Effect of Sheet Morphology on the Scalability of Graphene-Based Ultracapacitors

Graphene is considered a promising ultracapacitor material toward high power and energy density because of its high conductivity and high surface area without pore tortuosity. However, the two-dimensional (2D) sheets tend to aggregate during the electrode fabrication process and align perpendicular to the flow direction of electrons and ions, which can reduce the available surface area and limit the electron and ion transport. This makes it hard to achieve scalable device performance as the loading level of the active material increases. Here, we report a strategy to solve these problems by transforming the 2D graphene sheet into a crumpled paper ball structure. Compared to flat or wrinkled sheets, the crumpled graphene balls can deliver much higher specific capacitance and better rate performance. More importantly, devices made with crumpled graphene balls are significantly less dependent on the electrode mass loading. Performance of graphene-based ultracapacitors can be further enhanced by using flat graphene sheets as the binder for the crumpled graphene balls, thus eliminating the need for less active binder materials

Spontaneous and Selective Potassium Transport through a Suspended Tailor-Cut Ti₃C₂T_<i>x</i> MXene Film

Biological ion pumps selectively transport target ions against the concentration gradient, a process that is crucial to maintaining the out-of-equilibrium states of cells. Building an ion pump with ion selectivity has been challenging. Here we show that a Ti3C2Tx MXene film suspended in air with a trapezoidal shape spontaneously pumps K+ ions from the base end to the tip end and exhibits a K+/Na+ selectivity of 4. Such a phenomenon is attributed to a range of properties of MXene. Thanks to the high stability of MXene in water and the dynamic equilibrium between evaporation and swelling, the film keeps a narrow interlayer spacing of ∼0.3 nm when its two ends are connected to reservoirs. Because of the polar electrical structure and hydrophilicity of the MXene nanosheet, K+ ions experience a low energy barrier of ∼4.6 kBT when entering these narrow interlayer spacings. Through quantitative simulations and consistent experimental results, we further show that the narrow spacings exhibit a higher energy barrier to Na+, resulting in K+/Na+ selectivity. Finally, we show that the spontaneous ion transport is driven by the asymmetric evaporation of the interlayer water across the film, a mechanism that is similar to pressure driven streaming current. This work shows how ion transport properties can be facilely manipulated by tuning the macroscopic shape of nanofluidic materials, which may attract interest in the interface of kirigami technologies and nanofluidics and show potential in energy and separation applications

Designing a Quasi-Liquid Alloy Interface for Solid Na-Ion Battery

Solid-state sodium-ion batteries are attracting great attention due to their high energy density and high safety. However, the Na dendrite growth and poor wettability between sodium and electrolytes seriously limit its application. Herein, we designed a stable and dendrite-suppressed quasi-liquid alloy interface (C@Na–K) for solid sodium-ion batteries (SSIBs). The batteries exhibit excellent electrochemical performance thanks to better wettability and accelerated charge transfer and nucleation mode shifts. The thickness of the liquid phase alloy interface fluctuates along with the exotherm of the cell cycling process, which leads to better rate performance. The symmetrical cell can cycle steadily over 3500 h at 0.1 mA/cm2 at room temperature, and the critical current density can reach 2.6 mA/cm2 at 40 °C. The full cells with the quasi-liquid alloy interface also show outstanding performance; the capacity retention can reach 97.1%, and the average Coulombic efficiency can reach 99.6% of the battery at 0.5 C even after 300 cycles. These results proved the feasibility of using a liquid alloy interface of the anode for high-energy SSIBs, and this innovative approach to stabilizing the interface performance could serve as a basis for the development of next-generation high-energy SSIBs

Crumpled Graphene-Encapsulated Si Nanoparticles for Lithium Ion Battery Anodes

Submicrometer-sized capsules made of Si nanoparticles wrapped by crumpled graphene shells were made by a rapid, one-step capillary-driven assembly route in aerosol droplets. Aqueous dispersion of micrometer-sized graphene oxide (GO) sheets and Si nanoparticles were nebulized to form aerosol droplets, which were passed through a preheated tube furnace. Evaporation-induced capillary force wrapped graphene (a.k.a., reduced GO) sheets around the Si particles, and heavily crumpled the shell. The folds and wrinkles in the crumpled graphene coating can accommodate the volume expansion of Si upon lithiation without fracture, and thus help to protect Si nanoparticles from excessive deposition of the insulating solid electrolyte interphase. Compared to the native Si particles, the composite capsules have greatly improved performance as Li ion battery anodes in terms of capacity, cycling stability, and Coulombic efficiency

Nanoscale Graphene Oxide (nGO) as Artificial Receptors: Implications for Biomolecular Interactions and Sensing

