Mossy Structures Research Articles

Decoding the Mechanisms of Reversibility Loss in Rechargeable Zinc-Air Batteries.

Attaining high reversibility of the electrodes and electrolyte is essential for the longevity of secondary batteries. Rechargeable zinc-air batteries (RZABs), however, encounter drastic irreversible changes in the zinc anodes and air cathodes during cycling. To uncover the mechanisms of reversibility loss in RZABs, we investigate the evolution of the zinc anode, alkaline electrolyte, and air electrode through experiments and first-principles calculations. Morphology diagrams of zinc anodes under versatile operating conditions reveal that the nanosized mossy zinc dominates the later cycling stage. Such anodic change is induced by the increased zincate concentration due to hydrogen evolution, which is catalyzed by the mossy structure and results in oxide passivation on electrodes and eventually leads to low true Coulombic efficiencies and short life spans of batteries. Inspired by these findings, we finally present a novel overcharge-cycling protocol to compensate for the Coulombic efficiency loss caused by hydrogen evolution and significantly extend the battery life.

Effect of Li+ Addition during Initial Stage of Electrodeposition Process on Nucleation and Growth of Zn

Zn negative electrodes are expected to be used in next-generation batteries. However, irregular shape evolution, such as mossy structures, limits its practical applications. Cationic additive species are useful in suppressing this, and Li+ is a promising species. To identify the effect of Li+ on the nucleation and growth of Zn, this study analyzed the Zn aggregation behavior during electrodeposition with Li+ at the initial stage via experimental methods and theoretical calculations using density functional theory and kinetic Monte Carlo simulations. The results suggest that Li+ affected the surface diffusion of Zn adatoms, changing the nucleation and growth during the initial stage of deposition. Li+ allows Zn adatoms to diffuse rapidly owing to the mitigation of the solvation effect on surface diffusion by forming rigid solvation of Li+ in the vicinity of the surface. This results in two-dimensional nucleation of the Zn(0001) facet, which is supported by the X-ray diffraction measurements. Li+ mitigates protrusion as the initial structure of the mossy structure. This analysis provides valuable insight into the control of the behavior of Zn adatoms and their nucleation and growth.

(Digital Presentation) Electrochemical Noise Measurement in Batteries with Metallic Lithium Anode

Electrochemical noise measurements are well known in corrosion literature where the noise that is to be measured is appreciable in amplitude. From the measured noise, it is possible to identify the mode of corrosion and distinguish between localized corrosion types from the uniform ones. This is mainly because localized modes of corrosion are stochastic in nature, typically studied in conjunction with post-mortem studies.In recent years, the increase in the use of batteries demands that the tests to be performed on the batteries are faster, easier, cheaper and, if possible, non-destructive and non-perturbing. Although some electrochemical noise studies have begun to be carried out on batteries, the literature on this subject is scarce and questionable. Electrochemical noise measurement of Li batteries can be ultimately used as a non-invasive tool to diagnose the battery health and we have already shown that non-rechargeable batteries with Li/MnO2 chemistry shows increase in voltage noise after being exposed to a short circuit. On the other hand, if the battery is properly discharged, voltage noise does not increase. As a result, morphological changes on metallic lithium can be detected by electrochemical noise measurements and this method can be used as non-invasive diagnosis tool.[1]Lithium metal-based chemistries have a much higher capacity than rechargeable chemistries because of the use of Lithium-aluminum alloy or graphite in rechargeable chemistries, as opposed to metallic Lithium used at the anode. It is known that charging of lithium metal electrode to result in the formation of lithium dendrites and/or mossy structures. These end up creating safety and performance issues. For this reason, pre-detection is both academically interesting and industrially important. Some preliminary studies show that noise level increase drastically after charging. Moreover, the anodes of the charged batteries were also examined with SEM and serious deterioration was observed in the anode of the battery after charging. (Figure 1) Just like noise measurements on non-rechargeable batteries with lithium chemistry exposed to short circuits, it is worthy to study on and develop pre-detection method for in lithium batteries that are prone to form dendrite during charging and discharging cycles by using electrochemical noise measurements. For this reason, we also conduct noise studies with symmetrical and asymmetric cells (Li/Li, Cu/Cu and Li/Cu) prepared in the glove box and examine the details of the noise increase in a controlled and detailed manner.In this talk, how the electrochemical noise of metallic lithium-based batteries is measured, under what conditions it increases and what are the sources of the noise will be discussed both with noise measurements and imaging with optical microscope in situ and after death with spectroscopic analysis.

