Effect of expanders on NAM

The NAM contains some additives, i.e., 0.2 2.0wt.% BaS04 (most often 0.8wt.%), 

The dependence of plate potential on current density during the first and the second formation stages is presented in Fig. 3.55 [66]. The processes of PbO and 3BS reduction during the first stage of formation proceed at a lower potential in the plates without expander. The same tendency is observed with the reduction of PbS04 to Pb. This indicates that the expander impedes all the electrochemical reactions involved in the formation process. 

The morphology of the Pb crystals that build up the NAM skeleton for plates with or without expander is presented in Fig. 3.56. A more pronounced roughness of the skeleton surface is observed in expander-containing plates. Different expanders yield different NAM skeletons, which may serve as a memory of the technology of plate production [66]. 

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. 

In VRLA batteries, oxygen reaches the negative plates and is reduced to form water. It also oxidizes the expander and thereby produces carbon dioxide. Consequently, the morphology of the secondary Pb crystals is changed and they become dendrite-like, similar to those shown in Fig. 3.57(a). This results in capacity 

---