Dendrite Nucleation Research Articles

Current understanding of the origin of equiaxed grains in pure metals during ultrasonic solidification and a comparison of grain formation processes with low frequency vibration, pulsed magnetic and electric-current pulse techniques

The formation of fine, non-dendritic equiaxed grains throughout a casting without the addition of refiners (i.e. independent of alloy chemistry), is made possible by using ultrasonic, magnetic or pulsed magnetic and electric current pulse techniques. The dominant mechanisms proposed for the grain refinement produced during the application of an external field are cavitation phenomena assisted nucleation or fragmentation of dendrites (ultrasonic field), wall crystals arising from the cold surface of the mould (electric current pulse, magnetic and pulsed magnetic fields). In all these cases fluid flow provides an additional contribution (e.g. reduced temperature gradients, growth rate and remelting of dendrites) to maintaining an equiaxed grain structure. The origin of equiaxed grains under an external field also depends on the casting conditions (volume and shape of casting) and the type of alloy other than the mechanisms specific to a particular technique. The current work aims to provide a detailed understanding of the various factors and mechanisms that influence the grain refinement achieved during the solidification of pure metals (magnesium and zinc) subjected to UltraSonic Treatment (UST). The role of the temperature range of UST application, time duration and an unpreheated sonotrode are examined with respect to the origin, evolution of equiaxed grain structure, morphology and the columnar to equiaxed transition. The origin of grains was analysed from three fundamental aspects that contribute to refinement (i) heterogeneous nucleation (ii) fragmentation of existing dendrites and (iii) grains produced from the colder surfaces (arising from mould walls or vibrating surfaces as wall crystals). A comparison of UST refinement with mechanical, low-frequency vibration, electric current pulse and magnetic field solidification of pure metals has also been provided to highlight the importance of the cold surfaces (sonotrode and mould wall) in influencing grain refinement.

Phase-field simulation of Li dendrites with multiple parameters influence

Lithium metal has been regarded as a promising anode material for high theoretical energy batteries. However, the growth of lithium dendrites hinders its commercialization. Lithium dendrite may cause the battery internal short circuit, thermal runaway, even explosion, and other disasters. Here a phase-field model is established to investigate the influence of the Li-ions concentration, anisotropic strength, noise, and internal heat on the growth behaviors of the lithium dendrite. Increasing the consumption of Li-ions concentration, the growth rate of dendrite is faster and the morphology is close to mossy. Adjusting the appropriate anisotropic strength can effectively improve the dendrite structure and reduce the formation of dendrites. Large noise accelerates the formation of dendrite nucleation sites and disordered dendrites. The lithium dendrites grow in the direction of high temperature gradient caused by internal heat. The mechanism of pore evolution with time along with dendrite growth has also been studied.

Recent advances in the mitigation of dendrites in lithium-metal batteries

Utilizing lithium (Li) metal as the anode can enable lithium metal battery (LMB) systems to achieve energy densities of up to 1150 Wh Kg−1, approaching that of gasoline. However, the use of Li metal is plagued with a whole host of problems, the most serious of which is the issue of nucleation of Li metal dendrites. These dendrites grow inexorably on charge–discharge cycling, piercing through the battery separator membrane and eventually electrically shorting the battery. This can result in thermal runaway leading to an unacceptable fire hazard. In this review article, we focus on recent advances in mitigating, suppressing, and healing Li metal dendrites in LMBs with liquid electrolytes. We discuss a whole host of exciting approaches including electrolyte engineering, interface engineering, the use of stable Li hosts, homogenizing Li-ion flux, and the physical healing of dendrites using temperature fields. We discuss the pros and cons of these approaches and provide our perspectives for future research directions. In particular, it is important to consider dendrite mitigation in a full-cell setting and with a realistic form factor such as prismatic or cylindrical cells. Given the multifaceted nature of the dendrite challenge, it is unlikely that there is a single magic solution, making it necessary to explore a combination of strategies to work in synergy to make Li metal anodes viable in commercial systems.

