Hyung Mo Jeong

Nitrogen-Doped Graphene for High-Performance Ultracapacitors and the Importance of Nitrogen-Doped Sites at Basal Planes

Although various carbon nanomaterials including activated carbon, carbon nanotubes, and graphene have been successfully demonstrated for high-performance ultracapacitors, their capacitances need to be improved further for wider and more challenging applications. Herein, using nitrogen-doped graphene produced by a simple plasma process, we developed ultracapacitors whose capacitances (∼280 F/g(electrode)) are about 4 times larger than those of pristine graphene based counterparts without sacrificing other essential and useful properties for ultracapacitor operations including excellent cycle life (>200,000), high power capability, and compatibility with flexible substrates. While we were trying to understand the improved capacitance using scanning photoemission microscopy with a capability of probing local nitrogen-carbon bonding configurations within a single sheet of graphene, we observed interesting microscopic features of N-configurations: N-doped sites even at basal planes, distinctive distributions of N-configurations between edges and basal planes, and their distinctive evolutions with plasma duration. The local N-configuration mappings during plasma treatment, alongside binding energy calculated by density functional theory, revealed that the origin of the improved capacitance is a certain N-configuration at basal planes.

Nitrogen-Doped Multiwall Carbon Nanotubes for Lithium Storage with Extremely High Capacity

The increasing demands on high performance energy storage systems have raised a new class of devices, so-called lithium ion capacitors (LICs). As its name says, LIC is an intermediate system between lithium ion batteries and supercapacitors, designed for taking advantages of both types of energy storage systems. Herein, as a quest to improve the Li storage capability compared to that of other existing carbon nanomaterials, we have developed extrinsically defective multiwall carbon nanotubes by nitrogen-doping. Nitrogen-doped carbon nanotubes contain wall defects through which lithium ions can diffuse so as to occupy a large portion of the interwall space as storage regions. Furthermore, when integrated with 3 nm nickel oxide nanoparticles for a further capacity boost, nitrogen doping enables unprecedented cell performance by engaging anomalous electrochemical phenomena such as nanoparticles division into even smaller ones, their agglomeration-free diffusion between nitrogen-doped sites as well as capacity rise with cycles. The final cells exhibit a capacity as high as 3500 mAh/g, a cycle life of greater than 10,000 times, and a discharge rate capability of 1.5 min while retaining a capacity of 350 mAh/g.

Supercapacitors of Nanocrystalline Metal–Organic Frameworks

The high porosity of metal-organic frameworks (MOFs) has been used to achieve exceptional gas adsorptive properties but as yet remains largely unexplored for electrochemical energy storage devices. This study shows that MOFs made as nanocrystals (nMOFs) can be doped with graphene and successfully incorporated into devices to function as supercapacitors. A series of 23 different nMOFs with multiple organic functionalities and metal ions, differing pore sizes and shapes, discrete and infinite metal oxide backbones, large and small nanocrystals, and a variety of structure types have been prepared and examined. Several members of this series give high capacitance; in particular, a zirconium MOF exhibits exceptionally high capacitance. It has the stack and areal capacitance of 0.64 and 5.09 mF cm(-2), about 6 times that of the supercapacitors made from the benchmark commercial activated carbon materials and a performance that is preserved over at least 10000 charge/discharge cycles.

Extremely stable cycling of ultra-thin V2O5 nanowire–graphene electrodes for lithium rechargeable battery cathodes

Vanadium pentoxide (V2O5) has received considerable attention as a lithium battery cathode because its specific capacity (>250 mA h g−1) is higher than those (<170 mA h g−1) of most commercial cathode materials. Despite this conspicuous advantage, V2O5 has suffered from limited cycle life, typically below a couple of hundred cycles due to the agglomeration of its particles. Once V2O5 particles are agglomerated, the insulating phases continuously expand to an extent that ionic and electronic conduction is severely deteriorated, leading to the significant capacity decay. In this study, in order to overcome the agglomeration issue, the electrodes were uniquely designed such that ultrathin V2O5 nanowires were uniformly incorporated into graphene paper. In this composite structure, the dispersion of V2O5 nanowires was preserved in a robust manner, and, as a result, enabled substantially improved cycle life: decent specific capacities were preserved over 100000 cycles, which are 2–3 orders of magnitude larger than those of typical battery materials.

Three-Dimensional Metal-Catecholate Frameworks and Their Ultrahigh Proton Conductivity

A series of three-dimensional (3D) extended metal catecholates (M-CATs) was synthesized by combining the appropriate metal salt and the hexatopic catecholate linker, H6THO (THO(6-) = triphenylene-2,3,6,7,10,11-hexakis(olate)) to give Fe(THO)·Fe(SO4) (DMA)3, Fe-CAT-5, Ti(THO)·(DMA)2, Ti-CAT-5, and V(THO)·(DMA)2, V-CAT-5 (where DMA = dimethylammonium). Their structures are based on the srs topology and are either a 2-fold interpenetrated (Fe-CAT-5 and Ti-CAT-5) or noninterpenetrated (V-CAT-5) porous anionic framework. These examples are among the first catecholate-based 3D frameworks. The single crystal X-ray diffraction structure of the Fe-CAT-5 shows bound sulfate ligands with DMA guests residing in the pores as counterions, and thus ideally suited for proton conductivity. Accordingly, Fe-CAT-5 exhibits ultrahigh proton conductivity (5.0 × 10(-2) S cm(-1)) at 98% relative humidity (RH) and 25 °C. The coexistence of sulfate and DMA ions within the pores play an important role in proton conductivity as also evidenced by the lower conductivity values found for Ti-CAT-5 (8.2 × 10(-4) S cm(-1) at 98% RH and 25 °C), whose structure only contained DMA guests.

