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For this study, I11 was used as it offers both high-resolution (HR), time-resolved (TR) SXPD, and long-duration in situ studies. SXPD data (HR and TR) were collected in Experimental Hutch 1 (EH1) to study the evolution of the lattice parameters during the first charge, while in situ long-term studies (105 cycles) from two identical battery cells were performed on the recently commissioned Long Duration Experiment (LDE) facility4. SXPD patterns were taken once a week upon long-term cycling. Using pouch cells suitable for in situ SXPD, the first cycle was performed with a C-rate of C/20 to activate the material, and all subsequent cycles were performed at a rate of C/5 between 2.0 and 4.6 V, completing 14.5 cycles each week. From this data, it is possible to determine, and quantify, transition-metal (TM) migration upon cycling by detailed reflection profile analysis, and Difference-Fourier (DF) mapping. The relationship of reversible, and irreversible, TM disorder with the strain in the material can guide improvements in the cycling stability of lithium-rich cathodes, and potentially lead to the synthesis of new application-oriented materials. Although previous studies indicate only minor changes in the bulk material, long duration in situ SXPD measurements, in combination with difference Fourier analysis of the data, revealed an irreversible TM motion within the host structure.
TMs tend to occupy tetrahedral sites in the lithium layer (Fig. 2B) at highly delithiated (charged) states, since the occupation of tetrahedral sites in the TM layer is less favourable due to the occupation of neighbouring octahedral sites by cationic species. A TM in tetrahedral sites of the Li-layer can then either move back to the TM layer (Fig. 2A), or move on to occupy octahedral lithium sites (Fig. 2C), leading to a delithiated, disordered structure which is thermodynamically more stable and, therefore, energetically favourable5. This disorder is identified in diffraction patterns as a mismatch in the reflection intensities, rather than changes in positions or the presence/absence of reflections. While a minor mismatch in the refined reflection intensities of the first cycle was observed, this became more significant as cycling progressed. If analysed further, the discrepancy in the measured, and refined, reflection intensities can provide useful information, as it originates from the difference between the measured, and calculated, electron density in the crystallographic structure. Due to their relatively high number of electrons, and thus their significant contribution to the X-ray diffraction profiles, TMs are the only elements in the HE-NCM structure that can cause such an effect. DF analysis of the diffraction data was performed to investigate which sites are occupied by TMs upon cycling. The DF maps of the first discharged state (Fig. 3A and B) show that the electron density of octahedral sites (Lioc and TMoc) are poorly described by the ideal rhombohedral structure, and the differences become even more significant by the 103rd cycle (Fig. 3C and D). Li/TM disorder is not observed in the first cycle of conventional layered oxides, supporting the assumption that lithium vacancies in the TM layer, created during the first charge (the so-called activation), accelerate transition-metal migration. During subsequent cycles, the disorder increases gradually, reaching roughly 5% of the transition metals in Lioc sites after 100 cycles. This observed increase of disorder leads to the conclusion that this type of migration is to some extent irreversible. Equivalent DF maps of the 1.5 and 102.5 charged states show underestimated electron densities in Lite sites (tetrahedral sites in the lithium layer). This confirms the migration of TMs via these sites in the charged state. Nevertheless, these results support the migration pathway shown in Fig. 3, and suggest that TM motion takes place at high states of charge. Migration of TMs into tetrahedral sites has to be largely reversible because no occupation of tetrahedral sites was observed in the discharged state throughout the 100 charge/discharge cycles, while Ca. 8% of the TMs occupy tetrahedral sites in the charged state. At the end of discharge, 2-5% of the TMs are refined to be in octahedral Li-sites, while the tetrahedral sites are unoccupied, strongly indicating that 3-6% of the TMs migrate reversibly back from tetrahedral sites into octahedral TM sites upon lithiation.
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