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We used in situ neutron powder diffraction(NPD) to study the migrationof Li in Li4Ti5O12anodes with differentparticle sizes during battery cycling. The motivation of this workwas to uncover the mechanism of the increased capacity of the batterymade with a smaller-particle-sized anode. In real time, we monitoredthe anode lattice parameter, Li distribution, and oxidation stateof the Ti atom, and these suggested an increase in the rate of Liincorporation into the anode rather than a change in the migrationpathway as a result of the particle size reduction. The lattice ofthese anodes during continuous lithiation undergoes expansion followedby a gradual contraction and then expansion again. The measured latticeparameter changes were reconciled with Li occupation at specific siteswithin the Li4Ti5O12crystal structure,where Li migrates from the 8ato 16csites. Despite these similar Li-diffusion pathways, in larger-particle-sizedLi4Ti5O12the population of Li atthe 16csite is accompanied by Li depopulation fromthe 8asite, which is in contrast to the smaller-particle-sizedanode where our results suggest that Li at the 8asite is replenished faster than the rate of transfer of Li to the16csite. Fourier-difference nuclear density mapsof both anodes suggest that 32esites are involvedin the diffusion pathway of Li. NPD is again shown to be an excellenttool for the study of electrode materials for Li-ion batteries, particularlywhen it is used to probe real-time crystallographic changes of thematerials in an operating battery during charge–discharge cycling. [ABSTRACT FROM AUTHOR]