Positive active mass formation

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. 

Experimental values for the density of Pb02 are given in Table 2.6 [26], It can be seen that the density of a-PbOa is somewhat higher than that of p-Pb02- The data in the table evidence also that Pb02 in the active mass has a substantially lower density than that of the crystal deposit formed by electrochemical oxidation in the lead-ion-containing solutions. Micka et al. proposed the presence of amorphous PbCh in the active mass [66]. The formation of a third amorphous modification of Pb02 in the positive active mass of lead—acid batteries has been established by many researchers [67—70]. 

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. 

With the first curing method water wfll evaporate from the paste most quickly and the curing process will stop at a very early stage. The degree of active mass formation and the capacity of the positive plates will be low (Table 8.3). In the second method of curing, with increase of temperature to 55 °C and RH = 80%, water evaporation from the plates wfll be slowed down... 

Employing scanning electron microscopy and porometry we have examined the strueture of the positive active mass after formation and during cycling of the plates and have proposed a model for this structure [22—25]. Figure 10.16 presents SEM pictures of PAM at high magnifications. 

This loop is, however, affected by the availability of the reactant oxygen, which in surplus destroys the precursor VPO. Further, oxygen is positively needed to activate and re-oxidize the VxOy sites but leads also to more water formation that in turn hydrothermally deactivates the active mass. Likewise, water is needed to separate, via hydrolysis, the vanadium phosphate into VxOy and mobile phosphate. The multiplicity of the feedback loops is at first puzzling but explains the apparent stable steady state that can be reached with a catalyst undergoing so many chemical and microstructural transformations the multiplicity of controls prevents one single factor becoming dominant and thus potentially destabilizing the whole process. 

FORMATION OF LEAD-ACID BATTERIES AND STRUCTURE OF POSITIVE AND NEGATIVE ACTIVE MASSES... 

In this Chapter, the discussion is confined to the processes that occur during soaking and formation of the positive and negative plates, as well as to the structures of the two types of active mass obtained as a result of the formation procedure. These steps are identified in the dashed-line frame in Fig. 3.1. 

Recipes for the positive and negative active masses are well known and are aimed at achieving a positive mass that is as porous as possible and a negative mass that can withstand the formation process, which can involve very high temperatures. The majority of separators are pure glass-mats, but present research is focusing on the use of synthetic fibre additives to make manufacturing simpler and cheaper. 

The manufacture of positive battery plates is essentially a process of oxidation of 3Pb0 PbS04 H20 or 4Pb0 PbS04 and PbO to a-PbOi and P-Pb02. This technological process is called formation. Now let us discuss the structure of the Pb02 active mass obtained as a result of formation. 

---