Investigation Of Plates Soaking Process In The Formation Of Valve-regulated Lead-acid Batteries

The manufacture of lead-acid batteries includes a series of complex processes,in which the formation is one of the most important steps.The mechanism of the plates during soaking will directly affect the storage property and the cycle life of the batteries.This work is to study the soaking process and its effects on the lead dendrite growth in the separator,which will provide the basis for the later work such as choosing AGM separator and optimizing the content of sodium sulfate as additive in the electrolyte,to prevent the formation of lead dendrites.The conclusions are drawn as following:This work is to study the soaking process of 12V 12Ah VRLA batteries.XRD,SEM,AAS were used to analyze the compositions of the plates and the changes of the structures.The results indicate that:(1)At the initial stage of soaking,the interior temperature of the battery instantly rises up to over 60 ? because the rate of chemical reaction between H2SO4 and PbO is very fast and then the temperature drops quickly.The compositions at the positive and negative plates change quickly within the initial 30 min and then vary slowly.(2)The dissolution of PbSO4 in the AGM separators depends on its particle size on the plates,electrolyte temperature,H2SO4 concentration and common ion concentration such as Na2SO4.The cured plates,especially negative plate,have smaller PbSO4 particle size compared with commercial AR PbSO4.So the PbSO4 at cured plates has higher solubility.But the temperature is a dominant factor.(3)Since the H2SO4 diffusion into the interior of plate group is limited by its high assemble pressure,great changes take place in the H2SO4 concentration gradient in the plate group and even the middle of the separators can become neutral,which will make the Pb2+ ion concentration become high.The addition of Na2SO4 as common ion restrains the PbSO4 dissolution obviously.However,the concentration of Pb2+ ions is the lowest in the concentration range of 0.25-0.5%Na2S04.The simulated cells and failure batteries were used to study the formation of lead dendrites in the separators.The results indicate that:(1)The formation of lead dendrites depends on the battery temperature,Pb2+ ion concentration,AGM separator thickness and coarse fibers concent.The P2b ion concentration in the separators near the negative plate is higher than that near the positive plate because the former has lower curing temperature in the manufacture and so the particles are finer.(2)The Pb2+ ions react with SO42-ions to form PbSO4 which tends to deposit on the coarse glass fibers of the separators.And the fine PbSO4 crystals are continually transformed into large PbSO4 crystals via dissolution-deposition processes and then develop along the fibers.In the subsequent charge,these PbSO4 crystals near the negative plates can be reduced to the club-shaped lead dendrites which will easy to lead to short circuit.Therefore,the use of more fine glass fibers in separators can effectively inhibit the lead dendrite growth on the fibers and can reduce the pore diameter in the separators.The longer soaking time,the thinner separators and/or the defect in the manufacture like overpressing will raise the risk of short circuit of the battery.The investigations of hydrogen evolution reaction at Pb electrode in H2SO4 electrolyte by means of CV and EIS methods indicate that the addition of 0.25%-0.5%Na2SO4 increases polarization currents of hydrogen evolution,which will promote the hydrogen reaction at Pb electrode.At the same time,it makes the anode peak current at Pb electrode become low,which is due to fact that the formation of a relatively dense PbSO4 film on the Pb electrode surface.The Na2SO4 additive increases the dense impedance films at Pb electrode on low frequency,which will make polarization resistance become high.It can also increase the PbO2 peak current and thus promote the formation of PbO2.