A real space understanding of the short range order in disordered rocksalt cathode materials

Abstract text (incl. figure legends and references)

Layered cathode materials such as Li-rich NMCs (containing nickel, manganese and cobalt) show promise as new high capacity materials, as the layered structure allows the intercalation and deintercalation of lithium ions during the cycling of the battery. Many of the elements currently used in the cathode are expensive and have low natural abundance, the scope for introducing new more abundant materials is limited. Disordered rocksalt materials overcome with a much larger scope for different elements to be combined, with many examples have shown good performance using only more abundant transition metals such as manganese and titanium (1)(2).

The diffraction patterns of these materials contain clear diffuse scattering which takes many different forms depending on the composition, synthesis method and often exhibit changes on cycling of the material. This diffuse scattering is typical of materials with short range order, this type of diffuse scattering has previously been reported for many disordered rocksalt materials but to the best of our knowledge no precise understanding of the nature of this ordering has been achieved.

The disordered rocksalt Li1.2Mn0.4Ti0.4O2 (LMTO) has been studied because it is potentially commercially important material due its composition only containing relatively cheap and earth abundant elements. However, it shows poor capacity retention on cycling and poor ionic conductivity which leads to poor performance at room temperature and slow rate capability for cycling (3).

Electron diffraction has been used to study the nature of the short-range order in reciprocal space but does not give precise local information about the distribution of the ordering within a particle. In addition to TEM diffraction nanobeam electron diffraction has been used to gain real space information about the origin of the diffuse scattering.

The intensity of atomic columns scales with the atomic number in for ADF images, meaning that the Li and the transition metal in the real space images have very different intensities. While the disordered nature of these materials means that the atomic columns contain a mixture of the cations. However atomic columns with a higher number of transition metals will appear brighter than those with more lithium.

The ADF images of the particles show fluctuations in the intensity of the atomic columns, in what appears to be a random distribution. However, a Fourier transform of the ADF image reveals diffuse shapes which are like the diffuse scattering observed in the electron diffraction.

This link between the diffraction in the traditional sense and the spatial information which comes from the nanobeam diffraction with real space ADF images allows us to examine the precise nature of the local ordering in these materials and its spatial distribution.

Ji, H. et al. Nat. Commun. 10, (2019). Sato, T., Sato, K., Zhao, W., Kajiya, Y. & Yabuuchi, N, J. Mater. Chem. A 6, 13943–13951 (2018) Clement, R. J., Lun Z., Ceder, G., Energy and Enviro. Sci, 13, 345 (2020)

Figure 1: a) ADF image of LMTO b) Fourier transform of image a. c) Electron diffraction pattern of LMTO

Figure 2: (a-c) Radial integration profiles of the a) pristine LiMnO2, b )after 1 cycle and c)8 cycles. Insets show the area where the azimuthal integration profiles are generated from. (f-g) Show virtual dark-field images which show the small crystallites