Research
The availability of low-cost but intermittent renewable electricity (e.g., derived from solar and wind) underscores the grand challenge to store and dispatch energy so that it is available when and where it is needed. Redox-active materials, both organic and inorganic, promise the efficient transformation between electrical, chemical, and thermal energy, and are at the heart of carbon-neutral energy cycles. Understanding design rules that govern materials chemistry and architecture holds the key towards rationally optimizing technologies such as batteries, fuel cells, and electrolyzers. Lithium-ion batteries, for example, are transforming mobility through electric vehicles and electricity grid through the storage of intermittent renewables. Metrics such as energy density, lifetime and safety are controlled by phenomena that span enormous length scales, from individual atoms to full systems, and times scales, from picoseconds to decades. Despite the significant progress over the past three decades, we still lack a complete understanding of how each length and time scale connects to one another, and most importantly, controls the behavior of the device. We seek to understand and engineer redox reactions at the levels of electrons, ions, molecules, particles and devices using a bottom-up approach. Our approach integrates novel synthesis, fabrication, characterization, modeling and analytics to understand molecular pathways and interfacial structure, and to bridge fundamentals to energy storage and conversion technologies by establishing new design rules. While our focus and approach are fundamental, The Chueh group’s work paves the way towards achieving the following goals that underpin a sustainable future:
We have four scientific themes focused on characterizing, understanding and controlling redox-active solids. 
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Research Themes
1. Tool kit for probing and interpreting electrochemistry in-situ
Properties of electrochemically-active solids vary exponentially with electrical potential, and are heterogeneous both at the atomic length scale (i.e., point defects) and at the macroscale (i.e., microstructure). As a result, ion-insertion processes are sensitive to the environment, hysteretic and heterogeneous. The Chueh group has developed numerous in-situ X-ray spectroscopy and microscopy platforms to track electrochemically-active materials while reactions take place. To interpret these large datasets, we employ advanced data analysis techniques to translate experimental measurements to mechanistic understandings. Using CoOXHy as a model electrocatalyst for oxygen evolution reaction in an alkaline aqueous electrolyte (Mefford et al., Nature, 2021), the Chueh group directly observed the reaction at the 1-20 nm resolution using X-ray and scanning probe microscopy, identifying the reaction mechanism as well as pinpointing the active site. In another example (Zhao et al., Nature 2023), the Chueh group and collaborators imaged the phase transformation of LiXFePO4 at 20-nm spatial resolution and < 1-min time resolution and employed data-driven mathematical modeling to describe 200,000 pixels simultaneously. This combined experimental-modeling approach quantified the Gibbs free energy landscape, reaction kinetics and heterogeneities.
2. Point defect chemistry, mechanics, and molecular insights of ion-insertion reactions
Point defects such as vacancies and interstitials control the reaction pathways at solid/solid, solid/gas and solid/liquid interfaces. Combining interface- and chemistry-sensitive X-ray probes and theoretical modeling, the Chueh group is investigating point defect chemistry by carefully controlling the atomic nature of the interface and quantifying the defect concentrations. For the solid/solid interface, we recently recognized that mechanical forces exerted by points defects contribute significantly to the chemo-mechanical properties of the interface (Chen et al. Phys. Chem. Chem. Phys. 2021). Specifically, mechanical effects contribute to the formation of equilibrium space-charge regions and alter the interfacial charge carrier concentration profiles. This chemo-mechanical coupling also extends to larger length scales (i.e., dislocations and fracture), with one example being Li plating and stripping in a solid-state electrolyte battery (McConohy et al. Nature Energy 2023). At the solid/liquid interface, we have established the importance not only of isolated point defects but also defect-driven phase transformation at critical levels of ion insertion. Using epitaxial thin films of LaNiO3-X as a model electrocatalyst for alkaline oxygen evolution reaction, we have shown that oxygen loss from the surface is accompanied by the formation of a disordered, pseudo-two-dimensional NiO2 surface phase (Baeumer et al. Nature Mater. 2021). This phase transformation is necessary for catalytic activity. At the solid/gas interface, we have identified the rate-limiting step in oxygen incorporation reaction in Pr0.1Ce0.9O2-x by quantifying the chemical and electrostatic driving forces at the interface (Chen et al., Nature Catal., 2020). Specifically, the dissociation of neutral oxygen adsorbate mediated by oxygen vacancies limits the rate of oxygen incorporation. In a related example, we investigated CO2 dissociation, also on CeO2-X (Skafte et al. Nature Energy 2019). We demonstrated that the selectivity towards the desired 2-electron reduction to CO (relative to the undesired 4-electron reduction to C) is controlled by the stability of carbonate and carboxylates adsorbed on oxygen vacancies. Overall, these examples highlight the rich interplay between point defects, mechanics and chemistry at the molecular level to determine reactivity of solids.
