Mihui Park

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) .

High-Energy-Density Metal–Oxygen Batteries: Lithium–Oxygen Batteries vs Sodium–Oxygen Batteries

The development of next-generation energy-storage devices with high power, high energy density, and safety is critical for the success of large-scale energy-storage systems (ESSs), such as electric vehicles. Rechargeable sodium-oxygen (Na-O2 ) batteries offer a new and promising opportunity for low-cost, high-energy-density, and relatively efficient electrochemical systems. Although the specific energy density of the Na-O2 battery is lower than that of the lithium-oxygen (Li-O2 ) battery, the abundance and low cost of sodium resources offer major advantages for its practical application in the near future. However, little has so far been reported regarding the cell chemistry, to explain the rate-limiting parameters and the corresponding low round-trip efficiency and cycle degradation. Consequently, an elucidation of the reaction mechanism is needed for both lithium-oxygen and sodium-oxygen cells. An in-depth understanding of the differences and similarities between Li-O2 and Na-O2 battery systems, in terms of thermodynamics and a structural viewpoint, will be meaningful to promote the development of advanced metal-oxygen batteries. State-of-the-art battery design principles for high-energy-density lithium-oxygen and sodium-oxygen batteries are thus reviewed in depth here. Major drawbacks, reaction mechanisms, and recent strategies to improve performance are also summarized.

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.

Tailored Surface Structure of LiFePO4/C Nanofibers by Phosphidation and Their Electrochemical Superiority for Lithium Rechargeable Batteries

We offer a brand new strategy for enhancing Li ion transport at the surface of LiFePO4/C nanofibers through noble Li ion conducting pathways built along reduced carbon webs by phosphorus. Pristine LiFePO4/C nanofibers composed of 1-dimensional (1D) LiFePO4 nanofibers with thick carbon coating layers on the surfaces of the nanofibers were prepared by the electrospinning technique. These dense and thick carbon layers prevented not only electrolyte penetration into the inner LiFePO4 nanofibers but also facile Li ion transport at the electrode/electrolyte interface. In contrast, the existing strong interactions between the carbon and oxygen atoms on the surface of the pristine LiFePO4/C nanofibers were weakened or partly broken by the adhesion of phosphorus, thereby improving Li ion migration through the thick carbon layers on the surfaces of the LiFePO4 nanofibers. As a result, the phosphidated LiFePO4/C nanofibers have a higher initial discharge capacity and a greatly improved rate capability when compared with pristine LiFePO4/C nanofibers. Our findings of high Li ion transport induced by phosphidation can be widely applied to other carbon-coated electrode materials.

Pd-Impregnated NiCo2O4 nanosheets/porous carbon composites as a free-standing and binder-free catalyst for a high energy lithium–oxygen battery

A novel free-standing air electrode with various structural and electrochemical merits was designed for a highly reversible lithium–oxygen battery. Interconnected NiCo2O4 nanosheets were grown almost perpendicular to the surface of carbon foam acting as a gas diffusion layer via a hydrothermal method combined with low temperature calcination and then decorated with palladium (Pd). Basically, this novel class of heterostructured catalysts consists of hierarchical nanosheets that can provide enough catalytic surface and open space, which is advantageous for oxygen or lithium ion transfer. In addition, the intrinsic porous structure of carbon foam better facilitates barrier-free oxygen transport and electrolyte penetration, while the introduction of Pd can modify the electronic structure of NiCo2O4, thereby enhancing electron transport all over the electrode. Because Pd incorporation also evolves the surface oxygen vacancies, which helps the discharge product (Li2O2) grow into a flower-like form, its formation or decomposition in the free-standing Pd@NiCo2O4 electrode could be rendered extremely reversible, finally realizing low charge over-potential, high discharge capacity (the maximum capacity reaches about 4000 mA h g−1) and long cycle life (extremely stable cyclic retention almost up to 100 cycles under the capacity limitation of 1000 mA h g−1).

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.

CNT@Ni@Ni–Co silicate core–shell nanocomposite: a synergistic triple-coaxial catalyst for enhancing catalytic activity and controlling side products for Li–O2 batteries

A great challenge in the application of carbon-based materials to Li–O2 batteries is to prevent the formation of carbonate-based side products, thereby extending the cycle life of Li–O2 batteries. Herein, for the first time, CNT@Ni@NiCo silicate core–shell nanocomposite is designed and used as a cathode catalyst in Li–O2 batteries. This nanocomposite shows a promising electrochemical performance with a discharge capacity of 10 046 mA h gcat−1 and a low overpotential of 1.44 V at a current density of 200 mA gcat−1, and it can sustain for more than 50 cycles within the voltage range of 2–4.7 V. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) characterizations prove that the formation of Li2CO3 and other side products are prevented, likely due to the encapsulation of CNTs by NiCo silicates and Ni nanoparticles, which may help decompose the side products. Finally, the synergistic effects, which are contributed by the high electrical conductivity of CNTs, high surface area, the high oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities of NiCo silicate, and the simple decomposition of side products by Ni nanoparticles enable outstanding performance of the CNT@Ni@NiCo silicate core–shell nanocomposite as a cathode catalyst for Li–O2 batteries.