Resolving Fast Relative Kinetics in Inorganic Solid-State Synthesis.

Solid-state syntheses are generally regarded as being slow, limited by transport, and, as such, are often only stopped to check the products after many hours at high temperature. Here, using a custom-designed reactor to rapidly initiate solid-state syntheses, we are able to capture the earliest stages of a reaction using in situ X-ray scattering. For the reaction of TiO2 and Li2CO3 to form spinel lithium titanate (Li4Ti5O12)─an anode material for fast-charging applications─we capture two distinct kinetic regimes, including fast initial kinetics in the first seconds-minutes of the reaction that account for significant product formation. We use an Avrami model to compare the reaction at high temperatures (700-750 °C), which results in the rapid formation of Li4Ti5O12 within minutes, and lower temperatures (482 °C), consistent with conditions that might be chosen based on "Tamman's rule", a common heuristic. Our analysis reveals characteristic Avrami slopes (i.e., dimensionalities) for each step in the chemical transformation. We anticipate that the fast initial reaction kinetics found here are likely to be common in the synthesis of other materials used in battery electrodes, solid-state electrolytes, ion-conductive membranes, etc. where ion transport is a prerequisite for functionality.

Assessing Thermodynamic Selectivity of Solid-State Reactions for the Predictive Synthesis of Inorganic Materials.

Synthesis is a major challenge in the discovery of new inorganic materials. Currently, there is limited theoretical guidance for identifying optimal solid-state synthesis procedures. We introduce two selectivity metrics, primary and secondary competition, to assess the favorability of target/impurity phase formation in solid-state reactions. We used these metrics to analyze 3520 solid-state reactions in the literature, ranking existing approaches to popular target materials. Additionally, we implemented these metrics in a data-driven synthesis planning workflow and demonstrated its application in the synthesis of barium titanate (BaTiO3). Using an 18-element chemical reaction network with first-principles thermodynamic data from the Materials Project, we identified 82985 possible BaTiO3 synthesis reactions and selected 9 for experimental testing. Characterization of reaction pathways via synchrotron powder X-ray diffraction reveals that our selectivity metrics correlate with observed target/impurity formation. We discovered two efficient reactions using unconventional precursors (BaS/BaCl2 and Na2TiO3) that produce BaTiO3 faster and with fewer impurities than conventional methods, highlighting the importance of considering complex chemistries with additional elements during precursor selection. Our framework provides a foundation for predictive inorganic synthesis, facilitating the optimization of existing recipes and the discovery of new materials, including those not easily attainable with conventional precursors.

Direct Mechanochemical Synthesis, Phase Stability, and Electrochemical Performance of α-NaFeO2.

To better understand polymorph control in transition metal oxides, the mechanochemical synthesis of NaFeO2 was explored. Herein, we report the direct synthesis of α-NaFeO2 through a mechanochemical process. By milling Na2O2 and γ-Fe2O3 for 5 h, α-NaFeO2 was prepared without high-temperature annealing needed in other synthesis methods. While investigating the mechanochemical synthesis, it was observed that changing the starting precursors and mass of precursors affects the resulting NaFeO2 structure. Density functional theory calculations on the phase stability of NaFeO2 phases show that the α phase is stabilized over the ß phase in oxidizing environments, which is provided by the oxygen-rich reaction between Na2O2 and Fe2O3. This provides a possible route to understanding polymorph control in NaFeO2. Annealing the as-milled α-NaFeO2 at 700 °C has resulted in increased crystallinity and structural changes that improved electrochemical performance in terms of capacity over the as-milled sample.

Relative Kinetics of Solid-State Reactions: The Role of Architecture in Controlling Reactivity.

Countless inorganic materials are prepared via high temperature solid-state reaction of mixtures of reagents powders. Understanding and controlling the phenomena that limit these solid-state reactions is crucial to designing reactions for new materials synthesis. Here, focusing on topotactic ion-exchange between NaFeO2 and LiBr as a model reaction, we manipulate the mesoscale reaction architecture and transport pathways by changing the packing and interfacial contact between reagent particles. Through analysis of in situ synchrotron X-ray diffraction data, we identify multiple kinetic regimes that reflect transport limitations on different length scales: a fast kinetic regime in the first minutes of the reaction and a slow kinetic regime that follows. The fast kinetic regime dominates the observed reaction progress and depends on the reagent packing; this challenges the view that solid-state reactions are necessarily slow. Using a phase-field model, we simulated the reaction process and showed that particles without direct contact to the other reactant phases experience large reduction in the reaction rate, even when transport hindrance at particle-particle contacts is not considered.

