SIMES Publications

Abstract

Although liquid-solid interfaces are foundational in broad areas of science, characterizing this delicate interface remains inherently difficult because of shortcomings in existing tools to access liquid and solid phases simultaneously at the nanoscale. This leads to substantial gaps in our understanding of the structure and chemistry of key interfaces in battery systems. We adopt and modify a thin film vitrification method to preserve the sensitive yet critical interfaces in batteries at native liquid electrolyte environments to enable cryo–electron microscopy and spectroscopy. We report substantial swelling of the solid-electrolyte interphase (SEI) on lithium metal anode in various electrolytes. The swelling behavior is dependent on electrolyte chemistry and is highly correlated to battery performance. Higher degrees of SEI swelling tend to exhibit poor electrochemical cycling.

Abstract

Abstract

One in three people worldwide does not have access to safe drinking water. Notably, heavy-metal ions (HMIs) are major water pollutants threatening human health because of their severe toxicity, even at trace levels. Efficient HMI detection thus plays a major defense against metal poisoning by enabling early pollution warning and efficient regulatory enforcement. However, it remains a formidable challenge to accurately detect these pollutants on site at ultratrace levels in a cost- or time-effective manner. Here, we introduce an efficient, portable sensor to concurrently quantify five different HMIs down to the sub-nanomolar level by sulfiding them on a superhydrophobic surface. Sulfidation serves as a colorimetric reaction while the superhydrophobic surface concentrates analytes for sensitive visual detection. Our superhydrophobic concentrator (SPOT) sensor can be made portable by being integrated with a smartphone application to quantify HMIs in <8 min and at $0.02 per analysis. Decentralizing water monitoring by using our SPOT design is crucial to ensuring that clean water is accessible to everyone.

Abstract

Thin (≤20 μm) and free-standing Li metal foils would enable precise prelithiation of anode materials and high-energy-density Li batteries. Existing Li metal foils are too thick (typically 50 to 750 μm) or too mechanically fragile for these applications. Here, we developed a facile and scalable process for the synthesis of an ultrathin (0.5 to 20 μm), free-standing and mechanically robust Li metal foil within a graphene oxide host. In addition to low areal capacities of ~0.1 to 3.7 mAh cm⁻², this Li foil also has a much-improved mechanical strength over conventional pure Li metal foil. Our Li foil can improve the initial Coulombic efficiency of graphite (93%) and silicon (79.4%) anodes to around 100% without generating excessive Li residue, and increases the capacity of Li-ion full cells by 8%. The cycle life of Li metal full cells is prolonged by nine times using this thin Li composite anode.

Abstract

Abstract

Novel electrolyte designs to further enhance the lithium (Li) metal battery cyclability are highly desirable. Here, fluorinated 1,6‐dimethoxyhexane (FDMH) is designed and synthesized as the solvent molecule to promote electrolyte stability with its prolonged –CF₂– backbone. Meanwhile, 1,2‐dimethoxyethane is used as a co‐solvent to enable higher ionic conductivity and much reduced interfacial resistance. Combining the dual‐solvent system with 1 m lithium bis(fluorosulfonyl)imide (LiFSI), high Li‐metal Coulombic efficiency (99.5%) and oxidative stability (6 V) are achieved. Using this electrolyte, 20 µm Li||NMC batteries are able to retain ≈80% capacity after 250 cycles and Cu||NMC anode‐free pouch cells last 120 cycles with 75% capacity retention under ≈2.1 µL mAh⁻¹ lean electrolyte conditions. Such high performances are attributed to the anion‐derived solid‐electrolyte interphase, originating from the coordination of Li‐ions to the highly stable FDMH and multiple anions in their solvation environments. This work demonstrates a new electrolyte design strategy that enables high‐performance Li‐metal batteries with multisolvent Li‐ion solvation with rationally optimized molecular structure and ratio.

