Chanhoon Kim

Mass-scalable synthesis of 3D porous germanium–carbon composite particles as an ultra-high rate anode for lithium ion batteries

Electrode materials with three-dimensional (3D) mesoporous structures possess superior features, such as a shortened solid-phase lithium diffusion distance, a large pore volume, full lithium ion accessibility, and a high specific area, which can facilitate fast lithium ion transport and electron transfer between solid/electrolyte interfaces. In this work, we introduce a facile synthesis route for the preparation of a 3D nanoarchitecture of Ge coated with carbon (3D-Ge/C) via a carbothermal reduction method in an inert atmosphere. 3D-Ge/C showed excellent cyclability: almost 86.8% capacity retention, corresponding to a charge capacity of 1216 mA h g−1 even after 1000 cycles at a 2C-rate. Surprisingly, the high average reversible capacity of 1122 mA h g−1 was maintained at a high charge rate of 100C (160 A g−1). Even at an ultrahigh charge rate of 400C (640 A g−1), an average capacity of 429 mA h g−1 was attained. Further, the full cell composed of a 3D-Ge/C anode and an LiCoO2 cathode exhibited excellent rate capability and cyclability with 94.7% capacity retention over 50 cycles. 3D-Ge/C, which offers a high energy density like batteries as well as a high power density like supercapacitors, is expected to be used in a wide range of electrochemical devices.

Rational design of Sn-based multicomponent anodes for high performance lithium-ion batteries: SnO2@TiO2@reduced graphene oxide nanotubes

Ultra thin TiO2 layer (2 nm)-coated SnO2 nanotubes (NTs) wrapped by reduced graphene oxide (rGO) sheets (SnO2@TiO2@rGO) were synthesized as high capacity anode materials for lithium-ion batteries. The rationally designed anodes exhibited superior rate capability while maintaining a high discharge capacity of over 840 mA h g−1 at a current density of 500 mA g−1 after 50 cycles.

Formation of a Surficial Bifunctional Nanolayer on Nb2O5 for Ultrastable Electrodes for Lithium-Ion Battery

Safe and long cycle life electrode materials for lithium-ion batteries are significantly important to meet the increasing demands of rechargeable batteries. Niobium pentoxide (Nb2 O5 ) is one of the highly promising candidates for stable electrodes due to its safety and minimal volume expansion. Nevertheless, pulverization and low conductivity of Nb2 O5 have remained as inherent challenges for its practical use as viable electrodes. A highly facile method is proposed to improve the overall cycle retention of Nb2 O5 microparticles by ammonia (NH3 ) gas-driven nitridation. After nitridation, an ultrathin surficial layer (2 nm) is formed on the Nb2 O5 , acting as a bifunctional nanolayer that allows facile lithium (Li)-ion transport (10-100 times higher Li diffusivity compared with pristine Nb2 O5 microparticles) and further prevents the pulverization of Nb2 O5 . With the subsequent decoration of silver (Ag) nanoparticles (NPs), the low electric conductivity of nitridated Nb2 O5 is also significantly improved. Cycle retention is greatly improved for nitridated Nb2 O5 (96.7%) compared with Nb2 O5 (64.7%) for 500 cycles. Ag-decorated, nitridated Nb2 O5 microparticles and nitridated Nb2 O5 microparticles exhibit ultrastable cycling for 3000 cycles at high current density (3000 mA g-1 ), which highlights the importance of the surficial nanolayer in improving overall electrochemical performances, in addition to conductive NPs.

MOF derived ZnCo2O4 porous hollow spheres functionalized with Ag nanoparticles for a long-cycle and high-capacity lithium ion battery anode

Huge volume expansion during discharge is always a critical problem of high capacity conversion anodes for next generation lithium-ion (Li-ion) batteries. Although extensive efforts have been devoted to controlling the volume expansion using nanostructuring and surface engineering till now, more simple and facile approaches have to be considered due to the complicated and inefficient synthetic methods. Here, we report a straightforward synthesis of Ag coated ZnCo2O4 porous hollow spheres (ZnCo2O4@Ag HSs): (i) immobilization of metal–organic frameworks (MOFs) including Zn and Co metal nodes onto polystyrene sphere templates, (ii) calcination (∼450 °C) for the removal of core polystyrene sphere templates and oxidation of MOFs to produce a mesoporous ZnCo2O4 HSs, and (iii) a subsequent Ag-mirror reaction for 10 min, resulting in the formation of ZnCo2O4@Ag HSs. This porous hollow morphology not only effectively relieves the strain stemming from the volume expansion of transition metals, but also facilitates the efficient electron transport for Li+ diffusion by shortening the Li-ion diffusion path during a lithiation/delithiation process. Moreover, uniformly decorated Ag nanoparticles are beneficial to the formation of a stable solid electrolyte interface (SEI) layer as well as an increased electrical conductivity of ZnCo2O4. The MOF derived porous ZnCo2O4@Ag HSs exhibited remarkably stable cycling performance (a capacity value of 616 mA h g−1 after 900 cycles at a current density of 1 A g−1) and an excellent capacity retention of 80% at a very high current density of 20.0 A g−1.

