Solid-solid Interfaces Research Articles

Efficient Production of Graphene through a Partially Frozen Suspension Exfoliation Process: An Insight into the Enhanced Interaction Based on Solid-Solid Interfaces.

Liquid phase exfoliation (LPE) offers a promising path for scalable graphene production, but struggles with high energy consumption and low yield, with over 99.99% of the input energy wasted. Here, we present an energy-efficient approach for producing graphene via partially frozen-suspension exfoliation (PFE). As opposed to traditional liquid-solid interfaces, the solid-solid interface enhances shear strength between the frozen solvent and graphite from about 40 N m-2 to 105 N m-2. Additionally, the suspension flow transitions from turbulent to laminar, aligning graphite parallel to the flow direction and conducive to the effective utilization of shear force. Compared to conventional liquid-phase exfoliation (LPE), PFE improves energy efficiency by 102∼103 times. Furthermore, a production rate of 5 g h-1 has been achieved in a 10 L tank at an ultralow shear rate of 3 × 102 s-1.

Void Growth Dynamics in Solid-State Batteries

Solid-state batteries (SSBs) can potentially offer improved energy density and safety, when compared to current lithium-ion batteries. Despite their irrefutable theoretical promise, further advancement of SSBs relies on developing a comprehensive understanding of various fundamental aspects including electro-chemo-mechanics, transport interactions, morphological evolution and related failure modes at solid-solid interfaces. Amongst the wide range of challenges, the formation and evolution of interfacial voids at the lithium-metal/solid-electrolyte interface during stripping is critical, due to its associated effect on internal short-circuit and cell failure during subsequent plating. The void growth phenomenon in SSBs is jointly determined by different factors such as the self-diffusion kinetics and mechanical response of lithium, reaction distribution at the solid-solid interface, and operating conditions such as external pressure and temperature. In this presentation, the pivotal role of reaction heterogeneity on the contact evolution process, along with key descriptors such as the lithium microstructure, surface morphology and contact percolation are discussed. The underlying asymmetry in terms of the competing electrochemical/mechanical processes and contact evolution during plating and stripping is highlighted. Lastly, the influence of non-uniformities in external pressure and temperature on the occurrence of disparate void growth patterns and electrochemical response is examined in this presentation.

(Keynote) Thermo-Electrochemical Interactions in Solid-State Batteries

Solid-state batteries (SSBs) with a lithium metal anode hold tremendous promise to enable long-range electric vehicles and electric aviation. However, the successful realization of practical SSBs necessitates gaining a fundamental understanding of the complex nature of solid-solid interface evolution including the origin of various failure/degradation mechanisms such as lithium filament penetration, void growth and interphase formation. While the influence of chemo-mechanics at such interfaces has received significant research attention, the underpinnings of thermo-electrochemical coupling and the thermal stability of solid-solid interfaces still require critical investigation. In this presentation, we mechanistically delineate the spatio-temporal attributes and implications of thermal interactions in SSBs, including their critical role on interface stability of the anode, electrochemical-transport response in the cathode and crosstalk between the cathode/solid-electrolyte/anode interfaces. The effect of architectural asymmetry in a solid-state cell, consisting of a heat-generating cathode and a lithium-metal anode, on the occurrence of an intrinsic thermal gradient and associated degradation modes have been highlighted. A comprehensive description of mechanisms underlying the thermo-electrochemical stability of solid-solid interfaces in SSBs is presented.

Modulation of the Capillary Force Profile at the Solid-Solid Interface through Topographical Modifications.

