Operando Neutron Diffraction Analysis of Commercial Positive Electrodes of Lead Batteries

Abstract

We have analyzed by operando neutron diffraction the charge/discharge crystallographic transformation processes inside the positive active mass (PAM) of industrial lead batteries, getting information onthe spatial distribution of the phases. Our study was focused on the positive electrode because it is there where the main limitations to the charge/discharge efficiency are originated. The experiments were carried out in the VULCAN instrument at the Spallation Neutron Source of the Oak Ridge National Laboratory (Tennessee, USA) using the volume gauge technique. VULCAN is a high brightness time-of-flight diffractometer which provides fast volumetric mapping and, therefore, the possibility to study kinetic behaviors. Accordingly, both static and dynamic mapping experiments were performed during charge-discharge cycles, comparing fresh and cycled cells. The diffractograms obtained were analyzed using VDRIVE and GSAS software, together with dedicated Python scripts. In addition to compositional maps (Figure 1), information on lattice parameters, crystallinity, stoichiometry and hydration was also obtained as a function of the state of charge. Local inhomogeneities and differences in the behavior between fresh and cycled cells were observed. Moreover, we have used the Pb signal from the positive grid as internal calibrating element to obtain the absolute molar concentration of the component phases, comparing the PbSO4 PbO2 transformation with the effective charge driven by the cell during charge and discharge processes. The b-PbO2 mass utilization during the C5 100% to 20% discharge was about 0.3. In the same way, the charge factor was estimated to about 70%, from the start of charging until the PbO2 concentration reaches a near flat regime, and about 5% after reaching the flat regime. Giving for the full charging cycle a value about 50%.As far as we know, this is the first time that diffraction information has been obtained from inside commercial thick electrodes (3.4 mm in thickness) during real operation conditions, mapping the PbO2 and PbSO4 evolution in a 2.24 cm x 7.2 cm region of the positive electrode. Figure 1