Tuning the stability of Electrochemical Interfaces by Electron Transfer reactions

The morphology of interfaces is known to play fundamental role on the efficiency of energy-related applications, such light harvesting or ion intercalation. Altering the morphology on demand, however, is a very difficult task. Here, we show ways the morphology of interfaces can be tuned by driven electron transfer reactions. By using non-equilibrium thermodynamic stability theory, we uncover the operating conditions that alter the interfacial morphology. We apply the theory to ion intercalation and surface growth where electrochemical reactions are described using Butler-Volmer or coupled ion-electron transfer kinetics. The latter connects microscopic/quantum mechanical concepts with the morphology of electrochemical interfaces. Finally, we construct non-equilibrium phase diagrams in terms of the applied driving force (current/voltage) and discuss the importance of engineering the density of states of the electron donor in applications related to energy harvesting and storage, electrocatalysis and photocatalysis.Comment: 10 pages, 6 figure

Dielectric breakdown by electric-field induced phase separation

The control of the dielectric and conductive properties of device-level systems is important for increasing the efficiency of energy- and information-related technologies. In some cases, such as neuromorphic computing, it is desirable to increase the conductivity of an initially insulating medium by several orders of magnitude, resulting in effective dielectric breakdown. Here, we show that by tuning the value of the applied electric field in systems { with variable permittivity and electric conductivity}, e.g. ion intercalation materials, we can vary the device-level electrical conductivity by orders of magnitude. We attribute this behavior to the formation of filament-like conductive domains that percolate throughout the system, { which form only when the electric conductivity depends on the concentration}. We conclude by discussing the applicability of our results in neuromorphic computing devices and Li-ion batteries.Comment: 12 pages, 5 figure

Interplay of phase boundary anisotropy and electro-autocatalytic surface reactions on the lithium intercalation dynamics in LiX_XFePO4_4 platelet-like nanoparticles

Experiments on single crystal Li$_X$FePO$_4$ (LFP) nanoparticles indicate rich nonequilibrium phase behavior, such as suppression of phase separation at high lithiation rates, striped patterns of coherent phase boundaries, nucleation by binarysolid surface wetting and intercalation waves. These observations have been successfully predicted (prior to the experiments) by 1D depth-averaged phase-field models, which neglect any subsurface phase separation. In this paper, using an electro-chemo-mechanical phase-field model, we investigate the coherent non-equilibrium subsurface phase morphologies that develop in the $ab$- plane of platelet-like single-crystal platelet-like Li$_X$FePO$_4$ nanoparticles. Finite element simulations are performed for 2D plane-stress conditions in the $ab$- plane, and validated by 3D simulations, showing similar results. We show that the anisotropy of the interfacial tension tensor, coupled with electroautocatalytic surface intercalation reactions, plays a crucial role in determining the subsurface phase morphology. With isotropic interfacial tension, subsurface phase separation is observed, independent of the reaction kinetics, but for strong anisotropy, phase separation is controlled by surface reactions, as assumed in 1D models. Moreover, the driven intercalation reaction suppresses phase separation during lithiation, while enhancing it during delithiation, by electro-autocatalysis, in quantitative agreement with {\it in operando} imaging experiments in single-crystalline nanoparticles, given measured reaction rate constants

Vortices of Electro-osmotic Flow in Heterogeneous Porous Media

Traditional models of electrokinetic transport in porous media are based on homogenized material properties, which neglect any macroscopic effects of microscopic fluctuations. This perspective is taken not only for convenience, but also motivated by the expectation of irrotational electro-osmotic flow, proportional to the electric field, for uniformly charged surfaces (or constant zeta potential) in the limit of thin double layers. Here, we show that the inherent heterogeneity of porous media generally leads to macroscopic vortex patterns, which have important implications for convective transport and mixing. These vortical flows originate due to competition between pressure-driven and electro-osmotic flows, and their size are characterized by the correlation length of heterogeneity in permeability or surface charge. The appearance of vortices is controlled by a single dimensionless control parameter, defined as the ratio of a typical electro-osmotic velocity to the total mean velocity

