A battery in which both electrodes and the electrolyte are solids. Solid electrolytes are a class of materials also known as superionic conductors and fast ion conductors, and their study belongs to an area of science known as solid-state ionics. As a group, these materials are very good conductors of ions but are essentially insulating toward electrons, properties that are prerequisites for any electrolyte. The high ionic conductivity minimizes the internal resistance of the battery, thus permitting high power densities, while the high electronic resistance minimizes its self-discharge rate, thus enhancing its shelf life. Examples of such materials include Ag4RbI5 for Ag+ conduction, LiI/Al2O3 mixtures for Li+ conduction, and the clay and β-alumina group of compounds (NaAl11O17) for Na+ and other mono- and divalent ions. At room temperature the ionic conductivity of a single crystal of sodium β-alumina is 0.035 S/cm, comparable to the conductivity of a 0.1 M HCl solution. This conductivity, however, is reduced in a battery by a factor of 2–5, because of the use of powdered or ceramic material rather than single crystals. Of much interest are glassy and polymeric materials that can be readily made in thin-film form, thus enhancing the rate capability of the overall system. See also: Electrolyte; Electrolytic conductance; Ionic crystals
In some cases, solid-state batteries also make use of fast ion conductors for the electrodes. These materials are, however, good conductors for both ions and electrons. Examples include the layered-structure disulfides of titanium and vanadium, TiS2 and VS2, which can be used as the cathode or sink for lithium, and aluminum-lithium alloys, which can be used as the anode or source of lithium in lithium batteries. These materials can sustain high reaction rates because of their unique crystalline structures which allow the incorporation of ions into their crystalline lattices without destruction of those lattices (Fig. 1). An example of such a cell is given by notation (1), having anode reaction (2) and cathode reaction (3).
At the anode the face-centered cubic lattice of aluminum expands from 0.405 to 0.638 nanometer, whereas at the cathode the titanium disulfide sandwich structure expands only 10% on incorporation of the lithium. See also: Intercalation compounds; Solid-state chemistry
Potentially the largest use of solid electrolytes in the battery field is in the high-energy-density sodium/sodium-β-alumina/sulfur battery. This system is expected to find application for powering of automotive vehicles and for energy storage at central power plants, such as solar plants, provided the lifetime of the ceramic electrolyte can be improved. However, this battery operates at about 570–750°F (300–4000°C), so that both electrodes are in the liquid state. See also: Energy storage
Solid-state batteries generally fall into the low-power-density and high-energy-density category. The former limitation arises because of the difficulty of getting high currents across solid–solid interfaces. However, these batteries do have certain advantages that outweigh this disadvantage: They are easy to miniaturize (for example, they can be constructed in thin-film form), and there is no problem with electrolyte leakage. They tend to have very long shelf lives, and usually do not have any abrupt changes in performance with temperature, such as might be associated with electrolyte freezing or boiling. Being low-power devices, they are also inherently safer. The major applications of these batteries are in electronic devices such as cardiac pacemakers, cameras, electrochromic displays, watches, and calculators.
In a solid-state cell (Fig. 2), the polycrystalline pressed electrolyte is interspaced between a metallic anode and the solid cathode material. The electrodes are applied to the electrolyte by mechanically pressing the materials together, or in some cases the electrolyte is formed in place by reaction between the two electrodes. These cells are then stacked together to form a battery of the required voltage. A carbon current collector is often used on the cathode side, and this is frequently admixed with the cathode material. In some cases, where the cathode material is sufficiently conductive, for example titanium disulfide, no carbon conductor is needed. Although some of the earliest solid-state batteries were based on silver, these have not found any significant applications because of their low energy densities, their high cost, and competition from lithium batteries.
The lithium-iodine battery employed in many cardiac pacemakers is constructed with a lithium anode and an iodine–poly(vinyl pyridine) cathode. This cathode has a tarlike consistency in the fresh battery, then solidifies gradually as the battery is discharged. In this cell the solid electrolyte, lithium iodide, is formed in place by the reaction of the lithium and iodine. Because the electrolyte continues to form as the battery discharges, the overall resistance of the cell continually increases with discharge. This results in a drop in cell voltage for a given current drain. The initial cell voltage is around 2.8 V.
The most important characteristics of the lithium battery for this application are its high reliability and very low rate of self-discharge. A typical implantable battery has a volume of 0.4 in.3 (6 cm3), weighs 0.8 oz (23 g), and has a 2-amperehour capacity. The lifetime of this battery is 5–10 years since pacemakers typically draw only 15–30 microamperes. The battery is considered to be discharged when the output voltage drops to 1.8 V. This may be detected by monitoring the patient's pulse rate. See also: Medical control systems
A reversible lithium solid-state battery uses a glass as the electrolyte, pure lithium as the anode, and an intercalation compound as the cathode. These compounds, such as titanium disulfide, react reversibly with lithium, as discussed above. The overall resistance, unlike that of lithium-iodine batteries, does not increase with discharge, nor is a conductive element such as carbon necessary in the cathode. The emf of such a cell is around 2 V and decreases slightly but continuously with loss of capacity; this allows the cell voltage to be used as a fuel gage, unlike most conventional batteries where there is an abrupt loss of voltage without warning when the cell is depleted.
An advanced lithium battery uses a lithium carbon compound as anode and lithium cobalt oxide (LiCoO2) as cathode. The anode incorporates lithium between the layers of graphitic coke, thereby containing the lithium safely and making cell rechargeability much more feasible. The lithium cobalt oxide cathode has the same structure as lithium titanium disulfide (Fig. 1) and behaves in the same manner. However, it is much more oxidizing, leading to a cell emf of about 3.5 V. Since this is almost exactly three times that of Ni/Cd or Ni/hydride batteries, this battery can easily be used in their place. Moreover, since a single cell will suffice for such applications as portable computers, battery management is much easier.
Solid-state batteries based upon polymeric electrolytes are being actively investigated, particularly for use with lithium and sodium anodes. Energy densities of 5–10 times that of lead-acid cells have been calculated based upon very thin cells combined with high-voltage arrays. These cell designs could eventually lead to rechargeable solid-state batteries with energy densities of 100–200 watthours per kilogram and comparable power levels. They are most likely to be used with lithium anodes and conductive cathodes such as titanium disulfide. The polymer most under study is polyethylene oxide mixed with a lithium salt such as LiCF3SO3. See also: Battery