Negative 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. 

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

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. 

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. 

Fig. 3.42. Changes in phase compositions of paste and active mass of negative plates during formation [55].

The secondary lead structure of the completely formed plates with or without expander is shown in Fig. 3.57. In NAM without expander, the secondary lead structure covers the skeleton in the form of a smooth layer, whereas the secondary lead structure in the NAM of expander-containing plates comprises individual Pb crystals which are located over the skeleton structure. Hence, organic expanders regulate the processes involved in the formation of both types of structure in the lead active-mass during the formation of negative plates. 

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. 

An example of inhibitor treatment of negative plates to prevent oxidation of the sponge lead in the active mass after formation is as follows the washed plates are immersed in a 10—12 wt% solution of boric acid with addition of 1.0—1.5 wt% salicylic acid. The plates are left in this solution for about an hour and are then dried in a drying oven at a temperature of about 130 °C. 

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