Jiayan Luo

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.

Compression and Aggregation-Resistant Particles of Crumpled Soft Sheets

Unlike flat sheets, crumpled paper balls have both high free volume and high compressive strength, and can tightly pack without significantly reducing the area of accessible surface. Such properties would be highly desirable for sheet-like materials such as graphene, since they tend to aggregate in solution and restack in the solid state, making their properties highly dependent on the material processing history. Here we report the synthesis of crumpled graphene balls by capillary compression in rapidly evaporating aerosol droplets. The crumpled particles are stabilized by locally folded, π-π stacked ridges as a result of plastic deformation, and do not unfold or collapse during common processing steps. In addition, they are remarkably aggregation-resistant in either solution or solid state, and remain largely intact and redispersible after chemical treatments, wet processing, annealing, and even pelletizing at high pressure. For example, upon compression at 55 MPa, the regular flat graphene sheets turn into nondispersible chunks with drastically reduced surface area by 84%, while the crumpled graphene particles can still maintain 45% of their original surface area and remain readily dispersible in common solvents. Therefore, crumpled particles could help to standardize graphene-based materials by delivering more stable properties such as high surface area and solution processability regardless of material processing history. This should greatly benefit applications using bulk quantities of graphene, such as in energy storage or conversion devices. As a proof of concept, we demonstrate that microbial fuel electrodes modified by the crumpled particles indeed outperform those modified with their flat counterparts.

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.

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.

Graphene oxide as surfactant sheets

Graphite oxide sheet, now referred to as graphene oxide (GO), is the product of chemical oxidation and exfoliation of graphite powders that was first synthesized over a century ago. Interest in this old material has resurged in recent years, especially after the discovery of graphene, as GO is considered a promising precursor for the bulk production of graphene-based materials. GO sheets are single atomic layers that can readily extend up to tens of microns in lateral dimension. Therefore, their structure bridges the typical length scales of both chemistry and materials science. GO can be viewed as an unconventional type of soft material as it carries the characteristics of polymers, colloids, membranes, and as highlighted in this review, amphiphiles. GO has long been considered hydrophilic due to its excellent water dispersity, however, our recent work revealed that GO sheets are actually amphiphilic with an edge-to-center distribution of hydrophilic and hydrophobic domains. Thus, GO can adhere to interfaces and lower interfacial energy, acting as surfactant. This new property insight helps to better understand GO’s solution properties which can inspire novel material assembly and processing methods such as for fabricating thin films with controllable microstructures and separating GO sheets of different sizes. In addition, GO can be used as a surfactant sheet to emulsify organic solvents with water and disperse insoluble materials such as graphite and carbon nanotubes (CNTs) in water, which opens up opportunities for creating functional hybrid materials of graphene and other π-conjugated systems.

Two‐Dimensional Porous Carbon: Synthesis and Ion‐Transport Properties

Their chemical stability, high specific surface area, and electric conductivity enable porous carbon materials to be the most commonly used electrode materials for electrochemical capacitors (also known as supercapacitors). To further increase the energy and power density, engineering of the pore structures with a higher electrochemical accessible surface area, faster ion-transport path and a more-robust interface with the electrolyte is widely investigated. Compared with traditional porous carbons, two-dimensional (2D) porous carbon sheets with an interlinked hierarchical porous structure are a good candidate for supercapacitors due to their advantages in high aspect ratio for electrode packing and electron transport, hierarchical pore structures for ion transport, and short ion-transport length. Recent progress on the synthesis of 2D porous carbons is reported here, along with the improved electrochemical behavior due to enhanced ion transport. Challenges for the controlled preparation of 2D porous carbons with desired properties are also discussed; these require precise tuning of the hierarchical structure and a clarification of the formation mechanisms.

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.

A Metal‐Free Supercapacitor Electrode Material with a Record High Volumetric Capacitance over 800 F cm−3

A metal-free supercapacitor electrode material is prepared by the hybridization of graphene and polyaniline in a very compact way without sacrificing their gravimetric capacitance. It exhibits a record high volumetric capacitance over 800 F cm(-3).

Ultra-thick graphene bulk supercapacitor electrodes for compact energy storage

Compact energy storage with high volumetric performance is highly important. However, the state-of-the-art electrodes and devices remain far from the requirements due to the lack of consideration from a device perspective, which not only demands a high specific gravimetric capacity, but also needs to take into account operation voltage, material density and electrode thickness. We develop a novel approach to fabricate a monolithic ultra-thick and dense carbon electrode for symmetric supercapacitors, starting with graphene assembly and taking all the above factors into consideration. We found that zinc chloride is an ideal sacrificial pore former, and taken together with capillary drying can tune the specific surface area of the monolithic graphene from 370 to over 1000 m2 g−1 while the monoliths maintain a high density from 1.6 to 0.6 g cm−3. Having a good balance of porosity and density, the directly sliced graphene pellet electrode with a thickness up to 400 μm delivers a capacitance of 150 F cm−3 in an ionic liquid electrolyte, corresponding to a volumetric energy density of ∼65 W h L−1 for a symmetrical supercapacitor device, the highest value reported to date for supercapacitors. This study presents a design principle for electrode materials towards next-generation energy storage devices, not limited to supercapacitors, which are becoming smaller, lighter but more energetic.

Dynamics of Electrochemical Lithiation/Delithiation of Graphene-Encapsulated Silicon Nanoparticles Studied by In-situ TEM

The incorporation of nanostructured carbon has been recently reported as an effective approach to improve the cycling stability when Si is used as high-capacity anodes for the next generation Li-ion battery. However, the mechanism of such notable improvement remains unclear. Herein, we report in-situ transmission electron microscopy (TEM) studies to directly observe the dynamic electrochemical lithiation/delithiation processes of crumpled graphene-encapsulated Si nanoparticles to understand their physical and chemical transformations. Unexpectedly, in the first lithiation process, crystalline Si nanoparticles undergo an isotropic to anisotropic transition, which is not observed in pure crystalline and amorphous Si nanoparticles. Such a surprising phenomenon arises from the uniformly distributed localized voltage around the Si nanoparticles due to the highly conductive graphene sheets. It is observed that the intimate contact between graphene and Si is maintained during volume expansion/contraction. Electrochemical sintering process where small Si nanoparticles react and merge together to form large agglomerates following spikes in localized electric current is another problem for batteries. In-situ TEM shows that graphene sheets help maintain the capacity even in the course of electrochemical sintering. Such in-situ TEM observations provide valuable phenomenological insights into electrochemical phenomena, which may help optimize the configuration for further improved performance.