High-Entropy-Type Cation-Disordered Rocksalt Cathodes for Li-Ion Batteries with Enhanced Capacity and Rate Capability
Authors: Lun Zhengyan , Ouyang Bin , Kwon Deok-Hwang , Ceder Gerbrand 
Published: December 2020

The recent success of Li-excess cation-disordered rocksalt (DRX) cathodes is providing an avenue to develop high energy density cathodes with abundant and low-cost metals, such as Mn, Fe and Ti. As they have high energy density, these cathode materials are currently the most viable strategy to address the resource issues of Co / Ni that will arise as conventional layered-type Li-ion cathodes are scaled towards multiple TWh of annual production. In DRX cathodes, facile Li transport relies mostly on the so-called “0-TM” 3D percolation channel, in which the intermediate tetrahedral sites are coordinated only by Li (no transition metals, TMs). The Li migration barrier along this “0-TM” channel is significantly lower compared to that of “1-TM” channel, where the intermediate tetrahedral sites are coordinated by 3 Li and 1 TM.The complexity of DRX cathodes lies in the fact that in most cases, they present various types of cation short-range order (SRO), which influences the frequency and connectivity of “0-TM” channels, thus controls the capacity and rate capability of DRX cathodes.Monte Carlo simulations have suggested that the presence of SRO in DRX cathodes generally leads to reduced Li percolation, when compared to that of a random arrangement of TM species.
Inspired by the recent observations of nearly-random cation distribution in several high-entropy metal and oxide compounds, we have found that increasing the number of TM species in DRXs will improve Li transport properties by preventing the formation of a single dominant SRO type, resulting in improved capacity and high rate capability. We show that SRO in DRX cathodes systematically decreases as more TM components are added, and as a consequence, energy density and rate capability systematically increase. A DRX cathode with six TM species achieves 307 mAh g−1(955 Wh kg−1) at 20 mA g−1and retains more than 170 mAh g−1when cycling at 2,000 mA g−1. This success opens up a vast multi-dimensional chemical space to investigate, thus to facilitate further design in this high-entropy DRX space, we present a compatibility analysis of twenty-three different TM ions, and propose important design principles for advanced high-entropy DRX cathodes. The high compositional flexibility of these materials can also enable the use of less pure precursor materials.

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