Jihyeon Gim

High rate performance of a Na3V2(PO4)3/C cathode prepared by pyro-synthesis for sodium-ion batteries

A Na3V2(PO4)3/C cathode synthesized by a polyol-assisted pyro-synthetic reaction and subsequent sintering delivered a discharge capacity of 235 mA h g−1, corresponding to an extraction of 4 Na per formula with steady capacity retention and impressive rate capabilities that maintain 56% of theoretical capacity at 2.67 C.

Enhanced High-Rate Performance of Li4Ti5O12 Nanoparticles for Rechargeable Li-Ion Batteries

Li 4 Ti 5 O 12 was successfully synthesized by solvothermal techniques using cost-effective precursors in polyol medium. The x-ray diffraction (XRD) pattern of the sample (LTO-500) was clearly indexed to the spinel shaped Li 4 Ti 5 O 12 and in order to accurately determine the lattice parameters, synchrotron powder XRD pattern was fitted by the whole-pattern profile matching method using the model space group, Fd3m. The particle size, morphology, and crystallinity of LTO-500 were identified using field-emission scanning electron microscopy and transmission electron microscopy. The electrochemical performance of the sample revealed fairly high initial discharge/charge specific capacities of 230 and 179 mAh/g, respectively, and exhibited highly improved rate performances at C-rates as high as 30 and 60 C, when compared to Li 4 Ti 5 O 12 by the solid-state reaction method. This was attributed to the achievement of small particle sizes in nanoscale dimensions, a reasonably narrow particle size distribution and, hence, shorter diffusion paths combined with larger contact area at the electrode/electrolyte interface.

Particle Size Effect of Anatase TiO2 Nanocrystals for Lithium-Ion Batteries

Nanocrystalline anatase TiO 2 was synthesized from a triethylene glycol solution of titanium isopropoxide [Ti(O-iPr) 4 ] by refluxing at 270°C for 12 h. The thermal stability and effect of particle size on the corresponding electrochemical performances were investigated by annealing the prepared sample at various temperatures; namely, 300, 400, 500, 600, 700, 800, and 900°C. The X-ray diffraction patterns of the samples clearly revealed that the maximum temperature for the formation of pure anatase phase was 700°C beyond which the presence of rutile polymorph became significant. The field emission-transmission electron microscopy images of the obtained samples showed uniform and considerably dispersed particles with fairly homogeneous distributions and average sizes of 8-50 nm. The electrochemical measurements indicated considerable charge-discharge capacities devoid of major capacity fading during extended cycles, due to their electrochemically beneficial highly crystalline traits, nanosized particles, and uniform distribution.

Pyro-synthesis of a high rate nano-Li3V2(PO4)3/C cathode with mixed morphology for advanced Li-ion batteries.

A monoclinic Li3V2(PO4)3/C (LVP/C) cathode for lithium battery applications was synthesized by a polyol-assisted pyro-synthesis. The polyol in the present synthesis acts not only as a solvent, reducing agent and a carbon source but also as a low-cost fuel that facilitates a combustion process combined with the release of ultrahigh exothermic energy useful for nucleation process. Subsequent annealing of the amorphous particles at 800°C for 5 h is sufficient to produce highly crystalline LVP/C nanoparticles. A combined analysis of X-ray diffraction (XRD) and neutron powder diffraction (NPD) patterns was used to determine the unit cell parameters of the prepared LVP/C. Electron microscopic studies revealed rod-type particles of length ranging from nanometer to micrometers dispersed among spherical particles with average particle-sizes in the range of 20–30 nm. When tested for Li-insertion properties in the potential windows of 3–4.3 and 3–4.8 V, the LVP/C cathode demonstrated initial discharge capacities of 131 and 196 mAh/g (~100% theoretical capacities) at 0.15 and 0.1 C current densities respectively with impressive capacity retentions for 50 cycles. Interestingly, the LVP/C cathode delivered average specific capacities of 125 and 90 mAh/g at current densities of 9.6 C and 15 C respectively within the lower potential window.

Fully activated Li2MnO3 nanoparticles by oxidation reaction

Fully activated Li2MnO3 nanoparticles were prepared by a chemical based oxidation reaction. All of the diffraction peaks of the prepared samples were well matched to a monoclinic phase (space group: C2/m) with no impurity peaks and refined using the General Structure Analysis System (GSAS) program. The activated Li2MnO3 sample showed homogeneously well-dispersed nanoparticles with a size of ∼10 nm. The oxidation state of Mn was confirmed by XPS. The activated Li2MnO3 nanoparticles delivered a high charge capacity of 302 mA h g−1 above 4.5 V and discharge capacity of 236 mA h g−1 during the first cycle. Interestingly, the cycle performance of the activated Li2MnO3 nanoparticles during extended cycles exhibited somewhat stable discharge capacities without any drastic capacity fading, even when cycled in the high voltage range of 2.0–4.9 V and after the phase transition to spinel. In terms of the rate performance, the activated Li2MnO3 sample exhibited significantly superior properties compared to the bulk Li2MnO3 sample, probably due to the nano-size particles with high crystallinity.