The Influence of Solid Electrolyte Interface (SEI) on Li Dendrites Formation in Li-Metal Battery

The composition, structure and the formation mechanism of the solid-electrolyte interphase (SEI) in lithium-based (e.g., Li-ion and Li-metal) batteries have been widely explored in the literature. However, very little is known about the ions transport through the SEI, specifically through their grain boundaries (GBs) of the inorganic inner layers, and the corresponding mechanism for Li dendrites nucleation. Understanding the underlying ions diffusion processes across the inorganic components of SEI could lead to significant progress, enabling the performance increase and improving the mitigating strategies of dendritic growth and other safety aspects of these batteries. For this reason, in this work, we present the results of a combined ab initio Density Functional Theory (DFT) calculations to study Li diffusion across the SEI and Li nucleation at the Li metal/SEI junction and an extended phase-field model (PFM) for Li migration and electrodeposition that is a step forward toward better understanding of Li dendrites formation and growth.The present DFT calculations are performed using the VASP (Vienna Ab initio Simulation Package)1 code with the plane wave basis sets and the projector augmented wave (PAW)2 pseudo-potentials in the framework of Perdew-Burke-Ernzerhof sol (PBEsol)3 generalized gradient approximation (GGA)4. The migration barriers and diffusion coefficients are evaluated using the Nudged Elastic Band (NEB) method, as implemented in VASP.The major GB structures of SEI that are of interest are the ones between LiF/LiF, Li2O/Li2O and the mixed GBs between LiF/Li2O slabs. The energy barriers for the diffusion of Li vary significantly depending upon the structure of these channels, with the general trend being that Li diffusion in the GB is generally faster than in the neighboring crystalline regions within the grain interiors. The diffusion through LiF/Li2O slabs has the lowest barriers whereas GBs of LiF/LiF slabs has the highest. To understand the role of these GBs in dendrite nucleation, the SEI/electrode interface is analyzed for its stability. The energetics from the DFT analyses depend heavily on the grain structures, with (LiF/LiF)/Li grain structures being most stable and (LiF/Li2O)/Li being the least stable. The interfacial energies in the Li/SEI interfaces show that the interface between (LiF/Li2O)/Li has the least critical length (~1.6 Å) for stable dendritic growth and most favorable for the initiation of crack in the SEI.To quantify the influence of the SEI on the evolution of Li electrodeposits at the continuum scale, we performed the PFM calculations. The PFM is developed employing MOOSE framework5, and the SEI model includes the typical GB structures within 20 nm thickness of the SEI grains. The data from the DFT as described above is used for Li diffusion through the SEI and for mechanical properties of GB and grains. The results from PFM reveal that the anisotropic nature of Li presence at the metal/SEI interface leads to uneven electrodeposition at the Li metal surface. Also, the results allow for monitoring the stress field evolution and its influence on Li filament structure. The present results also show that significant stress is observed at the root of the Li electrodeposits and the triple junction between the GBs.

Amalgams Anodes for Alkali Metal Batteries

The dendrite issue and the associated cell degradation problems have proved to be a key challenge to the practical application of alkali metal anodes. We present a simple yet efficient approach to fabricate amalgam anodes that afford long-term stability cycling at practical current densities and capacities. The amalgam layer allows fast electron transfer as well as Li/Na/K migration at the electrolyte-electrode interphase, which significantly promote the cycling performance. In-situ optical microscopy analyses confirm the dendrite nucleation and growth can be remarkably suppressed with the amalgam-protected anodes. The prototype full cells demonstrate much improved rate and long-term cycling stability. These promising results provide new perspectives on the regulation of sodium electrodeposition and hence the elimination of the dendritic morphology for the practical development of alkali metal batteries.

