Chemomechanics of ionically conductive ceramics for electrical energy conversion and storage

Functional materials for energy conversion and storage exhibit strong coupling between electrochemistry and mechanics. For example, ceramics developed as electrodes for both solid oxide fuel cells and batteries exhibit cyclic volumetric expansion upon reversible ion transport. Such chemomechanical coupling is typically far from thermodynamic equilibrium, and thus is challenging to quantify experimentally and computationally. In situ measurements and atomistic simulations are under rapid development to explore how this coupling can be used to potentially improve both device performance and durability. Here, we review the commonalities of coupling between electrochemical and mechanical states in fuel cell and battery materials, illustrating with specific cases the progress in materials processing, in situ characterization, and computational modeling and simulation. We also highlight outstanding questions and opportunities in these applications – both to better understand the limiting mechanisms within the materials and to significantly advance the durability and predictability of device performance required for renewable energy conversion and storage.United States. Dept. of Energy (Basic Energy Sciences Division of Materials Sciences and Engineering, grant DE-SC0002633)United States. Dept. of Energy (Office of Science, Graduate Fellowship Program (DOE SCGF))United States. American Recovery and Reinvestment Act of 2009 (ORISE-ORAU, contract no. DE-AC05-06OR23100))United States. Dept. of Energy. Division of Materials Sciences and Engineering (MIT/DMSE Salapatas Fellowship)United States. Air Force Office of Scientific Research (Presidential Early Career Award in Science and Engineering (PECASE)

Effect of Electrochemical Charging on Elastoplastic Properties and Fracture Toughness of Li[subscript X]CoO[subscript 2]

Mechanical degradation of lithium-ion battery (LIB) electrodes has been correlated with capacity fade and impedance growth over repeated charging and discharging. Knowledge of how the mechanical properties of materials used in LIBs are affected by electrochemical lithiation and delithiation could provide insight into design choices that mitigate mechanical damage and extend device lifetime. Here, we measured Young's modulus E, hardness H, and fracture toughness K[subscript Ic] via instrumented nanoindentation of the prototypical intercalation cathode, Li[subscript X]CoO[subscript 2], after varying durations of electrochemical charging. After a single charge cycle, E and H decreased by up to 60%, while K[subscript Ic] decreased by up to 70%. Microstructural characterization using optical microscopy, Raman spectroscopy, X-ray diffraction, and further nanoindentation showed that this degradation in K[subscript Ic] was attributable to Li depletion at the material surface and was also correlated with extensive microfracture at grain boundaries. These results indicate that K[subscript Ic] reduction and irreversible microstructural damage occur during the first cycle of lithium deintercalation from polycrystalline aggregates of Li[subscript X]CoO[subscript 2], potentially facilitating further crack growth over repeated cycling. Such marked reduction in K[subscript Ic] over a single charge cycle also yields important implications for the design of electrochemical shock-resistant cathode materials.United States. Dept. of Energy. Office of Basic Energy Sciences. Division of Materials Sciences and Engineering (Award DE-SC0002633)United States. Dept. of Energy. Office of Science Graduate Fellowship (Contract DE-AC05-06OR23100)Massachusetts Institute of Technology (Salapatas Fellowship

Dynamic chemical expansion of thin-film non-stoichiometric oxides at extreme temperatures

Actuator operation in increasingly extreme and remote conditions requires materials that reliably sense and actuate at elevated temperatures, and over a range of gas environments. Design of such materials will rely on high-temperature, high-resolution approaches for characterizing material actuation in situ. Here, we demonstrate a novel type of high-temperature, low-voltage electromechanical oxide actuator based on the model material Pr[subscript x]Ce[subscript 1−x]O[subscript 2−δ] (PCO). Chemical strain and interfacial stress resulted from electrochemically pumping oxygen into or out of PCO films, leading to measurable film volume changes due to chemical expansion. At 650 °C, nanometre-scale displacement and strain of >0.1% were achieved with electrical bias values <0.1 V, low compared to piezoelectrically driven actuators, with strain amplified fivefold by stress-induced structural deflection. This operando measurement of films ‘breathing’ at second-scale temporal resolution also enabled detailed identification of the controlling kinetics of this response, and can be extended to other electrochemomechanically coupled oxide films at extreme temperatures.United States. Department of Energy. Office of Basic Energy Sciences (Award DE-SC0002633)United States. Department of Energy (Grant DE-AC05-06OR23100