Illuminating Polysulfide Distribution in Lithium Sulfur Batteries; Tracking Polysulfide Shuttle Using Operando Optical Fluorescence Microscopy.

High-energy-density lithium sulfur (Li-S) batteries suffer heavily from the polysulfide shuttle effect, a result of the dissolution and transport of intermediate polysulfides from the cathode, into the electrolyte, and onto the anode, leading to rapid cell degradation. If this primary mechanism of cell failure is to be overcome, the distribution, dynamics, and degree of polysulfide transport must first be understood in depth. In this work, operando optical fluorescence microscope imaging of optically accessible Li-S cells is shown to enable real-time qualitative visualization of the spatial distribution of lithium polysulfides, both within the electrolyte and porous cathode. Quantitative determinations of spatial concentration are also possible at a low enough concentration. The distribution throughout cycling is monitored, including direct observation of polysulfide shuttling to the anode and consequent dendrite formation. This was enabled through the optimization of a selective fluorescent dye, verified to fluoresce proportionally with concentration of polysulfides within Li-S cells. This ability to directly and conveniently track the spatial distribution of soluble polysulfide intermediates in Li-S battery electrolytes, while the cell operates, has the potential to have a widespread impact across the field, for example, by enabling the influence of a variety of polysulfide mitigation strategies to be assessed and optimized, including in this work the LiNO3 additive.

The Influence of Cathode Degradation Products on the Anode Interface in Lithium-Ion Batteries.

Degradation of cathode materials in lithium-ion batteries results in the presence of transition metal ions in the electrolyte, and these ions are known to play a major role in capacity fade and cell failure. Yet, while it is known that transition metal ions migrate from the metal oxide cathode and deposit on the graphite anode, their specific influence on anode reactions and structures, such as the solid electrolyte interphase (SEI), is still quite poorly understood due to the complexity in studying this interface in operational cells. In this work we combine operando electrochemical atomic force microscopy (EC-AFM), electrochemical quartz crystal microbalance (EQCM), and electrochemical impedance spectroscopy (EIS) measurements to probe the influence of a range of transition metal ions on the morphological, mechanical, chemical, and electrical properties of the SEI. By adding representative concentrations of Ni2+, Mn2+, and Co2+ ions into a commercially relevant battery electrolyte, the impacts of each on the formation and stability of the anode interface layer is revealed; all are shown to pose a threat to battery performance and stability. Mn2+, in particular, is shown to induce a thick, soft, and unstable SEI layer, which is known to cause severe degradation of batteries, while Co2+ and Ni2+ significantly impact interfacial conductivity. When transition metal ions are mixed, SEI degradation is amplified, suggesting a synergistic effect on the cell stability. Hence, by uncovering the roles these cathode degradation products play in operational batteries, we have provided a foundation upon which strategies to mitigate or eliminate these degradation products can be developed.

Investigating the mechanism of phosphorene nanoribbon synthesis by discharging black phosphorus intercalation compounds.

Phosphorene nanoribbons (PNRs) can be synthesised in intrinsically scalable methods from intercalation of black phosphorus (BP), however, the mechanism of ribbonisation remains unclear. Herein, to investigate the point at which nanoribbons form, we decouple the two key synthesis steps: first, the formation of the BP intercalation compound, and second, the dissolution into a polar aprotic solvent. We find that both the lithium intercalant and the negative charge on the phosphorus host framework can be effectively removed by addition of phenyl cyanide to return BP and investigate whether fracturing to ribbons occurred after the first step. Further efforts to exfoliate mechanically with or without solvent reveal that the intercalation step does not form ribbons, indicating that an interaction between the amidic solvent and the intercalated phosphorus compound plays an important role in the formation of nanoribbons.

The local ordering of polar solvents around crystalline carbon nitride nanosheets in solution.

