Equilibrium lithium transport between nanocrystalline phases in intercalated TiO2 anatase

Abstract

Microcrystalline TiO2 with an anatase crystal structure is used as an anode material for lithium rechargeable batteries1,2, and also as a material for electrochromic3,4,5,6 and solar-cell devices7,8. When intercalated with lithium, as required for battery applications, TiO2 anatase undergoes spontaneous phase separation into lithium-poor (Li0.01TiO2) and lithium-rich (Li0.6TiO2) domains on a scale of several tens of nanometres9. During discharge, batteries need to maintain a constant electrical potential between their electrodes over a range of lithium concentrations. The two-phase equilibrium system in the electrodes provides such a plateau in potential, as only the relative phase fractions vary on charging (or discharging) of the lithium. Just as the equilibrium between a liquid and a vapour is maintained by a continuous exchange of particles between the two phases, a similar exchange is required to maintain equilibrium in the solid state. But the time and length scales over which this exchange takes place are unclear. Here we report the direct observation by solid-state nuclear magnetic resonance of the continuous lithium-ion exchange between the intermixed crystallographic phases of lithium-intercalated TiO2. We find that, at room temperature, the continuous flux of lithium ions across the phase boundaries is as high as 1.2 × 1020 s-1 m-2.

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Figure 1: Central part of the 7Li magic-angle-spinning NMR spectrum of Li0.12TiO2 at 100 °C, showing the resonances of Li in the two coexisting phases.
Figure 2: Exchange of Li between the two coexisting phases in Li0.12TiO2 measured with 7Li two-dimensional exchange NMR.
Figure 3: Quantifying the diffusion process.

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Acknowledgements

This work is a contribution from the Delft Institute for Sustainable Energy (DISE).

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Correspondence to F. M. Mulder.

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Wagemaker, M., Kentgens, A. & Mulder, F. Equilibrium lithium transport between nanocrystalline phases in intercalated TiO2 anatase. Nature 418, 397–399 (2002). https://doi.org/10.1038/nature00901

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