A multimodal analytical toolkit to resolve correlated reaction pathways: the case of nanoparticle formation in zeolites.

Unraveling the complex, competing pathways that can govern reactions in multicomponent systems is an experimental and technical challenge. We outline and apply a novel analytical toolkit that fully leverages the synchronicity of multimodal experiments to deconvolute causal from correlative relationships and resolve structural and chemical changes in complex materials. Here, simultaneous multimodal measurements combined diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and angular dispersive X-ray scattering suitable for pair distribution function (PDF), X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) analyses. The multimodal experimental data was interpreted via multi-level analysis; conventional analyses of each data series were integrated through meta-analysis involving non-negative matrix factorization (NMF) as a dimensional reduction algorithm and correlation analysis. We apply this toolkit to build a cohesive mechanistic picture of the pathways governing silver nanoparticle formation in zeolite A (LTA), which is key to designing catalytic and separations-based applications. For this Ag-LTA system, the mechanisms of zeolite dehydration, framework flexing, ion reduction, and cluster and nanoparticle formation and transport through the zeolite are elucidated. We note that the advanced analytical approach outline here can be applied generally to multimodal experiments, to take full advantage of the efficiencies and self-consistencies in understanding complex materials and go beyond what can be achieved by conventional approaches to data analysis.

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 stable cathode-solid electrolyte composite for high-voltage, long-cycle-life solid-state sodium-ion batteries.

Rechargeable solid-state sodium-ion batteries (SSSBs) hold great promise for safer and more energy-dense energy storage. However, the poor electrochemical stability between current sulfide-based solid electrolytes and high-voltage oxide cathodes has limited their long-term cycling performance and practicality. Here, we report the discovery of the ion conductor Na3-xY1-xZrxCl6 (NYZC) that is both electrochemically stable (up to 3.8 V vs. Na/Na+) and chemically compatible with oxide cathodes. Its high ionic conductivity of 6.6 × 10-5 S cm-1 at ambient temperature, several orders of magnitude higher than oxide coatings, is attributed to abundant Na vacancies and cooperative MCl6 rotation, resulting in an extremely low interfacial impedance. A SSSB comprising a NaCrO2 + NYZC composite cathode, Na3PS4 electrolyte, and Na-Sn anode exhibits an exceptional first-cycle Coulombic efficiency of 97.1% at room temperature and can cycle over 1000 cycles with 89.3% capacity retention at 40 °C. These findings highlight the immense potential of halides for SSSB applications.

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.

Intrinsic Kinetic Limitations in Substituted Lithium-Layered Transition-Metal Oxide Electrodes.

Substituted Li-layered transition-metal oxide (LTMO) electrodes such as LixNiyMnzCo1-y-zO2 (NMC) and LixNiyCo1-y-zAlzO2 (NCA) show reduced first cycle Coulombic efficiency (90-87% under standard cycling conditions) in comparison with the archetypal LixCoO2 (LCO; ∼98% efficiency). Focusing on LixNi0.8Co0.15Al0.05O2 as a model compound, we use operando synchrotron X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) spectroscopy to demonstrate that the apparent first-cycle capacity loss is a kinetic effect linked to limited Li mobility at x > 0.88, with near full capacity recovered during a potentiostatic hold following the galvanostatic charge-discharge cycle. This kinetic capacity loss, unlike many capacity losses in LTMOs, is independent of the cutoff voltage during delithiation and it is a reversible process. The kinetic limitation manifests not only as the kinetic capacity loss during discharge but as a subtle bimodal compositional distribution early in charge and, also, a dramatic increase of the charge-discharge voltage hysteresis at x > 0.88. 7Li NMR measurements indicate that the kinetic limitation reflects limited Li transport at x > 0.86. Electrochemical measurements on a wider range of LTMOs including Lix(Ni,Fe)yCo1-yO2 suggest that 5% substitution is sufficient to induce the kinetic limitation and that the effect is not limited to Ni substitution. We outline how, in addition to a reduction in the number of Li vacancies and shrinkage of the Li-layer size, the intrinsic charge storage mechanism (two-phase vs solid-solution) and localization of charge give rise to additional kinetic barriers in NCA and nonmetallic LTMOs in general.

Quantifying Reaction and Rate Heterogeneity in Battery Electrodes in 3D through Operando X-ray Diffraction Computed Tomography.

In composite battery electrode architectures, local limitations in ionic and electronic transport can result in nonuniform energy storage reactions. Understanding such reaction heterogeneity is important to optimizing battery performance, including rate capability and mitigating degradation and failure. Here, we use spatially resolved X-ray diffraction computed tomography to map the reaction in a composite electrode based on the LiFePO4 active material as it undergoes charge and discharge. Accelerated reactions at the electrode faces in contact with either the separator or the current collector demonstrate that both ionic and electronic transport limit the reaction progress. The data quantify how nonuniformity of the electrode reaction leads to variability in the charge/discharge rate, both as a function of time and position within the electrode architecture. Importantly, this local variation in the reaction rate means that the maximum rate that individual cathode particles experience can be substantially higher than the average, control charge/discharge rate, by a factor of at least 2-5 times. This rate heterogeneity may accelerate rate-dependent degradation pathways in regions of the composite electrode experiencing faster-than-average reaction and has important implications for understanding and optimizing rate-dependent battery performance. Benchmarking multiscale continuum model parameters against the observed reaction heterogeneity permits extension of these models to other electrode geometries.

Ultraporous nanocrystalline TiO2-based films: synthesis, patterning and application as anti-reflective, self-cleaning, superhydrophilic coatings.

Crack-free, anatase-based optical coatings with a refractive index down to 1.27, a porosity up to 80 vol%, and a tunable thickness up to 1.5 µm were fabricated. The extraordinary stability of the porosity upon thermally induced crystallisation and template removal was attributed to the combined effects of the presence of 10% molar silica in the inorganic phase, a flash treatment at 500 °C, and the use of templates with different dimensions ranging from a few nanometers to 50 nm. The hierarchical porous system was directly patterned by UV lithography and used as multifunctional anti-reflective, self-cleaning coatings.