The crystalline graphitic carbon nitride, poly-triazine imide (PTI) is highly unusual among layered materials since it is spontaneously soluble in aprotic, polar solvents including dimethylformamide (DMF). The PTI material consists of layers of carbon nitride intercalated with LiBr. When dissolved, the resulting solutions consist of dissolved, luminescent single to multilayer nanosheets of around 60-125 nm in diameter and Li+ and Br- ions originating from the intercalating salt. To understand this unique solubility, the structure of these solutions has been investigated by high-energy X-ray and neutron diffraction. Although the diffraction patterns are dominated by inter-solvent correlations there are clear differences between the X-ray diffraction data of the PTI solution and the solvent in the 4-6 Å-1 range, with real space differences persisting to at least 10 Å. Structural modelling using both neutron and X-ray datasets as a constraint reveal the formation of distinct, dense solvation shells surrounding the nanoparticles with a layer of Br-close to the PTI-solvent interface. This solvent ordering provides a configuration that is energetically favourable underpinning thermodynamically driven PTI dissolution. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

Amphoteric dissolution of two-dimensional polytriazine imide carbon nitrides in water.

Crystalline two-dimensional carbon nitrides with polytriazine imide (PTI) structure are shown to act amphoterically, buffering both HCl and NaOH aqueous solutions, resulting in charged PTI layers that dissolve spontaneously in their aqueous media, particularly for the alkaline solutions. This provides a low energy, green route to their scalable solution processing. Protonation in acid is shown to occur at pyridinic nitrogens, stabilized by adjacent triazines, whereas deprotonation in base occurs primarily at basal plane NH bridges, although NH2 edge deprotonation is competitive. We conclude that mildly acidic or basic pHs are necessary to provide sufficient net charge on the nanosheets to promote dissolution, while avoiding high ion concentrations which screen the repulsion of like-charged PTI sheets in solution. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

Three-dimensional operando optical imaging of particle and electrolyte heterogeneities inside Li-ion batteries.

Understanding (de)lithiation heterogeneities in battery materials is key to ensure optimal electrochemical performance. However, this remains challenging due to the three-dimensional morphology of electrode particles, the involvement of both solid- and liquid-phase reactants and a range of relevant timescales (seconds to hours). Here we overcome this problem and demonstrate the use of confocal microscopy for the simultaneous three-dimensional operando measurement of lithium-ion dynamics in individual agglomerate particles, and the electrolyte in batteries. We examine two technologically important cathode materials: LixCoO2 and LixNi0.8Mn0.1Co0.1O2. The surface-to-core transport velocity of Li-phase fronts and volume changes are captured as a function of cycling rate. Additionally, we visualize heterogeneities in the bulk and at agglomerate surfaces during cycling, and image microscopic liquid electrolyte concentration gradients. We discover that surface-limited reactions and intra-agglomerate competing rates control (de)lithiation and structural heterogeneities in agglomerate-based electrodes. Importantly, the conditions under which optical imaging can be performed inside the complex environments of battery electrodes are outlined.

Electrode Materials for Enhancing the Performance and Cycling Stability of Zinc Iodide Flow Batteries at High Current Densities.

Aqueous redox flow battery systems that use a zinc negative electrode have a relatively high energy density. However, high current densities can lead to zinc dendrite growth and electrode polarization, which limit the battery's high power density and cyclability. In this study, a perforated copper foil with a high electrical conductivity was used on the negative side, combined with an electrocatalyst on the positive electrode in a zinc iodide flow battery. A significant improvement in the energy efficiency (ca. 10% vs using graphite felt on both sides) and cycling stability at a high current density of 40 mA cm-2 was observed. A long cycling stability with a high areal capacity of 222 mA h cm-2 is obtained in this study, which is the highest reported areal capacity for zinc-iodide aqueous flow batteries operating at high current density, in comparison to previous studies. Additionally, the use of a perforated copper foil anode in combination with a novel flow mode was discovered to achieve consistent cycling at exceedingly high current densities of >100 mA cm-2. In situ and ex situ characterization techniques, including in situ atomic force microscopy coupled with in situ optical microscopy and X-ray diffraction, are applied to clarify the relationship between zinc deposition morphology on the perforated copper foil and battery performance in two different flow field conditions. With a portion of the flow going through the perforations, a significantly more uniform and compact zinc deposition was observed compared to the case where all of the flow passed over the surface of the electrode. Results from modeling and simulation support the conclusion that the flow of a fraction of electrolyte through the electrode enhances mass transport, enabling a more compact deposit.

