Equilibrium lithium transport between nanocrystalline phases in intercalated TiO2 anatase

Microcrystalline TiO2 with an anatase crystal structure is used as an anode material for lithium rechargeable batteries(1,2), and also as a material for electrochromic(3-6) and solar-cell devices(7,8). 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 nanometres(9). 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 1.2x10(20) s(-1) m(-2).