Modulations of interfacial adhesion at solid-solid contacts are desired in many multidisciplinary applications. This study aims at the creation of reproducible solid-solid interfaces with significantly mitigated capillary adhesion through physical modifications. First, a continuum boundary element-based mathematical model to predict capillary forces at solid-solid contacts was developed and validated. Next, the model was utilized to simulate the capillary adhesion between a glass substrate with hypothetical surface topographies, in the form of nanopillars and nanowells, and silica particles of various sizes at varied humidity conditions. This study revealed that the nanopillar surface topography was much more effective than the nanowell in suppressing the capillary condensation and could lower the capillary forces by more than one order of magnitude for the micrometer-scale and nanoscale particulates. This study suggested that topographical tuning at the solid-solid interface can significantly reduce interfacial adhesion and promote the dust-resistance characteristic of a substrate. Finally, this simulation study can guide the fabrication of solid surfaces with reproducible topography and optimized geometrical parameters to yield an extremely reduced interfacial capillary adhesion.

Mechanism of Void Formation in Lithium Metal Solid-State Batteries

Solid-state batteries (SSBs) with lithium metal anodes promise higher energy and power density over conventional lithium-ion batteries. But the development of SSBs faces fundamental challenges related to the electro-chemo-mechanics and stability of solid-solid interfaces. Among these, the formation and growth of voids at the lithium metal-solid electrolyte interface during stripping persists to be a major limitation. In this work, we analyze the nature of metal electro-dissolution and its critical implication on the evolution of solid-solid point contacts. The competing nature of lithium self-diffusion, reaction kinetics and ionic transport at the solid-solid interface, dependent on factors such as temperature and surface roughness are examined. Hotspots in the dissolution morphologies are identified and systematically connected to the electrochemical signature and onset of failure in SSBs.

Fundamental Understanding and Research Progress on the Interfacial Behaviors for Potassium-Ion Battery Anode.

Potassium‐ion batteries (PIBs) exhibit a considerable application prospect for energy storage systems due to their low cost, high operating voltage, and superior ionic conductivity. As a vital configuration in PIBs, the two‐phase interface, which refers to K‐ion diffusion from the electrolyte to the electrode surface (solid–liquid interface) and K‐ion migration between different particles (solid–solid interface), deeply determines the diffusion/reaction kinetics and structural stability, thus significantly affecting the rate performance and cyclability. However, researches on two‐phase interface are still in its infancy and need further attentions. This review first starts from the fundamental understanding of solid–liquid and solid–solid interfaces to in‐depth analyzing the effect mechanism of different improvement strategies on them, such as optimization of electrolyte and binders, heterostructure design, modulation of interlayer spacing, etc. Afterward, the research progress of these improvement strategies is summarized comprehensively. Finally, the major challenges are proposed, and the corresponding solving strategies are presented. This review is expected to give an insight into the importance of two‐phase interface on diffusion/reaction kinetics, and provides a guidance for developing other advanced anodes in PIBs.

Characterising Ionic Transport Resistance at Solid-Solid Interfaces in SOFC Using Isotopic Exchange and Numerical Modelling