Deploying Proteins as Electrolyte Additives in Li-S Batteries: The Multifunctional Role of Fibroin in Improving Cell Performance.

It is widely accepted that the commercial application of lithium-sulfur batteries is inhibited by their short cycle life, which is primarily caused by a combination of Li dendrite formation and active material loss due to polysulfide shuttling. Unfortunately, while numerous approaches to overcome these problems have been reported, most are unscalable and hence further hinder Li-S battery commercialization. Most approaches suggested also only tackle one of the primary mechanisms of cell degradation and failure. Here, we demonstrate that the use of a simple protein, fibroin, as an electrolyte additive can both prevent Li dendrite formation and minimize active material loss to enable high capacity and long cycle life (up to 500 cycles) in Li-S batteries, without inhibiting the rate performance of the cell. Through a combination of experiments and molecular dynamics (MD) simulations, it is demonstrated that the fibroin plays a dual role, both binding to polysulfides to hinder their transport from the cathode and passivating the Li anode to minimize dendrite nucleation and growth. Most importantly, as fibroin is inexpensive and can be simply introduced to the cell via the electrolyte, this work offers a route toward practical industrial applications of a viable Li-S battery system.

Pseudohexagonal Nb2O5 Anodes for Fast-Charging Potassium-Ion Batteries.

High-rate batteries will play a vital role in future energy storage systems, yet while good progress is being made in the development of high-rate lithium-ion batteries, there is less progress with post-lithium-ion chemistry. In this study, we demonstrate that pseudohexagonal Nb2O5(TT-Nb2O5) can offer a high specific capacity (179 mAh g-1 ∼ 0.3C), good lifetime, and an excellent rate performance (72 mAh g-1 at ∼15C) in potassium-ion batteries (KIBs), when it is composited with a highly conductive carbon framework; this is the first reported investigation of TT-Nb2O5 for KIBs. Specifically, multiwalled carbon nanotubes are strongly tethered to Nb2O5 via glucose-derived carbon (Nb2O5@CNT) by a one-step hydrothermal method, which results in highly conductive and porous needle-like structures. This work therefore offers a route for the scalable production of a viable KIB anode material and hence improves the feasibility of fast-charging KIBs for future applications.

Black Phosphorus Degradation during Intercalation and Alloying in Batteries.

Numerous layered materials are being recognized as promising candidates for high-performance alkali-ion battery anodes, but black phosphorus (BP) has received particular attention. This is due to its high specific capacity, due to a mixed alkali-ion storage mechanism (intercalation-alloying), and fast alkali-ion transport within its layers. Unfortunately, BP based batteries are also commonly associated with serious irreversible losses and poor cycling stability. This is known to be linked to alloying, but there is little experimental evidence of the morphological, mechanical, or chemical changes that BP undergoes in operational cells and thus little understanding of the factors that must be mitigated to optimize performance. Here the degradation mechanisms of BP alkali-ion battery anodes are revealed through operando electrochemical atomic force microscopy (EC-AFM) and ex situ spectroscopy. Among other phenomena, BP is observed to wrinkle and deform during intercalation but suffers from complete structural breakdown upon alloying. The solid electrolyte interphase (SEI) is also found to be unstable, nucleating at defects before spreading across the basal planes but then disintegrating upon desodiation, even above alloying potentials. By directly linking these localized phenomena with the whole-cell performance, we can now engineer stabilizing protocols for next-generation high-capacity alkali-ion batteries.

Ex Situ Characterization of 1T/2H MoS2 and Their Carbon Composites for Energy Applications, a Review.

