Hidden Structural Order Controls Li-Ion Transport in Cation-Disordered Oxides

The development of cost-effective lithium-ion batteries depends on the discovery of high-energy-density cathode materials composed of nonprecious elements. Traditionally, lithium-ion cathodes are based on two categories, i.e., layered oxides and polyanionic compounds. However, most polyanionic compounds have low specific capacity due to their heavy polyanions. Layered oxides, though more favorable for energy density purposes, often contains the element cobalt, the supply of which is scarce and not sustainable. The discovery of cation-disordered rocksalts and their percolation rules unlocked an unprecedented chemical space for the exploration of high-capacity lithium-ion cathodes which do not contain Co and have high capacity[1]. The facile Li diffusion in these materials is enabled through a network of Li-rich environments (so-called 0-TM channels) created by excess Li. Following this insight, many new high-energy-density cathode materials that involve mostly earth-abundant elements have been developed, such as Li 1.2 Mn 0.4 Ti 0.4 O 2 [2], Li 1.2 Ni 1/3 Ti 1/3 Mo 2/15 O 2 [3], as well as their fluorinated variants[4, 5]. A prevailing assumption when studying disordered rocksalt cathodes is that all the cation species are randomly distributed. However, we will demonstrate that even minor deviations from randomness, not detectable by typical X-ray diffraction (XRD), can have profound influence on performance. We employ a combination of thorough experimental characterization and multi-scale computer simulations to reveal that cation short-range order is ubiquitous in these long-range disordered materials. More importantly, the short-range order controls Li transport by altering the distribution of local environments and the connectivity between them. By considering a variety of different chemistries, we explain the microscopic origin of short-range order and identify general guidelines for local-structure manipulation for the benefit of Li transport. This breakthrough in the fundamental understanding of structure-property relationship in disordered Li-ion cathodes sets an exciting new direction for future optimization. [1] J. Lee, et al, Science 343 (2014) 519-522. [2] N. Yabuuchi, et al, Nature communications 7 (2016) 13814. [3] J. Lee, et al, Energy & Environmental Science 8 (2015) 3255-3265. [4] R. Chen, et al, Advanced Energy Materials 5 (2015). [5] J. Lee, et al, Nature communications 8 (2017) 981.