Dendrite initiation and propagation in lithium metal solid-state batteries.

All-solid-state batteries with a Li anode and ceramic electrolyte have the potential to deliver a step change in performance compared with today's Li-ion batteries1,2. However, Li dendrites (filaments) form on charging at practical rates and penetrate the ceramic electrolyte, leading to short circuit and cell failure3,4. Previous models of dendrite penetration have generally focused on a single process for dendrite initiation and propagation, with Li driving the crack at its tip5-9. Here we show that initiation and propagation are separate processes. Initiation arises from Li deposition into subsurface pores, by means of microcracks that connect the pores to the surface. Once filled, further charging builds pressure in the pores owing to the slow extrusion of Li (viscoplastic flow) back to the surface, leading to cracking. By contrast, dendrite propagation occurs by wedge opening, with Li driving the dry crack from the rear, not the tip. Whereas initiation is determined by the local (microscopic) fracture strength at the grain boundaries, the pore size, pore population density and current density, propagation depends on the (macroscopic) fracture toughness of the ceramic, the length of the Li dendrite (filament) that partially occupies the dry crack, current density, stack pressure and the charge capacity accessed during each cycle. Lower stack pressures suppress propagation, markedly extending the number of cycles before short circuit in cells in which dendrites have initiated.

3D-printed λ/4 phase plate for broadband microwave applications.

An efficient design for a quarter-wave (λ/4) retardation plate (QWP) operating at microwave frequencies has been designed and manufactured using dual head fused deposition modelling (FDM) 3D printing. Exploiting a bespoke composite material feedstock filament with high dielectric permittivity ϵr = 10.8, the resulting 3D-printed QWP comprising alternative layers of high and low permittivity had a high artificial double refraction of Δϵ = 2.9. The QWP provided broadband conversion of linear to circular polarization and phase modulation of an incident plane electromagnetic wave at 12-18 GHz, and demonstrated the potential for optical devices via additive manufacture for use in the microwave frequency range.

Core-shell nanoparticles and enhanced polarization in polymer based nanocomposite dielectrics.

We present a detailed study of the evolution and nature of metallic core-oxide shell particles and the role of nanostructure in the physics of enhanced polarization in polymer-nanocomposite (PNC) based dielectrics. Nylon-6 based PNCs consisting of aluminium (core)-aluminium oxide (shell) nanoparticles were fabricated by a vacuum deposition technique. Their resulting high polarizability was closely related to the formation and chemistry of the core-shell structure that was revealed by transmission electron microscopy to comprise a highly-defective, strained and non-stoichiometric semi-crystalline/amorphous Al-oxide shell.

Spray fabrication of additive-free electrodes for advanced Lithium-Ion storage technologies.

Polymer binders and carbon conductivity enhancers are inevitably required to make improvements in structural durability and electrochemical performance of lithium-ion battery (LIB) electrodes, although these additive constituents incur weight and volume penalties on the overall battery capacity. Here, additive-free electrode architectures were successfully fabricated over 20 × 20 cm2 electrode areas using a layer-by-layer spray coating approach, with the ultimate goal to boost gravimetric/volumetric electrode capacity and to reduce the total cost of LIB cells. Initially, the binder fraction of spray-coated Li4Ti5O12 (LTO) electrodes was reduced progressively, from 40 to 0 wt%. The electrochemical behavior of electrodes was then re-optimized as a proportion of conductivity enhancers within the binder-free electrode decreased to zero. Further, the otherwise identical spray coating process was applied to manufacture LiFePO4 (LFP) positive electrodes, leading to all-additive-free full-cell LIB configurations with attractive energy density of âˆ¼310 Wh/kg and power performance of âˆ¼1500 W/kg.

Numerical Design of Microporous Carbon Binder Domains Phase in Composite Cathodes for Lithium-Ion Batteries.