Understanding the mechanisms governing ionic transport in ceramics is important for improving the performance and durability of Solid Oxide Fuel Cells (SOFCs). Properties such as the tracer diffusivity, D* , can be measured by Isotopic Exchange Depth Profiling (IEDP) as developed by Kilner et al. [1]. This method has, so far, primarily been used to characterise the material properties of single materials; however, SOFCs are multilayer devices with solid-solid interface that may also affect transport. Some investigations into the diffusion behaviour across multiple layers using the IEDP technique have been done in thin film samples [2][3]. Profiles obtained from these studies appear to show an abrupt concentration drop at the interface between certain materials, indicating an interface which significantly impacts the diffusion behaviour (and by extension the overall cell performance). However, no attempt was made to quantify this interface effect with a theoretical modelling approach.A finite-difference model for diffusion in a system containing multiple layers with interfaces has been developed. It numerically solves Fick’s second law of diffusion with various boundary conditions. This model can be used to fit experimental data obtained from tracer diffusion SIMS data, yielding a new way of quantifying interfacial resistance. A new interfacial resistance parameter, r int , has been defined, which quantifies the resistance to diffusion across an interface.The validity of the developed method has been experimentally demonstrated in a sample consisting of a layer of lanthanum strontium cobalt ferrite (LSCF) on a gadolinium-doped ceria (GDC) substrate, both common materials in SOFCs. Initial data from tracer diffusion experiments have shown the presence of a significant concentration drop at the interface of the LSCF-GDC stack, which can be fitted to the numerical model developed by the authors. This approach could be used to measure the changing interface properties under various ageing conditions, as well as the influence of interlayers and material selection on the interfacial resistances both in SOFC and other diffusion systems with interfaces, such as solid state batteries.[1] J. A. Kilner, B. C. H. Steele, and L. Ilkov. Oxygen self-dffusion studies using negative-ion secondary ion mass-spectrometry (sims). Solid State Ionics, 12(MAR):89-97, 1984.[2] K. Develos-Bagarinao, H. Yokokawa, H. Kishimoto, T. Ishiyama, K. Yamaji, and T. Horita. Elucidating the origin of oxide ion blocking effects at gdc/srzr(y)o-3/ysz interfaces. Journal of Materials Chemistry A, 5(18):8733-8743, 2017.[3] Katherine Develos-Bagarinao, Haruo Kishimoto, Jeffrey De Vero, Katsuhiko Yamaji, and Teruhisa Horita. Effect of la0.6sr0.4co0.2fe0.8o3-delta microstructure on oxygen surface exchange kinetics. Solid State Ionics, 288:6-9, 2016.Figure: Isotope Exchange Depth Profile Data taken by linescan from an LSCF-GDC multi-layer sample. Original image data and isotopic fraction profile, as well as the corresponding fit obtained using the numerical model is shown. Figure 1

Energetics and kinematics of undercooled nonequilibrium interfacial molten layer in cyclotetramethylene-tetranitramine crystal

The main objective of this work is to investigate the energetics and kinematics of non-equilibrium interfacial nanoscale melt, hundreds of degrees below melting temperature during thermally activated β → δ phase transformation in cyclotetramethylene-tetranitramine. The analytical solutions for energy, width, and mobility are derived in detail for each of the propagating solid-melt and solid-solid interfaces which can be used to calibrate and characterize interface material properties. Additionally, an approximate expression for energy and equilibrium width of solid-melt-solid interface has been proposed. Different parametric studies on the energy, width, and mobility of propagating interfacial melt have been explored. It is found out that for some specific limit of these parameters, mobility of solid-melt-solid interface can be higher than pure dry solid-solid interface. The present study reveals different scale effects of interface interactions and nontrivial phase transformation mechanisms.

(Invited) Theoretical Analysis of Microscopic Interface Ionics at Heterogeneous Solid-Solid Interfaces in Batteries

