The zinclbromine battery is a relatively high energy density system with the potential for bulk energy storage at a low cost. The system also has sufficient high power capability to make it a viable candidate for electric vehicle applications. Discharge tests following dlfferent protocols including constant current, SFUDS, and peak power, reveal a range of response depending on the conditions. Constant Current @ding iNTRODUCTlONThe zinclbromlne system has been under development by Johnson Controls, Inc. (JCi) since the earty 1980's.' it is an ambienttemperature flowing electrolyte battery, which uses an essentially allplastic, blpolar-electrode conshuction and reactants whkh are abundant in nature. Its high energy density (60-75 Whkg) and low cost make lt a candidate for electric vehicle propulsion, electric utility load management, and other bulk energy storage applications.2 8 On charge. the bromide ion is oxktized to bromine at the cathode.In dlscharge the reactions are reversed. Both anions and cations move across the separator between the electrode compartments to maintain charge neutrality during battery operation. in addition to the zinc bromide, quaternary ammonium salts are present to complex the bromine as It is produced. Complexation of the bromine formed on charge allows most of it to be stored extemally to the battery stack, and reduces its chemical activity. A schematic representation of the battery is shown in Figure 1.The bipolar electrodes are typically 0.025-0.030" thick and are made from carbonblack-filled polyolefln. The cathode slde uses a high surface area carbon coating in order to compensate for the relatively low exchange current density for the brominebromide reaction on carbon. The separator is a microporous silica/pdyethylene mposite.Optionally, for applications requiring high power, ammonium chloride may be added to the electrolyte to increase the conductivity so that high discharge rates (up to 200 m A / d ) are possible. Electrolyte to which a"onium chloride has been sdded is referred to as "supported electrolyte" to dmerentiate it from the standard "unsupported electrolyte" whlch contains no ammonium CMorlde.4Early zinc/bromine batteries used a bolt and adhesive-gasket sealing method and polypropylene-based electrodes. Problems reconnired e d v in the deveiODment of the system were electralvte leakage, expansion and warping of the electrodes, and high selfdischarge. Within the last year, JCI has been perfecting an aggressive new design whkh addresses these early design problems.The new design uses a thermal welding technique to achieve a hermetically-sealed battery stack. Electrodes in the new design are based on polyethylene. instead of polypropylene, since the latter was more susceptible to expansion and warpage in the presence of bromine.Embedded glass Hbers are also used to increase the tensile modulus of the electrodes in the new design. Improved separators and improved electrolyte flow dlsMbuton withln the battery stack have reduced the amount of self-discharga compared to ea...
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