Lithium-ion battery (LIB) performance can be significantly affected by the nature of the complex electrode microstructure. The carbon binder domain (CBD) present in almost all LIB electrodes is used to enhance mechanical stability and facilitate electronic conduction, and understanding the CBD phase microstructure and how it affects the complex coupled transport processes is crucial to LIB performance optimization. In this work, the influence of microporosity in the CBD phase has been studied in detail for the first time, enabling insight into the relationships between the CBD microstructure and the battery performance. To investigate the effect of the CBD pore size distributions, a random field method is used to generate in silico a multiple-phase electrode structure, including bimodal pore size distributions seen in practice and microporous CBD with a tunable pore size and variable transport properties. The distribution of macropores and the microporous CBD phase substantially affected simulated battery performance, where battery specific capacity improved as the microporosity of the CBD phase increased.

Sequential Deposition of Integrated Cathode-Inorganic Separator-Anode Multilayers for High Performance Li-Ion Batteries.

A porous, spray-deposited Al2O3-based separator was developed to enable the direct deposition of an electrode/separator/electrode Li-ion battery full cell assembly in a single operation. The optimized sprayed separator consisted of 50 nm Al2O3 particles, 1 wt % poly(acrylic acid), and 5 wt % styrene-butadiene rubber, deposited from an 80:20 vol % suspension of water and isopropanol. Separators between 5 and 22 µm thick had consistent and similar porosity of ∼58%, excellent wettability, thermal stability to at least 180 °C, adequate electrochemical stability and high effective ionic conductivity of ∼1 mS cm-1 at room temperature in an EC/DMC electrolyte, roughly double that of a conventional polypropylene separator. A sequentially deposited three-layer LiFePO4/Al2O3/Li4Ti5O12 full cell, the first of its kind, showed similar rate performance to an identical cell with a conventional polypropylene separator, with a capacity of ∼50 mAh g-1 at 30 C. However, after cycling at 2 C for 400 cycles, Al2O3 separator full cells retained 96.3% capacity, significantly more than conventional full cells with a capacity of 79.2% remaining.

X-ray Imaging of Alloy Solidification: Crystal Formation, Growth, Instability and Defects.

Synchrotron and laboratory-based X-ray imaging techniques have been increasingly used for in situ investigations of alloy solidification and other metal processes. Several reviews have been published in recent years that have focused on the development of in situ X-ray imaging techniques for metal solidification studies. Instead, this work provides a comprehensive review of knowledge provided by in situ X-ray imaging for improved understanding of solidification theories and emerging metal processing technologies. We first review insights related to crystal nucleation and growth mechanisms gained by in situ X-ray imaging, including solute suppressed nucleation theory of α-Al and intermetallic compound crystals, dendritic growth of α-Al and the twin plane re-entrant growth mechanism of faceted Fe-rich intermetallics. Second, we discuss the contribution of in situ X-ray studies in understanding microstructural instability, including dendrite fragmentation induced by solute-driven, dendrite root re-melting, instability of a planar solid/liquid interface, the cellular-to-dendritic transition and the columnar-to-equiaxed transition. Third, we review investigations of defect formation mechanisms during near-equilibrium solidification, including porosity and hot tear formation, and the associated liquid metal flow. Then, we discuss how X-ray imaging is being applied to the understanding and development of emerging metal processes that operate further from equilibrium, such as additive manufacturing. Finally, the outlook for future research opportunities and challenges is presented.

Design of Scalable, Next-Generation Thick Electrodes: Opportunities and Challenges.

Lithium-ion battery electrodes are on course to benefit from current research in structure re-engineering to allow for the implementation of thicker electrodes. Increasing the thickness of a battery electrode enables significant improvements in gravimetric energy density while simultaneously reducing manufacturing costs. Both metrics are critical if the transition to sustainable transport systems is to be fully realized commercially. However, significant barriers exist that prevent the use of such microstructures: performance issues, manufacturing challenges, and scalability all remain open areas of research. In this Perspective, we discuss the challenges in adapting current manufacturing processes for thick electrodes and the opportunities that pore engineering presents in order to design thicker and better electrodes while simultaneously considering long-term performance and scalability.

High Energy Density Single-Crystal NMC/Li6PS5Cl Cathodes for All-Solid-State Lithium-Metal Batteries.