All-solid-state battery (ASSB) has attracted considerable attention for decades, and the developments of many new materials appropriate for ASSB have been made. In contrast, less is known about the interfacial processes, especially at the solid electrolyte (SE)-electrode interfaces. The microscopic origins of interfacial resistance and degradation are still crucial issues for the efficient ASSBs, which are closely related to the interface chemistry and electrochemistry on the ionic and electronic scales. Therefore, understanding of the microscopic interface ionics and electronics is indispensable for the cutting-edge ASSB development.We have addressed establishing a universal calculation framework based on density functional theory (DFT) for heterogeneous solid-solid interfaces. Typical DFT geometry optimization or molecular dynamics does not work well for the atomistic structure search due to the coexistence of collective and local geometry relaxations inherent to the solid-solid interfaces. The methods with empirical force field are also insufficient because their parameters are usually fitted to bulk properties, not interfacial ones.To overcome these issues, we have developed a novel computational technique, “Heterogeneous Interface (HI)-CALYPSO method” [1], for systematic sampling of heterogeneous solid-solid interface structures. The breakthrough was achieved by applying CALYPSO structure prediction method for all types of interfacial geometry relaxations, allowing to explore a variety of interfacial disordered structures [1]. This approach is compatible to massive parallel calculations on the supercomputers like Fugaku.Then, the method was applied to a variety of solid-solid interfaces in ASSBs. As a representative model, we investigated the system with LiCoO2 (LCO), β-Li3PS4 (LPS), and LiNbO3 (LNO) acting as a cathode, a sulfide electrolyte, and a buffer layer, respectively [1-3]. For the LCO/LPS interfaces, we sampled over 20000 configurations and found several stable disordered structures involving mixing cations and anions, leading to the formation of a reaction layer. On the other hand, Li-ion sites that can be preferentially depleted upon charging always exist around the cathode-SE interfaces irrespective of the interfacial order/disorder. Therefore, we conclude that the dynamic Li-ion depletion is related to suppression of successive Li-ion transport, leading to the interfacial resistance. Through the electronic states analysis, we also deduced a probable origin for the interfacial Li-ion depletion and a mean to suppress this problematic behavior. Finally, we provided ionic potential surface across the interfaces, governing ion dynamics across the interfaces. In consequences, sampling probable interface structures enabled to analyze statistically probable characteristics of the ionics and electronics at the solid-solid interfaces and provide a possibility of space-charge layer as well as electric double layer.We have also applied the method to the interface between Li metal anode and Li7La3Zr2O12 electrolyte [4]. Again, we succeeded in sampling of the probable interface structures, which have many interfacial Li-O bonds and unsaturated (i.e. coordination number < 6) Zr sites. These undercoordinated Zr sites are reduced once the LLZO surface is in contact with the Li metal, leading to chemical instability of the LLZO/Li interface.Recently, the interfaces between the Li1.17Al0.17Ti1.83(PO4)3 SE and LCO cathode as well as Li metal anode have been investigated as well. Though it’s not related to ASSB, we examined the heterogeneous interfaces between the graphite anode and Li2CO3 model SEI layer [5]. All of these studies suggest a unified insight into the interfacial ionic transport in ASSBs. In this talk, I’ll introduce our recent methodological and application studies on the ASSB interfaces and discuss the unified insight obtained.Acknowledgements: These works were done in collaboration with Dr. Bo Gao, Dr. Hong-Kang Tian, and Dr. Randy Jalem in NIMS. The works were partly supported by MEXT as “Program for Promoting Researches on the Supercomputer Fugaku” (Fugaku Battery & Fuel Cell Project) and JSPS KAKENHI “Interface IONICS” Grant Number JP19H05815.

Mesoscale Elucidation of Electrodeposition in All-Solid-State Lithium Batteries

Lithium metal anodes, owing to their theoretical promise have been considered decisive towards the development of next-generation battery systems with higher power and energy densities. Compared to the liquid counterparts, solid electrolytes proffer enhanced safety and dendrite suppression features due to the non-flammability and mechanical stiffness. However, the electrodeposition dynamics at a solid-solid interface is quite intricate, involving coupled transport, mechanical and kinetic processes, which are closely related to the interfacial heterogeneities. Furthermore, the mechanical properties of the solid electrolyte, lithium metal and externally applied pressure influence the mechano-electrochemical interaction and hence the interfacial stability. In this work, a mechanistic understanding of the mesoscale underpinnings in the electrodeposition stability at solid-solid interfaces for all-solid-state lithium batteries will be presented.

Unique Orientation of the Solid-Solid Interface at the Janus Particle Boundary Induced by Ionic Liquids.

This study reveals the unique role on Janus particles of the solid-solid interface at the boundary in determining particle interactions and assembly. In an aqueous ionic liquid (IL) solution, Janus spheres adopt intriguing orientations with their boundaries pinned on the glass substrate. It was further discovered that the orientation was affected by the particle amphiphilicity as well as the chemical structure and concentration of the IL. Further characterization suggests that the adsorption on the hydrophilic side is due to both an electrostatic interaction and hydrogen bonding, while adsorption on the hydrophobic side is due to hydrophobic attraction. Through the concerted interplay of all these interactions, the amphiphilic boundary may attract an excessive amount of IL cations, which guide the unique orientations of the Janus spheres. The results highlight the importance of the Janus boundary that has not been recognized previously. Adsorption at the solid-solid interfaces may inspire new applications in areas such as separation and catalysis.

