Cation‐disordered rocksalt (DRX) cathodes have recently emerged as a promising class of cobalt‐free, high‐capacity cathodes for lithium‐ion batteries. To facilitate their commercialization, the development of scalable synthesis techniques providing control over composition and morphology is critical. To this end, a sol‐gel synthesis route to prepare Mn‐rich DRX cathodes with high capacities is presented here. Several compositions with varied Mn content and nominal F doping are successfully prepared using this technique. In‐situ X‐ray diffraction measurements demonstrate that DRX formation proceeds at moderate temperature (800 °C) through the sol‐gel route, which enables intimate mixing among reactive intermediate phases that form at lower temperatures. All synthesized compositions possess cation short‐range order, as evidenced by neutron pair distribution function and electron diffraction analysis. These DRX materials demonstrate promising electrochemical performance with reversible capacities up to 275 mAh g. Compared to the baseline oxide (Li1.2Mn0.4Ti0.4O2), the Mn‐rich compositions exhibit improved cycling stability, with some showing an increase in capacity upon cycling. Overall, this study demonstrates the feasibility of preparing high‐capacity DRX cathodes through a sol‐gel based synthesis route, which may be further optimized to provide better control over the product morphology compared to traditional synthesis methods.
Lithium‐rich transition metal oxides with a cation‐disordered rocksalt structure (disordered rocksalt oxides or DRX) are promising candidates for sustainable, next‐generation Li‐ion cathodes due to their high energy densities and compositional flexibility, enabling Co‐ and Ni‐free battery chemistries. However, current methods to synthesize DRX compounds require either high temperature (≈1000 °C) sintering for several hours, or high energy ball milling for several days in an inert atmosphere. Both methods are time‐ and energy‐intensive, limiting the scale up of DRX production. The present study reports the rapid synthesis of various DRX compositions in ambient air via a microwave‐assisted solid‐state technique resulting in reaction times as short as 5 min, which are more than two orders of magnitude faster than current synthesis methods. The DRX compounds synthesized via microwave are phase‐pure and have a similar short‐ and long‐range structure as compared to DRX materials synthesized via a standard solid‐state route, resulting in nearly identical electrochemical performance. In some cases, microwave heating allows for better particle size and morphology control. Overall, the rapid and energy‐efficient microwave technique provides a more sustainable route to produce DRX materials, further incentivizes the development of next‐generation DRX cathodes, and is key to accelerating their optimization via high‐throughput studies.
more » « less- NSF-PAR ID:
- 10393041
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Energy Materials
- Volume:
- 13
- Issue:
- 10
- ISSN:
- 1614-6832
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Fluorine substitution is a critical enabler for improving the cycle life and energy density of disordered rocksalt (DRX) Li‐ion battery cathode materials which offer prospects for high energy density cathodes, without the reliance on limited mineral resources. Due to the strong Li–F interaction, fluorine also is expected to modify the short‐range cation order in these materials which is critical for Li‐ion transport. In this work, density functional theory and Monte Carlo simulations are combined to investigate the impact of Li–F short‐range ordering on the formation of Li percolation and diffusion in DRX materials. The modeling reveals that F substitution is always beneficial at sufficiently high concentrations and can, surprisingly, even facilitate percolation in compounds without Li excess, giving them the ability to incorporate more transition metal redox capacity and thereby higher energy density. It is found that for F levels below 15%, its effect can be beneficial or disadvantageous depending on the intrinsic short‐range order in the unfluorinated oxide, while for high fluorination levels the effects are always beneficial. Using extensive simulations, a map is also presented showing the trade‐off between transition‐metal capacity, Li‐transport, and synthetic accessibility, and two of the more extreme predictions are experimentally confirmed.
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Abstract Understanding how various redox activities evolve and distribute in disordered rocksalt oxides (DRX) can advance insights into manipulating materials properties for achieving stable, high‐energy batteries. Herein, the authors present how the reaction kinetics and spatial distribution of redox activities are governed by the particle size of DRX materials. The size‐dependent electrochemical performance is attributed to the distinct cationic and anionic reaction kinetics at different sizes, which can be tailored to achieve optimal capacity and stability. Overall, the local charged domains in DRX particles display random heterogeneity caused by the isotropic delithiation pathways. Owing to the kinetic limitation, the micron‐sized particles exhibit a holistic “core‐shell” charge distribution, whereas sub‐micron particles show more uniform redox reactions throughout the particles and ensembles. Sub‐micron DRX particles exhibit increasing anionic redox activities yet inferior cycling stability. In summary, engineering particle size can effectively modulate how cationic and anionic redox activities evolve and distribute in DRX materials.
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Li‐rich rocksalt oxides are promising cathode materials for lithium‐ion batteries due to their large capacity and energy density, and their ability to use earth‐abundant elements. The excess Li in the rocksalt, needed to achieve good Li transport, reduces the theoretical transition metal redox capacity and introduces a labile oxygen state, both of which lead to increased oxygen oxidation and concomitant capacity loss with cycling. Herein, it is demonstrated that substituting the labile oxygen in Li‐rich cation‐disordered rocksalt materials with a vacancy is an effective strategy to inhibit oxygen oxidation. It is found that the oxygen vacancy in cation‐disordered lithium manganese oxide favors high Li coordination thereby reducing the concentration of unhybridized oxygen states, while increasing the theoretical Mn capacity. It is shown that in the vacancy‐containing compound, synthesized by ball milling, the Mn valence is lowered to less than +3, providing access to more than 300 mAh g−1capacity from the Mn2+/Mn4+redox reservoir. The increased transition metal redox and decreased O oxidation are found to improve the capacity and voltage retention, indicating that oxygen vacancy creation to remove the most vulnerable oxygen ions and reduce transition metal valence provides a new opportunity for the design of high‐performance Li‐rich rocksalt cathodes.