To match the high capacity of metallic anodes, all-solid-state batteries require high energy density, long-lasting composite cathodes such as Ni-Mn-Co (NMC)-based lithium oxides mixed with a solid-state electrolyte (SSE). However in practice, cathode capacity typically fades due to NMC cracking and increasing NMC/SSE interface debonding because of NMC pulverization, which is only partially mitigated by the application of a high cell pressure during cycling. Using smart processing protocols, we report a single-crystal particulate LiNi0.83Mn0.06Co0.11O2 and Li6PS5Cl SSE composite cathode with outstanding discharge capacity of 210 mA h g-1 at 30 °C. A first cycle coulombic efficiency of >85, and >99% thereafter, was achieved despite a 5.5% volume change during cycling. A near-practical discharge capacity at a high areal capacity of 8.7 mA h cm-2 was obtained using an asymmetric anode/cathode cycling pressure of only 2.5 MPa/0.2 MPa.

Amorphization in extreme deformation of the CrMnFeCoNi high-entropy alloy.

Ever-harsher service conditions in the future will call for materials with increasing ability to undergo deformation without sustaining damage while retaining high strength. Prime candidates for these conditions are certain high-entropy alloys (HEAs), which have extraordinary work-hardening ability and toughness. By subjecting the equiatomic CrMnFeCoNi HEA to severe plastic deformation through swaging followed by either quasi-static compression or dynamic deformation in shear, we observe a dense structure comprising stacking faults, twins, transformation from the face-centered cubic to the hexagonal close-packed structure, and, of particular note, amorphization. The coordinated propagation of stacking faults and twins along {111} planes generates high-deformation regions, which can reorganize into hexagonal packets; when the defect density in these regions reaches a critical level, they generate islands of amorphous material. These regions can have outstanding mechanical properties, which provide additional strengthening and/or toughening mechanisms to enhance the capability of these alloys to withstand extreme loading conditions.

SnS/PbS nanocrystal heterojunction photovoltaics.

We report advances in the growth, characterization and photovoltaic properties of SnS nanocrystals, with controlled < 10 nm size, and their inclusion into a lead chalcogenide solar cell. The SnS/PbS nanocrystalline film heterojunction is shown to display a type II band alignment, in which the direction of flow of the photocurrent depends on the order of the layers and not the relative work functions of the contacts. On placing the SnS layer next to the indium tin oxide (ITO) cathode we observe a dramatic increase in V(oc) to as much as 0.45 V. Our results suggest that SnS nanocrystal films can be used in multi-junction solar cells, that a SnS/PbS heterojunction on its own shows photovoltaic behaviour, and that a SnS layer in an ITO/SnS/PbS/Al device is acting to suppress the flow of an electron injection current.

Active Metamaterials with Negative Static Electric Susceptibility.

Although well-established textbook arguments suggest that static electric susceptibility χ(0) must be positive in "all bodies," it has been pointed out that materials that are not in thermodynamic equilibrium are not necessarily subject to this restriction. Media with inverted populations of atomic and molecular energy levels have been predicted theoretically to exhibit a χ(0) < 0 state, however the systems envisioned require reduced temperature, reduced pressure, and an external pump laser to maintain the population inversion. Further, the existence of χ(0) < 0 has never been confirmed experimentally. Here, a completely different approach is taken to the question of χ(0) < 0 and a design concept to achieve "true" χ(0) < 0 is proposed based on active metamaterials with internal power sources. Two active metamaterial structures are fabricated that, despite still having their power sources implemented externally for reasons of practical convenience, provide evidence in support of the general concept. Effective values are readily achieved at room temperature and pressure and are tunable throughout the range of stability -1 < χ(0) < 0, resulting in experimentally-determined magnitudes that are over one thousand times greater than those predicted previously. Since χ(0) < 0 is the missing electric analog of diamagnetism, this work opens the door to new technological capabilities such as stable electrostatic levitation.

4D Bragg Edge Tomography of Directional Ice Templated Graphite Electrodes.

