Electrochemical evaluation of Pb – Ca – Sn - Sr alloy for positive grid of lead-acid battery

The electrochemical behavior of positive grid alloy is a critical issueforlead-acid battery. In order to develop a favorable grid alloy, considerable efforts have been made in the way of adding various alloying elements.Due to high water consumption of batteries with antimony in grid alloys, positive grids containing Pb-Ca-Sn alloywere introduced as the first choice for lead acid batteriesdue to their effectson the mechanical and electrochemical properties of the lead.However, the grids containing calcium which enhance the age harden ability of grid suffers from early losing the capacity during cycling, called premature capacity loss (PCL). This phenomenon was found to be due to formation of a layer composed of PbO, PbOn (1 n 2) and PbO2 at the grid/positive active mass interface. As known, the PbO layer has high ohmic resistance. If the rate of PbO formation is higher than the rate of its oxidation to PbO2, a thick layer of PbO with high ohmic resistance formed on the grid surface which leads to high polarization of the plate on discharge and eventually to capacity loss.Therefore, the Sn with value≥1.2% wt is introduced to accelerate the oxidation of PbO to oxides of higher valency and higher electrical conductivity.In case of new application of lead acid batteries called EFB, thin full frames grids with high corrosion resistance is 13th Annual Electrochemistry Seminar of Iran Materials and Energy Research Center (MERC), 22- 23 Nov, 2017 224 neededthereforecalcium should be kept in a low rang to suppress the PCL effect and increase the corrosion resistance of thin grid by increasing sn/ca ratio in positive alloy. however, the lowered Cacontent will dropping the grid hardness and grids casting ability. Thus a forth element must be employed to supplied. The aim of this presentation is to investigate the electrochemical and mechanical behavior of a Pb-Ca-Sn-Sralloyfor positive grid of lead acid battery. To run the electrochemical experiments, the conventional three-electrode cell was used in which the saturated calomel electrode (SCE) was as reference electrode and platinum foil served as a counter electrode. All the electrochemical measurements were implemented using Gill AC potentiostat (ACM instruments). The electrochemical cell was a 250 ml beaker. All the electrochemical experiments were carried out at 25 ± 1°C in 4.8 M H2SO4 solution and after each test the solution was renewed. the cyclic voltammetry test were runed from +1800 mV/SCE to +2800 mV/SCE with scan rate of 1500 mV/min for 50 cycles, Additionally, battery tests were performed on the sample with the best mechanical and electrochemical properties. The idea of reducing the calcium content of positive plates grid alloy to reduce the PCL effect without losing hardness and castability properties was accomplished with introducing strontium as a new additive to the grid alloy. The results indicated that the effect of adding Sron electrochemical properties is more profound than only reducing calcium content. To sum up, the potentiostatictest results reveal more than four times lower passive current density for the first three samples, comparing to the standard sample. Indeed, cyclic voltammetry test results show about 40% lower PbO formation peak and about 67%reduced current density at 2400 mV/SCE for sample 1 and 2, which implies lower water consumption comparing to sample 5. Due to hardness test results, sample 2 was chosen to make 66 Ah batteries in Nirugostaran battery manufacturing company.The grid casting (gravity casting technology), paste making, 13th Annual Electrochemistry Seminar of Iran Materials and Energy Research Center (MERC), 22- 23 Nov, 2017 225 pasting, curing, assembly and formation processes were done in the same condition and time for sample 2 and standard sample (5) and thenthe manufactured batteries were tested according to B255210-D standard.The result shows about 10% improvement in cold cranking ability, 53% increased charge acceptance, 39% improved cycle life and 50% lower capacity loss after cycle life test which is in line with previously performed electrochemical tests.