Jung Kyoo Lee

Highly reversible conversion-capacity of MnOx-loaded ordered mesoporous carbon nanorods for lithium-ion battery anodes

An ordered mesoporous carbon (OMC) with a nanorod-shaped morphology and enhanced graphitic character was employed as an ideal support for MnOx (major phase of Mn3O4 with a small portion of MnO) nanocrystals which possess a high theoretical conversion capacity as a Li-ion battery anode. The MnOx/OMC nanocomposite was prepared by a simple wet-impregnation of Mn(NO3)2 aqueous solution onto OMC nanorods followed by thermal treatment at 450 °C in an Ar flow. The electrochemical properties of MnOx/OMC were investigated in comparison to those of bare OMC and a commercial graphite as an anode for Li-ion batteries. Transmission electron microscopy, scanning electron microscopy, X-ray diffraction, N2 adsorption–desorption analysis, X-ray photoelectron spectroscopy, and thermogravimetric analysis revealed that 3–30 nm MnOx nanocrystals at a high loading of 68.4 wt% were formed and well dispersed in the pore structure of OMC nanorods. The MnOx/OMC exhibited a high reversible capacity (>950 mAh g−1) after 50 deep charge–discharge cycles with excellent cycling stability, Coulombic efficiency and rate capability. As an anode for Li-ion batteries, the incorporation of insulating high density MnOx nanocrystals into OMC nanorods showed synergistic benefits of high volumetric capacity as well as specific capacity, and small redox voltage hysteresis compared to OMC nanorods.

Rational design of silicon-based composites for high-energy storage devices

Silicon-based composites are very promising anode materials for boosting the energy density of lithium-ion batteries (LIBs). These silicon-based anodes can also replace the dendrite forming lithium metal anodes in lithium metal-free Li–O2 and Li–S batteries, which can offer energy content far beyond that of current LIBs. However, it is challenging to design silicon-based materials for use as anodes in real energy storage devices. In this review, we discuss how to boost the energy content of LIBs, the pros and cons of silicon-based anodes, and challenges associated with silicon-based anodes. A major focus of this review is on the rational design of silicon-based composite anodes to address the outstanding issues. In addition, high energy LIBs and Li–S batteries that employ silicon-based anodes are introduced and discussed.

Self-Rearrangement of Silicon Nanoparticles Embedded in Micro-Carbon Sphere Framework for High-Energy and Long-Life Lithium-Ion Batteries

Despite its highest theoretical capacity, the practical applications of the silicon anode are still limited by severe capacity fading, which is due to pulverization of the Si particles through volume change during charge and discharge. In this study, silicon nanoparticles are embedded in micron-sized porous carbon spheres (Si-MCS) via a facile hydrothermal process in order to provide a stiff carbon framework that functions as a cage to hold the pulverized silicon pieces. The carbon framework subsequently allows these silicon pieces to rearrange themselves in restricted domains within the sphere. Unlike current carbon coating methods, the Si-MCS electrode is immune to delamination. Hence, it demonstrates unprecedented excellent cyclability (capacity retention: 93.5% after 500 cycles at 0.8 A g-1), high rate capability (with a specific capacity of 880 mAh g-1 at the high discharge current density of 40 A g-1), and high volumetric capacity (814.8 mAh cm-3) on account of increased tap density. The lithium-ion battery using the new Si-MCS anode and commercial LiNi0.6Co0.2Mn0.2O2 cathode shows a high specific energy density above 300 Wh kg-1, which is considerably higher than that of commercial graphite anodes.

MnO/C nanocomposite prepared by one-pot hydrothermal reaction for high performance lithium-ion battery anodes

Among various candidates to replace the low capacity graphitic carbon anode in current lithium ion batteries (LIBs), manganese oxides possess the advantages of high lithium storage capacity, low cost, high intrinsic density, environmental friendliness and low lithium storage voltage, i.e., 0.5 V Li+/Li. Manganese oxides, however, have to be incorporated with conducting and porous matrix due to poor electrical conductivity and large volume expansions associated with conversion reaction upon cycling. In this study, a facile one-pot route was attempted for the synthesis of MnO/C nanocomposite for which Mn3O4 nanoparticles were grown in aqueous medium followed by carbon gel formation in a one-pot reactor. Thus obtained Mn3O4/C carbon gel was transformed into MnO/C nanocomposite by thermal annealing in an Ar flow. The MnO nanoparticles (60wt%) of 20–50 nm in diameter were well dispersed throughout the MnO/C composite. The MnO/C composite delivered reversible capacity of 541mAh g−1 with an excellent cycling stability over 100 cycles, while parent Mn3O4 lost most of its capacity in 10 cycles. The MnO/C composite also exhibited much higher rate capability than a commercial graphite anode. Hence, the MnO/C composite based on low cost materials and facile synthetic process could be an attractive candidate for large-scale energy storage applications.