Amalgam Protection for Alkali Metal Anodes

The dendrite issue and the associated cell degradation problems have proved to be a key challenge to the practical application of alkali metal anodes.1,2 We present a simple yet efficient approach to fabricate amalgam anodes (Figure 1a) that afford long-term stability cycling at practical current densities and capacities.3 The amalgam layer allows fast electron transfer as well as Li/Na/K migration at the electrolyte-electrode interphase, which significantly promote the cycling performance. For example, Protected Li electrodes exhibit stable cycling over 750 hrs at high plating current and capacity of 8 mA cm-2 and 8 mAh cm-2 in symmetric cells (Figure 1b). Coupled with high-loading cathodes (~12 mg cm-2) such as LiFePO4 (LFP) and LiNi0.6Co0.2Mn0.2O2 (NCM), the prototype full cells also demonstrate much improved rate and long-term cycling stability (Figure 1c). In-situ optical microscopy analyses confirm the dendrite nucleation and growth can be remarkably suppressed with the Li-Hg protected anodes (Figure 1d). These promising results provide new perspectives on the regulation of sodium electrodeposition and hence the elimination of the dendritic morphology for the practical development of alkali metal batteries.

Crystal Structure Influences Migration along Li and Mg Surfaces.

Dendrite formation on Li metal anodes hinders commercialization of more energy-dense rechargeable batteries. Here, we use the migration energy barrier (MEB) for surface transport as a descriptor for dendrite nucleation and compare Li to Mg. Density functional theory calculations show that the MEB for the hexagonal close-packed structure is 40 and 270 meV lower than that of the body-centered cubic structure for Li and Mg, respectively. This is suggested as a reason why Mg surfaces are less prone to form dendrites than Li. We show that the close-packed facets exhibit lower MEBs because of smaller changes in atomic coordination during migration and thereby less surface distortion.

Microstructures and mechanical properties of WCP/Ti-6Al-4V composite coatings by laser melt injection and laser-induction hybrid melt injection

To improve the coating thickness and process efficiency of conventional laser melt injection (LMI), we introduced laser-induction hybrid melt injection (LIHMI) to prepare spherical WC particle (WCP)-reinforced metal matrix composite coatings on a Ti-6Al-4V substrate. The laser-induction hybrid melt injected coatings were 2.0–2.2 times thicker than that of the conventional laser melt injected coatings. The microstructure and mechanical properties of the WCP/Ti-6Al-4V composite coatings by LMI, LIHMI with induction preheating (pre-LIHMI) and LIHMI with induction preheating and post heating (pre/post-LIHMI) was investigated systemically. LIHMI inhibited the nucleation of primary TiC dendrites in the laser molten pool between adjacent WCP; therefore, the total amount of TiC in the laser-induction hybrid melt injected coatings was far less than that in the laser melt injected coatings. We found that (5.6–5.8)% TiC, β-Ti, and β-Ti/α-Ti formed in the laser-induction hybrid melt injected coatings, while 23% TiC, (Ti,W)C1-X and β-Ti formed in the laser melt injected coatings. In addition, pre/post-LIHMI led to a drastic WCP/Ti interfacial reaction due to the long dwell time and high temperature of the laser molten pool, which greatly deteriorated the mechanical properties of the coatings. Compared with those of the pre/post-LIHMI samples, the pre-LIHMI samples exhibited a shorter liquid residence time and lower temperature of the laser molten pool and a more moderate WCP/Ti interfacial reaction. In addition, tension strength of pre-LIHMI sample and pre/post-LIHMI sample is 34% and 12% higher than that of LMI sample.

Nanoscale observation of the solid electrolyte interface and lithium dendrite nucleation–growth process during the initial lithium electrodeposition

The influence of current density and capacity on lithium deposition was investigated, including the nucleation, growth and SEI evolution processes.

Impact of Salt Concentration on Lithium Dendritic Growth in a Rigid SEO Polymer Electrolyte

In a society increasingly concerned by its impact on the environment and the urgent issue of climate change, there is great demand for an energy storage device with higher energy density and power capability (especially for electric vehicles and power grid). The use of the high energy density lithium metal anode jointly with a solid polymer electrolyte could enable the development of large-scale battery applications. Yet, dendritic lithium formation leading to short circuit currently limits its practical applications. Therefore, understanding the fundamentals of dendrite nucleation and growth is necessary to design robust battery systems. In this work, the nucleation and growth of lithium dendrite is studied in function of salt concentration in lithium-lithium symmetric cells in a rigorous manner. Complete electrochemical characterization of a solid block copolymer polystyrene-polyethylene oxide mixed with lithium salt (LiTFSI) will be presented, as well as the cell lifetime under galvanostatic cycling. We have discovered an unexpected relationship between cell lifetime and salt concentration. X-ray micro-tomography was used to underpin the relationship between salt concentration and dendrite nucleation and morphology.