(Invited) Effect of Additive Species on the Nucleation and Growth Process on Zn Negative Electrode Surface

Zn is one of the promising candidates as negative electrode materials for next-generation secondary batteries, especially those for large capacity applications such as grid-scale energy storage. While the Zn has numbers of attracting features such as crustal abundance, applicability of aqueous system, etc., which enable to realize the secondary batteries with high performance and safety with lower cost, the major drawback is evolution of irregular morphology called mossy structure with highly filamentous features at the electrode surface during charge-discharge cycles. Unlike the conventional dendritic growth which proceeds under diffusion-limited conditions, formation of the mossy structure takes place without such a condition. At the same time, it has been reported that the growth of these irregular structures could be suppressed by applying metallic species as additives to form uniform electrodeposition [1]. We have investigated formation process of the mossy structure on the Zn anode surface, focusing on its initial stage in combination with the additive effects such as Pb and Sn [2]. In this presentation, we will introduce the results of our research on the nucleation and growth process of the mossy structure through experimental analysis and computational modeling. For the theoretical calculation, we attempted to develop multi-scale simulation model by employing density functional theory (DFT) and kinetic Monte Carlo (KMC) approaches [3], and the results will be described focusing on the effects of the metallic additives.This work is financially supported in part by MEXT/JSPS Grant-in-Aid for Scientific Research No. 21H01642.[1] For example, F. Mansfeld, S. Gilman, J. Electrochem. Soc. 117, 1328 (1970); Y. Ito, M. Nyce, R. Plivelich, M. Klein, D. Steingart, S. Banerjee, J. Power Sources, 196, 2340 (2011).[2] For example, T. Otani, M. Nagata, Y. Fukunaka, T. Homma, Electrochim. Acta, 206, 366 (2016); T. Otani, Y. Fukunaka, T. Homma, Electrochim. Acta, 242, 364 (2017).[3] Y. Onabuta, M. Kunimoto, S. Wang, Y. Fukunaka, H. Nakai, T. Homma, J. Phys. Chem.C, submitted.

Visualization of Mossy Zinc Electroplating Structure Evolution Via Operando Nanotomography

Dendrite growth during electrodeposition is a critical challenge faced by high-energy metallic electrodes (e.g. Li, Zn) in rechargeable batteries. Under typical battery operation conditions, mossy structure forms on Li and Zn anode surface, which may be described as colonies of intertwined and branched filaments with diameters of ~ 100 nm. Compared to the well-known diffusion-limited dendritic growth, which occurs at much higher current density, the mossy growth process is vaguely described as “kinetically limited” in literature and its mechanism remains poorly understood. Recent study shows that the detachment of mossy filaments from substrate during electrostripping, which forms “dead lithium”, is a major cause of the battery capacity fading. A better understanding of the structure evolution of mossy deposits during cycling is therefore of considerable importance for developing successful strategies to improve the cycling life of metallic anodes.In this study, we performed operando tomography of mossy zinc structure growth on Cu substrate during electrodeposition with nanoscale resolution at the National Synchrotron Light Source II. Zinc electroplating was carried out at a constant overpotential of 50mv in an in-situ electrodeposition cell made of a capillary tube with the working (Cu wire) and counter (Zn wire) electrodes inserted from opposite ends. Tomography datasets were collected every 2 minutes with an effective voxel size of 43 nm. The 3D structure of zinc deposits was reconstructed from the datasets using open-source code TomoPy.Three different nucleation sites were observed, and the Zn mossy structure was divided into three groups according to these sites for a more detailed analysis. The calculated average porosity of the three mosses was ~35% and this was supported by ex-situ analysis. These three groups were separated into shells from their cores, and the most significant growth was occurring at the outermost shell. Further, the porosity and the volume of the inner shell and the core reached a steady state after a certain amount of time of electrodeposition. This result showed the penetration of the electrolyte to the inner parts of the mossy Zn was limited. All three mossy structure showed the same growth pattern without a preferred growth direction; however, there was an apparent difference of volume growth rate between them, indicating a possible diffusion limitation or uneven electric field distribution, which was in contrast with the previous studies. From ex-situ focus-ion beam scanning electron microscopy examination, it was shown that the moss structure was comprised of individual filaments, which repeatedly branched and entangled to form a mossy Zn. Figure 1

Understanding Zn Electrodeposits Morphology in Secondary Batteries Using Multiscale Computational Approach