The growing interest in the development of next-generation net zero energy systems has led to the expansion of molybdenum disulfide (MoS2) research in this area. This activity has resulted in a wide range of manufacturing/synthesis methods, controllable morphologies, diverse carbonaceous composite structures, a multitude of applicable characterization techniques, and multiple energy applications for MoS2. To assess the literature trends, 37,347 MoS2 research articles from Web of Science were text scanned to classify articles according to energy application research and characterization techniques employed. Within the review, characterization techniques are grouped under the following categories: morphology, crystal structure, composition, and chemistry. The most common characterization techniques identified through text scanning are recommended as the base fingerprint for MoS2 samples. These include: scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Similarly, XPS and Raman spectroscopy are suggested for 2H or 1T MoS2 phase confirmation. We provide guidance on the collection and presentation of MoS2 characterization data. This includes how to effectively combine multiple characterization techniques, considering the sample area probed by each technique and their statistical significance, and the benefit of using reference samples. For ease of access for future experimental comparison, key numeric MoS2 characterization values are tabulated and major literature discrepancies or currently debated characterization disputes are highlighted.

Engineering Solution-Processed Non-Crystalline Solid Electrolytes for Li Metal Batteries.

Non-crystalline Li-ion solid electrolytes (SEs), such as lithium phosphorus oxynitride, can uniquely enable high-rate solid-state battery operation over thousands of cycles in thin film form. However, they are typically produced by expensive and low throughput vacuum deposition, limiting their wide application and study. Here, we report non-crystalline SEs of composition Li-Al-P-O (LAPO) with ionic conductivities > 10-7 S cm-1 at room temperature made by spin coating from aqueous solutions and subsequent annealing in air. Homogenous, dense, flat layers can be synthesized with submicrometer thickness at temperatures as low as 230 °C. Control of the composition is shown to significantly affect the ionic conductivity, with increased Li and decreased P content being optimal, while higher annealing temperatures result in decreased ionic conductivity. Activation energy analysis reveals a Li-ion hopping barrier of ≈0.4 eV. Additionally, these SEs exhibit low room temperature electronic conductivity (< 10-11 S cm-1) and a moderate Young's modulus of ≈54 GPa, which may be beneficial in preventing Li dendrite formation. In contact with Li metal, LAPO is found to form a stable but high impedance passivation layer comprised of Al metal, Li-P, and Li-O species. These findings should be of value when engineering non-crystalline SEs for Li-metal batteries with high energy and power densities.

Nanoscale Ultrafine Zinc Metal Anodes for High Stability Aqueous Zinc Ion Batteries.

Aqueous Zn batteries (AZBs) are a promising energy storage technology, due to their high theoretical capacity, low redox potential, and safety. However, dendrite growth and parasitic reactions occurring at the surface of metallic Zn result in severe instability. Here we report a new method to achieve ultrafine Zn nanograin anodes by using ethylene glycol monomethyl ether (EGME) molecules to manipulate zinc nucleation and growth processes. It is demonstrated that EGME complexes with Zn2+ to moderately increase the driving force for nucleation, as well as adsorbs on the Zn surface to prevent H-corrosion and dendritic protuberances by refining the grains. As a result, the nanoscale anode delivers high Coulombic efficiency (ca. 99.5%), long-term cycle life (over 366 days and 8800 cycles), and outstanding compatibility with state-of-the-art cathodes (ZnVO and AC) in full cells. This work offers a new route for interfacial engineering in aqueous metal-ion batteries, with significant implications for the commercial future of AZBs.

Ultrasmall, Coating-Free, Pyramidal Platinum Nanoparticles for High Stability Fuel Cell Oxygen Reduction.