Feasible Defect Engineering by Employing Metal Organic Framework Templates into One-Dimensional Metal Oxides for Battery Applications

Facile synthesis of rationally designed nanostructured electrode materials with high reversible capacity is highly critical to meet ever-increasing demands for lithium-ion batteries. In this work, we employed defect engineering by incorporating metal organic framework (MOF) templates into one-dimensional nanostructures by simple electrospinning and subsequent calcination. The introduction of Co-based zeolite imidazole frameworks (ZIF-67) resulted in abundant oxygen vacancies, which induce not only more active sites for Li storage but also enhanced electrical conductivity. Moreover, abundant mesoporous sites are formed by the decomposition of ZIF-67, which are present both inside and outside the resultant SnO2-Co3O4 nanofibers (NFs). Attributed to the creation of vacancy sites along with the synergistic effects of SnO2 and Co3O4, SnO2-Co3O4 NFs exhibit an excellent reversible capacity for 300 cycles (1287 mA h g-1 at a current density of 500 mA g-1) along with superior rate capabilities and improved initial Coulombic efficiency compared with pristine SnO2 NFs. This is an early report on utilizing MOF structures as the defect formation platform into one-dimensional nanostructures, which is expected to result in superior electrochemical performances required for advanced electrodes.

Ultrastable Graphene-Encapsulated 3 nm Nanoparticles by In Situ Chemical Vapor Deposition

Nanoscale materials offer enormous opportunities for catalysis, sensing, energy storage, and so on, along with their superior surface activity and extremely large surface area. Unfortunately, their strong reactivity causes severe degradation and oxidation even under ambient conditions and thereby deteriorates long-term usability. Here superlative stable graphene-encapsulated nanoparticles with a narrow diameter distribution prepared via in situ chemical vapor deposition (CVD) are presented. The judiciously designed CVD protocol generates 3 nm size metal and ceramic nanoparticles intimately encapsulated by few-layer graphene shells. Significantly, graphene-encapsulated Co3 O4 nanoparticles exhibit outstanding structural and functional integrity over 2000 cycles of lithiation/delithiation for Li-ion battery anode application, accompanied by 200% reversible volume change of the inner core particles. The insight obtained from this approach offers guidance for utilizing high-capacity electrode materials for Li-ion batteries. Furthermore, this in situ CVD synthesis is compatible with many different metal precursors and postsynthetic treatments, including oxidation, phosphidation, and sulfidation, and thus offers a versatile platform for reliable high-performance catalysis and energy storage/conversion with nanomaterials.

Stress-Tolerant Nanoporous Germanium Nanofibers for Long Cycle Life Lithium Storage with High Structural Stability

Nanowires (NWs) synthesized via chemical vapor deposition (CVD) have demonstrated significant improvement in lithium storage performance along with their outstanding accommodation of large volume changes during the charge/discharge process. Nevertheless, NW electrodes have been confined to the research level due to the lack of scalability and severe side reactions by their high surface area. Here, we present nanoporous Ge nanofibers (NPGeNFs) having moderate nanoporosity via a combination of simple electrospinning and a low-energetic zincothermic reduction reaction. In contrast with the CVD-assisted NW growth, our method provides high tunability of macro/microscopic morphologies such as a porosity, length, and diameter of the nanoscale 1D structures. Significantly, the customized NPGeNFs showed a highly suppressed volume expansion of less than 15% (for electrodes) after full lithation and excellent durability with high lithium storage performance over 500 cycles. Our approach offers effective 1D nanostructuring with highly customized geometries and can be extended to other applications including optoelectronics, catalysis, and energy conversion.

Ultrasmall titanium oxide/titanium oxynitride composite nanoparticle-embedded carbon nanofiber mats as high-capacity and free-standing electrodes for lithium sulfur batteries

Carbon nanofibers functionalized by ultrasmall titanium based nanoparticles were synthesized as high-performance free-standing sulfur electrodes. With their high affinity to polysulfides and high electrical conductivity, the free-standing lithium–sulfur battery electrodes showed superior cycle retention of 85% with high reversible capacity of 1107 mA h g−1 after 100 cycles at 1C.