Metal-ceramic adhesion and the Harris functional

Thermodynamics, Adhesion, and Wetting at Li/Cu(-Oxide) Interfaces: Relevance for Anode-Free Lithium-Metal Batteries

A rechargeable battery that employs a Li metal anode requires that Li be plated in a uniform fashion during charging. In "anode-free" configurations, this plating will occur on the surface of the Cu current collector (CC) during the initial cycle and in any subsequent cycle where the capacity of the cell is fully accessed. Experimental measurements have shown that the plating of Li on Cu can be inhomogeneous, which can lower the efficiency of plating and foster the formation of Li dendrites. The present study employs a combination of first-principles calculations and sessile drop experiments to characterize the thermodynamics and adhesive (i.e., wetting) properties of interfaces involving Li and other phases present on or near the CC. Interfaces between Li and Cu, Cu2O, and Li2O are considered. The calculations predict that both Cu and Cu2O surfaces are lithiophilic. However, sessile drop measurements reveal that Li wetting occurs readily only on pristine Cu. This apparent discrepancy is explained by the occurrence of a spontaneous conversion reaction, 2 Li + Cu2O → Li2O + 2 Cu, that generates Li2O as one of its products. Calculations and sessile drop measurements show that Li does not wet (newly formed) Li2O. Hence, Li that is deposited on a Cu CC where surface oxide species are present will encounter a compositionally heterogeneous substrate comprising lithiophillic (Cu) and lithiophobic (Li2O) regions. These initial heterogeneities have the potential to influence the longer-term behavior of the anode under cycling. In sum, the present study provides insights into the early stage processes associated with Li plating in anode-free batteries and describes mechanisms that contribute to inefficiencies in their operation.

Understanding and Engineering Interfacial Adhesion in Solid-State Batteries with Metallic Anodes

High performance alkali metal anode solid-state batteries require solid/solid interfaces with fast ion transfer that are morphologically and chemically stable upon electrochemical cycling. Void formation at the alkali metal/solid-state electrolyte interface during alkali metal stripping is responsible for constriction resistances and hotspots that can facilitate dendrite propagation and failure. Both externally applied pressures (35-400 MPa) and temperatures above the melting point of the alkali metal have been shown to improve the interfacial contact with the solid electrolyte, preventing the formation of voids. However, the extreme pressure and temperature conditions required can be difficult to meet for commercial solid-state battery applications. In this review, we highlight the importance of interfacial adhesion or 'wetting' at alkali metal/solid electrolyte interfaces for achieving solid-state batteries that can withstand high current densities without cell failure. The intrinsically poor adhesion at metal/ceramic interfaces poses fundamental limitations on many inorganics solid-state electrolyte systems in the absence of applied pressure. Suppression of alkali metal voids can only be achieved for systems with high interfacial adhesion (i. e. 'perfect wetting') where the contact angle between the alkali metal and the solid-state electrolyte surface goes to θ=0°. We identify key strategies to improve interfacial adhesion and suppress void formation including the adoption of interlayers, alloy anodes and 3D scaffolds. Computational modeling techniques have been invaluable for understanding the structure, stability and adhesion of solid-state battery interfaces and we provide an overview of key techniques. Although focused on alkali metal solid-state batteries, the fundamental understanding of interfacial adhesion discussed in this review has broader applications across the field of chemistry and material science from corrosion to biomaterials development.

Enhancing Wettability of Cu3P/Cu Systems through Doping with Si, Sn, and Zr Elements: Insights from First Principles Analysis

Explaining the wetting mechanism of Cu-P brazing materials and Cu remains challenging. This fundamental research aims to reveal the wettability mechanism of Si, Sn, and Zr doping on the interfacial bond strength of the Cu₃P/Cu system through the first principles study. We carried out several sets of calculations to test the validity of the result; included in the work are those used to establish the interfacial structure and to analyze the effect of doping on the wettability. Specific analysis was carried out in terms of three aspects: the work of adhesion (Wad), the charge density difference, and the density of states (DOS). The calculated results show that doping with Si, Sn, and Zr elements can effectively improve the wettability within the CuP/Cu interface with very high accuracy, and is particularly effective when doped with Zr. These results provide an insightful theoretical guide for enhancing the CuP/Cu system's wettability by adding active elements.

