Hansu Kim

Prospective materials and applications for Li secondary batteries

Li-ion batteries have been employed successfully in various small electronic devices for the last two decades, and the types of applications are currently expanding to include electric vehicles (EVs), power tools, and large electric power storage units. In order to be implemented in these emerging markets, novel materials for negative and positive electrodes as well as electrolytes need to be developed to achieve high energy density, high power, and safe lithium rechargeable batteries. Here, the trends of the market and development of materials for each application are introduced, and some of next generation Li-ion batteries are discussed.

The Insertion Mechanism of Lithium into Mg2Si Anode Material for Li‐Ion Batteries

The reaction mechanism of lithium insertion into Mg{sub 2}Si was studied using various analytic techniques including electrochemical measurements, X-ray diffraction (XRD), and Auger electron spectroscopy (AES). Electrochemical tests demonstrated that 1 mol Mg{sub 2}Si reacted with 3.9 mol Li from which the initial capacity obtained was approximately 1,370 mAh/g. Ex situ XRD and AES data showed that lithium intercalated into the Mg{sub 2}Si lattice first followed by alloying with Si and Mg. The degradation mechanism of Mg{sub 2}Si during cycling was investigated because the Mg{sub 2}Si materials degraded rapidly within ten cycles. The electrode material disintegrated and Li remained within the active material after ten cycles. The XRD and scanning electron microscope data suggested that the degradation mechanism of Mg{sub 2}Si was due to the volume change during the alloying/dealloying reaction, and the volume expansion/contraction made the Mg{sub 2}Si electrode materials electrically isolated.

Synthesis of Multilayer Graphene Balls by Carbon Segregation from Nickel Nanoparticles

Three-dimensional (3D) structured graphene is a material of great interest due to its diverse applications in electronics, catalytic electrodes, and sensors. However, the preparation of 3D structured graphene is still challenging. Here, we report the fabrication of multilayer graphene balls (GBs) by template-directed carbon segregation using nickel nanoparticles (Ni-NPs) as template materials. To maintain the ball shape of the template Ni-NPs, we used a carburization process using polyol solution as the carbon source and a thermal annealing process to synthesize graphene layers via carbon segregation on the outer surface of the Ni-NPs. The resulting GBs were hollow structures composed of multilayer graphene after the removal of core Ni-NPs, and the thickness of the graphene layers and the size of GBs were tunable by controlling the graphene synthesis conditions. X-ray diffraction analysis and in situ transmission electron microscope characterization revealed that carbon atoms diffused effectively into the Ni-NPs during the carburization step, and that the diffused carbon atoms in Ni-NPs segregated and successfully formed a graphene layer on the surface of the Ni-NPs during thermal annealing. We also performed further heat treatment at high temperature to improve the quality of the graphene layer, resulting in highly crystalline GBs. The unique hollow GBs synthesized here will be useful as excellent high-rate electrode materials for electrochemical lithium storage devices.

Highly Cyclable Lithium-Sulfur Batteries with a Dual-Type Sulfur Cathode and a Lithiated Si/SiOx Nanosphere Anode.

Lithium-sulfur batteries could become an excellent alternative to replace the currently used lithium-ion batteries due to their higher energy density and lower production cost; however, commercialization of lithium-sulfur batteries has so far been limited due to the cyclability problems associated with both the sulfur cathode and the lithium-metal anode. Herein, we demonstrate a highly reliable lithium-sulfur battery showing cycle performance comparable to that of lithium-ion batteries; our design uses a highly reversible dual-type sulfur cathode (solid sulfur electrode and polysulfide catholyte) and a lithiated Si/SiOx nanosphere anode. Our lithium-sulfur cell shows superior battery performance in terms of high specific capacity, excellent charge-discharge efficiency, and remarkable cycle life, delivering a specific capacity of ∼750 mAh g(-1) over 500 cycles (85% of the initial capacity). These promising behaviors may arise from a synergistic effect of the enhanced electrochemical performance of the newly designed anode and the optimized layout of the cathode.

Dual-Size Silicon Nanocrystal-Embedded SiOx Nanocomposite as a High-Capacity Lithium Storage Material

SiOx-based materials attracted a great deal of attention as high-capacity Li(+) storage materials for lithium-ion batteries due to their high reversible capacity and good cycle performance. However, these materials still suffer from low initial Coulombic efficiency as well as high production cost, which are associated with the complicated synthesis process. Here, we propose a dual-size Si nanocrystal-embedded SiOx nanocomposite as a high-capacity Li(+) storage material prepared via cost-effective sol-gel reaction of triethoxysilane with commercially available Si nanoparticles. In the proposed nanocomposite, dual-size Si nanocrystals are incorporated into the amorphous SiOx matrix, providing a high capacity (1914 mAh g(-1)) with a notably improved initial efficiency (73.6%) and stable cycle performance over 100 cycles. The highly robust electrochemical and mechanical properties of the dual-size Si nanocrystal-embedded SiOx nanocomposite presented here are mainly attributed to its peculiar nanoarchitecture. This study represents one of the most promising routes for advancing SiOx-based Li(+) storage materials for practical use.