Elastic NaxMoS2-Carbon-BASE Triple Interface Direct Robust Solid-Solid Interface for All-Solid-State Na-S Batteries.

The developments of all-solid-state sodium batteries are severely constrained by poor Na-ion transport across incompatible solid-solid interfaces. We demonstrate here a triple NaxMoS2-carbon-BASE nanojunction interface strategy to address this challenge using the β″-Al2O3 solid electrolyte (BASE). Such an interface was constructed by adhering ternary Na electrodes containing 3 wt % MoS2 and 3 wt % carbon on BASE and reducing contact angles of molten Na to ∼45°. The ternary Na electrodes exhibited twice improved elasticity for flexible deformation and intimate solid contact, whereas NaxMoS2 and carbon synergistically provide durable ionic/electronic diffusion paths, which effectively resist premature interface failure due to loss of contact and improved Na stripping utilization to over 90%. Na metal hosted via triple junctions exhibited much smaller charge-transfer resistance and 200 h of stable cycling. The novel interface architecture enabled 1100 mAh/g cycling of all-solid-state Na-S batteries when using advanced sulfur cathodes with Na-ion conductive PEO10-NaFSI binder and NaxMo6S8 redox catalytic mediator.

(Invited) Electrodeposition at Solid-Solid Interfaces

The development of next generation batteries is predicated on utilization of lithium metal anodes with solid electrolytes. Solid electrolytes offer attractive features of improved safety and reduced predilection for dendritic lithium growth through mechanical rigidity. However, the inherent solid-solid interfaces in these systems are not always stable necessitating fundamental understanding of the electrodeposition characteristics at solid-solid interfaces. In particular, coupled transport-kinetics-mechanics complexations arise with dependence on solid electrolyte structure, and interfacial stochasticity dependent on chemical and mechanical wettabilities. Furthermore, the formation of interphase layer complicates the phase-interface pairs with varying kinetic, transport and mechanical properties. In this presentation, the mechanistic underpinnings of the electrodeposition stability at the solid-solid interface for solid electrolyte and lithium metal systems will be examined.

Anisotropically Electrochemical-Mechanical Evolution in Solid-State Batteries and Interfacial Tailored Strategy.

All-solid-state batteries have attracted attention owing to the potential high energy density and safety; however, little success has been made on practical applications of solid-state batteries, which is largely attributed to the solid-solid interface issues. A fundamental elucidation of electrode-electrolyte interface behaviors is of crucial significance but has proven difficult. The interfacial resistance and capacity fading issues in a solid-state battery were probed, revealing a heterogeneous phase transition evolution at solid-solid interfaces. The strain-induced interfacial change and the contact loss, as well as a dense metallic surface phase, deteriorate the electrochemical reaction in solid-state batteries. Furthermore, the in situ growth of electrolytes on secondary particles is proposed to fabricate robust solid-solid interface. Our study enlightens new insights into the mechanism behind solid-solid interfacial reaction for optimizing advanced solid-state batteries.

Advanced Characterization of Solid-Solid Interfaces and Alkali Metal Anodes Via Cryogenic Focused Ion Beam and in Situ methodologies