The Stability and Electronic Structure of Cu(200)/AuCu(200) Interface: An Insight from First-Principle Calculation

AuCu phase had a significant effect on the bonding strength of Au80Sn20 alloy and Cu substrate. The formation of the AuCu(200)/Cu(200) interface significantly improves the shear strength of solder joints. Therefore, it is particularly important to analyze the strengthening mechanism of the AuCu phase in the Cu matrix. The atomic structure, interfacial stability, and interfacial bonding properties of the Cu(200)/AuCu(200) interface were investigated using first-principle calculation. The layer spacing convergence results show that seven layers of Cu(200) surface and seven layers of AuCu(200) surface are enough thick to be chosen for the interface model. The calculation shows that the surface energies are 1.463 J/m² and 1.081 J/m² for AuCu(200) surface and Cu(200) surface, respectively. Four interface combinations of Top sit, Long bridge, Short bridge, and Hollow were investigated by considering four stacking methods of AuCu(200). It is shown that the interfacial configuration of the Long bridge is the most stable and favorable structure, which has the largest adhesion work, the smallest interfacial energy, and the smallest interfacial spacing. The density of states and electron difference density were calculated for the four interfacial configurations, and the results showed that the main bonding mode of the Long bridge interface is composed of both Cu-Cu covalent bonds and Au-Cu covalent bonds.

Predicting Wettability and the Electrochemical Window of Lithium-Metal/Solid Electrolyte Interfaces

The development of solid electrolytes (SEs) is expected to enhance the safety of lithium-ion batteries. Additionally, a viable SE could allow the use of a Li-metal negative electrode, which would increase energy density. Recently, several antiperovskites have been reported to exhibit high ionic conductivities, prompting investigations of their use as an SE. In addition to having a suitable conductivity, phenomena at the interface between an SE and an electrode are also of great importance in determining the viability of an SE. For example, interfacial interactions can change the positions of the band edges of the SE, altering its stability against undesirable oxidation or reduction. Furthermore, the wettability of the SE by the metallic anode is desired to enable low interfacial resistance and uniform metal plating and stripping during cycling. The present study probes several properties of the SE/electrode interface at the atomic scale. Adopting the antiperovskite SE Li₃OCl (LOC)/Li-metal anode interface as a model system, the interfacial energy, work of adhesion, wettability, band edge shifts, and the electrochemical window are predicted computationally. The oxygen-terminated interface was determined to be the most thermodynamically stable. Moreover, the large calculated work of adhesion for this system implies that Li will wet LOC, suggesting the possibility for low interfacial resistance. Nevertheless, these strong interfacial interactions come at a cost to electrochemical stability: strong interfacial bonding lowers the energy of the conduction band minimum (CBM) significantly and narrows the local band gap by 30% in the vicinity of the interface. Despite this interface-induced reduction in electrochemical window, the CBM in LOC remains more negative than the Li/Li⁺ redox potential, implying stability against reduction by the anode. In sum, this study illustrates a comprehensive computational approach to assessing electrode/electrolyte interfacial properties in solid-state batteries.

Effects of dopants on the adhesion and electronic structure of a SnO2/Cu interface: a first-principles study

Doping has been adopted as a versatile approach for tuning the adhesion of metal oxide/metal interfaces. Understanding the mechanism of doping at the interface adhesion on the atomic and electronic scale is crucial for the rational design and optimization of metal oxide/metal composites. In this work, we have investigated the effects of dopants on the adhesion of SnO2/Cu interfaces through first-principles calculations. Firstly, O-terminated a SnO2(110)/Cu(111) interface (denoted as I) was considered and the work of separation values of the interfaces with various dopants (Mo, Sb, Ti, Zn and Cu) were calculated to evaluate the interface adhesion strength. It was demonstrated that low-valence dopants (Zn2+ and Cu2+) enhance the adhesion strength of interface I, while high-valence dopants (Mo6+ and Sb5+) play the opposite role. Secondly, the strengthening effects of low-valence dopants were further verified in four candidate interfacial models with different atomic structures (denoted as II-V). The work of separation values indicated that the adhesion of all of the interfaces involved could be enhanced by low-valence doping. The electronic structure of the interface was demonstrated through density of states, charge density and charge density difference analyses. The results revealed that upon low-valence doping, the holes facilitate charge transfer between Cu and SnO2, which generates strong covalent bonds across the interface and thus significantly enhances the interface adhesion. This work not only provides insight into rational doping to enhance the adhesion of SnO2/Cu composites but can also be expanded upon for the design of other metal oxide/metal composites with strong interface adhesion.