Plan-View Operando Video Microscopy of Li Metal Anodes

The reversible cycling of Li metal anodes is severely limited by poor cycle life and safety concerns. Dendrite growth has been linked to the poor performance of Li metal, but a comprehensive understanding of the mechanistic origins and dynamic evolution of Li metal morphology during electrodeposition and stripping is missing. Herein, a multifunctional operando visualization platform that synchronizes optical recordings with cycling electrochemistry is used to study nucleation on Li metal and dendrite reversibility. This platform was initially used to understand the origins of the “peaking” behavior exhibited in the voltage traces of Li symmetric cells [1-3]. However, by limiting the viewing angle to a cross-sectional perspective, the impact of electrode surface variations, including microstructure, flaws, and chemical inhomogeneities cannot be fully accounted for. To address this, a plan-view optical visualization cell was designed that maintains a highly uniform current distribution along the electrode surface. The top-down view provided by the updated cell is better suited to provide quantitative measurements of nucleation on Li metal and identify links between nucleation and microstructure. Utilizing this new cell, we are able to: 1) Quantitatively identify the spatial distribution of “hot spots” for both nucleation of both dendrites and pits; 2) Detail how these hot spots evolve during cycling; 3) Quantify dendrite and pit areal nucleation density as a function of current density; 4) Provide an understanding of why surface uniformity increases after the first cycle; 5) Elucidate the substantial impact that pits have on Li metal cycling; 6) Explore the dependency between dendrite size and reversibility; and 7) Explore the mechanisms that lead to the formation of dead Li. The knowledge from this study can be used to inform rational design of Li metal surface treatments and architectures to improve the reversibility and cycle life. 1. K. N. Wood, E. Kazyak, A. F. Chadwick, K. H. Chen, J. G. Zhang, K. Thornton, N. P. Dasgupta, ACS Cent. Sci. 2, 790 (2016) 2. K. N. Wood, M. Noked, N. P. Dasgupta, ACS Energy Lett. 2, 664 (2017) 3. K.-H. Chen, K. N. Wood, E. Kazyak, W. S. LePage, A. L. Davis, A. J. Sanchez, N. P. Dasgupta, J. Mater. Chem. A 5, 11671 (2017)

Recovery of Unwanted Lithium from Graphite Anodes Via Thermal Gradient

Thermal management is a known design consideration for safe and stable cycling of Li-ion batteries. Active thermal management systems attempt to maintain isothermal control to limit thermal excursions that can lead to premature cell performance loss and safety failures. Low temperature cycling is one example of unsafe Li-ion battery operation, which has been shown to limit cycle life by lithium plating and can also lead to dendrite nucleation and growth that can cause internal short circuits and battery failure. However, it can be assumed that these engineering controls cannot consistently maintain the safe isothermal condition and localized thermal gradients will develop within cells. We previously showed that the application of an external thermal gradient has a profound effect on cell electrochemical processes. For lithium metal symmetric cells, the direction of an interelectrode thermal gradient dictates lithium plating morphology and determines whether unstable, dendrite growth or stable plating occurs (1). We have also demonstrated that applying a thermal gradient (cold anode, warm cathode) allows plating lithium on graphite on-demand, dramatically reducing cell capacity by loss of lithium inventory (2). In this way, a thermal gradient rapidly incites realistic cell failure modes, accelerating testing to battery failure to enable improved understanding of battery failure. This talk focuses on the utility of reversing the direction of interelectrode thermal gradients to strip lithium from the graphite electrode. We use post mortem materials characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy to characterize texture and morphology of the plated and lithium metal on graphite and confirm its recovery. A thermal gradient effectively restores battery health by modulating plating to recover lithium inventory and battery capacity; this thermal gradient reversal prevents internal short circuits from dendrite growth and ultimately extends battery lifetime in NMC|graphite cells. In addition to post mortem materials characterization, we use electrochemical impedance spectroscopy (EIS) and incremental capacity analysis to monitor lithium plating and stripping modulation. References A. Mistry, C. Fear, R. Carter, C. T. Love and P. P. Mukherjee, ACS Energy Letters, 4, 156 (2018).R. Carter and C. T. Love, ACS Appl Mater Interfaces (2018).

