The PbO 2 agglomerate-of-spheres: investigation of four grid materials

During the past decade, the agglomerate-of-spheres (AOS) model has been developed to describe the behaviour of the PbO ₂ electrode during cycling. In the model, the formation of the AOS is described as an electro-metasomatic process that creates the so-called 'electroformative' force (EFF). This force causes an enlargement of the entire volume of the formed PbO ₂. PbO ₂ electrodes are investigated in a special electrochemical cell that is integrated into a tension testing machine. With this arrangement, the positive electrode can be cycled with simultaneous measurement of the EFF and the solid-state resistance (SSR) between the grid and the active material during formation, charge and discharge. From a technological point of view, it is of interest to investigate the influence of the grid alloy on the AOS parameters of the PbO ₂ electrode. Investigations have been performed on pure lead and Pb-Sb, Pb-Ca-Sn and Pb-Ca-Sn-Ag alloys. A very small amount of additive can greatly influence the mechanical, corrosive and electrochemical properties. The corrosion behaviour depends mainly on the properties of the corrosion layer (i.e., on its mechanical, electronic, and electrochemical characteristics) rather than on the properties of the alloy itself. The components of the grid alloys appear to influence the EFF, the highest values are observed for the Pb-Sb alloy. This alloy exhibits the smallest corrosion region of the tested materials. The rupture plane after 15 cycles for all four materials is located in the active material close to the 'grid'. The tensile strength differs markedly between the different materials. A very high tensile strength is displayed by the Pb-Ca-Sn-Ag alloy. This demonstrates the influence of silver on the behaviour of the active material, although the tin content is about 35% larger, too. Theoretical consideration is given to the formation of the corrosion layer. It is concluded that no significant layer of PbO exists between grid metal and the active material.