Zinc (Zn) aqueous rechargeable batteries (ZBs) have shown a tremendous success in various applications due to their environmental friendliness, multi-electron capacity, high abundance, safety and low cost; however, analogous to their Li-ion battery counterparts, they suffer from the dendrite formation leading to decrease in capacity and eventual failure. Despite many studies reporting a good performance and partial dendrites suppression1, there have been no systematic studies of Zn electrodeposits morphology analysis in ZBs.In this contribution, we present the results of a combined density functional theory (DFT) calculations of Zn diffusion through the ZnO surface layer (i.e., solid electrolyte interface – SEI) and the phase-field modeling (PFM) of Zn electrodeposition study. The DFT calculations are performed using commercial code VASP (Vienna Ab initio Simulation Package)2 with the projector augmented wave (PAW)3 Perdew-Burke-Ernzerhof (PBE)4 pseudo-potentials within generalized gradient approximation (GGA)5. The Zn path and migration barriers are evaluated using the Nudged Elastic Band (NEB) method. The PFM6,7 is developed in the open source code MOOSE and is formulated in continuum theory based upon diffusion-conservation laws using partial-differential equations and then discretized using finite element method8 and solved computationally.The DFT calculations are performed to reveal the dominant diffusion pathways, energetics and the corresponding diffusion coefficients associated with Zn diffusion through the SEI. The SEI surface layer on Zn electrode in ZBs primarily consists of ZnO. Correspondingly, two cases are considered, i.e. the Zn diffusion through the ZnO grain boundary (GB) formed between the different ZnO surface orientations and the Zn diffusion through the ZnO bulk grains. It is found that Zn diffusion occurs through the different nano-channels inside grains as well as through the GB. The path and the activation energies of Zn diffusion vary significantly depending upon the structure of grains and GB, as well as the presence of additional Zn atoms corresponding to the current density dependency. However, the general trend is that Zn diffusion in a grain is faster comparing to the ZnO/ZnO GB. Using the DFT calculations the appropriate elastic properties of SEI components are calculated as required by the PFM.The 2D PFM model predicts the Zn morphologies, the Zn ions concentration, the electrostatic potential, the stress and the equivalent plastic strain. Three regimes are investigated, specifically, low, intermediate and high current densities, where different Zn morphologies, such as boulders, mossy and dendritic shape are observed. It is found that the stress has a major influence on the electrodeposition, and vice versa the Zn electrodeposits growth is found to affect the stress distribution significantly. The plastic yielding occurs preferentially at the node of the boulders and through the filaments and mossy structures. In addition, based upon the present PFM results alternative strategies for the inclusion of the stress field in the mean-field electrochemical modeling will be discussed.Furthermore, both the DFT and the PFM results are supported by scanning electron (SEM) and scanning transmission electron microscopy (STEM) to reveal micro and nano-scopic details of Zn electrodeposits as well as the surface ZnO layer.The combined results also provide a basis for development and design of novel strategies against unwanted Zn dendrites formation.

Multiscale Simulation for the Initial Stage of the Zn Electrodeposition with Kinetic Monte Carlo Based on First-Principle Calculation

Zn negative electrodes for the secondary battery are attractive for the grid-scale energy storage. One of the main issues for its application is a morphological change of Zn during the electrodeposition on the cathode. In particular, formation mechanism of the mossy structure has not been clarified yet. Our previous study observed the layer-like Zn deposits before the evolution of the mossy structure[1]. However, cause of the layer-like structure is still unclear. To reveal further detailed mechanism, molecular level understanding is significant. Kinetic Monte Carlo (KMC) simulation with first-principle calculation is a powerful tool to analyze the morphological formation from the molecular level. This study analyses the initial stage of the Zn electrodeposition by KMC with the parameters derived from first-principle calculations to provide insights into the cause of the unique morphological changes of Zn.In the KMC simulation code, the surface diffusion and deposition were taken into account. (a) surface diffusion on pure facets, (b) edge diffusion along islands, (c) interlayer diffusion were included as the surface diffusion events. All surface diffusion rates were estimated by the transition state theory from first principle calculations. The deposition rate corresponds to current density set in the actual electrochemical measurements. First-principle calculations with the solvation were performed by Vienna ab initio simulation package (VASP) with the polarized continuum model (PCM) and Quantum-ESPRESSO code with the effective screening medium + the reference interaction site model (ESM-RISM)[2]. ESM-RISM can describe the electrochemical interface with the applied potential and the surrounding electrolyte. Nudged elastic band was used to calculate the transition state. Cu(111) and Zn(0001) were utilized as a typical substrate species.The transition state of the surface diffusion of the Zn adatom on Cu(111) and Zn(0001) were calculated in the gas phase initially. The activation energy on Cu was much higher than on Zn, which were 4.38 and 0.58 kJ/mol, respectively. This tendency showed a good agreement with the general discussion of the surface diffusion[3]. To realize the solvation effects on each surface, transition state of the surface diffusion with PCM were calculated. The activation energy on Cu(111) was 4.79 kJ/mol, which was 0.41 kJ/mol higher than the gas phase. On the other hand, the activation energy on Zn(0001) in solvent was only 0.19 kJ/mol higher than the gas phase. The Zn adatom behavior was affected by not only the simple solvation but the interfacial structure among the adatom, solvents and surfaces, indicating that the interaction between the surfaces and solvent species is a significant factor for the surface diffusion behavior. These differences could derive from the difference of the surface electronic state. Since the surface electronic state depends on the facets, the interfacial interaction would affect and change the balance of the surface diffusion among the facets. This effect will cause the change of the initial deposition behavior. Combining these activation energies with KMC as the parameters, we could find the relationship between the morphological features of metals and the electronic state of the surface.Fig. The transition state of the surface diffusion of the Zn adatom in gas and solvent phases[1] T. Otani, Y. Fukunaka, T. Homma, Electrochim. Acta, 242, 364-372 (2017).[2] S. Nishihara, M. Otani, Phys. Rev. B, 96, 115429-115433 (2017).[3] K. Iokibe, K. Azumi, H. Tachikawa, J. Phys. Chem. C, 111, 13510-13516 (2007). Figure 1

