Nature (Vol. 430, No. 6999, 29 July 2004)

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The high conductivity of aqueous solvent-based electrolytes is due to their dielectric constants, which favor stable ionic species, and the high solvating power, which favors formation of hydrogen bridge bonds and allows the unique Grotthus conductivity mechanism for protons. Thermodynamically, aqueous electrolytes show an electrochemical stability window of 1.

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Compared to water, most organic solvents have a lower solvating power and a lower dielectric constant. This favors ion pair formation, even at low salt concentration. Ion pair formation lowers the conductivity as the ions are no longer free and bound to each other. Organic electrolytes show lower conductivities and much higher viscosities than aqueous electrolytes.

Exceeding the voltage limit in the organic electrolytes results in polymerization or decomposition of the solvent system.

VI. Other National Practice

Solid electrolyte batteries have found limited use as the power source for heart pacemakers and for use in military applications. The basic principles described above apply to fuel cells and electrochemical capacitors as well as to batteries. A list of common commercial systems is found in Table 2.

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A graphical representation of the energy storage capability of common types of primary and secondary batteries is shown in Figures 13 and It is beyond the scope of this paper to discuss all systems in detail. Instead, we want to review the most common electrode mechanisms for discharge and charge depicted in Figure Figure 14 Energy storage capability of common rechargeable battery systems.

Table 2. Common Commercial Battery Systems.

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During the cell reaction, Cu is displaced by Li and segregates into a distinct solid phase in the cathode. The products of this displacement type of reaction, Li 2 S and Cu, are stable, and the reaction cannot be easily reversed. Hence, the electrode reactions cannot be recharged and the cell is considered to be a primary cell, as the discharge reaction is not reversible.

The Li electrode in Figure 15B is discharged by oxidation. The reaction is reversible by redeposition of the lithium. However, like many other metals in batteries, the redeposition of the Li is not smooth, but rough, mossy, and dendritic, which may result in serious safety problems.

This is in contrast to the situation with a lead electrode in Figure 15C, which shows a similar solution electrode. Figure 15D shows a typical electrochemical insertion reaction. Unlike displacement type electrodes Figure 15A and solution type electrodes Figure 15B , the insertion electrodes Figure 15D have the capability for high reversibility, due to a beneficial combination of structure and shape stability.

Many secondary batteries rely on insertion electrodes for the anode and cathode. A prerequisite for a good insertion electrode is electronic and ionic conductivity. However, in those materials with poor electronic conductivity, such as MnO 2 , good battery operation is possible. In this case, highly conductive additives such as carbon are incorporated in the electrode matrix, as in Figure 15E. The utilization of the MnO 2 starts at the surface, which is in contact with the conductive additive and continues from this site throughout the bulk of the MnO 2 particle. Most electrodes in batteries follow one of the basic mechanisms discussed in Figure Zinc manganese batteries consist of MnO 2 , a proton insertion cathode cf.

Figure 15E , and a Zn anode of the solution type. The discharge reaction of the MnO 2 electrode proceeds in two one-electron reduction steps as shown in the discharge curve Figure Starting at cell voltages of 1. This is consistent with the Gibbs phase rule that predicts the shape of the discharge curve for one- and two-phase reactions Figure If the values of two parameters, usually pressure, p , and temperature, T , are specified, there is no degree of freedom left and other parameters of the system such as voltage have to be constant.

Hence, the cell voltage stays constant for a two-phase discharge reaction. If there is a degree of freedom left, as in the case of a one-phase reaction, the cell voltage can be a variable and changes slopes-off during discharge. The detailed discharge reaction mechanism is shown in Figure It should be noted that local pH changes occur also during discharge of the MnO 2 electrode Figure 15E. The current version of the alkaline cell is mercury free. Instead, it uses a combination of alloying agents and corrosion inhibitors to lower the hydrogen gas generation from corrosion of the zinc anode and to compensate for the corrosion protection originally provided by the mercury.

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A synthetic gel holds the zinc powder anode together. The high energy density results from the cell design, as only the zinc powder anode is contained in the cell.

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The other reactant, oxygen, is available from the surrounding air. The air electrode is a polymer-bonded carbon, sometimes catalyzed with manganese dioxide. The electrode has a construction similar to that of fuel cell electrodes see section 3. Cells are available in sizes smaller than an aspirin tablet that fit into the ear to power the hearing aid. The cell operates with an alkaline electrolyte. The Zn electrode operates as discussed, whereas the Ag 2 O electrode follows a displacement reaction path cf. Figure 15A. Primary lithium cells use a lithium metal anode, the discharge reaction of which is depicted in Figure 15B.

Due to the strong negative potential of metallic lithium, cell voltages of 3. As the lithium metal is very reactive, the key to battery chemistry is the identification of a solvent system that spontaneously forms a very thin protective layer on the surface of metallic lithium, called the solid electrolyte interphase SEI layer. This electronically insulating film selectively allows lithium ion transport. Lithium batteries show higher energy density than the alkaline cells but have a lower rate capability because of the lower conductivity of the nonaqueous electrolyte and the low lithium cation transport rate through the SEI.

Commercial lithium primary batteries use solid and liquid cathodes. Chemical CMD or electrolytic manganese dioxide EMD is used as the cathode with high-temperature treatment to form a water-free active material.

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The CF x is made from the elements, and its cost is somewhat higher than that of the manganese dioxide cathode material. These cells are designed for relatively low-rate applications. The lithium thionyl chloride system employs a soluble cathode construction. The thionyl chloride acts as the solvent for the electrolyte and the cathode active material. The reaction mechanism of the cell is explained in Figure LiCl and S are also the products of the electrochemical discharge reaction at the carbon positive electrode, where the liquid cathode SOCl 2 is reduced.

The cell discharge stops when the electronically insulating discharge products block the carbon electrode. The SO 2 is dissolved in an organic solvent such as PC or acetonitrile. Alternatively, SO 2 is pressurized at several bars to use it in the liquid state.

The cell reaction is similar to that depicted in Figure 18, with electronically insulating Li 2 S 2 O 4 being the SEI component at the Li anode and the solid discharge product at the carbon cathode. Typical applications are military, security, transponder, and car electronics. Primary lithium cells have also various medical uses. Rechargeable cells generally have lower energy storage capability than primary cells.

The additional requirements for rechargeability and long operation limit the choice of chemical systems and constructions to those that are more robust than for primary batteries. The lead acid battery dominates the rechargeable market. In addition to the lead and lead oxide electrodes, sufficient amounts of sulfuric acid and water have to be provided for the cell reaction and formation of the battery electrolyte. For ionic conductivity in the charged and discharged states, an excess of acid is necessary.

Considering the limited mass utilization and the necessity of inactive components such as grids, separators, cell containers, etc. Due to the heavy electrode and electrolyte components used, the specific energy is low.