Unraveling the Nature and Role of Layered Cation Ordering in Cation-Disordered Rock-Salt Cathodes

Abstract

Cation-disordered rock salts (DRXs), a new class of cathode materials for Li-ion batteries, have attracted a great amount of attention in recent years due to their fascinatingly simple cubic structure, highly diverse composition, and great electrochemical performance. As cations in DRXs are randomly distributed in a long range, how the cations are spatially arranged is an intriguing question for the community of solid-state materials chemistry. In this work, we report the vibrational structure of a series of Mn- and Fe-based DRXs with well-controlled compositions and reveal significant layered-like cation ordering in the DRXs. A scheme is proposed to describe how the layered-like anisotropy could exist in rock salt structures with an overall cubic diffraction pattern. Furthermore, we raise a model of Li-ion transport based on the proposed scheme, which complements the theory of Li percolation in DRXs. The electrochemical behavior of the DRX cathodes used in the study supports the scheme and clearly demonstrates the role of layered anisotropy in the battery performance of DRXs.

Towards commercialization of fluorinated cation-disordered rock-salt Li-ion cathodes

“…Unlike the cracking of layered oxide cathodes, including Li-stoichiometric or Li/Mn-rich oxides, which gradually happens during the long-term cycling, immediate cracking happens in DRXs during their first cycle. Recently, Wang et al suggested that DRX is a set of randomly oriented O3-packing blocks based on layeredlike anisotropy (Wang et al, 2022). Although the study does not include any mechanical insights, the discovery of random anisotropy suggests that DRXs may weaker against the internal stress (Ortiz and Suresh, 1993;Lim et al, 2017).…”

Expandable Li Percolation Network: The Effects of Site Distortion in Cation-Disordered Rock-Salt Cathode Material

“…For example, Wang et al systematically studied the SRO in a series of Mn-and Fe-based DRX materials and revealed the widespread existence of layered-like domains. 36 The shuffled layer scheme they proposed for the DRX structure model may be beneficial for percolating Li diffusion. In current LTNNO material, the Ni-rich 4-TM clusters and Ti-rich 1-TM clusters are reminiscent of the NiO and Li 2 TiO 3 structure, respectively, implying the formation of NiO-and Li 2 TiO 3 -like domains.…”

“…The rules for SRO formation have not been fully elucidated. The ways of SRO pattern manifestation are various − and closely related to the DRX material composition. Some high-valence d 0 cations, like Ti 4+ , Nb 5+ , and Mo 6+ , were found critical in promoting cation mixing and breaking SRO .…”

“…Albeit important, the multiscale characterization of the DRX local structure is challenging. Local structure characterization methods, including nuclear magnetic resonance spectroscopy (NMR), extended X-ray absorption fine structure spectroscopy (EXAFS), and Raman spectroscopy, reveal the bonding environment only in the nearest coordination shells. ,, Electron microscopy techniques, such as high-resolution scanning transmission electron microscopy (HR-TEM), can identify partially ordered domains in a very small area of several hundreds of atoms . The electron diffraction technique could validate the existence of SRO on a larger scale yet only in a semiquantitative manner. , A method to quantitatively describe local structure variation throughout the global range is still needed.…”

Understanding the limits to Short-range order Suppression in Many-Component Disordered Rock Salt Lithium-ion Cathode Materials

“…The cause of the reduced lithium percolation in disordered rock salt materials relative to the random limit is understood to be a consequence of the short-range ordering preferences of the cations, which are comprised of contributions from bonding preferences and electrostatics [8,25,28,29]. Thus, finding ways to suppress unfavourable short-range ordering is crucial for improving the capacity and overall performance of disordered rock salt cathode materials.…”