Short-Range Order and Macroscopic Lithium Transport in Cation-Disordered Rocksalt Cathodes
Authors: Lun Zhengyan , Ouyang Bin , Kwon Deok-Hwang , Ceder Gerbrand 
Published: December 2020

The tremendous success and growth of Lithium(Li)-ion based energy storage in a broad range of applications is likely to strain our natural resources. Projected growth of Li-ion production towards 1 TWh/year will require more than a million tons of Co/Ni combined, which constitutes a very sizeable fraction of the annual production of these metals. The recent development of Li-excess cation-disordered rocksalt (DRX) cathodes, which require no separate Li layer and transition metal (TM) layer in the cation sublattice, is providing an avenue for the Li-ion battery field to develop high energy density cathodes with more abundant and less expensive metals, such as Mn, Fe and Ti. Unlike the traditional layered NCM cathodes in which Li ions transport within the well-defined Li layers, the Li ions diffuse through a “3-D” Li rich pathway (‘0-TM’ channel) in DRX cathodes, i.e., the feasible Li hops require that the intermediate tetrahedral sites along the Li diffusion path are coordinated only by Li (no transition metal). The complexity of this class of DRX cathodes mainly lies in the fact that lacking in long-range order though, DRX cathodes, in most cases, present various types of cation short-range order (SRO), which controls the frequency and connectivity of Li migration channels, thus, macroscopic Li transport.
Here we demonstrate two general strategies to engineer the cation SRO in DRX cathodes through compositional design, combining computational investigations, electrochemical tests, and advanced characterizations. (1) Fluorine doping on anion sublattices.The strong Li-F interaction modifies the cation SRO in DRX materials by forming Li-rich domains around F ions. We will present a design map which shows that the macroscopic Li transport is strongly controlled by the Li-excess level and F content within a fixed DRX chemical space and can be correlated to the observed capacities of a DRX cathode, as corroborated by electrochemical tests. (2) TM doping on cation sublattices.With a fixed Li and F content, TM doping can frustrate the cation SRO in DRX cathodes by increasing configurational entropy, thus improve the Li transport, i.e. capacity and rate capability in a significant way. The compatibility between various TM ions in DRX compounds is also investigated to facilitate future experimental design. We show that this can lead to DRX materials that combine high rate capability and high capacity.

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