Understanding Zn Electrodeposits Morphology in Secondary Batteries Using Phase-Field Model

Zinc (Zn) aqueous rechargeable batteries (ZBs) have shown a tremendous success in various applications due to their environmental friendliness, multi-electron capacity, high abundance, safety and low cost; however, analogous to their Li-ion battery counterparts, they suffer from the dendrite formation leading to decrease in capacity and eventual failure. Despite many studies reporting a good performance and partial dendrites suppression, there have been little theoretical systematic studies of Zn electrodeposits morphology analysis in ZBs. In the current work, we present the results from phase-field modeling (PFM) of Zn electrodeposition at different current densities. Different Zn morphologies, such as boulders, mossy and dendritic shape are modeled. The computational model predicts two-dimensional distribution of Zn electrodeposits, Zn2+ ions concentration, electrostatic potential, stress and equivalent plastic strain. It was found that the stress has a major influence on the electrodeposition, and vice versa the Zn electrodeposits growth is found to affect the stress distribution significantly. The plastic yielding occurs preferentially at the node of the boulders and through the filaments and mossy structures. The results also provide a basis for development and design of novel strategies against unwanted Zn dendrites formation.

Assessing Safety Properties of Lithium Metal Batteries

Currently, the so-called all-solid-state battery (ASSB) is getting enormous attention in academia and industry as a potential next-generation battery technology replacing the state-of-the-art lithium ion battery (LIB) based on a liquid electrolyte. Especially the high energy density (Wh/L) and specific energy (Wh/kg) are appealing [1] with regard to the application in electric vehicles and protable devices. However, the breakthrough of the ASSB technology is hindered by practical challenges in terms of processing, rate capability and cycle life. [2] Apart from high energy densities and a long cycle life, significantly enhanced safety properties are considered key for future battery technologies. Therein, the ASSB benefits from the absence of flammable liquid electrolyte as present in state-of-the-art LIBs. However, only very few studies were published showing the evaluation of the safety properties of lithium metal batteries. [3, 4] Therefore, this study aims on giving first insights into the factors influencing the thermal stability, hence the safety properties of different lithium metal battery (LMB) setups. As a baseline, lithium metal batteries based on liquid electrolyte were investigated in order to understand the influence of the lithium metal deposition behavior on the thermal stability of LMBs. Therein, during charging, lithium metal does not deposit homogeneously but forms dendritic or mossy structures often referred to as high surface area lithium (HSAL). [1, 5] At elevated temperatures, the high reactivity of HSAL can lead to fatal consequences, especailly in presence of liquid electrolyte (e.g. fire, explosion). Thus, the controlled deposition of lithium metal is crucial in order to guarantee safe operation of lithium metal batteries. This can be achieved by either modifying the charging process or the lithium metal surface (mechanically/chemically). Beyond that, lithium metal batteries based on either ceramic or polymer-based electrolytes are investigated. Therein, the cells are cycled to different sates of charge (SOC) and/or states of health (SOH) before analyzing the thermal stability by differential scanning calorimetry (DSC). This way, it is possible to determine distinct differences in the onset temperature for exothermic reactions and the evolving heat as a measure for the intensity of the reactions that might lead to a thermal runaway in large-scale cells. Moreover, the thermal stability of the cell components are characterized individually and in presence of each other du deduce interdependencies considering the chemical and thermal stability. In combination with comprehensive post-mortem analysis, it is possible to determine the factors influencing the thermal stability most. In summary, this study presents first results on the thermal stability of various LMB types to unravel the advantages/disadvantages compared to LIBs. [1] T. Placke, et al., J Solid State Electrochem 2017, 21, 1939-1964. [2] K. Kerman, et al., J Electrochem Soc 2017, 164, A1731-A1744. [3] T. Inoue, et al., ACS Appl Mater Interfaces 2017, 9, 1507-1515. [4] Uyama, et al., ACS Appl Energy Mater 2018, 1, 5712-5717. [5] M. Winter, et al., Adv Mater 1998, 10, 725-763.

