The effect of the expander upon the two types of negative active mass structure in lead-acid batteries
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The active mass of the negative plate in the lead-acid battery is organized in a skeleton (primary) and energetic (secondary) structure. With the aid of electrochemical measurements and SEM observations the effect of the expander upon these structures is investigated both during their formation and during the cycling of the cell under service conditions.
It is established, that in the absence of an expander, an active mass with dendritic secondary structure is formed, which ensures a relatively low capacity of the plates. In active masses containing an expander no dendrites are formed. The expander enhances the formation of a secondary structure containing separate lead crystals deposited on the surface of the skeleton. The morphology and size of these crystals depend on the nature of the expander. Plates containing expander display a higher capacity and a longer life. The effect of the expander upon the primary structure is clearly revealed during the service life of the battery under deep charge-discharge cycling conditions. Plates with no expander show a contraction of the skeleton and the active mass becomes more compact. A reverse effect is observed in the presence of a highly efficient expander, the skeleton structure becomes more loose and the plates become thicker. This is evidence that the skeleton partially participates in the cycling process and that its branches are oxidized, being formed again later at a different site, thus changing the density of the skeleton structure. This process is affected by the expander.
It is also established that the skeleton structure of the active mass is built either from compact, shapeless, interconnected crystals, or linked agglomerates of scale-form lead crystals.
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