Preparation And Application Of Pitch-based Carbon Foam Current Collectors For Lead Acid Batteries

Lead acid batteries are still the most prevalent rechargeable batteries in today's application markets.However,compared to other advanced rechargeable batteries,lead acid batteries have some drawbacks such as lower specific energy etc.To overcome these disadvantages,carbon foams attract much attention as substitutes for conventional lead-alloy grids in lead acid batteries.But due to the structure and production costs of carbon foams prepared by foaming method,these materials still face many difficulties in commercial application as current collectors in lead acid batteries.The aim of this paper is to develop a new method for preparation of carbon foams suitable for use as current collectors in lead acid batteries,and investigate the effects of the carbon foams as current collectors on properties of lead acid batteries.A water-based pitch powder slurry was prepared using ammonium lignosulfonate as the dispersant,and influence of the dispersant on the stability and fluidity of the slurry was investigated.The results show that pitch particles have good adsorbability for ammonium lignosulfonate.The Zata potential of pitch powder suspension varies obviously when 1wt%of ammonium lignosulfonate is added to the suspension,and the lowest sendimental volume can be observed in the suspension at pH=8.Slurries with 1.2%of dispersant and less than 60%of solid content at pH=8 exhibite the lowest viscosity and best fluidity.A new method for preparation of carbon foams using porous organic templates to form green pitch foam bodies was proposed.Pitch powder slurries were prepared using ammonium lignosulfonate as the dispersant,polyglycol as the binder and sodium carboxy methyl cellulose as the thickener.It was found that suface modification of polyurethane foam templates with a surfactant RPC can improve the loading of pitch powder slurry on the templates.The effects of the solid content,dosage of thickener and compressive strain of templates on the structure of green porous pitch bodies were evaluated.The results show that the obtained pitch bodies have 3D open-pore structure and suitable lignament thickness when the solid content,dosage of thickener and compressive strain are 55%,1.5%and 75%,respectively.The bodies can convert to carbon foams by preheating at 350℃in nitrogen for 30 minutes and then carbonizing at 1000℃for 3 hours.Cyclic voitamethy were performed on the pitch-based carbon foams.The results show that the carbon foam is electrochemically stable in the voltage range from -1.2 V to 0 V,while oxygen evolution at the foam occurs at ca.0.8 V which is below the voltage related to the oxidation of PbSO₄ to PbO₂(＞1.4V).As a result,it is proposed that the carbon foam is suitable for use as the negative current collectors but can not be directly used as the positive current collectors in lead acid batteries.The effect of carbon foam negative current collectors on the properties of lead acid batteries was also investigated.It was found that using pitch-based carbon foams as negative current collectors can improve not only the charge acceptance and negative mass utilization efficiencies but also partial-state-of-charge(PSoC) cyclibilities of lead acid batteries.Comparative charge-discharge tests show that the utilization efficiencies of the active material on the carbon foam negative current collector are 15%and 27% higher than those of the material on a lead grid at 1/20 C and 1/10 C,respectively. Through a PSoC window of 70%～40%,lead acid batteries with carbon foam negative current collectors can experience 50 cycles,and the end-of-discharge voltage is still above 2 V at the end of 50^th cycle,while the batteries with lead grids as the negative current collectors can only experience 30 cycles.Calculation shows that carbon foam current collectors have lowerαandγparameters when compared to conventional lead grids.Theγparameter of carbon foam collectors is 1/40～1/100 of conventional lead grids.The effect of carbon foams as negative current collectors on the impedance characteristics of lead acid batteries was evaluated.A mathematic model and an equivalent circuit model were proposed to describe the faradic and total impedance of lead acid battery negative electrodes,respectively.The obtained impedance spectrascopes and fitting results show that whether at fully charged or discharged state,both electrochemical reaction and diffusion steps play roles in determining the polarization of lead acid battery negative electrodes.The diffusion step plays a much more important role in determining the polarization of the electrodes at fully charged state,while the electrochemical reaction step is much more important at fully discharged state.At both states,carbon foam electrodes have much higher exchange current densities and double-layer capacities than lead grid ones.To use these carbon foams as positive current collectors in lead acid batteries,initial surface modification of the foam material was carried out.The CV and SEM test results show that electroplated polyaniline(PANI) is not compact,and thus it is difficult to increase the oxygen evolution potential at the carbon foams by electroplating a single layer of PANI on the surfaces of carbon foams.On the other hand,electroplating a lead layer on the lignment surfaces of carbon foams can increase the oxygen evolution potential effectively.Charge-discharge test results show that lead acid batteries using carbon foams electroplated with lead as the positive current collectors can improve the utilization efficiencies of positive active materials.The utilization efficiencies of the active material on lead coated carbon foam positive current collectors are 24.8%and 40%higher than those on lead grids at 1/25 C and 1/10 C,respectively.Calculation shows that Peuker coefficient for batteries with lead coated carbon foams as the positive current collectors is 1.18,which is much lower than 1.33 for today's commercial lead acid batteries.