Page 57 - ILLS Annual Report 2018

SCIENTIFIC HIGHLIGHTS
54-55
Figure 1
a) Top: Mn K-edge XANES of pristine (Li4) and charged (Li0) cathodes compared with the highly ordered MnO.
Bottom: experimental and simulated pre-edges with Mn in different configurations that might be expected in the presence of oxygen vacancies: square pyramid, square planar, a 1:1 ratio mixture of octahedral and square pyramid, and purely octahedral.
b) RMC fits of Li4 neutron (top) and X-ray (bottom) G(r) (left) and S(Q) (right).
c) Energy landscapes illustrating the 3D Li diffusion in the RMC refined model along a, b and c axes. Green, purple and red spheres denote Li, Mn and O atoms, respectively.
d) Li+ diffusion mechanism at the atomic scale. Black spheres denote empty tetrahedral interstices.
Up to ten RMC outputs were evaluated to quantify and ascertain the distribution and local environments of the
Mn- framework and the mobile Li ions. Moreover, the accessible Li+ sites in the RMC refined model were identified by calculating the mismatch of the BVS with respect to Li+ formal valence, and converted into energy units [3]. The energy landscapes revealed a plausible
3D Li diffusion pathway (figure 1c) connecting a large volume of 5-co-ordinated lithium clustered around oxygen vacancies. The diffusion mechanism in Li4 can be better understood when looking at vacancy-lithium octahedral environments (see figure 1d), which show that the transport of Li+ between three lithium sites within the octahedron’s faces could occur via the momentary occupation of the neighbouring vacant tetrahedral interstices.
Our results reveal how the combined use of multiple information sources increases the number of constraints for RMC refinements and allows us to reveal fine details in the local structure of advanced functional materials that would otherwise be unattainable.
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