Lead-acid batteries (LABs) find their wide-scale applications in automobiles, especially for start-stop and regenerative braking systems, where LABs mainly undergo partial state of charge (PSoC) operation with rapid charge and discharge cycling. The PSoC operation mode would easily lead to sulfation, where low solubility PbSO₄ crystals progressively replace battery active materials and impede conductive network pathway resulting in limited cycle life. One of the main strategies to suppress sulfation is through adding different forms of carbon additives into the negative active materials (NAM).

With the aim of suppressing sulfation, the use of graphene (Gr) was first validated by performing PSoC cycle test with various weight percentages of Gr in lead-graphene (PbG) electrode plates. T...[ Read more ]

Lead-acid batteries (LABs) find their wide-scale applications in automobiles, especially for start-stop and regenerative braking systems, where LABs mainly undergo partial state of charge (PSoC) operation with rapid charge and discharge cycling. The PSoC operation mode would easily lead to sulfation, where low solubility PbSO₄ crystals progressively replace battery active materials and impede conductive network pathway resulting in limited cycle life. One of the main strategies to suppress sulfation is through adding different forms of carbon additives into the negative active materials (NAM).

With the aim of suppressing sulfation, the use of graphene (Gr) was first validated by performing PSoC cycle test with various weight percentages of Gr in lead-graphene (PbG) electrode plates. The optimized Gr amount was identified at 0.2 wt% with a PSoC cycle life enhancement by more than 140% in comparison to the Pb plate.

In order to understand the sulfation control mechanism, the surface coverage evolution of PbSO₄ in negative plate during a discharge process was evaluated by measuring the double layer (DL) capacitance values that can be extracted from the electrochemical impedance spectroscopy (EIS) test. An electrochemical model, which takes into account of reduced interfacial resistance, improved charge transfer and enhanced electroactive surface area, was proposed to elucidate the role of Gr throughout the course of a PSoC cycle test. The 2D Gr additive served as a preferred charge transfer pathway with enhanced surface contact with Pb resulting in reduced interfacial resistance, which aligned with our proposed electrochemical model. A relationship between the adsorption rate constant for diffusion-controlled adsorption model and discharge current density was developed to predict the surface coverage evolution rate at different current densities. The modified porous electrode theory model with an accurate estimation of electroactive surface area change was validated with the experimental results and could be used to simulate battery performance.

With the purpose of identifying the Gr additive mechanism, interfacial study of the electrode/electrolyte interface via scanning electrochemical microscopy (SECM) was conducted to dynamically demonstrate the benefit of Gr in retarding the electroactive surface area reduction during discharge, which leads to longer PSoC cycling performance. Gr acts as the preferred charge transfer pathway with a slower degradation rate than active materials and is able to maintain a higher surface activity throughout a discharge process. It was also demonstrated that adsorption and migration of charges are enhanced on PbG electrode surface that is beneficial to the electrochemical reaction. Three mechanisms of Gr additive in improving cycle life of LABs are proposed with Gr additive enhancing the reaction kinetic through reduction of the charge transfer resistance as the dominant mechanism. Consequently, the formation of smaller PbSO₄ crystals on electrode surface helps to maintain stable electroactive surface area change throughout a discharge process. With the combination of the first two mechanisms, Gr additive enhances the stable electroactive surface area change and the adsorption of electrolyte on electrode surface, which aids in maintaining high sticking probability (S) and electrolyte flux (J). These beneficial effects would lead to prolonged PSoC cycle life.