Brent C. Melot

Lithium-conducting fluoride garnets exhibit ionic conductivities of ∼10–10 S cm–1, far below those of their oxide counterparts. Here, we investigate the origin of this poor transport in Na3Ga2Li3F12 and Na3Al2Li3F12 through a combination of structural characterization and first-principles calculations. Both compounds are highly ordered, with no significant antisite disorder, and exhibit intrinsic ionic conductivities of ∼2 × 10–11 S cm–1 at 100 °C. Nudged elastic band calculations reveal low Li-ion migration barriers (0.16 to 0.29 eV), indicating that the fluoride garnet framework does not inherently restrict Li-ion diffusion. However, the formation energies for lithium Frenkel and Schottky defect pairs are high (1.9 to 2.4 eV), indicating that the intrinsic concentrations of mobile Li-ion defects are negligibly small. These results establish that the low conductivity of stoichiometric fluoride garnets arises from high defect-formation energies, rather than high migration barriers.

Layered Na–Mn–Fe–O oxides are among the leading cathode candidates for Na-ion batteries owing to their low cost, earth-abundant constituents, and competitive electrochemical performance. Their rich structural chemistry, spanning O-type (octahedral Na) and P-type (prismatic Na) polymorphs with distinct stacking sequences, offers multiple avenues for property optimization. However, controllable access to specific polymorphs, particularly nonequilibrium ones, remains a synthetic challenge. Here we show that susceptor-assisted microwave heating can produce the P2, O3, and P3 phases of layered Na–Mn–Fe–O cathodes in minutes, with phase selectivity set by the nominal Na/M ratio of the precursor alone. The rapid reaction times suppress Na volatilization, preserving the target stoichiometry without the excess sacrificial Na precursors typically required by conventional solid-state routes. The high mobility of Na+ combined with the large size difference between Na and Mn/Fe, yields well-ordered frameworks with no evidence for antisite disorder between Na+ and the transition metals. Accessing all three structures from one rapid route enables a controlled comparison of how stacking sequence governs electrochemistry. On cycling, we observe Fe3+/Fe4+ redox and behavior consistent with Fe3+ migration into the Na layer at high voltage in all three polymorphs, with P2 showing the most stable high-voltage cycling. Operando diffraction shows that all three converge toward disordered O-type stacking on deep desodiation, and voltage-resolved distribution-of-relaxation-times analysis shows that Na+ diffusion kinetics are governed by stacking transitions and Na–vacancy ordering, with pronounced kinetic barriers at glide-type structural transitions. These results establish microwave synthesis as a versatile route to both equilibrium and nonequilibrium layered Na cathodes and clarify how stacking sequence and local disorder jointly control redox behavior and ion transport.

Bronze and bronze-derived, mixed metal oxides are materials of growing interest for lithium-ion battery anodes due to their high-rate capabilities and the potential for high energy densities via multielectron redox. We report on the synthesis and electrochemical properties of the tetragonal tungsten bronze-type phase Nb₈W₉O₄₇ and our investigation into the structural evolution of this phase upon lithium (de)-intercalation, with particular attention to how it relates to the fast-cycling capabilities. Electrochemical cycling shows Nb₈W₉O₄₇ can achieve greater than one Li⁺ per transition metal at rates of C/2 or slower and maintains a capacity equivalent to 0.65 Li⁺ per transition metal at a rate of 20C. Sequential Rietveld analysis of operando X-ray diffraction data reveals anisotropic changes to the unit cell parameters during discharge, accompanied by an off-centering of the transition metals within their octahedral environments. While higher capacities can be accessed below 1.2 V, cells discharged to 1.0 V exhibit a significant expansion of the unit cell volume and reduced Coulombic efficiencies. This report suggests that rigid crystal structures incapable of undergoing polyhedral rotational deformations can instead exploit displacive distortions of the transition metal redox center to enable fast Li-ion cycling with minimal atomic rearrangements.

Hybrid metal halides are a remarkably dynamic family of materials that offer a flexible platform for exploring the novel crystal chemistry that emerges at the intersection of organic and inorganic solids. Herein, we report the discovery of a hybrid that contains two molecules effectively adopting isostructural geometry, (1-NA)PbI₃ and (1-MQ)PbI₃, and our attempts to create solid solutions of the two beyond the 1:1 ratio. Single-crystal X-ray diffraction, combined with solid-state NMR measurements, clearly show that despite having nearly identical steric geometry, the only mixed phase attained was the composition (1-MQ)(1-NA)Pb₂I₆, which exhibits a high degree of order between the two molecules. We propose that this ordering is primarily driven by local molecular dipoles, which ultimately creates a band structure in the blended phase that is highly characteristic of the end members, with little sign of rehybridization between the organic or inorganic components.