Charge optimized many-body (COMB) potential for Al2O3 materials, interfaces, and nanostructures

This work presents the development and applications of a new empirical, variable charge potential for Al2O3 systems within the charge optimized many-body (COMB) potential framework. The potential can describe the fundamental physical properties of Al2O3, including cohesive energy, elastic constants, defect formation energies, surface energies and phonon properties of α-Al2O3 comparable to that obtained from experiments and first-principles calculations. The potential is further employed in classical molecular dynamics (MD) simulations to validate and predict the properties of the Al (1 1 1)-Al2O3 (0 0 0 1) interface, tensile properties of Al nanowires, Al2O3 nanowires, Al2O3-covered Al nanowires, and defective Al2O3 nanowires. The results demonstrate that the potential is well-suited to model heterogeneous material systems involving Al and Al2O3. Most importantly, the parameters can be seamlessly coupled with COMB3 parameters for other materials to enable MD simulations of a wide range of heterogeneous material systems.

First principles optical spectra of the β-SiC(0 0 1)/Al interface

The optical spectra of the β-SiC(0 0 1)/Al interface has been studied using first principles time-dependent density functional theory. We considered the bare random phase approximation as well as two different exchange-correlation kernels, i.e. the adiabatic-local-density-approximation and the jellium-with-gap kernel of Trevisanutto et al (2013 Phys. Rev. B 87 205143). We investigated the C-terminated interface with Al-C interaction which has quite good bond adhesion between the two materials. The absorption spectra of all methods are dependent on the electric field polarization, showing high anisotropy in these systems. When the electric field is parallel to the interface plane, all methods predict a metallic behavior, while enhanced semiconductor excitonic effects are present when the electric field is perpendicular to the interface plane. Between the considered methods, the jellium-with-gap kernel enhances the excitonic effects of the β-SiC(0 0 1)/Al interface with respect to the other methods.

Interfacial adhesive properties between a rigid-rod pyromellitimide molecular layer and a covalent semiconductor via atomistic simulations

We conducted a comprehensive atomistic simulation study of the adhesive properties of aromatic rigid-rod poly-[(4,4'diphenylene) pyromellitimide] on a dimer-reconstructed silicon surface. We describe the structural developments within the adherent's interfacial region at the atomistic scale, and evaluate the energetics of the adhesive interactions between bimaterial constituents. In particular, we observe a transition between noncontact and contact adhesion regimes as a function of the interfacial bonding strength between the polyimide repeat units and the silicon substrate. This transition is manifest by a three- to four-fold increase in adhesive energy, which is entirely attributable to structural relaxation in the organic layer near the interface, revealing the importance of accurately describing structural details at interfaces for reliable interfacial strength predictions. The underlying molecular reconfigurations in the pyromellitimide layer include preferred orientation of the rigid-rod molecules, molecular stacking, ordering, and the local densification. The role of each of these factors in the adhesive behavior is analyzed and conclusively described. Where possible, simulation results are compared with theoretical model predictions or experimental data.

Si/Cu interface structure and adhesion

An ab initio investigation of the Si(111)/Cu(111) interfacial atomic structure and adhesion is reported. Misfit dislocations appear naturally, as do hcp interfacial silicide phases that vary with temperature. The silicides form in the interface even at relatively low temperatures. These results are consistent with available experimental data.

Fundamental influence of C on adhesion of the Al2O3/Al interface

Our first-principles computations indicate that the clean Al2O3/Al interface is relatively weak-weaker than bulk Al. Fracture experiments reveal that the interface is relatively strong with observed failure in bulk Al, however. This paradox is resolved via doping effects of the common impurity C. We have found that only 1/3 of a monolayer of carbon segregated to the interface can increase the work of separation by a factor of 3. The resulting strong interface is consistent with fracture experiments. It arises due to void formation in the interface, which provides low-strain sites for the carbon to segregate to. The degree of void formation is consistent with the relatively high heat of oxide formation of Al.