Coherent Phase Change in Interstitial Solutions: A Hierarchy of Instabilities

Metal hydrides or lithium ion battery electrodes can take the form of interstitial solid solutions with a miscibility gap. This work discusses theory approaches for locating, in temperature-composition space, coherent phase transformations during the charging/discharging of such systems and for identifying the associated transformation mechanisms. The focus is on the simplest scenario, where instabilities derive from the thermodynamics of the bulk phase alone, considering strain energy as the foremost consequence of coherency and admitting for stress relaxation at free surfaces. The extension of the approach to include capillarity is demonstrated by an example. The analysis rests on constrained equilibrium phase diagrams that are informed by geometry- and dimensionality-specific mechanical boundary conditions and on elastic instabilities-again geometry-specific-as implied by the theory of open-system elasticity. It is demonstrated that some scenarios afford the analysis of chemical stability to be based entirely on a linear stability analysis of the mechanical equilibrium, which provides closed-form solutions in a straightforward manner. Attention is on the impact of the system geometry (infinitely extended or of finite size) and on the chemical (closed or open system) and mechanical (incoherent or coherent) boundary conditions. Transformation mechanism maps are suggested for documenting the findings. The maps reveal a hierarchy of instabilities, which depend strongly on each of the above characteristics. Specifically, realistic, finite-sized systems differ qualitatively from idealized systems of infinite extension. Among the transformation mechanisms exposed by the analysis are a uniform switchover to the other phase when the open system reaches its chemical spinodal, practical coherent nucleation, as well as chemo-elastically coupled spontaneous buckling modes, which may take the form of either, single-phase or dual-phase states.

Biotene: Earth-Abundant 2D Material as Sustainable Anode for Li/Na-Ion Battery

Natural ores are abundant, cost-effective, and environmentally friendly. Ultrathin (2D) layers of a naturally abundant van der Waals mineral, Biotite, have been prepared in bulk via exfoliation. We report here that this 2D Biotene material has shown extraordinary Li-Na-ion battery anode properties with ultralong cycling stability. Biotene shows 302 and 141 mAh g-1 first cycle-specific charge capacity for Li- and Na-ion battery applications with ∼90% initial Coulombic efficiency. The electrode exhibits significantly extended cycling stability with ∼75% capacity retention after 4000 cycles even at higher current densities (500-2000 mA g-1). Further, density functional theory studies show the possible Li intercalation mechanism between the 2D Biotene layers. Our work brings new directions toward designing the next generation of metal-ion battery anodes.

5D Analysis of Capacity Degradation in Battery Electrodes Enabled by Operando CT-XANES

For devices encountering long-term stability challenges, a precise evaluation of degradation is of paramount importance. However, methods for comprehensively elucidating the degradation mechanisms in devices, particularly those undergoing dynamic chemical and mechanical changes during operation, such as batteries, are limited. Here, a method is presented using operando computed tomography combined with X-ray absorption near-edge structure spectroscopy (CT-XANES) that can directly track the evolution of the 3D distribution of the local capacity loss in battery electrodes during (dis)charge cycles, thereby enabling a five-dimensional (the 3D spatial coordinates, time, and chemical state) analysis of the degradation. This paper demonstrates that the method can quantify the spatiotemporal dynamics of the local capacity degradation within an electrode during cycling, which has been truncated by existing bulk techniques, and correlate it with the overall electrode performance degradation. Furthermore, the method demonstrates its capability to uncover the correlation among observed local capacity degradation within electrodes, reaction history during past (dis)charge cycles, and electrode microstructure. The method thus provides critical insights into the identification of degradation factors that are not available through existing methods, and therefore, will contribute to the development of batteries with long-term stability.

Descriptor and Scaling Relations for Ion Mobility in Crystalline Solids

Ion mobility is a critical performance parameter not only in electrochemical energy storage and conversion but also in other electrochemical devices. On the basis of first-principles electronic structure calculations, we have derived a descriptor for the ion mobility in battery electrodes and solid electrolytes. This descriptor is entirely composed of observables that are easily accessible: ionic radii, oxidation states, and the Pauling electronegativities of the involved species. Within a particular class of materials, the migration barriers are connected to this descriptor through linear scaling relations upon the variation of either the cation chemistry of the charge carriers or the anion chemistry of the host lattice. The validity of these scaling relations indicates that a purely ionic view falls short of capturing all factors influencing ion mobility in solids. The identification of these scaling relations has the potential to significantly accelerate the discovery of materials with desired mobility properties.

Engineering Multiscale Coupled Electron/Ion Transport in Battery Electrodes

Coupled electron/ion transport is a defining characteristic of electrochemical processes, for example, battery charge/discharge. Analytical models that represent the complex transport and electrochemical processes in an electrode in terms of equivalent electrical circuits provide a simple, but successful framework for understanding the kinetics of these coupled transport phenomena. The premise of this review is that the nature of the time-dependent phase transitions in dynamic electrochemical environments serves as an important design parameter, orthogonal to the intrinsic mixed conducting properties of the active materials in battery electrodes. A growing body of literature suggests that such phase transitions can produce divergent extrinsic resistances in a circuit model (e.g., R_e, describing electron transport from an active electrode material to the current collector of an electrode, and/or R_ion, describing ion transport from a bulk electrolyte to the active material surface). It is found that extrinsic resistances of this type play a determinant role in both the electrochemical performance and long-term stability of most battery electrodes. Additionally, successful suppression of the tendency of extrinsic resistances to accumulate over time is a requirement for practical rechargeable batteries and an important target for rational design. We highlight the need for battery electrode and cell designs, which explicitly address the specific nature of the structural phase transition in active materials, and for advanced fabrication techniques that enable precise manipulations of matter at multiple length scales: (i) meso-to-macroscopic conductive frameworks that provide contiguous electronic/ion pathways; (ii) nanoscale uniform interphases formed on active materials; and (iii) molecular-level structures that promote fast electron and/or ion conduction and mechanical resilience.