Mihee Jeong

Porous V2O5/RGO/CNT hierarchical architecture as a cathode material: Emphasis on the contribution of surface lithium storage

A three dimensional vanadium pentoxide/reduced graphene oxide/carbon nanotube (3D V2O5/RGO/CNT) composite is synthesized by microwave-assisted hydrothermal method. The combination of 2D RGO and 1D CNT establishes continuous 3D conductive network, and most notably, the 1D CNT is designed to form hierarchically porous structure by penetrating into V2O5 microsphere assembly constituted of numerous V2O5 nanoparticles. The highly porous V2O5 microsphere enhances electrolyte contact and shortens Li+ diffusion path as a consequence of its developed surface area and mesoporosity. The successive phase transformations of 3D V2O5/RGO/CNT from α-phase to ε-, δ-, γ-, and ω-phase and its structural reversibility upon Li+ intercalation/de-intercalation are investigated by in situ XRD analysis, and the electronic and local structure reversibility around vanadium atom in 3D V2O5/RGO/CNT is observed by in situ XANES analysis. The 3D V2O5/RGO/CNT achieves a high capacity of 220 mAh g−1 at 1 C after 80 cycles and an excellent rate capability of 100 mAh g−1 even at a considerably high rate of 20 C. The porous 3D V2O5/RGO/CNT structure not only provides facile Li+ diffusion into bulk but contributes to surface Li+ storage as well, which enables the design of 3D V2O5/RGO/CNT composite to become a promising cathode architecture for high performance LIBs.

From grass to battery anode: agricultural biomass hemp-derived carbon for lithium storage

Biomass-derived carbon, as a low-cost material source, is an attractive choice to prepare carbon materials, thus providing an alternative to by-product and waste management. Herein, we report the preparation of carbon from hemp stem as a biomass precursor through a simple, low-cost, and environment-friendly method with using steam as the activating agent. The hemp-derived carbon with a hierarchically porous structure and a partial graphitization in amorphous domains was developed, and for the first time, it was applied as an anode material for lithium-ion battery. Natural hemp itself delivers a reversible capacity of 190 mA h g−1 at a rate of 300 mA g−1 after 100 cycles. Ball-milling of hemp-derived carbon is further designed to control the physical properties, and consequently, the capacity of milled hemp increases to 300 mA h g−1 along with excellent rate capability of 210 mA h g−1 even at 1.5 A g−1. The milled hemp with increased graphitization and well-developed meso-porosity is advantageous for lithium diffusion, thus enhancing electrochemical performance via both diffusion-controlled intercalation/deintercalation and surface-limited adsorption/desorption. This study not only demonstrates the application of hemp-derived carbon in energy storage devices, but also guides a desirable structural design for lithium storage and transport.

Further utilization of a Mn redox reaction via control of structural disorder in olivine systems

Olivine-type phosphates have become cathode materials of great interest in Li rechargeable batteries because of their low cost, high energy density and thermal safety. Strategies such as introduction of non-stoichiometric character, doping, carbon coating, and reduction of particle size have been shown to improve the electrochemical performances of olivine materials. However, the underlying mechanism responsible for electrochemical enhancement by introduction of non-stoichiometric character and doping is not yet well understood at the structure level. In this study, we investigated the structural and electrochemical properties of the equi-sized LiFe0.5Mn0.5PO4, non-stoichiometric LiFe0.5−xMn0.5−xPO4−σ and Cr-doped LiFe0.5−xMn0.5−xCryPO4−σ materials in depth by using synchrotron X-rays, neutrons and electrochemical techniques in order to explore the underpinning science responsible for improved electrochemistry of olivines as a result of non-stoichiometry and supervalent doping. Our neutron diffraction study revealed that anti-site defects are a critical factor for improving the electrochemical performance of olivines, and these defects can be decreased by introducing non-stoichiometry in the crystal structure. The X-ray absorption near edge structure results show that the improved electrochemical performance obtained in non-stoichiometric LiFe0.5−xMn0.5−xPO4−σ and Cr-doped LiFe0.5−xMn0.5−xCryPO4−σ is achieved by a selective further oxidation of Mn, and there is no effect of non-stoichiometry and doping on the Fe2+/Fe3+ redox couple.

Characterization and Control of Irreversible Reaction in Li-Rich Cathode during the Initial Charge Process

Li-rich layered oxide has been known to possess high specific capacity beyond the theoretical value from both charge compensation in transition metal and oxygen in the redox reaction. Although it could achieve higher reversible capacity due to the oxygen anion participating in electrochemical reaction, however, its use in energy storage systems has been limited. The reason is the irreversible oxygen reaction that occurs during the initial charge cycle, resulting in structural instability due to oxygen evolution and phase transition. To suppress the initial irreversible oxygen reaction, we introduced the surface-modified Li[Li0.2Ni0.16Mn0.56Co0.08]O2 prepared by carbon coating (carbonization process), which was verified to have reduced oxygen reaction during the initial charge cycle. The electrochemical performance is improved by the synergic effects of the oxygen-deficient layer and carbon coating layer formed on the surface of particles. The sample with suitable carbon coating exhibited the highest structural stability, resulting in reduced capacity fading and voltage decay, which are attributed to the mitigated layered-to-spinel-like phase transition during prolonged cycling. The control over the oxygen reaction of Li2MnO3 by surface modification affects the activation reaction above 4.4 V in the initial charge cycle and structure changes during prolonged cycling. X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy analyses as well as electrochemical performance measurement were used to identify the correlation between reduced oxygen activity and structural changes.