High rate performance of a NaTi2(PO4)3/rGO composite electrode via pyro synthesis for sodium ion batteries

The present study reports on a highly rate capable NASICON-structured NaTi2(PO4)3/reduced graphene oxide (NTP/rGO) composite electrode synthesized by polyol-assisted pyro synthesis for Na-ion batteries (NIBs). X-ray diffraction (XRD) studies confirmed the presence of a rhombohedral NaTi2(PO4)3 phase in the composite while Raman spectroscopy studies helped to identify the existence of rGO in the composite. Electron microscopy studies established that NaTi2(PO4)3 nanoparticles of average sizes ranging between 20 and 30 nm were uniformly distributed and embedded in the GO sheets. When tested for sodium storage properties, the obtained NTP/rGO composite electrode registered high rate capacities (95 mA h g−1 at 9.2C and 78 mA h g−1 at 36.8C) when compared to that of the NTP/C electrode (∼1 mA h g−1 at 9.2 and 36.8C). Further, the NTP/rGO composites delivered a reversible capability of 62 mA h g−1 at 20C after 1000 cycles. The enhanced performance of the composite electrode can be attributed to the nano-sized NaTi2(PO4)3 particles with shorter diffusion path lengths. These particles embedded in the rGO sheets with enhanced electrolyte/electrode contact areas ultimately lead to an improvement in the electrical conductivity at high current densities. Ex situ XANES studies confirmed reversible Na-ion intercalation/de-intercalation into/from NTP/rGO. The study thus demonstrates that the NaTi2(PO4)3/rGO nanocomposite electrode is a promising candidate for the development of high power/energy density anodes for NIBs.

Pyro-Synthesis of Functional Nanocrystals

Despite nanomaterials with unique properties playing a vital role in scientific and technological advancements of various fields including chemical and electrochemical applications, the scope for exploration of nano-scale applications is still wide open. The intimate correlation between material properties and synthesis in combination with the urgency to enhance the empirical understanding of nanomaterials demand the evolution of new strategies to promising materials. Herein we introduce a rapid pyro-synthesis that produces highly crystalline functional nanomaterials under reaction times of a few seconds in open-air conditions. The versatile technique may facilitate the development of a variety of nanomaterials and, in particular, carbon-coated metal phosphates with appreciable physico-chemical properties benefiting energy storage applications. The present strategy may present opportunities to develop “design rules” not only to produce nanomaterials for various applications but also to realize cost-effective and simple nanomaterial production beyond lab-scale limitations.

One‐Step Pyro‐Synthesis of a Nanostructured Mn3O4/C Electrode with Long Cycle Stability for Rechargeable Lithium‐Ion Batteries

A nanostructured Mn3 O4 /C electrode was prepared by a one-step polyol-assisted pyro-synthesis without any post-heat treatments. The as-prepared Mn3 O4 /C revealed nanostructured morphology comprised of secondary aggregates formed from carbon-coated primary particles of average diameters ranging between 20 and 40 nm, as evidenced from the electron microscopy studies. The N2 adsorption studies reveal a hierarchical porous feature in the nanostructured electrode. The nanostructured morphology appears to be related to the present rapid combustion strategy. The nanostructured porous Mn3 O4 /C electrode demonstrated impressive electrode properties with reversible capacities of 666 mAh g-1 at a current density of 33 mA g-1 , good capacity retentions (1141 mAh g-1 with 100 % Coulombic efficiencies at the 100th cycle), and rate capabilities (307 and 202 mAh g-1 at 528 and 1056 mA g-1 , respectively) when tested as an anode for lithium-ion battery applications.

Enhanced electrochemical performance of novel K-doped Co3O4 as the anode material for secondary lithium-ion batteries

K-doped Co3O4 was prepared by a solvothermal method in polyol medium, followed by annealing at a low temperature of 400 °C for 5 h. The obtained samples were characterized by the synchrotron X-ray diffraction pattern, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, field-emission transmission electron microscopy and high-resolution transmission electron microscopy. Synchrotron XRD analysis demonstrates that the K+ ion doping caused no change in the phase structure, and a highly crystalline KxCo3−xO4−δ (x = 0.08) powder without any impurity was obtained. When applied as the anode material, the K+-doped Co3O4 electrode exhibits a much better rate capability and cycling stability, and could retain a charge capacity of 351.3 mA h g−1 at 3 C, while undoped Co3O4 exhibits only 144.3 mA h g−1 at the same rate. In addition, the electrochemical impedance spectroscopy also reveals that the K+-doped Co3O4 electrode has the highest electronic conductivity compared to an undoped sample. However, the improvement in the doped sample is due to the influence of K+ ions on the increased electronic conductivity, diffusion efficiency, and kinetic properties of Co3O4 during the lithiation and delithiation process. This material shows promising potential for use in high-rate anodes for lithium-ion batteries.

Electrochemical lithium storage of a ZnFe2O4/graphene nanocomposite as an anode material for rechargeable lithium ion batteries

In the present work, a graphene-based ZnFe2O4 nanocomposite has been synthesized using urea-assisted auto combustion synthesis followed by an annealing step. Urea synthesis is attractive, as it can rapidly synthesize materials with a high degree of control of particle size and morphology at low cost. The microstructure images clearly show that the ZnFe2O4 nanoparticles are homogeneously anchored on the surface of the graphene nanosheets. The average nanoparticle size ranges from 25–50 nm for both samples. As anode materials for lithium ion batteries, the obtained nanocomposite electrode shows significantly improved lithium storage properties with a high reversible capacity, excellent cycling stability and higher rate capability compared to the pure ZnFe2O4 nanoparticle electrode. The enhanced electrochemical performance of the nanocomposite sample can be attributed to the synergistic interaction between the uniformly dispersed ZnFe2O4 nanoparticles and the graphene nanosheets, which offers a large number of accessible active sites for the fast diffusion of Li+ ions, low internal resistance and more importantly accommodates the large volume expansion/contraction during cycling.