Nanostructure Transformation as a Signature of Oxygen Redox in Li-Rich 3d and 4d Cathodes.

Lithium-rich nickel manganese cobalt oxide (LRNMC) is being explored as an alternative to stoichiometric nickel manganese cobalt oxide (NMC) cathode materials due to its higher, initially accessible, energy-storage capacity. This higher capacity has been associated with reversible O oxidation; however, the mechanism through which the change in O chemistry is accommodated by the surrounding cathode structure remains incomplete, making it challenging to design strategies to mitigate poor electrode performance resulting from extended cycling. Focusing on LRNMC cathodes, we identify nanoscale domains of lower electron density within the cathode as a structural consequence of O oxidation using small-angle X-ray scattering (SAXS) and operando X-ray diffraction (XRD). A feature observed in the small angle scattering region suggests the formation of nanopores, which first appears during O oxidation, and is partially reversible. This feature is not present in traditional cathode materials, including stoichiometric NMC and lithium nickel cobalt aluminum oxide (NCA) but appears to be common to other Li-rich systems tested here, Li2RuO3 and Li1.3Nb0.3Mn0.4O2.

A thermal-gradient approach to variable-temperature measurements resolved in space.

Temperature is a ubiquitous environmental variable used to explore materials structure, properties and reactivity. This article reports a new paradigm for variable-temperature measurements that varies the temperature continuously across a sample such that temperature is measured as a function of sample position and not time. The gradient approach offers advantages over conventional variable-temperature studies, in which temperature is scanned during a series measurement, in that it improves the efficiency with which a series of temperatures can be probed and it allows the sample evolution at multiple temperatures to be measured in parallel to resolve kinetic and thermodynamic effects. Applied to treat samples at a continuum of tem-peratures prior to measurements at ambient temperature, the gradient approach enables parametric studies of recovered systems, eliminating temperature-dependent structural and chemical variations to simplify interpretation of the data. The implementation of spatially resolved variable-temperature measurements presented here is based on a gradient-heater design that uses a 3D-printed ceramic template to guide the variable pitch of the wire in a resistively heated wire-wound heater element. The configuration of the gradient heater was refined on the basis of thermal modelling. Applications of the gradient heater to quantify thermal-expansion behaviour, to map metastable polymorphs recovered to ambient temperature, and to monitor the time- and temperature-dependent phase evolution in a complex solid-state reaction are demonstrated.

Rewriting Electron-Transfer Kinetics at Pyrolytic Carbon Electrodes Decorated with Nanometric Ruthenium Oxide.

Platinum is state-of-the-art for fast electron transfer whereas carbon electrodes, which have semimetal electronic character, typically exhibit slow electron-transfer kinetics. But when we turn to practical electrochemical devices, we turn to carbon. To move energy devices and electro(bio)analytical measurements to a new performance curve requires improved electron-transfer rates at carbon. We approach this challenge with electroless deposition of disordered, nanoscopic anhydrous ruthenium oxide at pyrolytic carbon prepared by thermal decomposition of benzene (RuOx@CVD-C). We assessed traditionally fast, chloride-assisted ([Fe(CN)6]3-/4-) and notoriously slow ([Fe(H2O)6]3+/2+) electron-transfer redox probes at CVD-C and RuOx@CVD-C electrodes and calculated standard heterogeneous rate constants as a function of heat treatment to crystallize the disordered RuOx domains to their rutile form. For the fast electron-transfer probe, [Fe(CN)6]3-/4-, the rate increases by 34× over CVD-C once the RuOx is calcined to form crystalline rutile RuO2. For the classically outer-sphere [Fe(H2O)6]3+/2+, electron-transfer rates increase by an even greater degree over CVD-C (55×). The standard heterogeneous rate constant for each probe approaches that observed at Pt but does so using only minimal loadings of RuOx.