Batteries, Other Secondary Cells (2000)

Abstract The proliferation of portable electronic devices has fueled rapid market growth for the rechargeable battery industry. Miniaturization of electronics coupled with consumer demand for lightweight batteries providing ever longer run times continues to spur interest in advanced battery systems. Interest also continues to run strong in electric vehicles (EVs) and the large auto manufacturers continue to develop prototype EVs. Advanced batteries continue to play a strong role in other applications such as load leveling for the electric utility industry and satellite power systems for aerospace. Secondary battery systems have been based on aqueous electrolytes. The use of water imposes a fundamental limitation on battery voltage because of the electrolysis of water. The application of nonaqueous electrolytes affords a significant advantage in terms of achievable battery voltages. By far the most actively researched field in nonaqueous battery systems has been the development of practical rechargeable lithium batteries based on the use of lithium metal, Li, or a lithium alloy, as the negative electrode. The use of lithium as a negative electrode for secondary batteries offers a number of advantages. Lithium has the lowest equivalent weight of any metal and affords very negative electrode potentials when in equilibrium with solvated lithium ions, resulting in very high theoretical energy densities for battery couples. These high theoretical energy densities have prompted a wealth of research activity in a wide variety of experimental battery systems. However, realization of the technology to commercialize these systems has been slow. A key technical problem in developing practical lithium batteries has been poor cycle life attributable to the lithium electrode. The highly reactive nature of freshly plated lithium leads to reactions with electrolyte and impurities to form passivating films that electrically isolate the lithium metal. The formation of surface films on the lithium electrode imparts the apparent stability of the electrolyte to the electrode. In addition to providing a stable film in the presence of lithium, the electrolyte must satisfy additional requirements, including good conductivity, being in the liquid range over the battery operating temperature, and electrochemical stability over a wide voltage range. In order to satisfy the various electrolyte system requirements, the use of mixed solvent electrolytes has become common in practical cells. Examples are tetrahydrofuran, C 4 H 8 O, ‐based electrolytes or ethylene carbonate C 2 H 4 O 3 ,–propylene carbonate, C 4 H 6 O 3 , mixed solvent systems. A second class of important electrolytes for rechargeable lithium batteries are solid electrolytes. Of particular importance is the class known as solid polymer electrolytes (SPEs), polymers capable of forming complexes with lithium salts to yield ionic conductivity. The best known of the SPEs are the lithium salt complexes of poly(ethylene oxide) (PEO), —(CH 2 CH 2 O h ) n —, and poly(propylene oxide) (PPO). The lithium or lithium alloy negative electrode systems employing a liquid electrolyte can be categorized as having either a solid positive electrode or a liquid positive electrode. Systems employing a solid electrolyte employ solid positive electrodes to provide a solid‐state cell. The most important rechargeable lithium batteries are those using a solid positive electrode within which the lithium ion is capable of intercalating. These intercalation, or insertion, electrodes function by allowing the interstitial introduction of the ion into a host lattice. Intercalation electrodes have found wide application in systems employing both solid or liquid electrolytes. The use of high temperature lithium cells for electric vehicle applications has been under development since the 1970s. Advances in the development of lithium alloy–metal sulfide batteries have led to lithium–aluminum/metal sulfide batteries, ie, the Li–Al/FeS system. The cell employs a molten salt electrolyte, most commonly a lithium chloride/potassium chloride, LiCl–KCl eutectic mixture. The negative electrode is composed of lithium–aluminum alloy, which operates at about 300 mV positive of pure lithium. The positive electrode is composed of iron sulfide mixed with a conductive agent such as carbon or graphite. Electrodes are constructed by cold pressing powder onto current collectors. The best known of the high temperature batteries is the sodium–sulfur, Na–S, battery. The cell is constructed using a solid electrolyte typically consisting of β‐alumina, β‐Al 2 O 3 , ceramic, although borate glass fibers have also been used. The negative electrode consists of molten sodium metal and the positive electrode of molten sulfur. Because sulfur is not conductive, a current collection network of graphite is required. The cell is operated at about 350°C. The Na–S battery couple is a strong candidate for applications in both EVs and aerospace. The Na–S system is expected to provide significant increases in energy density for satellite battery systems. A battery system closely related to Na–S is the Na–metal chloride cell. The cell design is similar to Na–S; however, in addition to the β‐alumina electrolyte, the cell also employs a sodium chloroaluminate, NaAlCl 4 , molten salt electrolyte. The positive electrode active material consists of a transition metal chloride such as iron(II) chloride, FeCl 2 , or nickel chloride, NiCl 2 , in lieu of molten sulfur. This technology is in a younger state of development than the Na–S. Rechargeable cells employing aluminum, Al, as a negative electrode in room temperature molten salts have been investigated. Redox flow batteries, under development since the early 1970s, are still of interest primarily for utility load leveling applications. Examples of this technology include the iron–chromium, Fe–Cr, battery and the vanadium redox cell. Commercialization of advanced battery systems is limited. Efforts to develop commercially viable EV versions of advanced battery systems continue. The ultimate goal is to develop battery technology suitable for practical, consumer‐acceptable electric vehicles.