When the lead-antimony grids in lead/acid batteries were substituted by lead-calcium ones, battery cycle life was dramatically shortened. This phenomenon was called first 'antimony-free effect' and later 'premature capacity loss' (PCL), 'early capacity decline' or 'relaxable insufficient mass utilization' (RIMU). PCL is encouraged by the following conditions: (i) lack of certain alloying additives as antimony, tin in the positive grid; (ii) high utilization coefficient of the positive active mass; (iii) low active mass density; (iv) no stack pressure on the positive plates in the cells, and (v) no capacity limiting role of H2SO4 in the cells. Discharge of the positive active mass (PAM) proceeds through two successive reactions: PbO2 + 2H+ + 2e- --> Pb(OH)2-'double-injection process' (1) Pb(OH)2 + H2SO4 --> PbSO4 + 2H2O (2) For reaction (i) to proceed, equivalent amounts of H+ ions from the bulk of the electrolyte and electrons from the plate grid should reach PAM. It has been established that PCL is due to the impeded transport of electrons when passing from the grid through the corrosion layer (CL) and PAM. A survey of the concepts of various authors for the reasons causing PCL has been made. PAM and CL have been treated as solid-state systems. Two general concepts have been distinguished. The first one assumes that PCL is due to the changes in PAM during cycling which result from the following phenomena: (i) impaired contact between PbO2 crystals in the structure of PAM (Kugelhaufen model). (ii) changes in the intrinsic electrochemical activity of PAM (hydrogen-loss model), and (iii) changes in the alpha/beta-PbO2 ratio. The second concept assumes that the changes in the electrical properties of the corrosion layer are the reason for the PCL effect. The latter is due to: (i) formation of a PbSO4 barrier layer, and (ii) formation of a alpha-PbO semiconductor layer. The present paper suggests a new approach to the structure of PAM which seems to unite all above concepts. PAM and CL are viewed as gel/crystal systems. Their particles and agglomerates are composed of alpha- and beta-PbO2 crystal zones which are in equilibrium with amorphous gel zones. The latter are built up of hydrated linear polymer chains along which electrons move between the crystal zones. Crystal zones have high electron conductivity. The electron conductivity of PAM is determined by the conductivity of the gel zones. To asses the effect of various conditions on this conductivity a tubular PbO2 powder electrode was used. The surface of PAM particles are hydrated. Hence the electrode capacity will be determined by those particles whose hydrated surfaces are in electronic contact with one another and with the CL. The electrode capacity during the first and second cycles is a measure for the conductivity ot gel zones at the CL/PAM interface. After 15 cycles the structure of the electrode is restored and its capacity depends on the conductivity of gel zones in PAM. It has been established that the capacity of the tubular powder electrode depends on the density of PAM and on the additive to the grid alloy, to the solution and to PAM. A critical PAM density has been found to exist below which no restoration of the structure is possible. On plate cycling the volume of PAM pulsates, decreasing the density of PAM and of the CL. Gel zones play the role of 'hinges' in the skeleton of PAM making it elastic. When the active-mass density in a given region of PAM or in the outer CL sublayer falls below the critical value, this region is excluded from the current generation process and PCL may occur. It has been established that antimony lowers the critical value of the density and thus prolongs the life of the plate. Antimony ions get into the polymer chains improving the conductivity of the gel zones. The appearance of the PCL effect has been explained by the impact of dopants on the ratio gel/crystal zones and on the conductivity of gel zones in PAM and in the CL.