All-solid-state batteries are a potentially safer and more energy dense alternative to the modern lithium ion battery, replacing flammable organic liquid electrolytes and enabling lithium metal anodes. While solid electrolytes have been developed with ionic conductivities far in excess of modern liquid organic electrolytes, electrolyte-electrode interfacial resistance remain an issue. In spite of their fundamental importance, the nature of these interfaces remains obscured, primarily due to the limited methods of characterization of both lithium metal and buried interfaces alike. Recently, cryogenic microscopy techniques—traditionally employed by biological field—have been employed to lithium ion batteries, elucidating the nature of solid-electrolyte interfaces and lithium metal morphologies alike. Of similar difficulty, observation of solid-solid interface dynamics is limited to very few techniques, and has only been demonstrated recently, primarily via in situ transmission electron microscopy (TEM) methodologies. We present developments in the application of cryogenic focused ion beam (FIB) and in situ TEM to enable fundamental characterization of solid-solid interfaces for lithium ion battery applications. Use of cryogenic temperatures is shown to clearly reduce damage induced in the lithium metal, preserving the morphology and structure, as demonstrated by both three-dimensional FIB slice-and-view and the observation of crystallinity in cryogenic-TEM. This technique is further extended to polymer and sulfide electrolytes, overcoming instability during FIB milling and permitting further characterization via cryo-TEM. Such cryo-prepared samples enable lithium metal for in situ TEM cycling of metal-anode-based solid-state batteries. Further, progress in in situ characterization of solid-state interfaces by TEM will be presented, showing developments in the extraction of electrochemically active nanobatteries for in situ testing applications, providing a platform for dynamic solid-solid interface characterization.

First Principles Studies of Solid-Solid Interfaces between LiMn2O4 and Protective Coatings

Numerous studies have shown that protective surface coatings are effective in reducing the well-documented problem of Mn ion dissolution from the surface of the spinel LiMn2O4 (LMO) Li-ion battery cathode.[1] Although recent computational work has identified many promising coating chemistries based on bulk thermochemical properties,[2,3] there remains much to be understood regarding the interface formed between protective coatings and the LMO electrode and how such interfacial properties may influence electrochemical performance. In this work, first principles density functional theory calculations are performed on models for both oxide and metallic coatings on the LMO surface. First, we elucidate the mechanism for the onset of Al2O3 film growth by atomic layer deposition (ALD) and show that precursor decomposition in early ALD pulses leads to adsorption site blocking and sub-monolayer film growth.[4] During ALD, the trimethylaluminum [Al(CH3)3] precursor loses its -CH3 groups to the LMO surface, where the Al(CH3)x* and CH3* surface adsorbates each behave as Lewis bases (electron donors), where near-surface Mn ions are the Lewis acidic adduct (electron acceptor). These studies are extended to various thermodynamically stable low- and high-index LMO surface models,[5] demonstrating that ALD on LMO is structure-sensitive toward stepped sites, which are more electron accepting as suggested by calculated oxygen vacancy formation energies. Moreover, we find that the thermodynamics of decomposed intermediates are linearly related to the corresponding oxygen vacancy formation energy, or the relative Lewis acidity of near-surface Mn ions, of each given surface facet and termination. We propose that selective ALD leads to partial coatings formed at defect sites which may be particularly vulnerable to Mn dissolution. Next, we model the process of delithiation near the surface of an LMO coated with metallic Au, which effectively mitigates Mn dissolution. Moreover, Au coatings have been shown to improve kinetics and do not strip under the potentials where LMO electrochemistry occurs.[6] DFT calculations show that a Li+-deficient near-surface is thermodynamically favored for Au-coated LMO. We demonstrate through local density of states and chemical bonding analyses that Au hybridizes with LMO upon delithiation, leading to partial oxidation on Au electronic states rather than full localization of holes on Mn ions. The Au coating has a lower work function than LMO, driving electron transfer from Au to LMO. This is also associated with downward bending of the LMO conduction band in response to Fermi level pinning at the LMO/Au interface. Operando X-ray diffraction experiments show that these interfacial effects may translate to a reduction in the amount of Li+ that can intercalate into the bulk of Au-coated LMO. These studies help elucidate the structure-property relationships of protective coatings at the LMO surface. In particular, this work highlights the driving forces for nucleation and growth of protective coatings by ALD, as well as the influence of interfacial electronic structure on intercalation chemistry of coated electrode materials. This research is supported as part of the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

New Approach of Dendrite Suppression Using Solid Electrolyte to Enable Li Metal Anodes

