Deniz Gunceler

Nanoscale imaging of lithium ion distribution during in situ operation of battery electrode and electrolyte.

A major challenge in the development of new battery materials is understanding their fundamental mechanisms of operation and degradation. Their microscopically inhomogeneous nature calls for characterization tools that provide operando and localized information from individual grains and particles. Here, we describe an approach that enables imaging the nanoscale distribution of ions during electrochemical charging of a battery in a transmission electron microscope liquid flow cell. We use valence energy-loss spectroscopy to track both solvated and intercalated ions, with electronic structure fingerprints of the solvated ions identified using an ab initio nonlinear response theory. Equipped with the new electrochemical cell holder, nanoscale spectroscopy and theory, we have been able to determine the lithiation state of a LiFePO4 electrode and surrounding aqueous electrolyte in real time with nanoscale resolution during electrochemical charge and discharge. We follow lithium transfer between electrode and electrolyte and image charging dynamics in the cathode. We observe competing delithiation mechanisms such as core-shell and anisotropic growth occurring in parallel for different particles under the same conditions. This technique represents a general approach for the operando nanoscale imaging of electrochemically active ions in the electrode and electrolyte in a wide range of electrical energy storage systems.

The importance of nonlinear fluid response in joint density-functional theory studies of battery systems

Delivering the full benefits of first-principles calculations to battery materials demands the development of accurate and computationally efficient electronic structure methods that incorporate the effects of the electrolyte environment and electrode potential. Realistic electrochemical interfaces containing polar surfaces are beyond the regime of validity of existing continuum solvation theories developed for molecules, due to the presence of significantly stronger electric fields. We present an ab initio theory of the nonlinear dielectric and ionic response of solvent environments within the framework of joint density-functional theory, with precisely the same optimizable parameters as conventional polarizable continuum models. We demonstrate that the resulting nonlinear theory agrees with the standard linear models for organic molecules and metallic surfaces under typical operating conditions. However, we find that the saturation effects in the rotational response of polar solvent molecules, inherent to our nonlinear theory, are crucial for a qualitatively correct description of the ionic surfaces typical of the solid electrolyte interface.

Structure of the Photo-catalytically Active Surface of SrTiO3

A major goal of energy research is to use visible light to cleave water directly, without an applied voltage, into hydrogen and oxygen. Although SrTiO3 requires ultraviolet light, after four decades, it is still the gold standard for the photo-catalytic splitting of water. It is chemically robust and can carry out both hydrogen and oxygen evolution reactions without an applied bias. While ultrahigh vacuum surface science techniques have provided useful insights, we still know relatively little about the structure of these electrodes in contact with electrolytes under operating conditions. Here, we report the surface structure evolution of a n-SrTiO3 electrode during water splitting, before and after training with an applied positive bias. Operando high-energy X-ray reflectivity measurements demonstrate that training the electrode irreversibly reorders the surface. Scanning electrochemical microscopy at open circuit correlates this training with a 3-fold increase of the activity toward the photo-induced water splitting. A novel first-principles joint density functional theory simulation, constrained to the X-ray data via a generalized penalty function, identifies an anatase-like structure as the more active, trained surface.

Electroless Formation of Hybrid Lithium Anodes for Fast Interfacial Ion Transport

Rechargeable batteries based on metallic anodes are of interest for fundamental and application-focused studies of chemical and physical kinetics of liquids at solid interfaces. Approaches that allow facile creation of uniform coatings on these metals to prevent physical contact with liquid electrolytes, while enabling fast ion transport, are essential to address chemical instability of the anodes. Here, we report a simple electroless ion-exchange chemistry for creating coatings of indium on lithium. By means of joint density functional theory and interfacial characterization experiments, we show that In coatings stabilize Li by multiple processes, including exceptionally fast surface diffusion of lithium ions and high chemical resistance to liquid electrolytes. Indium coatings also undergo reversible alloying reactions with lithium ions, facilitating design of high-capacity hybrid In-Li anodes that use both alloying and plating approaches for charge storage. By means of direct visualization, we further show that the coatings enable remarkably compact and uniform electrodeposition. The resultant In-Li anodes are shown to exhibit minimal capacity fade in extended galvanostatic cycling when paired with commercial-grade cathodes.

Weighted-density functionals for cavity formation and dispersion energies in continuum solvation models

Continuum solvation models enable efficient first principles calculations of chemical reactions in solution, but require extensive parametrization and fitting for each solvent and class of solute systems. Here, we examine the assumptions of continuum solvation models in detail and replace empirical terms with physical models in order to construct a minimally-empirical solvation model. Specifically, we derive solvent radii from the nonlocal dielectric response of the solvent from ab initio calculations, construct a closed-form and parameter-free weighted-density approximation for the free energy of the cavity formation, and employ a pair-potential approximation for the dispersion energy. We show that the resulting model with a single solvent-independent parameter: the electron density threshold (nc), and a single solvent-dependent parameter: the dispersion scale factor (s6), reproduces solvation energies of organic molecules in water, chloroform, and carbon tetrachloride with RMS errors of 1.1, 0.6 and 0.5 kcal/mol, respectively. We additionally show that fitting the solvent-dependent s6 parameter to the solvation energy of a single non-polar molecule does not substantially increase these errors. Parametrization of this model for other solvents, therefore, requires minimal effort and is possible without extensive databases of experimental solvation free energies.

