Battery separators stacked cells

The narrow tolerances to be maintained for the total separator thickness are tightened even further by the trend towards high-performance batteries with many thin electrodes, and therefore many separators also. One can easily calculate that for, say, ten or more electrodes and an equal number of separators per cell, the permitted tolerances become very small for fitting the electrodes/separators stack into the cell container. With electrodes and separators... 

Another feature of AGM separators is their compressibility. With compression of the plate and separator stack, this AGM property guarantees good plate-separator contact, even if the plates are not perfectly smooth. Also, battery assembly is facilitated since the stack can be easily inserted into the cell after compression to a thickness lower than the cell dimension. An undesirable result of the compressibility is that the AGM separator does not exert sufficient resistance against expansion of the positive plate during battery cycle-life. This expansion is particularly prevalent in deep-cycle applications and can cause the battery to suffer premature capacity loss (PCL) via reduced inter-particle conductivity — a phenomenon known as PCL-2 [7]. In the literature, two additional characteristics, which are related to the PCL-2 failure mode, are discussed, namely, AGM separators shrink when first wetted with electrolyte and their fibres can be crushed at high pressure levels [8-10]. These features result in a loss of separator resilience, i.e., a lessening of the ability to display a reversible spring effect. 

Battery assembly. Dried plates are stacked in active blocks, so that positive and negative plates alternate with separators in between. Plates of like polarity are interconnected into semi-blocks by welding together through the plate lugs. The active blocks are then introduced into battery containers, the cells are connected and the batteries are covered with lids and tested for air-tightness. The vents are closed to eliminate access of air from the surroundings, and the batteries are packed and ready for delivery. 

Figure 1.39 shows the principle of the zinc/bromine battery. In Fig. 1.39, the cell stack consists of only three cells. Actual batteries contain stacks of 50 or more cells. Except the two end plates, all electrodes are bipolar. They consist of an electron-conducting plate of carbon plastic enframed by insulating plastic. On the positive side, a porous layer of carbon increases the surface to increase the reaction rate of bromine. In the center of each cell, a microporous separator is positioned as used in lead-acid batteries (e.g. Daramic ) to suppress the direct contact between bromine and zinc as far as possible. 

The requirements for a battery separator can best be understood in the context of how the separator is used. Currently there are two major designs spirally wound or stacked plate. There are two types of stacked-plate designs, one relying on stack pressure to maintain good interfacial contact between the electrodes and separators, and a second which uses an adhesive to bond the electrodes and separators. The manufacturing processes for both spiral and stack cell designs are reviewed by Brodd and Tagawa [7]. Both processes put stress on the separator. 

The actual flotation phenomenon occurs in flotation cells usually arranged in batteries (12) and in industrial plants and individual cells can be any size from a few to 30 m in volume. Column cells have become popular, particularly in the separation of very fine particles in the minerals industry and coUoidal precipitates in environmental appHcations. Such cells can vary from 3 to 9 m in height and have circular or rectangular cross sections of 0.3 to 1.5 m wide. They essentially simulate a number of conventional cells stacked up on top of one another (Fig. 3). Microbubble flotation is a variant of column flotation, where gas bubbles are consistently in the range of 10—50 p.m. 

The bipolar plate with multiple functions, also called a flow field plate or separation plate (separator), is one of fhe core components in fuel cells. In reality, like serially linked batteries, fuel cells are a serial connection or stacking of fuel cell unifs, or so-called unif cells fhis is why fuel cells are normally also called sfacks (Figure 5.1) [2]. The complicated large fuel cells or module can consist of a couple of serially connecfed simple fuel cells or cell rows. Excepf for the special unit cells at two ends of a simple stack or cell row, all the other unit cells have the same structure, shape, and functions. 

The electrolyte is very conductive. In my example of 7 separate containers wired in series, there s no charge applied to the middle ones either because they are in series If you wire 7 resistors in series you have 1/7th of the total voltage across each resistor. I wanted to get about 2V dc per cell, and with 7 cells you can use a 14V dc power supply (for example a battery charger). You could scale up to any number of plates, but the voltage across the stack would be higher. If 7 cells produce 7 units of gas with a given current then 100 cells would produce 100 units of gas with the same current, but the voltage across the whole stack would need to be about 2V multiplied by the number of cells. Thus the power consumption increases approximately linearly with the number of plates. 

Some commonly used batteries are shown in Table 15.5, and two are drawn schematically in Fig. 15.10. From these it can be seen that important components are the container, the anode/cathode compartment separators, current collectors to transport current from the electrode material (usually a porous, particulate paste), the electrode material itself, and the electrolyte. It should be noted that the electrode reactions can be significantly more complex than those indicated in Table 15.5, and there will probably be parallel reactions. By stacking the batteries in series, any multiple of the cell potential can be obtained. 

It should be noted that in practical batteries such as coin cell (parallel plate configuration) or AA, C, and D (jelly-roll configuration), there is a stack pressure on the electrodes (the Li anodes are pressed by the separator), and the ratio between the solution volume and the electrode s area is usually much lower than in laboratory testing. Both factors may considerably increase the Li cycling efficiency obtained in practical cells, compared with values measured for the same electrolyte solutions in the Li half-cell testing described above. It has already been proven that stack pressure suppresses Li dendrite formation and thus improves the uniformity of Li deposition-dissolution processes [107], The low ratio between the solution volume and the electrode area in practical batteries decreases the detrimental effects of contaminants such as Lewis acids, water, etc., on Li passivation. 

Figure 8-5 shows the different cooling configurations used to study the thermal conductivity along and across the stack. The electrical load and UU curves were measured using a battery test system. The voltage of each cell was monitored separately. Figure 8-6 shows a typical C///curve for the stack. Five fuel cell stacks were used for the experiments described here. 

---