PbO 2 active material as an electrocrystalline network

According to the 'agglomerate-of-spheres' (AOS) model the PbO ₂ electrode is considered as an electrocrystalline network of agglomerates of interconnected small particles with a common surface. Its electrical and mechanical properties are determined by the behaviour of the narrow contact zones ('necks'). Therefore, the apparent electronic conductivity of the PbO ₂ network is the effective sensor for the behaviour of the necks. In order to achieve a better understanding of the PbO ₂ electrode by direct measuring the physical characteristics, which are involved in the AOS model, we have developed a new type of experiment and the corresponding equipment. It consists of a special electrochemical cell integrated into a tension-testing machine. It enables us to cycle the positive electrode and to measure simultaneously the resistance between the grid and the active material, as well as the apparent conductivity of the electrocrystalline PbO ₂ network during formation, charge and discharge. The registration of the resistance and of the force (strength) during the formation process allows us to observe the creation of the AOS structure at the very first time, when the mechanic and electronic connections are formed, that is immediately after the current is switched on. Then a negative force is generated. We call this force 'electroformative'. It is associated with the expansion of the PbO ₂ electrode as a cause of the formation of the ball-like AOS structure. The registration of the same parameters during charge and discharge shows, that the increase of the solid-state resistance of the PbO ₂ limits the discharge capacity of the positive electrode. But during discharge one can observe a decrease of the resistance at the beginning; the discharge process proceeds at lower values, until the resistance increases rapidly at the end of discharge. This is in accordance with the AOS model. This states that the apparent conductance of the electrocrystalline network is governed by the constriction resistances of the neck regions and not so much by the volume of the spheres. But the necks do also determine the strength of the PbO ₂ network. Therefore, after cycling of the electrode, a rupture process is performed and the electronic resistance, the force (strength), and the dilatation are registered simultaneously again. The results enable us to obtain the tensile strength of the necks.