Morphology Control of Zinc Electrodeposition By Surfactant Addition for Alkaline-Based Rechargeable Batteries

Critical challenges have remained in practical realization of Zn-air batteries. Focusing on negative electrode, Zn grows in the form of mossy and dendrite structures in low and high current densities, respectively. The non-uniform Zn morphology prevents homogeneous Zn dissolution (discharge) reaction, which is responsible for deterioration of battery performance. In the present study, we used trimethyloctadecylammonium chloride (STAC / surfactants) as an inhibitor of the formation of mossy and dendrite Zn structures. The addition of surfactant to an electrolyte is not necessarily a new approach for improving Zn deposition morphology. However, the new insight in this study is that we found a leveling agent enabling to suppress the formation of not only mossy structure but also dendrite structure and highly flatten deposit in wide current densities (5−40 mA cm−2). The mechanism was systematically investigated by means of X-ray diffraction and measurements of zeta potential and kinetic current density for Zn-deposition. The obtained results indicate that quaternary ammonium cations (STAC) with bulky size adsorb to protrusions which are cause of shape change and suppress Zn deposition in the region to promote lateral growth. In charge−discharge test using an asymmetric Cu|Zn cell, Coulombic efficiency in an additive free electrolyte was less than 95%, whereas the addition of STAC with 1 mM achieved superior cycling performance without any capacity loss originated from the generation of dead Zn. These results demonstrate that the addition of STAC is a promising way of controlling Zn morphology and allows the efficient utilization of Zn metal electrode in large-scale batteries for important next-generation applications. Figure 1

Insights into the Safety Properties of Lithium Metal Batteries

With regard to current trends, the so-called all-solid-state battery (ASSB) is getting enormous attention in academia and industry as a potential next-generation battery technology replacing the state-of-the-art lithium ion battery (LIB). Especially the high energy density (Wh/L) and specific energy (Wh/kg) are appealing properties of the ASSB [1] potentially enabling longer driving ranges for electric vehicles compared to the state-of-the-art lithium ion technology. However, the breakthrough of the ASSB technology is facing practical challenges in terms of processing, rate capability and cycle life. [2] In addition to high energy densities and a long cycle life, significantly enhanced safety properties are considered key for future battery technologies. Regarding the latter, the ASSB benefits from the absence of flammable liquid electrolyte compared to LIBs. However, only very few studies were published showing the evaluation of the safety properties of lithium metal batteries. [3] Therefore, this study aims on giving first insights into the factors influencing the thermal stability, hence the safety properties of different lithium metal battery setups. As a starting point, lithium metal batteries based on liquid electrolyte were investigated in order to understand the influence of the lithium metal deposition behavior on the thermal stability of the cell. Therein, it has to be considered that during charging lithium metal does not deposit homogeneously but forms dendritic or mossy structures often referred to as high surface area lithium (HSAL). [1, 4] At elevated temperatures, the high reactivity of HSAL in presence of liquid electrolyte can lead to fatal consequences (e.g. fire, explosion). Thus, the controlled deposition of lithium metal is crucial in order to guarantee safe operation of lithium metal batteries. This can be achieved by either modifying the charging process or the lithium metal surface (mechanically/chemically). Beyond that, lithium metal batteries based on either ceramic or polymer-based electrolytes are investigated. Therein, the cells are cycled to different sates of charge (SOC) and/or states of health (SOH) before analyzing the thermal stability by differential scanning calorimetry (DSC). This way, it is possible to determine distinct differences in the onset temperature for exothermic reactions and the evolving heat as a measure for the intensity of the reactions that might lead to a thermal runaway in large-scale cells. Moreover, the thermal stability of the cell components are characterized individually and in presence of each other. In combination with comprehensive post-mortem analysis, it is possible to determine the factors influencing the thermal stability most. In summary, this study presents first results on the thermal stability of various lithium metal battery types to unravel the advantages/disadvantages compared to LIBs. The authors acknowledge the BMBF for funding the project “BCT” (03XP019I). [1] T. Placke, et al., J Solid State Electrochem 2017, 21, 1939-1964. [2] K. Kerman, et al., J Electrochem Soc 2017, 164, A1731-A1744. [3] T. Inoue, et al., ACS Appl Mater Interfaces 2017, 9, 1507-1515. [4] M. Winter, et al., Adv Mater 1998, 10, 725-763.