Hydrogen Silsequioxane-Derived Si/SiOx Nanospheres for High-Capacity Lithium Storage Materials

Si/SiOx composite materials have been explored for their commercial possibility as high-performance anode materials for lithium ion batteries, but suffer from the complexity of and limited synthetic routes for their preparation. In this study, Si/SiOx nanospheres were developed using a nontoxic and precious-metal-free preparation method based on hydrogen silsesquioxane obtained from sol-gel reaction of triethoxysilane. The resulting Si/SiOx nanospheres with a uniform carbon coating layer show excellent cycle performance and rate capability with high-dimensional stability. This approach based on a scalable sol-gel reaction enables not only the development of Si/SiOx with various nanostructured forms, but also reduced production cost for mass production of nanostructured Si/SiOx.

Electrochemical characteristics of Mg–Ni alloys as anode materials for secondary Li batteries

Abstract The electrochemical characteristics of Mg and several Mg–Ni alloys were studied as alternatives to anode materials for secondary Li batteries. Li was alloyed and dealloyed reversibly with Mg at very low voltage region (below 100 mV vs. Li/Li+), and the initial capacity obtained was approximately 3070 mA h/g. Alloys of Mg50Ni50, Mg67Ni33 and Mg75Ni25 were prepared by mechanical alloying and characterized using X-ray diffraction (XRD) and Auger electron spectroscopy (AES). Mg50Ni50 and Mg67Ni33 were found amorphous and crystalline, respectively, while Mg75Ni25 was a mixture of Mg and Mg2Ni phases. Electrochemical tests with these Mg–Ni alloys demonstrated that only Mg75Ni25 reacted significantly with Li at room temperature while Mg67Ni33 reacted with Li at high temperature. Mg75Ni25 showed enhanced cycle performance compared to that of pure Mg.

Nano-propping effect of residual silicas on reversible lithium storage over highly ordered mesoporous SnO2 materials

Highly ordered mesoporous SnO2 materials with residual silica species were successfully synthesized from a mesoporous silica template (SBA-15) via nano-replication and simple etching processes. A tin precursor, SnCl2·2H2O, was infiltrated spontaneously within the mesopores of the silica templates by melting the precursor at 353 K without using a solvent. After the heat-treatment of composite materials at 973 K under static air conditions, the controlled removal of silica templates using NaOH or HF solutions with different concentrations results in the successful preparation of mesoporous SnO2 materials, where the amounts of residual silica species are in the range 0.9–17.4 wt%. The residual silica species induce a nano-propping effect enabling the mesoporous SnO2 material (containing 6.0 wt% of silica species) to remain stable up to 973 K without any significant structural collapse. More importantly, the optimum amount of residual silica species (3.9–6.0 wt%) results in a dramatic reduction in capacity fading after prolonged charging–discharging cycles in Li-ion battery. The mesoporous SnO2 material with 3.9 wt% of silica species still exhibits a large capacity (about 600 mAh g−1) after the 30th cycle, which is probably because the residual silica species act as a physical barrier to suppress the aggregation of Sn clusters formed in the mesoporous SnO2 materials during the reversible lithium storage.

Porous carbon spheres as a functional conducting framework for use in lithium–sulfur batteries

Porous carbon spheres with hybrid pore structure have been designed as a promising conducting framework to be used in cathode material for lithium–sulfur batteries. By creating three-dimensionally interconnected micropores and mesopores, sufficient space for sulfur storage, as well as electrolyte pathways, can be secured in the carbon spheres. Sulfur is mainly confined in mesopores with diameters of a few tens of nanometers in the carbon spheres and separated on the mesoscopic domain, which is advantageous for enhancing charge transfer and effectively accommodating volume expansion of sulfur during electrochemical reactions with Li+. The important role of the micropores, with diameters of less than 2 nm, is to extend effective interfacial contact between the sulfur and electrolyte, leading to enhancement of Li+ mobility. The sulfur-porous carbon sphere composite exhibits excellent cyclic performance and rate capability without significant capacity degradation caused by the loss of soluble Li polysulfides or electrical isolation of the active sulfur in the cathode. Importantly, the shape of the porous carbon spheres is advantageous for building robust electrodes with high-energy density. Our observations, based on various structural and electrochemical analyses, will be helpful for understanding and consolidating the fundamental aspects of the electrochemistry of sulfur. Furthermore, our approach is expected to be helpful in designing and tailoring advanced cathode materials with improved performance for lithium–sulfur batteries.

Electrochemical Characteristics of Ti–P Composites Prepared by Mechanochemical Synthesis

Titanium phosphide composites with various Ti/P molar ratios (1:1, 1:2, and 1:4) were synthesized by a mechanochemical method and their potential use as an alternative anode material for Li secondary batteries was investigated. The titanium phosphide composites with Ti/P molar ratios of 1:1 and 1:2 did not show any electrochemical reactivity with Li, while both the TiP 2 and P in the composite with a Ti/P molar ratio of 1:4 reacted with Li, yielding a reversible capacity of 1422 mAh/g. Ex situ X-ray diffraction analyses and X-ray absorption spectroscopy showed that the TiP 2 -P composite transformed into cubic Li 10.5 TiP 4 phase without decomposing into Ti and Li 3 P during the initial Li insertion. Subsequent cycling showed a highly reversible insertion/ extraction process of Li to/from Li 10.5 TiP 4 phase, accompanied by the loss/recovery of the long-range cubic order. The cubic Li 10.5 TiP 4 phase showed excellent cycling performance with a large capacity of 625 mAh/g for 80 cycles when the amount of lithium reacted was suitably controlled.