Publication date: 21st July 2025
In an operating battery, injected electrons hybridize with the electrode lattice to form lattice-charge coupled species known as polarons [1]. The mobility of these polarons is critical to the overall conductivity of the electrode, often limiting material performance. While conventional 4-point probe measurements provide equilibrium conductivities, they fail to capture the intrinsically out-of-equilibrium dynamics during battery operation. To unravel these ultrafast, nanoscale processes, techniques capable of resolving polaron dynamics on sub-100 ps timescales and sub-10 nm spatial scales are required.
Here, we address this challenge using quantitative transient reflection microscopy (TRM) with picosecond temporal resolution and wide-field optical access [2,3]. We apply this technique to pulsed laser deposited thin films of LiₓMn₂O₄ (LMO) (100 facet), across a range of lithium contents (~0.2 < x < ~0.9). By capturing pump-probe images at various time delays, we visualize polaron transport across multiple lithiation states.
Our imaging reveals spatially resolved transient reflectivity changes, from which we can extract carrier diffusion profiles and model mean squared displacement (MSD) dynamics. This is enabled by an optical model accounting for transient changes in the dielectric function in the reflection geometry.
We establish links between the lithiation state and the transport dynamics, and establish transport timescales. Notably, we observe differences between the early (< 1 ns) and late-time (> 10 ns) behaviour, suggesting a dynamic evolution of the transport regime. We rationalise our results using electronic structure calculations.
Altogether, our results demonstrate the potential of ultrafast spectroscopy to probe nonequilibrium charge dynamics in battery materials, offering new pathways.
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