Ultrasmall (<5 nm diameter) noble metal nanoparticles with a high fraction of {111} surface domains are of fundamental and practical interest as electrocatalysts, especially in fuel cells; the nanomaterial surface structure dictates its catalytic properties, including kinetics and stability. However, the synthesis of size-controlled, pure Pt-shaped nanocatalysts has remained a formidable chemical challenge. There is an urgent need for an industrially scalable method for their production. Here, a one-step approach is presented for the preparation of single-crystal pyramidal nanocatalysts with a high fraction of {111} surface domains and a diameter below 4 nm. This is achieved by harnessing the shape-directing effect of citrate molecules, together with the strict control of oxidative etching while avoiding polymers, surfactants, and organic solvents. These catalysts exhibit significantly enhanced durability while, providing equivalent current and power densities to highly optimized commercial Pt/C catalysts at the beginning of life (BOL). This is even the case when they are tested in full polymer electrolyte membrane fuel cells (PEMFCs), as opposed to rotating disk experiments that artificially enhance electrode kinetics and minimize degradation. This demonstrates that the {111} surface domains in pyramidal Pt nanoparticles (as opposed to spherical Pt nanoparticles) can improve aggregation/corrosion resistance in realistic fuel cell conditions, leading to a significant improvement in membrane electrode assembly (MEA) stability and lifetime.

Disentangling water, ion and polymer dynamics in an anion exchange membrane.

Semipermeable polymeric anion exchange membranes are essential for separation, filtration and energy conversion technologies including reverse electrodialysis systems that produce energy from salinity gradients, fuel cells to generate electrical power from the electrochemical reaction between hydrogen and oxygen, and water electrolyser systems that provide H2 fuel. Anion exchange membrane fuel cells and anion exchange membrane water electrolysers rely on the membrane to transport OH- ions between the cathode and anode in a process that involves cooperative interactions with H2O molecules and polymer dynamics. Understanding and controlling the interactions between the relaxation and diffusional processes pose a main scientific and critical membrane design challenge. Here quasi-elastic neutron scattering is applied over a wide range of timescales (100-103 ps) to disentangle the water, polymer relaxation and OH- diffusional dynamics in commercially available anion exchange membranes (Fumatech FAD-55) designed for selective anion transport across different technology platforms, using the concept of serial decoupling of relaxation and diffusional processes to analyse the data. Preliminary data are also reported for a laboratory-prepared anion exchange membrane especially designed for fuel cell applications.

Aquaporin-like water transport in nanoporous crystalline layered carbon nitride.

Designing next-generation fuel cell and filtration devices requires the development of nanoporous materials that allow rapid and reversible uptake and directed transport of water molecules. Here, we combine neutron spectroscopy and first-principles calculations to demonstrate rapid transport of molecular H2O through nanometer-sized voids ordered within the layers of crystalline carbon nitride with a polytriazine imide structure. The transport mechanism involves a sequence of molecular orientation reversals directed by hydrogen-bonding interactions as the neutral molecules traverse the interlayer gap and pass through the intralayer voids that show similarities with the transport of water through transmembrane aquaporin channels in biological systems. The results suggest that nanoporous layered carbon nitrides can be useful for developing high-performance membranes.

Operando Electrochemical Atomic Force Microscopy of Solid-Electrolyte Interphase Formation on Graphite Anodes: The Evolution of SEI Morphology and Mechanical Properties.

Understanding and ultimately controlling the properties of the solid-electrolyte interphase (SEI) layer at the graphite anode/liquid electrolyte boundary are of great significance for maximizing the performance and lifetime of lithium-ion batteries (LIBs). However, comprehensive in situ monitoring of SEI formation and evolution, alongside measurement of the corresponding mechanical properties, is challenging due to the limitations of the characterization techniques commonly used. This work provides a new insight into SEI formation during the first lithiation and delithiation of graphite battery anodes using operando electrochemical atomic force microscopy (EC-AFM). Highly oriented pyrolytic graphite (HOPG) is investigated first as a model system, exhibiting unique morphological and nanomechanical behavior dependent on the various electrolytes and commercially relevant additives used. Then, to validate these findings with respect to real-world battery electrodes, operando EC-AFM of individual graphite particles like those in commercial systems are studied. Vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are shown to be effective additives to enhance SEI layer stability in 1 M LiPF6/ethylene carbonate/ethyl methyl carbonate (EC/EMC) electrolytes, attributed to their role in improving its structure, density, and mechanical strength. This work therefore presents an unambiguous picture of SEI formation in a real battery environment, contributes a comprehensive insight into SEI formation of electrode materials, and provides a visible understanding of the influence of electrolyte additives on SEI formation.

