Understanding short range order in disordered rocksalt cathodes from diffuse scattering and ADF images

Session

Poster Session One

Authors

Ms Emma Hedley (1), Mr Liquan Pi (1), Dr Nikolaj Roth (1), Dr Mikkel Juelsholt (1), Dr Robert House (1), Prof Peter G. Bruce (1, 2, 3), Prof Peter D. Nellist (1)

Affiliations

1. University of Oxford
2. Faraday Institution
3. Henry Royce Institute

Keywords

Batteries, Energy Materials, Annular Dark Field, Diffuse Scattering

Abstract text

Disordered Rocksalt (DRS) cathodes are proposed as cobalt free alternative to current state of the art layered cathodes. DRS cathodes have a rocksalt-like fcc structure with an anion lattice of oxygen, an excess of lithium cations and the possibility for various combinations of transition metals. The lithium and transition metals, commonly, manganese and titanium are predominantly disordered in their occupation of the cation positions in the cation sub-lattice. Understanding the nature of the short range order of the cations is essential to our understanding of the rate limitations in current DRS cathodes and the structural changes which occur on voltage cycling.  It is has been demonstrated that the nature of the short range order can affect the percolation channels, through which lithium is required to migrate in the absence of the typical layered cathode structure [1].

Structured diffuse scattering has been observed in electron diffraction patterns for a variety of materials and recently disordered rocksalt cathodes. Diffuse scattering in disordered rock salt cathodes has been widely reported and attributed to the presence of short range order, however very little work has been able to elucidate the nature of this ordering.

We have obtained atomic resolution annular dark field (ADF) images of the disordered rocksalt cathodes which show fluctuations in the atomic column intensity. The structure of the diffuse scattering in the Fourier transform correlated very strongly with the diffraction pattern, as can be observed in Fig 1 b. Taking the inverse Fourier transform of the masked FT allows us to isolate contrast associate with the short range order. We demonstrate that this information can be used to obtain spatial information on the distribution and nature of short range order.

Figure 1

a) HAADF STEM image of Li_1.2Mn_0.4Ti_0.4O₂ b) Fourier transform of a with smoothing filter applied across Bragg peaks (inset selected area electron diffraction pattern from Li_1.2Mn_0.4Ti_0.4O₂) c) Inverse Fourier transform of b showing regions of contrast attributed to the diffuse scattering.

References

[1]         H. Ji et al., ‘Hidden structural and chemical order controls lithium transport in cation-disordered oxides for rechargeable batteries’, Nat. Commun., vol. 10, no. 1, p. 592, Dec. 2019, doi: 10.1038/s41467-019-08490-w.