Batteries active masses

Active mass — The portions of a -> battery or -> accumulator which are participating in electrode reactions, i.e., in the transformation of chemical into electrical -> energy or vice versa. In a -> lead-acid battery active masses are lead dioxide and lead, with the lead or lead alloy grid serving as -> current collector and mechanical holder and all other components are not active masses. For maximum -> energy density the fraction of active mass in the overall cell weight should be as large as possible. 

Skeleton and Energetic Structure of Battery Active Masses... 

FIGURE 3.3 Structure of the lead dioxide active mass. (FromD. Pavlov, andE. Bashtavelova, J. Electrochem. Soc., 131,1468, 1984 D. Pavlov, and E. Bashtavelova, J. Electrochem. Soc., 133, 241, 1986 D. Pavlov et al., Structure of the Lead-Acid Battery Active Masses, in Proc. Int. Symp. Advances in Lead-Acid Batteries, Vol. 84-14, p. 16, Electrochemical Society, Pennington, NJ, 1984.)... 

D. Pavlov, E. Bashtavelova, and V. Ihev, Structure of the Lead-Acid Battery Active Masses, in Proc. Int. Symp. Advances in Lead-Acid Batteries, Vol. 84-14, p. 16, Electrochemical Society, Pennington, NJ, 1984. 

Uses. Nickel nitrate is an intermediate in the manufacture of nickel catalysts, especially those that are sensitive to sulfur and therefore preclude the use of the less expensive nickel sulfate. Nickel nitrate also is an intermediate in loading active mass in nickel—alkaline batteries of the sintered plate type (see Batteries, SECONDARY cells). Typically, hot nickel nitrate symp is impregnated in the porous sintered nickel positive plates. Subsequendy, the plates are soaked in potassium hydroxide solution, whereupon nickel hydroxide [12054-48-7] precipitates within the pores of the plate. 

When nickel hydroxide is oxidized at the nickel electrode in alkaline storage batteries the black trivalent gelatinous nickel hydroxide oxide [12026-04-9], Ni(0H)0, is formed. In nickel battery technology, nickel hydroxide oxide is known as the nickel active mass (see Batteries, secondary cells). Nickel hydroxide nitrate [56171-41-6], Ni(0H)N02, and nickel chloride hydroxide [25965-88-2], NiCl(OH), are frequently mentioned as intermediates for the production of nickel powder in aqueous solution. The binding energies for these compounds have been studied (55). 

The performance of a battery is often designed to be limited by one electrode ia order to achieve special performance characteristics, such as overcharge protection and safety. The coulombic efficiency of the active mass is of particular iaterest ia battery design and performance. 

Most battery electrodes are porous stmctures in which an interconnected matrix of soHd particles, consisting of both nonconductive and electronically conductive materials, is filled with electrolyte. When the active mass is nonconducting, conductive materials, usually carbon or metallic powders, are added to provide electronic contact to the active mass. The soHds occupy 50% to 70% of the volume of a typical porous battery electrode. Most battery electrode stmctures do not have a well defined planar surface but have a complex surface extending throughout the volume of the porous electrode. MacroscopicaHy, the porous electrode behaves as a homogeneous unit. 

Typical pore size distributions result in mean pore diameters of around 15 //m. Even long and intensive efforts did not succeed in decreasing this value decisively in order to enable production of micropo-rous pocketing material resistant to penetration [65, 66], In practice PVC separators prove themselves in starter batteries in climatically warmer areas, where the battery life is however noticeably reduced because of increased corrosion rates at elevated temperature and vibration due to the road condition. The failure modes are similar for all leaf separator versions shedding of positive active mass fills the mud room at the bottom of the container and leads to bottom shorts there, unless — which is the normal case — the grids of the positive electrodes are totally corroded beforehand. 

There is no question that the development and commercialization of lithium ion batteries in recent years is one of the most important successes of modem electrochemistiy. Recent commercial systems for power sources show high energy density, improved rate capabilities and extended cycle life. The major components in most of the commercial Li-ion batteries are graphite electrodes, LiCo02 cathodes and electrolyte solutions based on mixtures of alkyl carbonate solvents, and LiPF6 as the salt.1 The electrodes for these batteries always have a composite structure that includes a metallic current collector (usually copper or aluminum foil/grid for the anode and cathode, respectively), the active mass comprises micrometric size particles and a polymeric binder. 