The high costs and geopolitical challenges inherent to the lithium-ion (Li-ion) battery supply chain have driven a rising interest in the development of sodium-ion (Na-ion) batteries as a potential alternative. Unfortunately, the larger ionic radius of Na limits the reversibility of cycling because of the extensive atomic rearrangements that accompany Na-ion insertion, which in turn limit diffusion and charging speed, and lead to rapid degradation of the electrodes. The Center for Strain Optimization for Renewable Energy (STORE) was established to address these challenges and develop new electrode materials for Na-ion cells. This article discusses the current state-of-the-art materials used in Na-ion cells and several directions that STORE believes are critical to understand and control the structural and volumetric changes during the reversible (de)insertion of large cations. Graphical Highlights: Understanding the fundamental way materials respond to localized strains at the atomic length-scale is a critical first step in the development of highly reversible, long cycle life, Na-ion insertion hosts. This perspective explores a variety of methods that can be employed to mitigate the detrimental effects of large strain. The insights gained from these investigations should help lay the foundation for the creation of more economical and sustainable batteries that could have immediate impact on global energy infrastructure. Discussion: Although there is near universal agreement that electrochemical energy storage must be an integral part of a green-energy future, there is less agreement about how to reduce the cost of energy storage. Replacing high-cost lithium-ion cells with lower-cost sodium-ion batteries is one option frequently considered in future energy models, but the details of what can be achieve with optimized sodium cell performance remains unclear. Here we posit that developing methods to mitigating strain on the electrode particle length scale is a key factor for achieving long-cycle-life sodium-ion batteries. Mitigating strain on the atomic scale suppress electrode-level volume change. Allowing for fast cycling in materials without the problems of electrode cracking or delamination. We further posit that understanding volume change in sodium-ion electrodes at a fundamental level will lead to the designing new sodium-ion electrode materials that will allow for efficient, stable, lower-cost energy storage.

The lithium-carbon monofluoride (Li-CFx) couple has the highest specific energy of any practical battery chemistry. However, the large polarization associated with the CFx electrode (>1.5 V loss) limits it from achieving its full discharge energy, motivating the search for new CFx reaction mechanisms with reduced overpotential. Here, using a liquid fluoride (F)-ion conducting electrolyte at room temperature, we demonstrate for the first time the electrochemical defluorination of CFx cathodes, where metal fluorides form at a metal anode instead of the CFx cathode. F-ion primary cells were developed by pairing CFx cathodes with either lead (Pb) or tin (Sn) metal anodes, which achieved specific capacities of over 700 mAh g(-1) and over 400 mAh g(-1), respectively. Solid-state F-19 and Sn-119{F-19} nuclear magnetic resonance (NMR), X-ray diffraction (XRD), Raman, inductively coupled plasma (ICP), and X-ray fluorescence (XRF) measurements establish that upon discharge, the CFx cathode defluorinates while Pb forms PbF2 and Sn forms both SnF4 and SnF2. Technological development of F-ion metal-CFx cells based on this concept represents a promising avenue for realizing primary batteries with high specific energy.

Conventional Li-ion battery intercalation cathodes leverage charge compensation that is formally associated with redox on the transition metal. Employing the anions in the charge compensation mechanism, so-called anion redox, can yield higher capacities beyond the traditional limitations of intercalation chemistry. Here, we aim to understand the structural considerations that enable anion oxidation and focus on processes that result in structural changes, such as the formation of persulfide bonds. Using a Li-rich metal sulfide as a model system, we present both first-principles simulations and experimental data that show that cation vacancies are required for anion oxidation. First-principles simulations show that the oxidation of sulfide to persulfide only occurs when a neighboring vacancy is present. To experimentally probe the role of vacancies in anion redox processes, we introduce vacancies into the Li₂TiS₃ phase while maintaining a high valency of Ti. When the cation sublattice is fully occupied and no vacancies can be formed through transition metal oxidation, the material is electrochemically inert. Upon introduction of vacancies, the material can support high degrees of anion redox, even in the absence of transition metal oxidation. The model system offers fundamental insights to deepen our understanding of structure-property relationships that govern reversible anion redox in sulfides and demonstrates that cation vacancies are required for anion oxidation, in which persulfides are formed.