Spatially Resolved Imaging of Inhomogeneous Charge Transfer Behavior in Polymorphous Molybdenum Oxide. II. Correlation of Localized Coloration/Insertion Properties Using Spectroelectrochemical Microscopy

Abstract

A newly developed spectroelectrochemical imaging approach for directly assessing lithium ion insertion energetics and kinetics in mixed-phase, polymorphous MoO3 is reported. Two variants of spectroelectrochemical microscopy were used to monitor insertion dynamics and to follow electrochemically induced phase transformations at specifically identified structural and compositional domains. Cyclovoltoabsorptometric (dOD/dE) measurements carried out in LiClO4/propylene carbonate solutions reveal that the lithium insertion is nonuniform and can be directly correlated with phase-segregated domains comprising α-MoO3, β-MoO3, and intermixed α-/β-MoO3. Lithium insertion is found to proceed by a staging process where each phase displays energetically distinct insertion behaviors. Chronoabsorptometric imaging measurements allow for the simultaneous estimation of lithium diffusion coefficients, ionic conductivities, and lithium capacities at isolated phases within the polymorphous material. The lithium diffusion coefficient and ionic conductivity is largest for domains comprising intermixed α-/β-MoO3, whereas it is smallest at domains consisting of β-MoO3. The higher diffusion coefficient observed for intermixed α-/β-MoO3 domains is most likely due to larger thermodynamic enhancement factors for the mixed phase domains than for domains consisting of either α-MoO3 or β-MoO3. Estimation of capacity values within each uniquely identified domain reveals that the lithium insertion capacity is about 4 times greater in α-MoO3 than in β-MoO3. The discrepancies between the lithium insertion capacities can be rationalized in terms of lattice oxygen defects, which effectively reduce the number of available lithium insertion sites in β-MoO3 as compared to α-MoO3