The study of dendritic growth and roughening during electrodeposition has gained renewed interest in light of the safety concerns associated with dendritic shorting in the current Li-ion batteries. A precise understanding of the growth of dendrites during electrodeposition could enable the use of metal anodes, especially Li, which could lead to significantly higher energy density batteries. Solid electrolytes present a new avenue for attacking this problem of stable electrodeposition, besides improving the safety of energy storage technology. Replacing the organic liquid electrolyte by a solid electrolyte creates a solid-solid interface whose properties alter the kinetics of electrodeposition. Monroe and Newman analyzed the electrodeposition stability of Li/solid polymer electrolyte system within linear elasticity theory and showed using a kinetic model that solid polymer electrolytes with a sufficiently high modulus are capable of suppressing dendrite growth [1]. However, the propagation of the interface is often accompanied by a change in density of the metal which is thus, an important order parameter that should affect the stability of electrodeposition at the interface. In the theory of stress-driven phase transformation at solid-solid interfaces studied in geological systems, it has been shown that interfacial stability or roughening condition depends on the density change at the interface [2]. In this work, we analyze the stability of electrodeposition at the interface between a solid electrolyte and a metal anode, both of which are assumed to be linearly elastic. Based on the derived stability criteria, we show that the condition for stability is sensitive to the mechanical response (shear modulus) and relative density of metal in the two phases [3]. We illustrate this using the constructed stability diagram which shows stable regimes of shear modulus and molar volume of metal ion. This stability diagram qualitatively resembles the stability diagram for stress-driven phase transition at solid-solid interfaces (encountered in, for example, stylolites) studied by Angheluta et al. [2]. Our analysis shows that it is possible to use a soft solid electrolyte provided the density of metal (e.g. Li) is greater in the solid electrolyte than the metal anode. This finding provides new ways to suppress dendrite growth at metal electrode/solid electrolyte interfaces. Two out of the four regions in the stability diagram represent stable electrodeposition regimes. To determine electrodeposition stability for candidate materials for solid electrolytes, we use first-principles calculations of the elastic tensor [4] and estimate of molar volume from ionic radii tabulated by Shannon [5] and Marcus et al. [6]. We find that typical inorganic solid electrolytes have higher shear modulus, but lower molar volume than that required for stable electrodeposition. On the other hand, solid polymer electrolytes have higher molar volume but lower shear modulus than the requirement, leading once again to unstable electrodeposition. Our results suggest that a composite material with a combination of high (low) Li molar volume and high (low) shear modulus is required to suppress dendrites. Recently, the high anisotropy of Li was proposed to play an important role in determining stability of electrodeposition at different Li surfaces [7]. To investigate anisotropy effects, we performed linear stability analysis of electrodeposition at different metal surfaces in contact with solid electrolyte. The deformation equations were solved using the Stroh formalism with the same boundary conditions. Results show that both high density and low density stable regimes exist, with the critical shear modulus curves shifted depending on the Li surface in contact with the solid electrolyte. This presents an opportunity to realize safer high energy density solid-state batteries with metal anodes. [1] C. Monroe and J. Newman, J. Electrochem. Soc. 152, A396 (2005). [2] L. Angheluta, E. Jettestuen, and J. Mathiesen, Phys. Rev. E 79, 031601 (2009). [3] Z. Ahmad and V. Viswanathan, arXiv:1702.08406. [4] Z. Ahmad and V. Viswanathan, Phys. Rev. B 94, 064105 (2016). [5] R. D. Shannon, Acta Crystallogr., Sect. A: Found. Adv. 32, 751 (1976). [6] Y. Marcus, H. Donald Brooke Jenkins, and L. Glasser, J. Chem. Soc., Dalton Trans., 3795 (2002). [7] C. Xu, Z. Ahmad, A. Aryanfar, V. Viswanathan, and J. R. Greer, Proc. Natl. Acad. Sci. USA 114, 57 (2017). Figure 1: Stability diagram showing stable and unstable electrodeposition regimes depending on shear modulus and molar volume ratio of metal [3]. Change in the critical shear modulus curves due to Poisson's ratio is also shown. Figure 1

Stability of Electrodeposition at Solid-Solid Interfaces and Implications for Metal Anodes.

