Electron work function - a probe for interfacial diagnosis

Operando Characterization and Theoretical Modeling of Metal|Electrolyte Interphase Growth Kinetics in Solid-State Batteries. Part II: Modeling

Understanding the interfacial dynamics of batteries is crucial to control degradation and increase electrochemical performance and cycling life. If the chemical potential of a negative electrode material lies outside of the stability window of an electrolyte (either solid or liquid), a decomposition layer (interphase) will form at the interface. To better understand and control degradation at interfaces in batteries, theoretical models describing the rate of formation of these interphases are required. This study focuses on the growth kinetics of the interphase forming between solid electrolytes and metallic negative electrodes in solid-state batteries. More specifically, we demonstrate that the rate of interphase formation and metal plating during charge can be accurately described by adapting the theory of coupled ion-electron transfer (CIET). The model is validated by fitting experimental data presented in the first part of this study. The data was collected operando as a Na metal layer was plated on top of a NaSICON solid electrolyte (Na3.4Zr2Si2.4P0.6O12 or NZSP) inside an XPS chamber. This study highlights the depth of information which can be extracted from this single operando experiment and is widely applicable to other solid-state electrolyte systems.

Dependence of Interfacial Adhesion between Substances on Their Electron Work Functions

In this article, we demonstrate the dependence of the adhesive force (F_Ad) between two different substances on their electron work functions (EWF or φ) without atomic diffusion involved. The adhesive forces between Si₃N₄ and a number of metals were measured using an atomic force microscope. It is shown that the larger the difference in φ between the two substances in contact, the larger the F_Ad. F_Ad is also influenced by the electron freedom and density (related to the charge availability). An analytical model is proposed to elucidate the underlying mechanism and quantify the adhesive interaction.

Electron work function: an indicative parameter towards a novel material design methodology

Electron work function (EWF) has demonstrated its great promise in materials analysis and design, particularly for single-phase materials, e.g., solute selection for optimal solid-solution strengthening. Such promise is attributed to the correlation of EWF with the atomic bonding and stability, which largely determines material properties. However, engineering materials generally consist of multiple phases. Whether or not the overall EWF of a complex multi-phase material can reflect its properties is unclear. Through investigation on the relationships among EWF, microstructure, mechanical and electrochemical properties of low-carbon steel samples with two-level microstructural inhomogeneity, we demonstrate that the overall EWF does carry the information on integrated electron behavior and overall properties of multiphase alloys. This study makes it achievable to develop "electronic metallurgy"-an electronic based novel alternative methodology for materials design.

Tuning the Conductivity and Electron Work Function of a Spin-Coated PEDOT:PSS/PEO Nanofilm for Enhanced Interfacial Adhesion

We report a novel phenomenon of increasing the adherence of a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS/PEO) nanofilm for Si₃N₄ through cosolvent treatment by DMSO. By varying the w/w% ratio of DMSO, nanofilms with different conductivities were produced. Atomic force microscopy (AFM) analysis showed that the adhesive force between the AFM's Si₃N₄ probe and the nanofilm increased by 35.8% as the conductivity of the nanofilm increased. The conductivity became saturated after the PEDOT:PSS-to-DMSO ratio reached a certain level. This study demonstrates that the variations in the adhesive force are determined by two factors: (1) the difference in EWF between the nanofilm and the counter-body Si₃N₄ and (2) the electrical conductivity of the materials involved. The former is used for establishing a dipole layer at the interface, while the latter determines the degree of ease to achieve the dipole layer. This study demonstrates an approach to tailor interfacial bonding for different types of materials without atomic diffusion, which is promising for applications in various fields such as control of biomedical films on implants and functional films for electronic devices.

Understanding the Strength of the Selenium-Graphene Interfaces for Energy Storage Systems

We present comprehensive first-principles density functional theory (DFT) analyses of the interfacial strength and bonding mechanisms between crystalline and amorphous selenium (Se) with graphene (Gr), a promising duo for energy storage applications. Comparative interface analyses are presented on amorphous silicon (Si) with graphene and crystalline Se with a conventional aluminum (Al) current collector. The interface strengths of monoclinic Se (0.43 J m^-2) and amorphous Si with graphene (0.41 J m^-2) are similar in magnitude. While both materials (c-Se, a-Si) are bonded loosely by van der Waals (vdW) forces over graphene, interfacial electron exchange is higher for a-Si/graphene. This is further elaborated by comparing the potential energy step and charge transfer (Δq) across the graphene interfaces. The interface strength of c-Se on a 3D Al current collector is higher (0.99 J m^-2), suggesting a stronger adhesion. Amorphous Se with graphene has comparable interface strength (0.34 J m^-2), but electron exchange in this system is slightly distinct from monoclinic Se. The electronic characteristics and bonding mechanisms are different for monoclinic and amorphous Se with graphene as they activate graphene via surface charge doping divergently. The implications of these interfacial physicochemical attributes on electrode performance have been discussed. Our findings highlight the complex electrochemical phenomena in Se interfaced with graphene, which may profoundly differ from their "free" counterparts.

Evaluation of interfacial stability and strength of cermets based on work function

A new method based on work function to analyze the interfacial stability and strength of ceramic-metal composites was proposed in this work. The interfacial work function gradient and interfacial elastic modulus were evaluated experimentally using WC-Co and TiC-Co as the examples. It found that a stable and strongly bonded interface had a gradually changing interfacial work function, while a weak interface exhibited a steep work function changing across the interface. The spatial resolution of the experimental analysis could be down to 10 nm with a high work function sensitivity. First-principles calculations were conducted to analyze the electronic configurations across the interfaces. They revealed the potential distribution across the interfaces in the sub-nano scale. They demonstrated that the interface with a smaller interfacial work function gradient had smaller interface energy and stronger interfacial bonds, and thus the interface was more stable and stronger. The calculation disclosed the mechanism of the experimental observations of the interfacial work function. Both the experimental and theoretical studies confirmed that the interfacial work function gradient could be a measure of the interactions across the interfaces. The effectiveness of the established model was demonstrated by analyzing the stability of thin films at WC/Co interfaces. This study provides a new method to evaluate the interfacial stability and bonding strength for cermets.