Effects of Porous Separators on Dendrite Growth in Lithium Batteries

Dendrite growth on the surface of the anode in lithium batteries can diminish their energy storage capacity and increase safety concerns, i.e. risks of a short circuit. Dendrite growth consists of two stages, nucleation and growth. Favorable sites for the nucleation of dendrites are difficult to avoid and so suppressing dendrites after nucleation is a promising research direction. Previous work has suggested that the local mass transport in the electrolyte is closely related to the structure of dendrites. In this research, a mesoscale diffusion-reaction model is used to study the effects of various porous separator geometries and configurations on dendrite formation. The porous separators act to restrict the local mass transport near the anode surface, which effects dendrite structure and growth rates. The porous separator structures considered in this study highlight the importance of both porosity and tortuosity. These attributes are often simplified when reporting characteristics in experimental tests or ignored in other simulations. In this study, we directly observe the impact of separator characteristics on the local mass transport gradient through the separator and the subsequent dendrite growth. By taking into account the impact of both overall and local porosity and tortuosity on dendrite growth, battery design can be improved.

Atomic Layer 2D MoS2 Coated Li-Metal for Advanced Li-Metal Rechargeable Batteries

Rapidly increasing markets for electric vehicles (EVs), energy storage for backup support systems and high-power portable electronics demand batteries with higher energy density, higher power density, and longer cycle life. The success of Li-S cells depends in large part to the utilization of metallic lithium as anode material. Metallic lithium, however, is prone to grow parasitic dendrites and is highly reactive to several electrolytes; it is also susceptible to polysulfides when used in Li-S cells. Here, we show that a ~10 nm thick 2D MoS2 can act as a protective layer for Li-metal anodes, greatly improving the performances of Li-S batteries [2, 3]. In particular, we observe stable Li electrodeposition and the suppression of dendrite nucleation sites. The deposition and dissolution process of a symmetric MoS2 coated Li-metal cell operates at a current density of 10 mA cm-2 with low voltage hysteresis, and a three-fold improvement in cycle-life than using bare Li-metal. In a Li-S full cell configuration, using the MoS2 coated Li as anode and a 3D carbon nanotube-sulfur cathode, we obtain a specific energy density of ~600 Wh kg-1 and a Coulombic efficiency of ~98% for over 1200 cycles at 0.5 C. Our approach can lead to the realization of high energy density and safe Li-metal based rechargeable batteries. Figure. Long-term cycling performance of Li–S battery with the MoS2-coated Li anode measured at 0.5 C for 1,200 cycles [3]

Precipitation Mechanism and Reduction of Amount of Primary Carbides During Electroslag Remelting of 8Cr13MoV Stainless Steel

The precipitation and growth of primary carbides in 8Cr13MoV steel during electroslag remelting (ESR) were studied. The effects of melting rate and fill ratio of ESR on the reduction of primary carbides were clarified. Primary carbides M7C3 precipitated from liquid steel at the final stage of solidification, which is attributed to an enrichment of carbon and chromium in liquid steel. The carbon content in the residual liquid steel was the determining factor for the formation of primary carbides. The growth of primary carbides was affected mainly by the chromium-concentration and temperature-field gradients. An appropriate melting rate facilitated the nucleation and growth of dendrites, which caused dendrite closure and prevented the further enrichment of carbon atoms. Primary carbides were refined and their amount was reduced when using a lower ESR melting rate. The fill ratio can affect the uniformity of the temperature field in the liquid metal pool and heat transfer from the slag pool to the liquid metal pool, and an optimal fill ratio resulted in a minimum mushy zone and less primary carbides. The volume fraction of primary carbides was reduced by 23 pct when the melting rate decreased from 150 to 133 kg/h and the fill ratio increased from 0.23 to 0.33.