The role of conventional graphene-oxide in biosensing has been limited to that of a quenching substrate or signal transducer due to size inconsistencies and poor supramolecular response. We overcame these issues by using nanoscale GOs (nGO) as artificial receptors. Unlike conventional GO, nGOs are sheets with near uniform lateral dimension of 20 nm. Due to its nanoscale architecture, its supramolecular response was enhanced, with demonstrated improvements in biomacromolecular affinities. This rendered their surface capable of detecting unknown proteins with cognizance not seen with conventional GOs. Different proteins at 100 and 10 nM concentrations revealed consistent patterns that are quantitatively differentiable by linear discriminant analysis. Identification of 48 unknowns in both concentrations demonstrated a >95% success rate. The 10 nM detection represents a 10-fold improvement over analogous arrays. This demonstrates for the first time that the supramolecular chemistry of GO is highly size dependent and opens the possibility of improvement upon existing GO hybrid materials

Strong Solvent and Dual Lithium Salts Enable Fast-Charging Lithium-Ion Batteries Operating from −78 to 60 °C

Current lithium-ion batteries degrade under high rates and low temperatures due to the use of carbonate electrolytes with restricted Li+ conduction and sluggish Li+ desolvation. Herein, a strong solvent with dual lithium salts surmounts the thermodynamic limitations by regulating interactions among Li+ ions, anions, and solvents at the molecular level. Highly dissociated lithium bis(fluorosulfonyl)imide (LiFSI) in dimethyl sulfite (DMS) solvent with a favorable dielectric constant and melting point ensures rapid Li+ conduction while the high affinity between difluoro(oxalato)borate anions (DFOB–) and Li+ ions guarantees smooth Li+ desolvation within a wide temperature range. In the meantime, the ultrathin self-limited electrode/electrolyte interface and the electric double layer induced by DFOB– result in enhanced electrode compatibility. The as-formulated electrolyte enables stable cycles at high currents (41.3 mA cm–2) and a wide temperature range from −78 to 60 °C. The 1 Ah graphite||LiCoO2 (2 mAh cm–2) pouch cell achieves 80% reversible capacity at 2 C rate under −20 °C and 86% reversible capacity at 0.1 C rate under −50 °C. This work sheds new light on the electrolyte design with strong solvent and dual lithium salts and further facilitates the development of high-performance lithium-ion batteries operating under extreme conditions

Building an Interfacial Framework: Li/Garnet Interface Stabilization through a Cu₆Sn₅ Layer

Various artificial interlayers like metal/metallic oxides have been introduced to improve Li wettability through alloy reaction for the Li/garnet interface. However, huge volume change during the continuous alloying/dealloying process is detrimental to the rigid solid-to-solid contact of Li/garnet and subsequently leads to instability of the polarization voltage. Herein, we demonstrate an improved artificial interlayer of Cu6Sn5 to simultaneously restrict the volume change and ensure the intimate contact of the Li/garnet interface. It is proved that the Cu atom in Cu6Sn5 cannot only mitigate the volume change but also restrain the diffusion of Sn. Intimate solid contact of Li with garnet can still be realized after Li stripping due to upholding of the Li2+xCu1–xSn framework. Moreover, the Li/garnet/Li symmetric cell with Cu6Sn5 modification displays smaller voltage polarization and impedance change than the pure Sn modified counterpart

A novel wood identification method for <i>Pterocarpus santalinus</i> L.f. species based on fluorescence features

The precise identification of wood plays a vital role in protecting rare timber species. Among the various wood identification techniques, the fluorescence characteristics of Pterocarpus santalinus L.f. species have rarely been investigated. In this study, the types, number of fluorescent compounds, maximum excitation and emission wavelengths, and effects of solvent and pH were characterized by fluorescence photographs, fluorescence emission spectra and fluorescence contour spectra, respectively. The results indicate that the fluorescence of P. santalinus species is relatively strong, and the fluorescent substances extracted in different solvents exhibit different color photographs under irradiation. The spectrum is also affected by the polarity of the solvent and the pH of the solution. In addition, the method for extracting the fluorescent compounds in P. santalinus samples was optimized. Based on the reported results, a good correlation between the fluorescence characteristics and the wood species was obtained as a potentially new identification method for precious rosewoods at the species level.</p

Graphene Oxide Nanocolloids

Graphene oxide (GO) nanocolloidssheets with lateral dimension smaller than 100 nmwere synthesized by chemical exfoliation of graphite nanofibers, in which the graphene planes are coin-stacked along the length of the nanofibers. Since the upper size limit is predetermined by the diameter of the nanofiber precursor, the size distribution of the GO nanosheets is much more uniform than that of common GO synthesized from graphite powders. The size can be further tuned by the oxidation time. Compared to the micrometer-sized, regular GO sheets, nano GO has very similar spectroscopic characteristics and chemical properties but very different solution properties, such as surface activity and colloidal stability. Due to higher charge density originating from their higher edge-to-area ratios, aqueous GO nanocolloids are significantly more stable. Dispersions of GO nanocolloids can sustain high-speed centrifugation and remain stable even after chemical reduction, which would result in aggregates for regular GO. Therefore, nano GO can act as a better dispersing agent for insoluble materials (e.g., carbon nanotubes) in water, creating a more stable colloidal dispersion