We generalize the conditions for stable electrodeposition at isotropic solid-solid interfaces using a kinetic model which incorporates the effects of stresses and surface tension at the interface. We develop a stability diagram that shows two regimes of stability: a previously known pressure-driven mechanism and a new density-driven stability mechanism that is governed by the relative density of metal in the two phases. We show that inorganic solids and solid polymers generally do not lead to stable electrodeposition, and provide design guidelines for achieving stable electrodeposition.

Liquid-Solid and Solid-Solid Interfaces in Electrocatalysis from First Principles

Understanding the electrochemical reactions at Liquid-Solid interfaces has been a great and long-term interest but challenge for both experimentalists and theorists. Even though tremendous advances have made through numerous techniques, significant uncertainties relating to the reaction intermediates and reaction mechanism remain. The problem is further complicated when a Solid-Solid interface has been formed either through electrodeposition or de-alloying, as the corresponding interface structures and consequently surface structures of the catalysts are still largely unknown. In this talk, we will review the methodologies we have recently developed to de-convolute those effects, including a synergistic strategy towards highly accurate prediction of Pourbaix diagram of transition metal (hydroxy)oxides, a simple Morie pattern technique for screening Solid-Solid interface, ab initio molecular simulation to sample Liquid-Solid interfaces and X-ray Photoelectron Spectroscopy simulation to identify the oxygenated species at Liquid-Solid interface. Next, using monolayer layer (hydroxy)oxide-metal interface as an example, we will demonstrate how the combination of the above techniques could identify the stable phases under various electrochemical conditions. As a case study, we will show the bi-functional role of the interface in promoting hydrogen evolution reaction (HER). As another example, we will show the development of the metal-metal interface during the de-alloying process of Pt-based alloy electrocatalysts. Finally, we will show the applications of these understandings for the design of new catalysts with improved performances for electrolysis and fuel cell applications. [1] L. Wang, Y. Zhu, Z. Zeng, C. Lin, M. Giroux, L. Jiang, Y. Han, J. Greeley, C. Wang, and J. Jin, Nano Energy 31, 456 (2017). [2] Z. Zeng and J. Greeley, Nano Energy 29, 369 (2016). [3] Y. Ha, Z. Zeng, Y. Cohen, J. Greeley, and A. A. Gewirth, J. Phys. Chem. C 120, 8674 (2016). [4] Z. Zeng, M. K. Y. Chan, Z.-J. Zhao, J. Kubal, D. Fan, and J. Greeley, J. Phys. Chem. C 119, 18177 (2015).

Eutectic colony formation in systems with interfacial energy anisotropy: A phase field study

Instability of a binary eutectic solidification front to morphological perturbations due to rejection of a ternary impurity leads to the formation of eutectic colonies. Whereas, the instability dynamics and the resultant microstructural features are reasonably well understood for isotropic systems, several experimental observations point to the existence of colonies in systems with anisotropic interfaces. In this study, we extend the understanding of eutectic colonies to anisotropic systems, where certain orientations of the solid-liquid or solid-solid interfaces are associated with a lower free energy than the others. Through phase field simulations in 2D and 3D, we have systematically probed the colony formation dynamics and the resulting microstructures, as functions of the pulling velocity and the relative orientation of the equilibrium interfaces with that of the imposed temperature gradient. We find that in 2D, stabler finger spacings are selected with an increase in the magnitude of anisotropy introduced, either in the solid-liquid or in the solid-solid interface. The fingers have a well-defined orientation for the case of anisotropy in the solid-liquid interface, with no fixed orientations for the lamellae constituting the colony. For the case where anisotropy exists in the solid-solid interface, the lamellae tend to orient themselves along the direction of the imposed temperature gradient, with tilted solid-liquid interfaces from the horizontal. The 3D simulations reveal existence of eutectic spirals which might become tilted under certain orientations of the equilibrium interfaces. Our simulations are able to explain several key features observed in our experimental studies of solidification in Ni-Al-Zr alloy.