Dendrite nucleation in lithium-conductive ceramics.

Solid-state lithium batteries cannot achieve reasonable power densities because of dendrites, whose formation mechanisms remain uncertain. This paper applies principles of chemomechanics to investigate the critical current above which dendrites form in lithium-conductive ceramics. Applied voltage induces stress in solid electrolytes; dendrites appear to nucleate in the exemplary garnet-oxide material Li7La3Zr2O12 (LLZO) when the interfacial pressure exceeds a particular value. The critical pressure of polycrystalline LLZO correlates well with the surface-energy changes incurred by lithium plating in its grain boundaries. A derived formula, validated by experiments, predicts quantitatively how critical current varies with properties including interfacial impedance, bulk permittivity, and grain size. As well as suggesting novel strategies to create more resilient ion-conductive ceramics, the proposed mechanism rationalizes experimental observations of bulk lithium plating and explains how LLZO exhibits an electrically activated transition from stable low-current cyclability to high-current dendrite nucleation.

The effect of local lithium surface chemistry and topography on solid electrolyte interphase composition and dendrite nucleation

High resolution analysis shows localized organic-rich impurities in the native Li surface that promote preferential lithium deposition, leading to dendrite growth.

Single-ion conducting artificial solid electrolyte interphase layers for dendrite-free and highly stable lithium metal anodes

A single-ion conducting artificial SEI layer guides lithium ions to uniformly distribute and deposit and suppresses the growth of lithium dendrites.

Modeling of microstructure and microsegregation formation during solidification of Al-Si-Mg alloys

A two-dimensional (2-D) cellular automaton (CA) model considering phase equilibria according to the Calphad method is proposed for the simulation of solidification microstructures and microsegregation of Al-Si-Mg alloys. The present model encompasses the essential aspects of thermodynamics and kinetics of phase transformation during solidification of ternary Al-rich alloys, particularly nucleation and growth of primary dendrites, irregular binary and ternary eutectic phases, as well as the time-dependent mass transport of two solutes. The proposed CA model is applied to simulate the evolution of primary dendrites and irregular eutectic microstructures of Al-Si-Mg alloys. The influence of the cooling rate on the solidified microstructures and microsegregation is investigated. The following features are reproduced well by the CA simulations: microstructural length scales correlate with the cooling rate; the homogeneity of the primary α-phase is quantified depending on the cooling rate and deviates considerably from Scheil-type calculations; back diffusion is accompanied by a decrease in the eutectic fraction, considering that some amount of eutectic β-phase is dissolved to maintain solute conservation and adjustment of equilibrium in the eutectic.

First‐Principles Studies of Li Nucleation on Double‐Layered Defective Graphene

AbstractLithium‐dendrite growth is one of the most challenging problems affecting the safety performance of Li‐ion batteries. Understanding the evolution process of Li‐dendrite growth at the atomic scale is critical for solving this problem. In this paper, the adsorption processes, geometrical configurations, and electronic structures of Li clusters on double‐layered graphene with two types of defects were investigated by first‐principles calculations. It was found that single vacancy (SV) defects tend to promote the nucleation of Li dendrites, and the adsorption energy of Li clusters near the SV defect decreases with increasing number of Li atoms. Meanwhile, the Li atoms accumulated on the surface of SV‐defect graphene with distances between the Li atoms similar to those in bulk metallic Li. However, in the case of double vacancy (DV) defects, the Li atoms could diffuse freely in the direction perpendicular to the graphene sheets through DV defects at the top‐layer graphene, thus hindering nucleation and dendrite growth. Density of states analyses suggested that the Li atoms on SV‐defect graphene transform from the fully ionized state to the metallic state with continuously increasing number, while the ionic properties of the Li atoms remain and even increase on DV‐defect graphene. Our conclusions can help to understand the Li‐graphite interaction from an atomistic point of view and provide theoretical hints for the development of graphite anodes with high charge‐rate properties.