3D Si/C particulate nanocomposites internally wired with graphene networks for high energy and stable batteries

It is challenging to design silicon anodes exhibiting stable cycling behavior, high volumetric and specific capacity, and low volume expansion for Li-based batteries. Herein, we designed Si/C-IWGN composites (Si/C composites internally wired with graphene networks). For this purpose, we used simple aqueous sol–gel systems consisting of varying amounts of silicon nanoparticles, resorcinol–formaldehyde, and graphene oxide. We found that a small amount of graphene (1–10 wt%) in Si/C-IWGNs efficiently stabilized their cycling behavior. The enhanced cycling stability of Si/C-IWGNs could be ascribed to the following facts: (1) ideally dispersed graphene networks were formed in the composites, (2) these graphene networks also created enough void spaces for silicon to expand and contract with the electrode thickness increase comparable to that of graphite. Furthermore, properly designed Si/C-IWGNs exhibited a high volumetric capacity of ∼141% greater than that of commercial graphite. Finally, a hybrid sample, Si–Gr, consisting of a high capacity Si/C-IWGN and graphite was prepared to demonstrate a hybrid strategy for a reliable and cost-effective anode with a capacity level required for high-energy Li-ion cells. The Si–Gr hybrid exhibited not only high capacity (800–900 mA h g−1 at 100 mA g−1) but also a high electrode volumetric capacity of 161% greater than that of graphite.

Manganese oxides nanocrystals supported on mesoporous carbon microspheres for energy storage application

Mesoporous carbon microspheres (MCM) with a uniform size distribution (1–2 μm in diameter) were replicated from mesoporous silica microspheres (MSM) by using sucrose as a carbon source. MCM (BET surface area=1,001 m2/g, total pore volume=0.82 cc/g, average pore size=3.4 nm) was used as the support of MnOx nanocrystals (Mn3O4 with MnO as a minor phase). The MnOx/MCM composite was prepared by pore-filling wet-impregnation of Mn nitrate solution followed by a moderate annealing under Ar flow. Thus obtained MnOx/MCM composite was characterized as a high capacity anode for lithium ion battery (LIB). The electrochemical responses of MnOx/MCM were investigated in comparison with those of commercial graphite. The MnOx/MCM composite exhibited the reversible capacity of ∼720 mAh g−1 at the current density of 200 mA g−1 with an excellent cycling stability up to 100 cycles. The MnOx/MCM composite also showed much higher volumetric capacity and better rate capability than the state of the art graphite anode, suggesting its potential use as a new anode material for LIBs.

Enhanced Li–S battery performance based on solution-impregnation-assisted sulfur/mesoporous carbon cathodes and a carbon-coated separator

The widespread use of lithium–sulfur (Li–S) batteries is still hindered by the low electrochemical activity of sulfur-species, and a short cycle life owing to anode instability coupled with polysulfide shuttle effects. As a first measure to counteract these issues, a new and simple sulfur-loading method consisting of solution impregnation and subsequent melt-diffusion (IM) is demonstrated and compared to the conventional method of physical mixing followed by melt-diffusion (PM). Using the IM method, sulfur is well encapsulated and highly dispersed in conducting mesoporous carbons (MCs) that possess an ideal pore diameter (around 10 nm) and a large pore volume (∼2.8 cm3 g−1). S/MC cathodes prepared by the IM method deliver much higher capacities, better rate responses and cycling stabilities up to 300 cycles (the fading rate was as low as −0.037% per cycle) with less concerns in lithium polysulfide (LPS) shuttling and impedance build up than S/MC cathodes prepared by the PM method. The S/MC cathodes prepared by the IM method has a pseudo-optimum sulfur content of 65 wt%, and when coupled with a new type of carbon-coated separator (CCS), a high areal capacity of >2.5 mA h cm−2 is successfully achieved, combined with excellent cycling stability and rate capability.

Zeolite-Templated Mesoporous Silicon Particles for Advanced Lithium-Ion Battery Anodes

For the practical use of high-capacity silicon anodes in high-energy lithium-based batteries, key issues arising from the large volume change of silicon during cycling must be addressed by the facile structural design of silicon. Herein, we discuss the zeolite-templated magnesiothermic reduction synthesis of mesoporous silicon (mpSi) (mpSi-Y, -B, and -Z derived from commercial zeolite Y, Beta, and ZSM-5, respectively) microparticles having large pore volume (0.4-0.5 cm3/g), wide open pore size (19-31 nm), and small primary silicon particles (20-35 nm). With these appealing mpSi particle structural features, a series of mpSi/C composites exhibit outstanding performance including excellent cycling stabilities for 500 cycles, high specific and volumetric capacities (1100-1700 mAh g-1 and 640-1000 mAh cm-3 at 100 mA g-1), high Coulombic efficiencies (approximately 100%), and remarkable rate capabilities, whereas conventional silicon nanoparticles (SiNP)/C demonstrate limited cycle life. These enhanced electrochemical responses of mpSi/C composites are further manifested by low impedance build-up, high Li ion diffusion rate, and small electrode thickness changes after cycling compared with those of SiNP/C composite. In addition to the outstanding electrochemical properties, the low-cost materials and high-yield processing make the mpSi/C composites attractive candidates for high-performance and high-energy Li-ion battery anodes.