(Invited) Mapping Electrode and Electrolyte Heterogeneity with High Energy Synchrotron Diffraction

Lead acid batteries are often portrayed as a mature technology with limited capacity and room for innovation. In reality, lead batteries have significant room for growth since they only utilize a fraction of their active material and have been engineered for SLI applications that lack a battery management system for controlled charge and discharge. In large part these issues are related to chemical heterogeneity that exists at both the pack level and within the battery electrode microstructure. Unlike a lithium ion battery, lead acid electrodes are typically larger laterally and millimeters thick with significant porosity to allow acid penetration. At the particle level, the growth and dissolution of PbSO4 during discharge from the Pb and PbO2 charge species also leads to significant volume change and swings in local acid concentration within the electrode pores. Recently, we have begun analyzing the spatial variation of charge acceptance in lead acid cells using high energy x-ray diffraction. At energies approaching the lead K-edge (88 keV), x-rays can penetrate through several millimeters of active material. This allows chemical mapping across 2 V cells during formation and cycling and even depth profiling on miniaturized batteries. In situ diffraction provides valuable new information on microstructural changes in phase and crystallite size during these precipitation and growth reactions. Surprisingly, scatter from noncrystalline phases – i.e. the electrolyte – can also be measured by analyzing changes in the broad background underlying the strong Bragg peaks. While the average state of charge (SOC) computed from diffraction species closely matches the electrochemistry, diffraction mapping shows that the charge state and the degree of charge acceptance can vary substantially throughout each electrode. In general, we find strong gradients in SOC and electrolyte concentration that grow from the lead current collector during formation and initial cycling. In larger cells, we also find vertical heterogeneity that develops over repeated cycling. Combined with ongoing electrochemical modeling and innovations in electrode architectures, these results are helping improve utilization and cyclability, developing lead acid into a stronger contender for emerging stationary storage applications.