Sungun Wi

Preparation and Exceptional Lithium Anodic Performance of Porous Carbon-Coated ZnO Quantum Dots Derived from a Metal–Organic Framework

Hierarchically porous carbon-coated ZnO quantum dots (QDs) (~3.5 nm) were synthesized by a one-step controlled pyrolysis of the metal-organic framework IRMOF-1. We have demonstrated a scalable and facile synthesis of carbon-coated ZnO QDs without agglomeration by structural reorganization. This unique microstructure exhibits outstanding electrochemical performance (capacity, cyclability, and rate capability) when evaluated as an anode material for lithium ion batteries.

Effective wrapping of graphene on individual Li4Ti5O12 grains for high-rate Li-ion batteries

An effective way of synthesizing graphene-wrapped Li4Ti5O12 particles was developed by solid-state reaction between graphene oxide-wrapped P25 (TiO2) and Li2CO3. Compared to the previously reported graphene/Li4Ti5O12 composites, prior wrapping of TiO2 with subsequent chemical lithiation led to more effectively confined Li4Ti5O12. The Li4Ti5O12 tightly bound by graphene exhibited a remarkable specific capacity of 147 mA h g−1 at a rate of 10 C (1 C = 175 mA g−1) after 100 cycles. This rate capability is one of the highest values among reported Li4Ti5O12 with 150 ± 50 nm grains. The improved rate capability was attributed to the enhanced electronic conductivity of each Li4Ti5O12 grain via uniform graphene wrapping, with single-grain growth during annealing from the initial ∼25 nm TiO2 nanoparticles enclosed by outer graphene sheets. Graphene-eliminated Li4Ti5O12 by thermal decomposition was also directly compared to the graphene-coated sample, to clarify the role of graphene with nearly equivalent particle size/morphology distributions.

Nanoscale interface control for high-performance Li-ion batteries

AbstactLi-ion batteries have attracted great interest for the past decades, and now are one of the most important power sources for portable electronic devices, store electricity, hybrid electric vehicles (HEV), etc. However, Li-ion-battery technologies still have several problems related to the electrochemical performance (cycle-life performance and power density) or safety of the active electrode materials. There have been numerous breakthrough challenges to overcome these problems by extensive research. Among the various methods to improve the battery’s electrochemical properties, nanoscale coating on active materials and control of the nanostructured morphology were proven as effective approaches over the last decade. In this review paper, enhanced electrochemical properties of the cathode and anode materials via nanoscale interface modification and the respective enhancing mechanisms will be discussed.

Reduced graphene oxide/carbon double-coated 3-D porous ZnO aggregates as high-performance Li-ion anode materials

The reduced graphene oxide (RGO)/carbon double-coated 3-D porous ZnO aggregates (RGO/C/ZnO) have been successfully synthesized as anode materials for Li-ion batteries with excellent cyclability and rate capability. The mesoporous ZnO aggregates prepared by a simple solvothermal method are sequentially modified through distinct carbon-based double coating. These novel architectures take unique advantages of mesopores acting as space to accommodate volume expansion during cycling, while the conformal carbon layer on each nanoparticle buffering volume changes, and conductive RGO sheets connect the aggregates to each other. Consequently, the RGO/C/ZnO exhibits superior electrochemical performance, including remarkably prolonged cycle life and excellent rate capability. Such improved performance of RGO/C/ZnO may be attributed to synergistic effects of both the 3-D porous nanostructures and RGO/C double coating.

Evaluation of graphene-wrapped LiFePO4 as novel cathode materials for Li-ion batteries

Graphene-wrapped LiFePO4 (LiFePO4/G) is introduced as a cathode material for Li-ion batteries with an excellent rate capability. A straightforward solid-state reaction between graphene oxide-wrapped FePO4 and a lithium precursor resulted in highly conducting LiFePO4/G composites, which feature ∼70 nm-sized LiFePO4 crystallites with robust connection to the external graphene network. This unique morphology enables all LiFePO4 particles to be readily accessed by electrons during battery operation, leading to a remarkably enhanced rate capability. The in situ electrochemical impedance spectra were studied in detail throughout charge and discharge processes, by which enhanced electronic conductance and thereby reduced charge transfer resistance was confirmed as the origin of the superior performance in the novel LiFePO4/G.

Optimum Morphology of Mixed-Olivine Mesocrystals for a Li-Ion Battery

In this present work, we report on the synthesis of micron-sized LiMn0.8Fe0.2PO4 (LMFP) mesocrystals via a solvothermal method with varying pH and precursor ratios. The morphologies of resultant LMFP secondary particles are classified into two major classes, flakes and ellipsoids, both of which are featured by the mesocrystalline aggregates where the primary particles constituting LMFP secondary particles are crystallographically aligned. Assessment of the battery performance reveals that the flake-shaped LMFP mesocrystals exhibit a specific capacity and rate capability superior to those of other mesocrystals. The origin of the enhanced electrochemical performance is investigated in terms of primary particle size, pore structure, antisite-defect concentration, and secondary particle shape. It is shown that the shape of the secondary particle has just as much of a significant effect on the battery performance as the crystallite size and antisite defects do. We believe that this work provides a rule of design for electrochemically favorable meso/nanostructures, which is of great potential for improving battery performance by tuning the morphology of particles on multilength scales.