Active mass is the material that generates electrical current by means of a chemical reaction within the battery. 

Mechanical and Chemical Stability. The materials must maintain their mechanical properties and their chemical structure, composition, and surface over the course of time and temperature as much as possible. This characteristic relates to the essential reliability characteristic of energy on demand. Initially, commercial systems were derived from materials as they are found in nature. Today, synthetic materials can be produced with long life and excellent stability. When placed in a battery, the reactants or active masses and cell components must be stable over time in the operating environment. In this respect it should be noted that, typically, batteries reach the consumer 9 months after their original assembly. Mechanical and chemical stability limitations arise from reaction with the electrolyte, irreversible phase changes and corrosion, isolation of active materials, and local, poor conductivity of materials in the discharged state, etc. 

Therefore, passivation of the positive electrode by poorly conducting PbS04 can be reduced [348]. The porosity is important because it enables the expansion during the solid phase volume increase, which accompanies the transformation of Pb02 to PbS04. In the most popular construction, the electrode paste material (mixture of metallic lead with lead oxides) is held in a framework composed of lead alloys with additions of tin, antimony, selenium, and calcium [348]. Antimony improves the mechanical stability however, it increases the resistance and facilitates the selfdischarge of the battery. Better results are obtained for low antimony content and/or for lead-calcium alloys [203]. Methods of positive electrodes improvement, from the point of view of lead oxide technology have been discussed [350]. Influence of different factors on life cycle, nature, and composition of the positive active mass has been studied by Pavlov with coworkers [200, 351, 352]. 

SLI batteries are also supplied in a dry charged state and are activated simply by filling with electrolyte. Plates for such batteries have extra additives, such as antioxidants in the negative active mass, and forming is followed by one of a number of controlled drying processes. 

In the following we adopt the usual distinction between devices that convert chemical in electrical energy but cannot be electrically charged again (primary batteries), metabolistic cells in which active masses are continually supplied and removed by gas flow (fuel cells) and cells that can be recharged electrically (secondary batteries). In all these cases great technological advances have been made and there exists a rich literature on these topics. 

The topic of this book is focused on active masses containing carbon, either as an active mass (e.g., negative mass of lithium-ion battery or electrical double layer capacitors), as an electronically conducting additive, or as an electronically conductive support for catalysts. In some cases, carbon can also be used as a current collector (e.g., Leclanche cell). This chapter presents the basic electrochemical characterization methods, as applicable to carbon-based active materials used in energy storage and laboratory scale devices. 

Each active mass contains a redox couple. For example, the negative mass (Reaction 1.1) of a Ni/MH battery contains a metallic hydride MH ... 

Current collectors in Ni/MH batteries are usually made of nickel foam, passivated by a NiOOH layer at the positive side. Separators are usually microporous plastic films impregnated with liquid electrolyte. It is, therefore, necessary that the active masses be insoluble, or at least sparingly soluble, in order to avoid mixing of components via diffusion through the separator, thus leading to self-discharge. 

Partial dissolution of an active mass often leads to poor cyclability by compaction and loss of the double percolation, electronic and ionic conductivity, necessary for a successful operation. This is especially true for PbS04, in diluted H2S04 (discharged battery). Metallic deposition in the separator of a dissolved metal may cause short circuits, accelerated self-discharge, and may prevent subsequent charging. 

Electrochemical generators are the most common way for the storage of electrical energy. The energetic yield is greater than 50%, usually around 80%. Electrochemical methods are used for the assessment of the batteries as a whole or of their components active masses (electrodes), electrolytes, separators, and current collectors. The most important are, of course, electrodes, and their behavior can be analyzed using different electrochemical techniques. The most common techniques are... 

A similar effect can be expected for the carbon conductive diluent used in cathode formulations for all commercial cells. In all cases, it is necessary to take into account the particular cell design and the electrical resistivity of the electrode-active mass, perpendicular to the current collector, to optimize cell performance [8], One cannot standardize on any one type of carbon for all battery environments. Fortunately, since carbon is a versatile material, one can find a unique form for each application. 

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