Communication—Alkyl-Chain-Length Dependence of Quaternary Ammonium Cation on Zn Deposition Morphology in Alkaline-Based Electrolytes

The Influence of alkyl chain length in quaternary ammonium cations on electrochemical Zn growth morphology in alkaline-based electrolytes was systematically investigated under constant current density. Mossy structures that typically developed at low current density were mitigated in an electrolyte with 1 mM octyltrimethylammonium chloride. By changing from octyl to decyl group in the surfactant, completely smooth deposition morphology was achieved. Electrochemical impedance spectroscopic analysis revealed that charge transfer resistance associated with Zn deposition increased with alkyl chain length. The obtained results indicate that bulky ammonium cations suppress excess electrodeposition due to the steric hindrance effect and thereby result in smooth morphology.

Mesoscale Understanding of Lithium Electrodeposition for Intercalation Electrodes

Stringent performance and operational requirements in electric vehicles can push lithium-ion batteries toward unsafe conditions. Electroplating and possible dendritic growth are a cause for safety concern as well as performance deterioration in such intercalation chemistry-based energy storage systems. There is a need for better understanding of the morphology evolution because of electrodeposition of lithium on the graphite anode surface and the interplay between material properties and operating conditions. In this work, a mesoscale analysis of the underlying multimodal interactions is presented to study the evolution of morphology due to lithium deposition on typical graphite electrode surfaces. It is found that electrodeposition is a complex interplay between the rate of reduction of Li ions and the intercalation of Li in the graphite anodes. The morphology of the electrodeposited film changes from dendritic to mossy structures because of the surface diffusion of lithium on the electrodeposited film.

Electrochemical and Mechanical Driving Forces for Lithium Electrodeposition

It is well documented in the scientific literature that lithium electrodeposits grow into elongated "dendritic" structures from the top (tip-controlled) at high current densities and into mossy structures from the base (base-controlled) under low current densities [1,2]. In order to physically understand base-controlled growth, first an analytical model has been developed to incorporate the thermodynamic and kinetic driving forces of lithium to describe the multiphysical electrodeposition kinetics of lithium. Given the charging current density, the following regimes of lithium growth behavior are readily predicted from the analytical framework: (i) thermodynamic suppression regime, (ii) incubation regime, (iii) tip-controlled regime, (iv) base-controlled regime, and (v) mixed growth regime. These regimes explain the experimental observations and provide the kinetic boundaries between tip- and base-controlled growth [3]. Further, numerical calculations demonstrate that high von Mises stresses in localized regions of the lithium whisker extrude lithium and lead to base-controlled growth. In addition, the complex multiphysical dendrite-dendrite electro-chemo-mechanical interactions are rationalized through local electric fields, electrodeposition rates, mechanical stresses, and plastic velocities that are otherwise difficult to visualize in-situ in experiments. Overall, the combined analytical and the numerical frameworks provide multiphysical insight and serve as a design guideline for dendrite-free rechargeable batteries. [1] O. Crowther, A. C. West. “Effect of electrolyte composition on lithium dendrite growth.” Journal of the Electrochemical Society, 155, A806-A811 (2008). [2] P. Bai, J. Li, F.R. Brushett, M.Z. Bazant. “Transition of lithium growth mechanisms in liquid electrolytes.” Energy & Environmental Science, 9, 3221-3229 (2016). [3] A. Jana, R.E. García. “Lithium dendrite growth mechanisms in liquid electrolytes.” Nano Energy, 41, 552-565 (2017).

