Paste structure and its influence on the agglomerate-of-spheres parameters of the PbO_2 electrode

摘要

During the past decade, the agglomerate-of-spheres (AOS) model has been developed to describe the behaviour of the PbO_2 electrode during cycling. Recently, the creation of the AOS has been described as an electrometasomatic process. In this process, a single ''sphere'' at the end of an electronic path is a result of the action of the surface tension and its tendency to construct a sphere. This process can continue as long as the surface tension is sufficient to act against the internal friction. The so-called ''electroformative force'' leads to an extension of the volume of the positive paste during the formation process. From a technological point of view, it is of interest to investigate the influence of the paste structure on the AOS parameters of the PbO_2 electrode. For this purpose, measurements are performed on PbO_2 produced from 3PbO • PbSO_4 • H_2O (3BS) and 4PbO·PbSO_4 (4BS) pastes. During formation of PbO_2, the electroformative forces cause an expansion of the electrode volume of the 3BS and 4BS pastes. Due to the ''pommes frites''-like structure of the 4BS mass, the formation process is different with respect to the velocity of building of the electronic conducting bridges within the electrode body. The formation process converts PbSO_4 on the outer surface of the ''pommes frites'' to PbO_2, and leaves the internal region unaffected at first. The formation process bridges the cross points of the ''pommes frites''. Thus, an electronic and mechanic network is constructed. This leads to an electroformative force that is 3—4 times larger for 4BS than for 3BS paste. The apparent specific resistance of PbO_2 is twice as large for 4BS than for 3BS precursor paste. At the beginning of cycling, the ''unusual'' increase of the force during discharge of the 4BS material shows that the reaction product, PbSO_4, is deposited within the PbO_2 electrode under formation of a mechanical stress in the AOS network. The cause for this is a difference in pore-size distribution between the ''4BS'' and ''3BS'' PbO_2. On average, the diffusion path for the lead ions is longer in the ''4BS'' material. The simultaneous measurement of polarization resistance and electronic-path resistance reveals the current distribution within the electrode during charge and discharge. It is concluded that during discharge and charge, the current distribution is almost homogeneous throughout the electrode, except at the end of discharge and the beginning of recharge when the current is constricted to the region near to the grid. The capacity of the constriction range is about 10% of the total capacity.