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Soft chemistry techniques, such as ion exchange, hold great potential for the development of battery electrode materials that cannot be stabilized via conventional equilibrium synthesis methods. Nevertheless, the intricate mechanisms governing ion exchange remain elusive. Herein, we investigate the evolution of the long-range and local structure, as well as the ion (de)intercalation mechanism during electrochemical Li-to-Na ion exchange initiated from an O3-type lithium-layered oxide cathode. The in situ-formed mixed-cation electrolyte leads to competitive intercalation of Li and Na ions. While Li ion intercalation predominates at the beginning of initial discharge, Na ion cointercalation into a different layer results in ion redistribution and phase separation, with the emergence of a P3–Na phase alongside an O3–Li phase. Further, this study spatially resolves the heterogeneous nature of electrochemical ion exchange reactions within individual particles and provides insights into the correlations between local Ni redox processes and phase separation. Overall, electrochemical ion exchange leads to a mixed-phase cathode and alters its reaction kinetics. Those findings have important implications for the development of new metastable materials for renewable energy devices and ion separation applications.

Weberite-type sodium transition metal fluorides (Na2M2+M′3+F7) have emerged as potential high-performance sodium intercalation cathodes, with predicted energy densities in the 600–800 W h/kg range and fast Na-ion transport. One of the few weberites that have been electrochemically tested is Na2Fe2F7, yet inconsistencies in its reported structure and electrochemical properties have hampered the establishment of clear structure–property relationships. In this study, we reconcile structural characteristics and electrochemical behavior using a combined experimental–computational approach. First-principles calculations reveal the inherent metastability of weberite-type phases, the close energetics of several Na2Fe2F7weberite polymorphs, and their predicted (de)intercalation behavior. We find that the as-prepared Na2Fe2F7samples inevitably contain a mixture of polymorphs, with local probes such as solid-state nuclear magnetic resonance (NMR) and Mössbauer spectroscopy providing unique insights into the distribution of Na and Fe local environments. Polymorphic Na2Fe2F7exhibits a respectable initial capacity yet steady capacity fade, a consequence of the transformation of the Na2Fe2F7weberite phases to the more stable perovskite-type NaFeF3phase upon cycling, as revealed by ex situsynchrotron X-ray diffraction and solid-state NMR. Overall, these findings highlight the need for greater control over weberite polymorphism and phase stability through compositional tuning and synthesis optimization.

Conventional Li-ion cathode materials are dominated by well-ordered structures, in which Li and transition metals occupy distinct crystallographic sites. We show in this paper that profoundly new degrees of freedom for the optimization of electrochemical properties may be accessed if controllable cation disorder is introduced. In a class of high-capacity spinel-type cathode materials, we identify cation to anion ratio in synthesis as a key parameter for tuning the structure continuously from a well-ordered spinel, through a partially ordered spinel, to rocksalt. We find that the varying degree of cation disorder modifies the voltage profile, rate capability, and charge-compensation mechanism in a rational and predictable way. Our results indicate that spinel-type order is most beneficial for achieving high-rate performance as long as the cooperative 8a to 16c phase transition is suppressed, while more rocksalt-like disorder facilitates O redox, which can increase capacity. Our findings reveal an important tuning handle for achieving high energy and power in the vast space of partially ordered cathode materials.

High-entropy (HE) ceramics, by analogy with HE metallic alloys, are an emerging class of solid solutions composed of a large number of species. These materials offer the benefit of large compositional flexibility and can be used in a wide variety of applications, including thermoelectrics, catalysts, superionic conductors and battery electrodes. We show here that the HE concept can lead to very substantial improvements in performance in battery cathodes. Among lithium-ion cathodes, cation-disordered rocksalt (DRX)-type materials are an ideal platform within which to design HE materials because of their demonstrated chemical flexibility. By comparing a group of DRX cathodes containing two, four or six transition metal (TM) species, we show that short-range order systematically decreases, whereas energy density and rate capability systematically increase, as more TM cation species are mixed together, despite the total metal content remaining fixed. A DRX cathode with six TM species achieves 307?mAh?g-1(955?Wh?kg-1) at a low rate (20?mA?g-1), and retains more than 170?mAh?g-1when cycling at a high rate of 2,000?mA?g-1. To facilitate further design in this HE DRX space, we also present a compatibility analysis of 23 different TM ions, and successfully synthesize a phase-pure HE DRX compound containing 12 TM species as a proof of concept.