3D mossy structures of zinc filaments: A facile strategy for superamphiphobic surface design

The superamphiphobic surfaces with extreme repellency to liquids are very attractive in many fields, but their fabrication processes are always low effective and expensive. So it is still a challenge to create the superamphiphobic surfaces by simple, time saving and universal method. In this work, the mossy zinc (Zn) filaments, a promising re-entrant structure, was rapidly constructed on various metal surfaces by electrochemical deposition approach. After modification by 1H,1H,2H,2H- perfluorodecyltrichlorosilane (PFDTCS), the Zn@PFDTCS coating exhibited superamphiphobicity in air. The correlation between the morphology of Zn filaments and electrochemical deposition parameters has been studied. The superamphiphobic surface with contact angle higher than 154°, sliding angle lower than 5° and adhesive force lower than 0.043 mN to water and hexadecane was obtained, when the current density was 1.78 A ·dm−2, the mass fraction of zinc was 0.71 wt% and the deposition time was 40 min. Furthermore, the Zn@PFDTCS 2D-meshes were used to collect oil droplets under water and cut water droplet in oil due to their superoleophilicity under water and superhydrophobicity under oil. We anticipated that the simple and rapid method guides the design of perfect artificial superamphiphobic surfaces in practical application.

Electrodeposited TiO2 Film with Mossy Nanostructure for Efficient Compact Layer in Scaffold‐Type Perovskite Solar Cell

A quality compact layer (CL) with a thin and pinhole‐free structure is essential for an efficient perovskite solar cell (PSC). This study offers a new perspective regarding a criterion of a quality CL, namely three‐dimensional interfacial contact. In an examination of the deposition mechanisms, film morphologies, and optical and electronic properties of CLs fabricated using different methods, an electrodeposited (ED) TiO2 CL outperformed other TiO2 CLs. This is not only because of its ultrathin, fully covered morphology but also because of its mossy structure, which provides high shunt resistance and low ohmic resistance in ED‐TiO2‐based PSCs. In addition, our results confirm that in scaffold‐type PSCs, the CL provides a charge extraction function in addition to the mesoporous scaffold, which is designed as the main electron transporting layer.

Characterization of Lithium Inventory in Lithium Metal Anodes during Cycling

A rising interest in batteries has resulted from a need for improved energy storage systems for transportation and portable electronics applications. Lithium is of particular interest as an anode due to its low redox potential and high specific capacity. However, these systems have not been practically realized due to the nonuniform plating of lithium. Many studies have modeled this formation of dendritic or mossy structures as a function of parameters such as temperature, electrolyte composition, current density and overpotentials [1]. However, the majority of these efforts have focused on the influence of these parameters on the nucleation and growth of lithium at early stages; the processes for deplating of lithium during discharge and the continued growth of dendritic structures, once formed, are poorly understood. In this work, we use commercially available primary cells as a basis for understanding the movement of lithium during cycling. Specifically, we discharge both coin and pouch cells at a range of C rates to specified states of charge (SOC). During discharge, acoustic time of flight studies are used to provide insight on how mass is transferred between layers [2]. Once the cell has been discharged to the specified SOC, electrochemical impedance spectroscopy is conducted before dismantling the cell and observing the lithium electrode using scanning electron microscopy. The same characterization methods are subsequently used for cells that have been discharged and charged for a range of cycles. Through these techniques, we track how lithium is transferred between electrodes, as well as how lithium is spatially utilized across the anode.

Effect of lead and tin additives on surface morphology evolution of electrodeposited zinc

Effects of Pb and Sn additives on electrodeposition of Zn were investigated for the applications in Zn secondary battery, focused on their roles on morphological evolution. Similarly to well-known effect of Pb addition to smoothly electrodeposit Zn film surface, Sn also exhibited to suppress the formation of mossy structures, which were highly filamentous Zn electrodeposits to cause frequently battery failure. Pb significantly shifted deposition potential of Zn to negative value at concentration less than 1.0mmoldm−3, while Sn showed no substantial effect even at 50mmoldm−3. The morphological evolution analysis demonstrated that the addition of 1.0mmoldm−3 Pb significantly altered nucleation behavior of Zn at initial stages of deposition; stacking of layer-like structures (microsteps) were altered to discrete and uniform nuclei of Zn preferentially oriented to (002) direction. In contrast, with Sn additive, morphological features at the initial stage of deposition were similar to that without additive. However, microsteps evolution was gradually suppressed and relatively strong (002) texture of electrodeposited Zn was mitigated by Sn addition. These results suggested that the beneficial roles of Pb and Sn addition on Zn morphological evolution were ascribed to different reasons; Pb diminished the active growth sites of Zn surface by suppressing the deposition reaction of Zn, while Sn suppressed microsteps evolution by different texture evolution of Zn.