Theory of coupled ion-electron transfer kinetics

The microscopic theory of chemical reactions is based on transition state theory, where atoms or ions transfer classically over an energy barrier, as electrons maintain their ground state. Electron transfer is fundamentally different and occurs by tunneling in response to solvent fluctuations. Here, we develop the theory of coupled ion-electron transfer, in which ions and solvent molecules fluctuate cooperatively to facilitate electron transfer. We derive a general formula of the reaction rate that depends on the overpotential, solvent properties, the electronic structure of the electron donor/acceptor, and the excess chemical potential of ions in the transition state. For Faradaic reactions, the theory predicts curved Tafel plots with a concentration-dependent reaction-limited current. For moderate overpotentials, our formula reduces to the Butler-Volmer equation and explains its relevance, not only in the well-known limit of large electron-transfer (solvent reorganization) energy, but also in the opposite limit of large ion-transfer energy. The rate formula is applied to Li-ion batteries, where reduction of the electrode host material couples with ion insertion. In the case of lithium iron phosphate, the theory accurately predicts the concentration dependence of the exchange current measured by {\it in operando} X-Ray microscopy without any adjustable parameters. These results pave the way for interfacial engineering to enhance ion intercalation rates, not only for batteries, but also for ionic separations and neuromorphic computing

Electrochemical and Transport processes in Ion Intercalation materials

Moving towards an environmentally sustainable society, energy storage becomes increasingly important. As has been widely recognized with the recent Nobel prize in chemistry, ion intercalation materials play an essential role in the state-of-the-art energy storage technologies, such as Li-ion batteries, as they are used everywhere around us; from the phones we use to the cars we drive. Intercalation-based energy storage devices can be thought as an electrochemical plant where processes related to transport phenomena (solid/liquid diffusion) and kinetics (ion insertion/extraction) take place. Even though extensive research has been done in understanding and optimizing the solid and liquid diffusion properties of the active materials and electrolytes, very few studies have been conducted in elucidating the fundamentals of ion insertion kinetics that take place at interfaces. Additionally, most ion intercalation materials tend to phase separate into ion–rich and –poor phases under ion insertion/extraction. Our understanding, however, on the effects of intercalation rates, applied electric fields and the microscopic nature of the reactions on the resulting phase morphologies is not complete. The main goal of this thesis is to comprehend, from a very basic level, the reasons that limit the performance of energy (e.g. Li-ion batteries) and information (memristors) storage technologies, and provide insights and engineering solutions for next generation intercalation-based devices that deliver high efficiency and optimal performance. In the first part of the thesis, I develop the theory of coupled ion-electron transfer, where both the ion and the electron have to be transferred in a concerted way. By using simulations and experiments I demonstrate the use of the theory on ion intercalation, the fundamental process of Li-ion batteries. In the second part of this thesis, I derive a simple theory that unifies the behavior of all phase-separating electrode materials under driven ion insertion. The proposed criterion predicts the non-equilibrium phase morphology during ion insertion, which is validated by phase-field simulations of single LiCoO2 (LCO) particles, in situ optical imaging of single LiC6 (graphite) particles undergoing transitions between stage 1 ( = 1) and 2 ( = 0.5) at different rates, and collapse of all the available literature data for single-particle imaging of 3 LCO, graphite and LiFePO4. In the third part, I investigate the influence of different electron transfer kinetics on the thermodynamic phase stability of open driven systems. By using non-equilibrium thermodynamics, I demonstrate different ways to control the morphology of interfaces during operation, e.g. ion intercalation, electroplating, corrosion, and conclude that the electronic density of states of the electron donor is key on engineering the morphology of electrochemical interfaces. In the last part of the thesis, I focus on the effects of electric fields on the thermodynamic stability of ion intercalation materials. There, I develop the theory that describes how electric fields induce metal-to-insulator transitions and lead to effective dielectric breakdown, which is essential in (non-)volatile memristive devices. In summary, this thesis constitutes a comprehensive study of how kinetics, phase separation and transport phenomena affect the non-equilibrium response of ion intercalation materials. The theoretical, computational, and experimental results of the present work can serve as a guideline for engineering ion intercalation systems suitable for fast-charging energy storage devices and non-volatile neuromorphic chips. In addition to its practical aspect, the present thesis expands the boundaries of what is known in terms of reaction kinetics, phase-separating driven open systems, and non-equilibrium thermodynamics.Ph.D