SERS-Active Cu Nanoparticles on Carbon Nitride Support Fabricated Using Pulsed Laser Ablation.

We report a single-step route to co-deposit Cu nanoparticles with a graphitic carbon nitride (gCN) support using nanosecond Ce:Nd:YAG pulsed laser ablation from a Cu metal target coated using acetonitrile (CH3CN). The resulting Cu/gCN hybrids showed strong optical absorption in the visible to near-IR range and exhibited surface-enhanced Raman or resonance Raman scattering (SERS or SERRS) enhancement for crystal violet (CV), methylene blue (MB), and rhodamine 6G (R6G) used as probe analyte molecules adsorbed on the surface. We have characterized the Cu nanoparticles and the nature of the gCN support materials using a range of spectroscopic, structural, and compositional analysis techniques.

Production of phosphorene nanoribbons.

Phosphorene is a mono-elemental, two-dimensional (2D) substance with outstanding, highly directional properties and a bandgap that depends on the number of layers of the material1-8. Nanoribbons, meanwhile, combine the flexibility and unidirectional properties of one-dimensional nanomaterials, the high surface area of 2D nanomaterials and the electron-confinement and edge effects of both. The structures of nanoribbons can thus lead to exceptional control over electronic band structure, the emergence of novel phenomena and unique architectures for applications5,6,9-24. Phosphorene's intrinsically anisotropic structure has motivated numerous theoretical calculations of phosphorene nanoribbons (PNRs), predicting extraordinary properties5,6,12-24. So far, however, discrete PNRs have not been produced. Here we present a method for creating quantities of high-quality, individual PNRs by ionic scissoring of macroscopic black phosphorus crystals. This top-down process results in stable liquid dispersions of PNRs with typical widths of 4-50 nm, predominantly single-layer thickness, measured lengths of up to 75 µm and aspect ratios of up to 1,000. The nanoribbons are atomically flat single crystals, aligned exclusively in the zigzag crystallographic orientation. The ribbons have remarkably uniform widths along their entire lengths, and are extremely flexible. These properties-together with the ease of downstream manipulation via liquid-phase methods-should enable the search for predicted exotic states6,12-14,17-19,21, and an array of applications in which PNRs have been predicted to offer transformative advantages. These applications range from thermoelectric devices to high-capacity fast-charging batteries and integrated high-speed electronic circuits6,14-16,20,23,24.

Formation of an ion-free crystalline carbon nitride and its reversible intercalation with ionic species and molecular water.

The development of processes to tune the properties of materials is essential for the progression of next-generation technologies for catalysis, optoelectronics and sustainability including energy harvesting and conversion. Layered carbon nitrides have also been identified as of significant interest within these fields of application. However, most carbon nitride materials studied to date have poor crystallinity and therefore their properties cannot be readily controlled or easily related to their molecular level or nanoscale structures. Here we report a process for forming a range of crystalline layered carbon nitrides with polytriazine imide (PTI) structures that can be interconverted by simple ion exchange processes, permitting the tunability of their optoelectronic and chemical properties. Notable outcomes of our work are (a) the creation of a crystalline, guest-ion-free PTI compound that (b) can be re-intercalated with ions or molecules using "soft chemistry" approaches. This includes the intercalation of HCl, demonstrating a new ambient pressure route to the layered PTI·xHCl material that was previously only available by a high-pressure-high-temperature route (c). Our work also shows (d) that the intercalant-free (IF-) PTI material spontaneously absorbs up to 10 weight% H2O from the ambient atmosphere and that this process is reversible, leading to potential applications for membranes and water capture in dry environments.