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Unraveling Ultrafast Charge Transport in Battery Electrodes Using X-ray Free Electron Lasers
Understanding ultrafast charge transport mechanisms in battery electrodes is essential for optimizing energy storage performance. A critical challenge in battery research is the lack of experimental techniques capable of capturing the simultaneous electronic and structural dynamics that govern charge and ion transport on ultrafast timescales and atomic length scales. Conventional methods, such as electrochemical impedance spectroscopy, provide valuable but limited insight, as they cannot resolve dynamics on ultrafast timescales, such as polaron formation during electron transport or the resulting structural reorganization which occurs during ion migration. The ultrashort X-ray pulses available from X-ray Free Electron Lasers (XFELs) offer a unique opportunity to overcome this limitation, providing the potential for element-specific, ultrafast, and atomically resolved insights into charge transport in battery materials [1].
Recent advancements in optical pump-probe spectroscopy have demonstrated that polaron formation in LiMn₂O₄ (LMO) after optical excitation occurs on timescales ranging from 500 fs to 2 ps. This process likely involves a complex cascade of charge transfer events within the Mn 3d orbitals. Optical measurements, however, lack the ability to probe structural distortions, orbital occupation, or spin-state transitions with element specificity. As polaronic-hopping is the most likely charge-transport mechanism in LMO cathodes, resolving these ultrafast charge transport mechanisms is crucial for bridging the gap between theoretical models and real-world battery performance.
To address this, transient X-ray absorption (trXAS) and X-ray emission spectroscopy (trXES) would allow to track the coupled electronic-structural evolution of polarons in LMO with sub-100 fs resolution. Preliminary Mn K-edge trXAS at the FXE Instrument of the European XFEL has confirmed a multi-step charge transfer process occurring in the Mn crystal field, revealing transitions between the t₂g and eg orbitals and evidence of multiple polaron relaxation pathways.
To further refine our understanding, complementary X-ray scattering experiments could capture the predicted Jahn-Teller distortions accompanying polaron formation, which involve asymmetric Mn–O bond elongations of ~0.2 Å [4].
XFELs have the potential to play a transformative role in battery research by performing ultrafast X-ray spectroscopy and scattering measurements. These measurements will allow us to resolve the fundamental polaron transport mechanisms that dictate charge mobility and efficiency in battery electrodes, providing critical insights for the design of next-generation energy storage materials.
References
[1] Li, B., Sougrati, M. T., Rousse, G., Morozov, A. V., Dedryvère, R., Iadecola, A., ... & Tarascon, J. M. (2021). Correlating ligand-to-metal charge transfer with voltage hysteresis in a Li-rich rock-salt compound exhibiting anionic redox. Nature Chemistry, 13(11), 1070-1080.
[2] Goodenough, J. B. (1971). Metallic oxides. Progress in solid state chemistry, 5, 145-399.
[3] Nishizawa, M., & Yamamura, S. (1998). Irreversible conductivity change of Li1–xCoO2 on electrochemical lithium insertion/extraction, desirable for battery applications. Chemical Communications, (16), 1631-1632.
[4] Ouyang, C., Du, Y., Shi, S., & Lei, M. (2009). Small polaron migration in LixMn2O4: From first principles calculations. Physics Letters A, 373(31), 2796-2799.
