Positive active mass conductivity

One of possible ways of improving the specific characteristics of LIB electrodes is the use of thermally expanded graphites (TEG) as electrically conductive additives of positive active mass. The efficiency of TEG s use may be illustrated by analyzing a simple theoretical model proposed by us AM - electrically conductive additive (Figure 4). 

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]. 

Positive plates need much more time to form than negatives. The reason for this is the dielectric behaviour of the cured positive paste. Oxidation of the bivalent lead compounds in the paste and formation of the Pb02 positive active mass passes through a number of chemical reactions, some of which proceed at a low rate, which retards the technological process of formation of the positive plate. In an attempt to accelerate the formation process, additives to the positive paste have been looked for, which are characterised by electro-conductive properties and stability in sulfuric acid. These additives create an electro-conductive network in the paste and the process of oxidation proceeds simultaneously within a large paste volume, thus accelerating plate formation. 

Table 7.4 Materials proposed to enhance the conductivity of the positive active mass [52].

Positive and negative active-mass formation. The cured pastes of both positive and negative plates comprise identical mixtures of bivalent lead compounds (3BS, 4BS, PbO), which cannot create electromotive forces when the pasted plates are assembled into cells. The purpose of the formation step is to convert the cured pastes into electrochemically active porous materials — Pb02 in the positive plates and Pb in the negative plates — which are connected mechanically and electrically to the grids. The process of formation can be conducted via two basic schemes, as shown in Fig. 3.1. 

The delivery of too much overcharge can also result in damage to the positive active-material [24]. It has been suggested that the vigorous evolution and movement of gas as a result of this process can disrupt the internal structure of the active material. The related decrease in performance has been explained in terms of a loss of electrical conductivity within the positive plate due to progressive expansion of the active mass. 

The active substance of the positive electrode is the polymer of fluorinated carbon with the overall formula of (CFj ) . As a rule, subscript x in this formula is close to unity, polymerization degree n exceeds 1(X)0. The polymer of fluorinated carbon is a layered compound obtained by fluorination of carbon (graphitized or nongraphitized) in the form of a powder, fibers, or even fabrics by elementary fluorine at the temperatures of 350 - 600°C. As polyfluorocarbon is characterized by negligible electron conductivity, a certain amount of a conductive additive (carbon black) is introduced into the active mass of cathodes. Elementary carbon is formed in the course of discharge and the overall conductivity of the cathode increases. 

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