Practical batteries

A finite time is required to reestabUsh the ion atmosphere at any new location. Thus the ion atmosphere produces a drag on the ions in motion and restricts their freedom of movement. This is termed a relaxation effect. When a negative ion moves under the influence of an electric field, it travels against the flow of positive ions and solvent moving in the opposite direction. This is termed an electrophoretic effect. The Debye-Huckel theory combines both effects to calculate the behavior of electrolytes. The theory predicts the behavior of dilute (<0.05 molal) solutions but does not portray accurately the behavior of concentrated solutions found in practical batteries. 

When a battery produces current, the sites of current production are not uniformly distributed on the electrodes (45). The nonuniform current distribution lowers the expected performance from a battery system, and causes excessive heat evolution and low utilization of active materials. Two types of current distribution, primary and secondary, can be distinguished. The primary distribution is related to the current production based on the geometric surface area of the battery constmction. Secondary current distribution is related to current production sites inside the porous electrode itself. Most practical battery constmctions have nonuniform current distribution across the surface of the electrodes. This primary current distribution is governed by geometric factors such as height (or length) of the electrodes, the distance between the electrodes, the resistance of the anode and cathode stmctures by the resistance of the electrolyte and by the polarization resistance or hinderance of the electrode reaction processes. 

Solid Electrolyte Systems. Whereas there has been considerable research into the development of soHd electrolyte batteries (18—21), development of practical batteries has been slow because of problems relating to the low conductivity of the soHd electrolyte. The development of an all sohd-state battery would offer significant advantages. Such a battery would overcome problems of electrolyte leakage, dendrite formation, and corrosion that can be encountered with Hquid electrolytes. 

The variety of practical batteries has increased during the last 20 years. Applications for traditional and new practical battery systems are increasing, and the market for lithium-ion batteries and nickel-metal hydride batteries has grown remarkably. This chapter deals with consumer-type batteries, which have developed relatively recently. 

The ideal nonaqueous electrolyte for practical batteries would possess the following properties ... 

PP/PE/PP trilayer separators 556 practical batteries 19-61 precipitation, solid electrolytes 540 precursors... 

The organization of the Handbook of Battery Materials is simple, dividing between aqueous electrolyte batteries and alkali metal batteries and further in anodes, cathodes, electrolytes and separators. There are also three more general chapters about thermodynamics and mechanistics of electrode reactions, practical batteries and the global competition of primary and secondary batteries. 

In practical batteries and fuel cells, the influence of the current rate on the cell voltage is controlled... 

The dependence of cell emf on the concentrations of reactants and products of the cell reaction is of fundamental importance in the understanding and design of certain practical battery systems. As a cell undergoes discharge, reactants are steadily converted to products until one of the reactants species is completely exhausted. Consider the cell reaction A + B -> C + D, where A-D are all solution species. For this system,... 

Knowledge of the amount by which the voltage of a cell, delivering a particular level of current, deviates from its equilibrium value is of central importance in assessing the performance of a practical battery system. This polarization voltage, Ep0], can be associated with two principal causes ... 

In practical batteries, especially those employing porous phases, it is not always possible to separate ohmic and electrode losses clearly. 

It is common in many practical battery designs to immobilize a liquid electrolyte phase within a porous solid insulator. The electrolyte conductivity and ohmic loss in such a system are determined by the number of pores, their size, shape and tortuosity. The tortuosity coefficient, /3, is defined as the ratio of the mean distance covered by an ion traversing a porous matrix, to the direct distance of one side of the matrix to the other. The relative reduction in the conductivity of an electrolyte solution caused by confining it in a porous solid is called the conductivity attenuation, 0. For a matrix of uniform cylindrical pores it is given by... 

In most modern practical batteries, a major part of polarization loss at moderately high current densities is due to ohmic potential drop. Considerable attention is therefore given during the design of a battery to ... 

In addition to mass transport from the bulk of the electrolyte phase, electroactive material may also be supplied at the electrode surface by homogeneous or heterogeneous chemical reaction. For example, hydrogen ions required in an electrode process may be generated by the dissociation of a weak acid. As this is an uncommon mechanism so far as practical batteries are concerned (but not so for fuel cells), the theory of reaction overvoltage will not be further developed here. However, it may be noted that Tafel-like behaviour and the formation of limiting currents are possible in reaction controlled electrode processes. 

The coulombic efficiency of a cell is defined as Qp/Qj. It is often more useful to determine the capacity of each half-cell separately, since for operational reasons, most practical batteries do not have an equal number of equivalents of anodic and cathodic reactants. 

Fig. 4.1 Power density-energy density curves for practical battery systems...

In addition to carbon-based systems, other intercalation compounds are also currently being proposed as alternative lithium ion cell negative plates. Examples include LiJtTiS2, Li/TiC, L /H-sO and, more recently, a family of Li SnOv compounds. However, the applicability of these materials in practical batteries has not yet been established, and coke and graphite are still the materials used in all commercial lithium ion cells. 

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