Designing solid-liquid interphases for sodium batteries

Secondary batteries based on earth-abundant sodium metal anodes are desirable for both stationary and portable electrical energy storage. Room-temperature sodium metal batteries are impractical today because morphological instability during recharge drives rough, dendritic electrodeposition. Chemical instability of liquid electrolytes also leads to premature cell failure as a result of parasitic reactions with the anode. Here we use joint density-functional theoretical analysis to show that the surface diffusion barrier for sodium ion transport is a sensitive function of the chemistry of solid–electrolyte interphase. In particular, we find that a sodium bromide interphase presents an exceptionally low energy barrier to ion transport, comparable to that of metallic magnesium. We evaluate this prediction by means of electrochemical measurements and direct visualization studies. These experiments reveal an approximately three-fold reduction in activation energy for ion transport at a sodium bromide interphase. Direct visualization of sodium electrodeposition confirms large improvements in stability of sodium deposition at sodium bromide-rich interphases.The chemistry at the interface between electrolyte and electrode plays a critical role in determining battery performance. Here, the authors show that a NaBr enriched solid–electrolyte interphase can lower the surface diffusion barrier for sodium ions, enabling stable electrodeposition.

JDFTx: Software for joint density-functional theory

Density-functional theory (DFT) has revolutionized computational prediction of atomic-scale properties from first principles in physics, chemistry and materials science. Continuing development of new methods is necessary for accurate predictions of new classes of materials and properties, and for connecting to nano- and mesoscale properties using coarse-grained theories. JDFTx is a fully-featured open-source electronic DFT software designed specifically to facilitate rapid development of new theories, models and algorithms. Using an algebraic formulation as an abstraction layer, compact C++11 code automatically performs well on diverse hardware including GPUs (Graphics Processing Units). This code hosts the development of joint density-functional theory (JDFT) that combines electronic DFT with classical DFT and continuum models of liquids for first-principles calculations of solvated and electrochemical systems. In addition, the modular nature of the code makes it easy to extend and interface with, facilitating the development of multi-scale toolkits that connect to ab initio calculations, e.g. photo-excited carrier dynamics combining electron and phonon calculations with electromagnetic simulations.

Towards a generalized iso-density continuum model for molecular solvents in plane-wave DFT

Implicit electron-density solvation models offer a computationally efficient solution to the problem of calculating thermodynamic quantities of solvated systems from first-principles quantum mechanics. However, despite much recent interest in such models, to date the applicability of such models in the plane-wave context to non-aqueous solvents has been limited because the determination of the model parameters requires fitting to a large database of experimental solvation energies for each new solvent considered. This work presents a simple approach to quickly find approximations to the non-electrostatic contributions to the solvation energy, allowing for development of new iso-density models for a large class of protic and aprotic solvents from only simple, single-molecule ab initio calculations and readily available bulk thermodynamic data. Finally, to illustrate the capabilities of the resulting theory, we also calculate the surface solvation energies of crystalline LiF in various different non-aqueous solvents, and discuss the observed trends and their relevance to lithium battery technology.

A study of the density functional methods on the photoabsorption of bodipy dyes

Abstract Tunability of the photoabsorption and directional charge injection characteristics of Bodipy-based dye molecules with different carbonyl groups make them promising candidates for photovoltaic applications. In order to study the effect of screening in the Coulomb interaction on the electronic and optical properties of two Bodipy derivatives, we have used linear response time-dependent and exact exchange hybrid density functional approaches. The effect of linear and non-linear solvation models on the electrochemical properties of the dyes has also been discussed.

Anatase TiO2 nanowires functionalized by organic sensitizers for solar cells: A screened Coulomb hybrid density functional study

The adsorption of two different organic molecules cyanidin glucoside (C21O11H20) and TA-St-CA on anatase (101) and (001) nanowires has been investigated using the standard and the range separated hybrid density functional theory calculations. The electronic structures and optical spectra of resulting dye–nanowire combined systems show distinct features for these types of photochromophores. The lowest unoccupied molecular orbital of the natural dye cyanidin glucoside is located below the conduction band of the semiconductor while, in the case of TA-St-CA, it resonates with the states inside the conduction band. The wide-bandgap anatase nanowires can be functionalized for solar cells through electron-hole generation and subsequent charge injection by these dye sensitizers. The intermolecular charge transfer character of Donor-π-Acceptor type dye TA-St-CA is substantially modified by its adsorption on TiO2 surfaces. Cyanidin glucoside exhibits relatively stronger anchoring on the nanowires through its hydroxyl groups. The atomic structures of dye–nanowire systems re-optimized with the inclusion of nonlinear solvation effects showed that the binding strengths of both dyes remain moderate even in ionic solutions.