Bragg edge tomography was carried out on novel, ultra-thick, directional ice templated graphite electrodes for Li-ion battery cells to visualise the distribution of graphite and stable lithiation phases, namely LiC12 and LiC6. The four-dimensional Bragg edge, wavelength-resolved neutron tomography technique allowed the investigation of the crystallographic lithiation states and comparison with the electrode state of charge. The tomographic imaging technique provided insight into the crystallographic changes during de-/lithiation over the electrode thickness by mapping the attenuation curves and Bragg edge parameters with a spatial resolution of approximately 300 µm. This feasibility study was performed on the IMAT beamline at the ISIS pulsed neutron spallation source, UK, and was the first time the 4D Bragg edge tomography method was applied to Li-ion battery electrodes. The utility of the technique was further enhanced by correlation with corresponding X-ray tomography data obtained at the Diamond Light Source, UK.

Spray deposition of steam treated and functionalized single-walled and multi-walled carbon nanotube films for supercapacitors.

Steam purified, carboxylic and ester functionalized single-walled carbon nanotube (SWNT) and multi-walled carbon nanotube (MWNT) films with homogeneous distribution and flexible control of thickness and area were fabricated on polymeric and metallic substrates using a modified spray deposition technique. By employing a pre-sprayed polyelectrolyte, the adhesion of the carbon nanotube (CNT) films to the substrates was significantly enhanced by electrostatic interaction. Carboxylic and ester functionalization improved electrochemical performance when immersed in 0.1 M H(2)SO(4) and the specific capacitance reached 155 and 77 F g(-1) for carboxylic functionalized SWNT and MWNT films respectively. Compared with existing techniques such as hot pressing, vacuum filtration and dip coating, the ambient pressure spray deposition technique is suggested as particularly well suited for preparing CNT films at large scale for applications including providing electrodes for electrochemical supercapacitors and paper batteries.

Scalable, Large-Area Printing of Pore-Array Electrodes for Ultrahigh Power Electrochemical Energy Storage.

Through-electrode thickness honeycomb architectures were layer-by-layer self-assembled directly through a scalable printing process for ultrapower hybrid lithium-ion capacitor applications. Initially, the electrochemical performance of the pore-array electrodes was investigated as a function of the active material type (graphene plates, carbon nanofibers, and activated carbon). Inactive components (conductive carbon and polymer binder) were then minimized to 5 wt %. Finally, an optimized activated carbon-based cathode was paired with a spray-printed Li4Ti5O12-based anode and a range of anode-to-cathode mass ratios in a lithium-ion capacitor arrangement were investigated. A 1:5 anode/cathode mass ratio provided an attractive energy density comparable with a Li4Ti5O12/LiFePO4 lithium-ion battery but with outstanding power capability that was an order of magnitude greater than typical for lithium-ion batteries. The pore-array electrode was reproduced over areas of 20 cm × 15 cm in a double-sided coated configuration, and the option for selectively patterning electrodes was also demonstrated.

Spray-Printed and Self-Assembled Honeycomb Electrodes of Silicon-Decorated Carbon Nanofibers for Li-Ion Batteries.

Directional, micron-scale honeycomb pores in Li-ion battery electrodes were fabricated using a layer-by-layer, self-assembly approach based on spray-printing of carbon nanofibers. By controlling the drying behavior of each printed electrode layer through optimization of (i) the volume ratio of fugitive bisolvent carriers in the suspension and (ii) the substrate temperature during printing, self-assembled, honeycomb pore channels through the electrode were created spontaneously and reliably on current collector areas larger than 20 cm × 15 cm. The honeycomb pore structure promoted efficient Li-ion dynamics at high charge/discharge current densities. Incorporating an optimum fraction (2.5 wt %) of high-energy-density Si particulate into the honeycomb electrodes provided a 4-fold increase in deliverable discharge capacity at 8000 mA/g. The spray-printed, honeycomb pore electrodes were then investigated as negative electrodes coupled with similar spray-printed LiFePO4 positive electrodes in a full Li-ion cell configuration, providing an approximately 50% improvement in rate capacity retention over half-cell configurations of identical electrodes at 4000 mA/g.

Ultrasonic liquid metal processing: The essential role of cavitation bubbles in controlling acoustic streaming.