Nitrogen-doped open pore channeled graphene facilitating electrochemical performance of TiO2 nanoparticles as an anode material for sodium ion batteries

We report that titanium dioxide nanoparticles anchored on nitrogen-doped open pore channeled graphene exhibit high performance as anode materials for sodium ion batteries with a high reversible capacity of 405 mA h g−1 at a current density of 50 mA g−1, excellent cycle stability with a capacity of 250 mA h g−1 over 100 charge–discharge cycles at a current density of 100 mA g−1, and superior rate capability. Also, it shows that high electrochemical performance is attributed to the facilitated ion diffusion by their open pore channels, in addition to the promoted electron transfer in electrochemical reactions by nitrogen-doping.

Nickel oxide encapsulated nitrogen-rich carbon hollow spheres with multiporosity for high-performance pseudocapacitors having extremely robust cycle life

We report nickel oxide encapsulated nitrogen-rich carbon hollow spheres with multiporosity, where micropores increase the number of active sites to store redox ions and mesopores enhance the ionic diffusivity of encapsulated nickel oxide, which lead to high capacitance and robust cycle life. Moreover, a high-performance full capacitor showing excellent energy and power densities is realized.

Silicon@porous nitrogen-doped carbon spheres through a bottom-up approach are highly robust lithium-ion battery anodes

Due to its excellent capacity, around 4000 mA h g−1, silicon has been recognized as one of the most promising lithium-ion battery anodes, especially for future large-scale applications including electrical vehicles and utility power grids. Nevertheless, Si suffers from a short cycle life as well as limitations for scalable electrode fabrication. Herein, we report a novel design for highly robust and scalable Si anodes: Si nanoparticles embedded in porous nitrogen-doped carbon spheres (NCSs). The porous nature of NCSs buffers the volume changes of Si nanoparticles and thus resolves critical issues of Si anode operations, such as pulverization, vulnerable contacts between Si and carbon conductors, and an unstable solid-electrolyte interphase. The unique electrode structure exhibits outstanding performance with a gravimetric capacity as high as 1579 mA h g−1 at a C/10 rate based on the mass of both Si and C, a cycle life of 300 cycles with 94% capacity retention, as well as a discharge rate capability of 6 min while retaining a capacity of 702 mA h g−1. Significantly, the coulombic efficiencies of this structure reach 99.99%. The assembled structure suggests a design principle for high capacity alloying electrodes that suffer from volume changes during battery operations.

Rescaling of metal oxide nanocrystals for energy storage having high capacitance and energy density with robust cycle life

Significance The combined study of experiments and molecular dynamics simulations demonstrates that metal oxide nanocrystals on graphene can be rescaled into atomic clusters. It is notable that the capacitance of 3,023 F per the mass of NiO, matching the measured capacitance of 2,231 per the total electrode mass, exceeds the theoretical gravimetric capacitance of 2,618 F available via ion-to-atom redox reactions. This approach thus provides a new pathway to realize full capacitance via ion-to-atom Faradaic redox reactions. Furthermore, assembly with a rescaled metal oxide positive electrode shows that further development of high-capacity negative counter electrode materials can pave a new route to address challenging energy storage issues. Nanocrystals are promising structures, but they are too large for achieving maximum energy storage performance. We show that rescaling 3-nm particles through lithiation followed by delithiation leads to high-performance energy storage by realizing high capacitance close to the theoretical capacitance available via ion-to-atom redox reactions. Reactive force-field (ReaxFF) molecular dynamics simulations support the conclusion that Li atoms react with nickel oxide nanocrystals (NiO-n) to form lithiated core–shell structures (Ni:Li2O), whereas subsequent delithiation causes Ni:Li2O to form atomic clusters of NiO-a. This is consistent with in situ X-ray photoelectron and optical spectroscopy results showing that Ni2+ of the nanocrystal changes during lithiation–delithiation through Ni0 and back to Ni2+. These processes are also demonstrated to provide a generic route to rescale another metal oxide. Furthermore, assembling NiO-a into the positive electrode of an asymmetric device enables extraction of full capacitance for a counter negative electrode, giving high energy density in addition to robust capacitance retention over 100,000 cycles.

Hierarchical Si hydrogel architecture with conductive polyaniline channels on sulfonated-graphene for high-performance Li ion battery anodes having a robust cycle life

We report a unique hierarchical Si hydrogel architecture with conductive polyaniline polymers connected on a sulfonated-graphene nanosheet, which gives high capacity, stable cycling performance of capacity with almost 100% Coulombic efficiency over 5000 cycles of Li-ion insertion/desertion especially at high current densities, and excellent rate reversibility.