In-plane
tungsten oxide nanostructures, including hexagonally patterned cylinders and holes in a matrix, were fabricated via sequential infiltration synthesis (SIS) on self-assembled block copolymer templates. Using the tailored morphology and porosity of these model
electrodes with in situ grazing incidence small-angle X-ray scattering, the intrinsic structural change of nanoscale active materials during the
conversion reaction of WO3 + 6Li ↔ W + 3Li2O was investigated at controlled electrochemical conditions. Reversible
electrode volume expansion and contraction was observed during lithiation and delithiation cycles, respectively. The potential where the
electrode's thickness expansion started was ∼1.6 V, which is close to the thermodynamically expected one for the
conversion reaction of WO3 with
lithium (1.65 V). The temporal evolution of the
electrode volume at constant
electrode potentials revealed high overpotential for bulk lithiation and slow
conversion reaction kinetics, despite the tailored porosity of the SIS
electrodes.
Oxide cylinders showed a smaller overall
electrode thickness change, likely due to unconstrained lateral volume change, as compared to a matrix with holes. On the other hand, better connectivity and guided volume change of the latter
electrode morphology provided improved cycling stability. In addition, heterogeneity in an
electrode, from internal pores and density gradients, was found to aggravate the fragmentation of the
electrode during the
conversion reaction. Insights into
oxide conversion reaction kinetics and the relationship between
electrode mesostructure and cycling behavior obtained from this study can help guide the more rational design of conversion
electrodes for high-performing batteries.