Abstract

Transition metal (oxy)hydroxides are promising electrocatalysts for the oxygen evolution reaction^1,2,3. The properties of these materials evolve dynamically and heterogeneously⁴ with applied voltage through ion insertion redox reactions, converting materials that are inactive under open circuit conditions into active electrocatalysts during operation⁵. The catalytic state is thus inherently far from equilibrium, which complicates its direct observation. Here, using a suite of correlative operando scanning probe and X-ray microscopy techniques, we establish a link between the oxygen evolution activity and the local operational chemical, physical and electronic nanoscale structure of single-crystalline β-Co(OH)₂ platelet particles. At pre- catalytic voltages, the particles swell to form an α-CoO₂H_1.5·0.5H₂O-like structure—produced through hydroxide intercalation—in which the oxidation state of cobalt is +2.5. Upon increasing the voltage to drive oxygen evolution, interlayer water and protons de-intercalate to form contracted β-CoOOH particles that contain Co³⁺ species. Although these transformations manifest heterogeneously through the bulk of the particles, the electrochemical current is primarily restricted to their edge facets. The observed Tafel behaviour is correlated with the local concentration of Co³⁺ at these reactive edge sites, demonstrating the link between bulk ion-insertion and surface catalytic activity.

Abstract

Thermoelectric devices convert thermal energy to electrical energy and are particularly well-suited for energy harvesting from waste heat. Even as the number of electronic devices used in daily life proliferates, technical advances diminish the average power such devices require to perform a given function. Localized thermal gradients that abound in our living environments, despite having modest energy densities, are therefore becoming increasingly viable and attractive to power such devices. With this motivation, we report the design, fabrication, and characterization of single-wall carbon nanotube thermoelectric devices (CNT-TDs) on flexible polyimide substrates as a basis for wearable energy converters. Our aqueous-solution-based film fabrication process could enable readily scalable, low-cost TDs; here, we demonstrate CNT-hydroxypropyl cellulose (HPC) composite thermoelectric films by aerosol jet printing. The electrical conductivity of the composite films is controlled through the number of CNT/HPC layers printed in combination with control of the annealing conditions. The HPC initially disperses the CNTs in deionized water, the greenest of solvents, and is subsequently partially eliminated from the film by annealing, with concomitant morphological changes that we characterized by TEM. HPC removal is key to obtaining good electrical conductivity (0.94 to 1.10 × 105 S/m) and Seebeck coefficients (36 to 43 μV/K). We also report a power factor of 208 μW m−1 K−2 for a CNT-TD composed of 15 layers of CNT/HPC, promising performance for CNT-based flexible TDs that are deposited from aqueous solution, stable in air, and require no additional doping or sorting processes.

Abstract

The lithium–sulfur (Li–S) battery is an attractive high‐energy‐density technology for future flexible and wearable electronics, but it is a challenge to simultaneously realize adequate flexibility, stability, and energy density. Recent studies show that fibrous materials are promising to achieve flexible batteries with high energy density, by taking advantages of their intrinsic flexibility, lightweight, large surface area, and cost‐effectiveness. With a good tunability of the structure and function, fibrous materials can be suitable to construct various components of a working battery, including current collectors, buffer layers, interlayers, and solid‐state electrolytes. This review summarizes the recent development of fibrous materials for flexible Li–S batteries, with an emphasis on the discussion of the synthesis and preparation of materials, design of fibrous structures and functionalities, and layout of battery cells for achieving high Coulombic efficiency, long cycle life, and good flexibility.

Abstract

Investigating the impact of nanoscale heterogeneity on heat transport requires a spatiotemporal probe of temperature on the length and time scales intrinsic to heat navigating nanoscale defects. Here, we use stroboscopic optical scattering microscopy to visualize nanoscale heat transport in disordered films of gold nanocrystals. We find that heat transport appears subdiffusive at the nanoscale. Finite element simulations show that tortuosity of the heat flow underlies the subdiffusive transport, owing to a distribution of nonconductive voids. Thus, while heat travels diffusively through contiguous regions of the film, the tortuosity causes heat to navigate circuitous pathways that make the observed mean-squared expansion of an initially localized temperature distribution appear subdiffusive on length scales comparable to the voids. Our approach should be broadly applicable to uncover the impact of both designed and unintended heterogeneities in a wide range of materials and devices that can affect more commonly used spatially averaged thermal transport measurements.