips that are wound to produce a tube. One end is crimped and the active material poured into the open end in layers and crimped again to close the tube. A machine automaticaUy introduces the active material and tamps it into the pockets. After filling, the tubes are pressed into openings in the nickel... 

Steel electrode frame. The pocket plate follows a similar process. Rectangular pockets of perforated nickel-plated iron strips are filled with active material, crimped, closed, and fixed/bolted in a nickel-plated steel frame. The assembled cells are placed in polyethylene containers and filled with KOH electrolyte. Spacing of the negative and positive electrodes is maintained by the internal assembly structure. There are no separators as are common in the lead acid battery structure. 

The Ni-Fe batteries range in size from 5 Ah to over 1,200 Ah. The cell open circuit voltage of the Ni-Fe cell is 1.4 which drops quickly to 1.2 V during discharge. Tubular or pocket plate COTistructiOTis are used. Active materials of high purity are contained inside the perforated nickel-plated steel tubes or rectangular pockets. The active materials are irrm for the negative electrode and nickel oxide for the cathode and a KOH electrolyte. 

Figures 7.9, 7.10, and 7.11 show examples for single cells and bloc batteries with lead and lead-dioxide electrodes in figure 7.12 a nickel/cadmium cell with pocket plates is shown housed in a steel container.

A cutaway view of a modern pocket-plate cell is shown in Fig. 26.2. The active material for the positive electrodes consists of nickel hydroxide mixed with graphite for conductivity and... 

The positive and negative electrodes of pocket-plate nickel-cadmium batteries are made using the same basic design to hold the active materials. The pocket plates are buUt up of flat pockets of perforated steel strips holding the active materials. The thin steel strips are perforated by hardened steel needles or by a technique using profiled roller dies. The specific hole area is between 15 and 30%. The strips are nickel-plated to prevent iron poisoning of the positive active material. 

The electrochemically active material of the nickel electrode is nickel hydroxide. This material is an amorphous colloid and is only semiconductive at best. It must be supported and contained by a structural component which provides mechanical support, conductivity and current collection for the electrode. Standard types of nickel electrodes can be used in the nickel-zinc system. They can be classified by the type of electrode substrate used and by the method of preparation. These electrodes consist of two basic types, sintered and nonsin-tered. Each type has different advantages and disadvantages and may be selected based on the application. Other types of nickel electrodes, such as pocket plate, are generally not in common use. 

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