Processes at the Negative Electrode of Alkaline Zinc-Air Batteries

Secondary alkaline Zinc-Air batteries hold several distinct advantages over other large scale systems for energy storage. Such batteries use materials that are low cost (Zinc is abundant in the earth’s crust and alkaline aqueous solutions are cheap and well understood) and have a low toxicity associated with them. To make Zn-air batteries viable for large-scale use, issues at the Zn electrode need to be addressed. Allowance must be made to prevent dendrite growth that eventually leads to cell shorting during charging and electrode passivation during discharge. Additionally electrode morphology changes between each cycle produce inconsistent performance. During the oxidation of Zn occurring in the discharge process in the battery, a sequence of chemical steps occur resulting in eventual production of ZnO on the electrode. Here we report detailed studies of the Zn plating and stripping processes in alkaline environments. These processes are investigated by in situ electrochemical STEM (in situ ec-STEM) measurements that allow us to directly observe nucleation and growth processes as well as the behavior of Zn structures during stripping. These are correlated with electrochemical QCM measurements (EQCM). These methods give us insight into the relative rates of different processes. In situ ec-STEM experiments show that deposition and dissolution processes occurred at different rates and directionality. During deposition, deposits are initially observed grow quickly in all directions consuming available Zinc ions near the electrode surface. In the experiment whose video frames are shown in figure 1, 10,000 µm2 of observed Zinc deposit area was added in the first 30 seconds of deposition and was half of the total observed deposit area for a 3 minute, 250mV overpotential deposition experiment. After this, growth follows the concentration gradient and grows at a linear rate of 3,500 µm2 of observed deposit area per minute. Additionally, the overpotential employed during deposition appeared to impact deposit morphology dramatically, as 250mV deposition produced large boulder type deposits while a 300mV deposition test produced a mossy structure that covered the electrode surface. Dissolution processes observed via in situ ec-STEM with a 300mV underpotential step to anodically strip deposits showed only half of the material area was removed over a 3 min experiment with only a third reduction of the max particle height. Figure 2 shows the video frames from this experiment which began immediately after the experiment from figure 1. Further material removal appears to occur universally along the surface of the deposit, leading to the possibility of the deposit being detached from the electrode surface. EQCM was used to observe the formation and removal of the ZnO passive layer described above. The formation of the passivation layer was observed as a deviation from the rate of Zinc removal in the massograms generated by the QCM as a ZnO layer forms on the electrode surface. EQCM confirmed that the passive layer was the result of a precipitation reaction occurring in the solution near the interface. Further EQCM indicated that during dissolution multiple pathways of removing Zn were possibly occurring at once when looking at the charge transfer with actual mass removed, while deposition steps appeared to have only one reaction path of Zinc deposition occurring. This indicates that the forward and reverse pathways of the Zn reaction are not the same. Motional resistance to the crystal oscillation was collected as well to determine relative roughness of the deposits formed during deposition to correlate to the variations of in deposition seen in the in situ ec-STEM work. To overcome macroscopic issues associated with the Zn electrode, we are exploring using porous carbon negative electrodes similar to those used as air catalyst support materials. These provide a conductive and permanent structure in which Zn can be deposited and removed for battery operation. Operation of this electrode under various circumstances will be discussed. We gratefully acknowledge the support of this work by the U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability (Dr. Imre Gyuk). A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User facility. Figure 1

Morphological evolution of mossy structures during the electrodeposition of zinc from an alkaline zincate solution

The morphological evolution of zinc during electrodeposition from an alkaline zincate solution, i.e., corresponding to charging operation in secondary zinc batteries, was investigated. The initiation and propagation of mossy structures were particularly focused; mossy structures represent one of the undesired, irregular morphologies at low cathodic overpotential. Scanning electron microscopy observations showed that mossy structures formed transiently from layer-like morphology followed by the formation of protrusions with a size of submicrometers. These protrusions still exhibited layer-like features in the initial stage. In addition to this transition behavior, the propagation of mossy structures was monitored in situ by optical microscopy. At −5.0mAcm−2, mossy structures formed successively and propagated, while there was no clear change in layer-like morphology during deposition outside these characteristic structures. At −20mAcm−2, the layer-like growth proceeded for longer duration and the population of the mossy structures apparently decreased. However, they continuously increased in size followed by successive formation of mossy structures once initiated. From these results, growth mode of zinc at low cathodic overpotential transiently changed from layer-like growth to mossy structure evolution accompanied with dynamic variations in local current density.