Abstract
Structure plays a vital role in determining materials properties. In lithium ion cathode materials, the crystal structure defines the dimensionality and connectivity of interstitial sites, thus determining lithium ion diffusion kinetics. In most conventional cathode materials that are well-ordered, the average structure as seen in diffraction dictates the lithium ion diffusion pathways. Here, we show that this is not the case in a class of recently discovered high-capacity lithium-excess rocksalts. An average structure picture is no longer satisfactory to understand the performance of such disordered materials. Cation short-range order, hidden in diffraction, is not only ubiquitous in these long-range disordered materials, but fully controls the local and macroscopic environments for lithium ion transport. Our discovery identifies a crucial property that has previously been overlooked and provides guidelines for designing and engineering cation-disordered cathode materials.
Highlights
Structure plays a vital role in determining materials properties
We reveal through a combination of electron diffraction, neutron pair distribution function measurements, and cluster-expansion Monte Carlo simulation that the difference in the performance of LMTO and LMZO is due to different cation short-range order (SRO), which controls the population and connectivity of Limigration channels
The upper cutoff voltage for LMZO is set at 4.5 V, while that of LMTO is set at 4.7 V. This is because we find that cycling LMZO to 4.7 V at 50 °C provides little extra reversible capacity compared to a 4.5 V-cutoff and causes a side reaction above 4.5 V which seems to lead to increased polarization and subsequent discharge voltage fade
Summary
Structure plays a vital role in determining materials properties. In lithium ion cathode materials, the crystal structure defines the dimensionality and connectivity of interstitial sites, determining lithium ion diffusion kinetics. We demonstrate in this paper that in the recently discovered class of Li-excess cation-disordered rocksalt (DRX) cathodes, the chemistry of non-redox-active stabilizers plays a critical role in performance through subtle structural changes. DRX materials were recently shown to have facile Li transport enabled by a percolating network of Li-rich environments[3] Their ability to function without requiring cation ordering has enabled novel cathodes with remarkable chemical diversity[3,4]. We compare two Li-excess DRXs, Li1.2Mn0.4Ti0.4O2 (LMTO) and Li1.2Mn0.4Zr0.4O2 (LMZO) Based on their chemical similarity, these materials would be expected to have comparable electrochemical properties, as Zr4+ and Ti4+ are isoelectronic and their sole role is to charge compensate for the excess Li. If anything, the larger ionic radius of Zr4+ should result in a larger lattice parameter for LMZO, which is generally considered beneficial for Li mobility[16,17,18]. These results indicate the importance of SRO and provide another important handle to tailor the performance of DRX cathode materials, in addition to the already large compositional flexibility
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