The acoustic streaming behaviour below an ultrasonic sonotrode in water was predicted by numerical simulation and validated by experimental studies. The flow was calculated by solving the transient Reynolds-Averaged Navier-Stokes equations with a source term representing ultrasonic excitation implemented from the predictions of a nonlinear acoustic model. Comparisons with the measured flow field from Particle Image Velocimetry (PIV) water experiments revealed good agreement in both velocity magnitude and direction at two power settings, supporting the validity of the model for acoustic streaming in the presence of cavitating bubbles. Turbulent features measured by PIV were also recovered by the model. The model was then applied to the technologically important area of ultrasonic treatment of liquid aluminium, to achieve the prediction of acoustic streaming for the very first time that accounts for nonlinear pressure propagation in the presence of acoustic cavitation in the melt. Simulations show a strong dependence of the acoustic streaming flow direction on the cavitating bubble volume fraction, reflecting PIV observations. This has implications for the technological use of ultrasound in liquid metal processing.

Crystal nucleation in metallic alloys using x-ray radiography and machine learning.

The crystallization of solidifying Al-Cu alloys over a wide range of conditions was studied in situ by synchrotron x-ray radiography, and the data were analyzed using a computer vision algorithm trained using machine learning. The effect of cooling rate and solute concentration on nucleation undercooling, crystal formation rate, and crystal growth rate was measured automatically for thousands of separate crystals, which was impossible to achieve manually. Nucleation undercooling distributions confirmed the efficiency of extrinsic grain refiners and gave support to the widely assumed free growth model of heterogeneous nucleation. We show that crystallization occurred in temporal and spatial bursts associated with a solute-suppressed nucleation zone.

Multiscale Engineered Si/SiO x Nanocomposite Electrodes for Lithium-Ion Batteries Using Layer-by-Layer Spray Deposition.

Si-based high-capacity materials have gained much attention as an alternative to graphite in Li-ion battery anodes. Although Si additions to graphite anodes are now commercialized, the fraction of Si that can be usefully exploited is restricted due to its poor cyclability arising from the large volume changes during charge/discharge. Si/SiO x nanocomposites have also shown promising behavior, such as better capacity retention than Si alone because the amorphous SiO x helps to accommodate the volume changes of the Si. Here, we demonstrate a new electrode architecture for further advancing the performance of Si/SiO x nanocomposite anodes using a scalable layer-by-layer atomization spray deposition technique. We show that particulate C interlayers between the current collector and the Si/SiO x layer and between the separator and the Si/SiO x layer improved electrical contact and reduced irreversible pulverization of the Si/SiO x significantly. Overall, the multiscale approach based on microstructuring at the electrode level combined with nanoengineering at the material level improved the capacity, rate capability, and cycling stability compared to that of an anode comprising a random mixture of the same materials.

Fabrication of Composite Filaments with High Dielectric Permittivity for Fused Deposition 3D Printing.

Additive manufacturing of complex structures with spatially varying electromagnetic properties can enable new applications in high-technology sectors such as communications and sensors. This work presents the fabrication method as well as microstructural and dielectric characterization of bespoke composite filaments for fused deposition modeling (FDM) 3D printing of microwave devices with a high relative dielectric permittivity ϵ = 11 in the GHz frequency range. The filament is composed of 32 vol % of ferroelectric barium titanate (BaTiO 3 ) micro-particles in a polymeric acrylonitrile butadiene styrene (ABS) matrix. An ionic organic ester surfactant was added during formulation to enhance the compatibility between the polymer and the BaTiO 3 . To promote reproducible and robust printability of the fabricated filament, and to promote plasticity, dibutyl phthalate was additionally used. The combined effect of 1 wt % surfactant and 5 wt % plasticizer resulted in a uniform, many hundreds of meters, continuous filament of commercial quality capable of many hours of uninterrupted 3D printing. We demonstrate the feasibility of using the high dielectric constant filament for 3D printing through the fabrication of a range of optical devices. The approach herein may be used as a guide for the successful fabrication of many types of composite filament with varying functions for a broad range of applications.