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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]

The mechanism of capacity fade of the Li2MnO3·LiMO2 (M = Li, Ni, Co, Mn) composite positive electrode within a full cell was investigated using a combination of operando neutron powder diffraction and transmission X-ray microscopy methods, enabling the phase, crystallographic, and morphological evolution of the material during electrochemical cycling to be understood. The electrode was shown to initially consist of 73(1) wt % R3m LiMO2 with the remaining 27(1) wt % C2/m Li2MnO3 likely existing as an intergrowth. Cracking in the Li2MnO3·LiMO2 electrode particle under operando microscopy observation was revealed to be initiated by the solid-solution reaction of the LiMO2 phase on charge to 4.55 V vs Li+/Li and intensified during further charge to 4.7 V vs Li+/Li during the concurrent two-phase reaction of the LiMO2 phase, involving the largest lattice change of any phase, and oxygen evolution from the Li2MnO3 phase. Notably, significant healing of the generated cracks in the Li2MnO3·LiMO2 electrode particle occurred during subsequent lithiation on discharge, with this rehealing being principally associated with the solid-solution reaction of the LiMO2 phase. This work reveals that while it is the reduction of lattice size of electrode phases during charge that results in cracking of the Li2MnO3·LiMO2 electrode particle, with the extent of cracking correlated to the magnitude of the size change, crack healing is possible in the reverse solid-solution reaction occurring during discharge. Importantly, it is the phase separation during the two-phase reaction of the LiMO2 phase that prevents the complete healing of the electrode particle, leading to pulverization over extended cycling. This work points to the minimization of behavior leading to phase separation, such as two-phase and oxygen evolution, as a key strategy in preventing capacity fade of the electrode. [ABSTRACT FROM AUTHOR]