Lithium-excess cation-disordered rocksalt oxides (DRXs) are a new class of high energy Li-ion cathode materials. A DRX cathode can deliver an exceptionally high capacity of >250 mA h g–1, implying the participation of cationic transition metal and anionic oxygen redox in the electrochemistry. Understanding the roles of these distinct redox processes in the newly developed cathode is of critical importance for the further development of these materials. Herein, we synthesize a representative Mn redox-based fluorinated DRX material, Li1.2Mn0.625Nb0.175O1.95F0.05(LMNOF). Using a comprehensive suite of electroanalytical methods, we probe the cationic and anionic redox processes and their evolution upon cycling exclusively from an electrochemical point of view. This work sheds light on understanding the cationic and anionic redox reactions and their roles in the capacity loss for this high-capacity fluorinated DRX cathode, meanwhile showcases a thorough approach to studying the electrochemical properties of novel electrode materials using nondisruptive electroanalytical methods.

Topochemical reactions are powerful pathways to modify inorganic extended structures, but the present approaches are limited by the degrees of freedom to tune the structural connectivity and dimensionality. In this work, we unveil a new topochemical bottom-up approach to tailor three-dimensional (3D) iron fluoride frameworks from the same one-dimensional (1D) FeF3.3H2O (IF) precursor upon reacting with iodide-based reagents (AI; A+= Na+, K+, and NH4+). To elucidate their formation mechanism, a series of topochemical reactions are performed by varying the concentration of precursors, reaction temperatures, and durations, and their corresponding products are analyzed through X-ray diffraction, nuclear magnetic resonance, and Mössbauer spectroscopy techniques. Although the lower molar ratio of AI:IF (≈0.25:1.0) produces the same hexagonal tungsten bronze (HTB)–type AxFeF3, the topochemical reactions with higher AI:IF ratios yield weberite-Na1.95Fe2F7, tetragonal tungsten bronze (TTB)-K0.58FeF3, and pyrochlore-NH4Fe2F6phases. Our density functional theory calculations attribute the formation of iron fluoride phases to their thermodynamic stability. Moreover, kinetics also play an important role in enabling weberite and HTB fluorides with high purity, while the pyrochlore-NH4Fe2F6retains a minor HTB-(NH4)xFeF3impurity. Overall, this work shows a new possibility of modulating the low-dimensional precursor to attain 3D frameworks with different structural arrangements viatopochemical approaches.

Sodium (Na)-ion conducting polyanionic structures are among the most promising cathode materials to enable more sustainable Na-based battery chemistries to replace current Li-based energy storage systems. Materials adopting the alluaudite structure have been found to reversibly intercalate Na, with phosphate and sulfate derivatives exhibiting moderate to high capacities when used as Na-ion cathodes. However, the development of alluaudite cathodes has been hampered by the fact that largely only Fe2+/3+redox has been observed in this structure. Herein, we show that the Mn2+/3+redox couple can be activated through compositional tuning of alluaudite compounds. Specifically, vanadate compounds were explored to increase the electrical conductivity as compared to the phosphates and sulfates. Al substitution was also employed to buffer the Jahn–Teller distortions of Mn(III)O6octahedra, further facilitating electron transfer and redox processes. We report the synthesis of a series of new Na2Mn3–xAlx(VO4)3alluaudite cathodes and employ synchrotron X-ray diffraction, solid-state nuclear magnetic resonance, scanning electron microscopy, density functional theory, X-ray absorption spectroscopy, and magnetometry to characterize the structure, morphology, electronic, and magnetic properties of the as-synthesized materials. Electrochemical Na de(intercalation) in Na2Mn3–xAlx(VO4)3(x= 0, 0.05, 0.2) was probed through galvanostatic cycling, galvanostatic intermittent titration technique, and ex situsynchrotron X-ray diffraction. While these materials are all redox active, Al substitution results in a more than 2-fold increase in discharge capacity. The successful activation of a higher voltage Mn2+/3+redox couple opens up a new compositional space for alluaudite-type Na-ion cathodes.