Gi-Hyeok Lee

Cobalt‐Doped FeS2 Nanospheres with Complete Solid Solubility as a High‐Performance Anode Material for Sodium‐Ion Batteries

Considering that the high capacity, long-term cycle life, and high-rate capability of anode materials for sodium-ion batteries (SIBs) is a bottleneck currently, a series of Co-doped FeS2 solid solutions with different Co contents were prepared by a facile solvothermal method, and for the first time their Na-storage properties were investigated. The optimized Co0.5 Fe0.5 S2 (Fe0.5) has discharge capacities of 0.220 Ah g(-1) after 5000 cycles at 2 A g(-1) and 0.172 Ah g(-1) even at 20 A g(-1) with compatible ether-based electrolyte in a voltage window of 0.8-2.9 V. The Fe0.5 sample transforms to layered Nax Co0.5 Fe0.5 S2 by initial activation, and the layered structure is maintained during following cycles. The redox reactions of Nax Co0.5 Fe0.5 S2 are dominated by pseudocapacitive behavior, leading to fast Na(+) insertion/extraction and durable cycle life. A Na3 V2 (PO4 )3 /Fe0.5 full cell was assembled, delivering an initial capacity of 0.340 Ah g(-1) .

A reduced graphene oxide-encapsulated phosphorus/carbon composite as a promising anode material for high-performance sodium-ion batteries

As a promising anode for sodium ion batteries, phosphorus has captured remarkable interest due to its highest sodium-storage capacity. However, phosphorus suffers from huge volume expansion during discharging and its typical low conductive nature. So, herein, we have realized a phosphorus/carbon composite encapsulated by reduced graphene oxide (P/C@rGO) through a simple spray drying process combined with in situ oxidation of the P/C composite. The rGO coating layer alleviates the volume expansion of phosphorus and improves its electrical conductivity. In addition, a small amount of phosphate species in P/C@rGO enables strong interaction between rGO nanosheets and P/C particles, finally promoting sodium ion transfer and cycling stability. As a result, P/C@rGO shows a high reversible capacity of 2445 mA h gp−1 and maintains 95% of its initial capacity even after 100 cycles.

Controlling Solid-Electrolyte-Interphase Layer by Coating P-Type Semiconductor NiOx on Li4Ti5O12 for High-Energy-Density Lithium-Ion Batteries

Li4Ti5O12 is a promising anode material for rechargeable lithium batteries due to its well-known zero strain and superb kinetic properties. However, Li4Ti5O12 shows low energy density above 1 V vs Li(+)/Li. In order to improve the energy density of Li4Ti5O12, its low-voltage intercalation behavior beyond Li7Ti5O12 has been demonstrated. In this approach, the extended voltage window is accompanied by the decomposition of liquid electrolyte below 1 V, which would lead to an excessive formation of solid electrolyte interphase (SEI) films. We demonstrate an effective method to improve electrochemical performance of Li4Ti5O12 in a wide working voltage range by coating Li4Ti5O12 powder with p-type semiconductor NiOx. Ex situ XRD, XPS, and FTIR results show that the NiOx coating suppresses electrochemical reduction reactions of the organic SEI components to Li2CO3, thereby promoting reversibility of the charge/discharge process. The NiOx coating layer offers a stable SEI film for enhanced rate capability and cyclability.

Cobalt phosphide nanoparticles embedded in nitrogen-doped carbon nanosheets: Promising anode material with high rate capability and long cycle life for sodium-ion batteries

Cobalt phosphide (CoP) nanoparticles which were uniformly embedded in N-doped C nanosheets (CNSs) were fabricated via the simple one-step calcination of a Co-based metal–organic framework (MOF) and red P and exhibited a high capacity, fast kinetics, and a long cycle life. This CoP/CNS composite contained small CoP particles (approximately 11.3 nm) and P–C bonds. When its electrochemical properties were evaluated by testing CoP/Na coin cells, the composite delivered a Na-storage capacity of 598 mAh·g−1 at 0.1 A·g−1 according to the total mass of the composite, which means that the capacity of pure CoP reached 831 mAh·g−1. The composite also exhibited a high rate capability and long-term cyclability (174 mAh·g−1 at 20 A·g−1 and 98.5% capacity retention after 900 cycles at 1 A·g−1), which are commonly attributed to robust P–C bonding and highly conductive CNSs. When the reaction mechanism of the CoP/CNS composite was investigated, a conversion reaction expressed as CoP + 3Na+ + 3e− ↔ Co + Na3P was observed. The outstanding Na-storage properties of the CoP/CNS composite may suggest a new strategy for developing high-performance anode materials for Na-ion batteries.

The synergistic effect of nitrogen doping and para-phenylenediamine functionalization on the physicochemical properties of reduced graphene oxide for electric double layer supercapacitors in organic electrolytes

The presence of nitrogen atoms in reduced graphene oxide (RGO) sheets considerably modulates their intrinsic physical and chemical properties to finally improve their electrochemical properties in electric double layer supercapacitors. However, this also accelerates the restacking phenomena of RGO, which results in a decreased active surface area and pore volume. To solve this problem, we fabricated para-phenylenediamine (p-PDA) functionalized nitrogen doped RGO (NRGO) to inhibit the restacking phenomenon and thus preserve the active surface area and pore volume via chemical bonding between RGO and p-PDA. Finally, we realized an impressive electrochemical performance through the synergistic effect of nitrogen doping and p-PDA functionalization.