Page 56 - ILLS Annual Report 2018

MATERIALS SCIENCE
Maria Diaz-Lopez. Spanish
ISIS and Diamond (UK)
‘I am a post-doctoral researcher working
on the structural characterisation of crystallographically challenged materials
for energy applications, using a combination of neutron and X-ray total scattering.’
Unravelling the lithium diffusion mechanisms in a highly disordered and nanostructured high-capacity cathode
Disordered materials diffractometer D4
New cathode designs involve more and
more disordered/nanosized materials for enhanced Li+ cation diffusion and larger specific surfaces. This trend poses new challenges for the structural investigation methods employed, which mostly rely on the periodic and long-range ordered nature of the compounds under study. This is especially the case for the recently discovered nanostructured Li4Mn2O5 high-capacity cathode material, which shows record reversible capacities
and a strongly disordered structure. Here, we demonstrate that a thorough description of such a disordered structure can be achieved by a combination of neutron and synchrotron powder diffraction experiments analysed using the pair distribution function method.
AUTHORS
M. Diaz-Lopez (ISIS and Diamond, Harwell, UK)
C.V. Colin and P. Bordet (Grenoble Alpes University and Institute Néel, CNRS Grenoble, France)
V. Pralong (Caen University and CRISMAT, CNRS Caen, France)
H.E. Fischer (ILL)
ARTICLE FROM
Chem. Mater. 30 (2018) 3060—doi: 10.1021/acs.chemmater.8b00827
REFERENCES
[1] M. Freire et al., Nat. Mater. 15 (2015) 173
[2] H. Playford et al., Ann. Rev. Mater. Res. 44 (2014) 429 [3] M.O. Filsø et al., Chem. Eur. J. 19 (2013) 15535
Although reported to be efficient [1], the migration of Li in the nonstoichiometric Li4Mn2O5 (Li4) synthesised by mechanical alloying is not yet understood, due to a lack of detailed knowledge of this disordered structure at the atomic scale. Previous studies based on laboratory X-ray diffraction data revealed Li4 to show on average an MnO-type rock-salt structure, distorted at the local scale as a result of the substitution of 2/3 manganese for lithium and the presence of 1/6 oxygen vacancies. Therefore, its full structural description is greatly challenged by its high degree of intrinsic disorder and nanostructuration. In this article, we summarise the complex structural modelling approach that successfully describes, for the first time, the intricate Li-diffusion pathways in this intriguing compound.
First, near-edge X-ray absorption spectroscopy (XANES)
at the Mn K-edge was used to probe the co-ordination environment of the manganese, which remained octahedral despite the large number of oxygen vacancies (figure 1a). This information was used to build the starting model for the analysis of the total scattering data, where Li atoms are clustered around the O vacancies. Then, a combined neutron and X-ray total scattering refinement was carried out using the Reverse Monte Carlo (RMC) modelling approach performed on a ‘large-box’ [2] with dimensions similar to the structural coherence length of Li4. Both neutron and X-ray reduced pair distribution G(r), while total scattering S(Q) functions were simultaneously fitted (figure 1b) and soft-chemical bond valence sum (BVS) constraints applied.
ANNUAL REPORT 2018