3. Coupling between local structure and redox – beyond the dilute limit
In many electrochemically-active solids, point defect concentrations can reach tens of atomic percent, such as for Li vacancies in LiXTMO2 layered oxides, a ubiquitous material for lithium-ion batteries. In this regime, defects can no longer be approximated as a dilute solid solution. The Chueh group has shown the interaction between these point defects (and the accompanying redox-structure couplings) can explain many of the mysteries at the macroscopic scale. Using layered oxides as a model system, we examined the coupling between Li defects, transition metal defects, and the Li redox potential. Specifically, through the analysis of local structure via X-ray diffraction and pair-distribution function analysis, we discovered that electron count in LiXTMO2 (established by the Li concentration) dramatically affects the stability of TM vacancies (Hong et al. Nature Mater. 2019). Under very oxidizing conditions, one expects that electrons are depopulated from the nonbonding O 2p state, leading the instability of the lattice oxygen. Rather than forming such oxygen vacancies, defect-defect interaction provides a more stable configuration, namely, the migration of TM from the TM layer to the Li layer and the subsequent decoordination of oxygen (Gent et al., Joule 2021). The increased bond order (i.e., covalency) between the TM and oxygen as well as between oxygen (i.e., peroxo) splits and raises some of the O 2p state, and quantitatively explains why TM disorder accompanies the stable oxidation of oxygen. We confirmed that this mechanism is general across 3d, 4d and 5d layered oxides. Following this line of inquiry, we also posed the question: what happens if the TM cannot migrate due to a kinetic barrier? In Na2-XMn3O7, the ordered Mn vacancies are immobile (Abate et al., Energy Environ. Sci. 2021). In this material, unlike other layered oxides, there is no pathway for oxygen decoordination to stabilize the high oxidation state. Instead, the ordered Mn vacancies electrostatically template the electron holes generated by de-sodiation and forms isolated O- species. These coulombic interactions rather covalent bonding disables the redox-structure coupling.
4. Electrochemically-induced heterogeneity across length scales
Variations in rate constants and composition contribute significantly to electrochemical heterogeneity, as do phase transformations. These so-called electrochemical hot spots drive degradation in devices such as lithium-ion batteries. The Chueh group has three major directions within this theme: (1) the development of heterogeneity under non-steady-state conditions, (2) coupling between many-particle population dynamics and autocatalysis (i.e., reactivity changes with the extent of reaction), and (3) heterogeneity across long timescales. In the first direction, we recognize that most electrochemical devices operate under dynamic, non-steady-state conditions. However, most studies focus on constant current operations. Using LiXFePO4 as a model system, the Chueh group examined how pulse charging and discharging modifies heterogeneity (Deng et al. ACS Nano, 2023). Surprisingly, contrary to the Fickian diffusion picture, we discovered that electrochemical pulses remove rather than enhance heterogeneity. Mechanistically, these pulses increase the rate of nucleation events, increasing uniformity. At the same time, these pulses also provide the driving force to bypass some phase transformation events, making it possible to sustain a metastable solid solution. Taken together, electrochemical pulses lead to significantly more uniform ion insertion. In the second direction, we recognize that reaction rate during ion insertion is not constant, arising from its compositional dependence. In fact, the reactivity can vary by several orders of magnitude. This variation in reactivity has significant implications on heterogeneity, especially in a many-particle system. Using a solid-solution solid (LiXTMO2) as a model system, we demonstrate that autocatalysis amplifies and (and also retards) heterogeneity (Park et al., Nature Mater. 2021). Specifically, incidental compositional heterogeneity in a population, for example, arising from particle size distribution, leads to variation in reaction rates between particles. When reactivity increases with the extent of reaction (as it is the case when de-lithiating LiXTMO2), particles with slightly less Li de-lithiates faster, thus amplifying the heterogeneity. The opposite is true when the reaction is reversed: heterogeneity is retarded during lithiation because reactivity decreases with the extent of reaction. In the third and final direction, we recognize that heterogeneity develop not solely from the reaction and transport of the majority ion but also of minority ions. One example is oxygen diffusion in LiXTMO2 (Csernica et al. Nature Energy 2022). Because the oxygen chemical potential is inversely coupled to the Li chemical potential, migration of one is coupled to the other. However, the timescale of transport is dramatically different, given the extremely low diffusivity of oxygen (~ 10-17 cm2 s-1). We show that an oxygen diffusion gradient emerges on the hundreds to thousands hour timescale, and has significant implication on charge balancing in the material. Overall, we are establishing mechanistic and predictive understandings of heterogeneity in ion-insertion material, enabling the engineering of more uniform and longer-lasting electrochemical devices.
Our research is supported by:
