Shibi Fang

Mechanism of lithium storage in low temperature carbon

Abstract Through measurement of the intensity of the EPR signal of carbon anodes at different discharge and charge potentials, a micropore mechanism is suggested for the storage of lithium in low temperature carbons (LTCs), and it is further confirmed by results from the addition of pore-genic agent and introduction of crosslinker DVB into addition polymers PAN and P(4-VP). The size of micropores acting effectively as ‘reservoirs’ for lithium storage is suggested to be below 100 nm. The phenomena, which are characteristic in LTCs such as voltage hysteresis and capacity fading, are explained through the suggested mechanism.

Effects of nitrogen on the carbon anode of a lithium secondary battery

Nitrogen-containing carbons have been made from different polymer precursors at 600°C. Their composition and structure have been studied by chemical analysis, X-ray powder diffraction and X-ray photoelectron spectroscopy. These results show that this kind of carbon is disordered, and nitrogen exists as two kinds of forms in the polymeric carbons: graphene nitrogen (N1s binding energy 398.5 eV) and conjugated nitrogen (N1s binding energy 400.2 eV). The discharge and charge process suggests that these two kinds of nitrogen are bonded satisfactorily and could not result in irreversible reaction with Li. The increase of reversible capacity mainly results from the graphene nitrogen, and the higher the content of nitrogen, the higher the charge capacity. Part of the irreversible capacity is derived from the formed lithium carbide and lithium atoms which are intercalated and could not be deintercalated.

Carbon anode materials based on melamine resin

We have obtained carbon anode materials based on a melamine resin. Through elemental analysis, X-ray powder diffraction, BET measurement and X-ray photoelectron spectroscopy, the effects of heat-treatment temperature and doping of phosphorus were investigated. Temperature affects mainly the nitrogen content, the size of graphite crystallites and the number of micropores in the carbons, and thus their reversible capacity changes with temperature. The highest reversible capacity occurs at 600 °C. The addition of phosphoric acid can affect the carbon structure and relative content of graphene nitrogen. Its behaviour changes with temperature. Only at high temperature, can the doped phosphorus favour the enhancement of reversible capacity.

Improvements of photocurrent by using modified SiO2 in the poly(ether urethane)/poly(ethylene oxide) polymer electrolyte for all-solid-state dye-sensitized solar cells

Nanocomposite polymer electrolytes containing poly(ether urethane) (PEUR)/poly(ethylene oxide) (PEO)/modified SiO(2) were prepared for all-solid-state dye-sensitized solar cells with a high efficiency of 4.86% and an active area of 0.25 cm(2) under AM1.5 conditions at 100 mW cm(-2) irradiation.

Nano-scale copper-coated graphite as anode material for lithium-ion batteries

Nano-scale copper particles were homogeneously deposited on the surface of natural graphite through electroless plating. The co-intercalation of solvated lithium ion and reduction of the electrolyte were effectively depressed after coating of copper particles. Consequently, the graphite showed a significant improvement in charge–discharge properties such as coulombic efficiency, cycle characteristics, and high rate performance as an anode material for lithium ion batteries.

Carbon anodes for a lithium secondary battery based on polyacrylonitrile

Abstract Carbon anode materials for a lithium secondary battery based on polyacrylonitrile (PAN) are studied by using elemental analysis, X-ray powder diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy. The reversible lithium capacity and charging voltage curves of carbons from PAN are affected by the heat-treatment temperature the rate of temperature rise and the soak time. These factors lead to a change in nitrogen content, cyclization and cross-linking processes, the carbon structure, and the number of micropores. The reversible capacity reaches 426 mAh g−1 at 600°C; the lower the rate of temperature rise, the higher the reversible capacity. The addition of phosphoric acid can favour the cyclization process of PAN, and can increase the number of micropores in the resulting carbon. It can also act as setting agent for graphene molecules and can improve the regularity of the carbon structure. In addition, the doped phosphorus is bonded with C and O, and dispersed homogeneously in the bulk carbon structure. This results in an increase in d002. Such doping can enhance the reversible capacity above and below 0.9 V.

Nitrogen‐containing polymeric carbon as anode material for lithium ion secondary battery

Nitrogen-containing polymeric carbon as anode materials for the lithium ion secondary battery is prepared from polyacrylonitrile (PAN) and melamine–formaldehyde resin (MF) at 600 and 800°C. Its physicochemical properties were investigated through elemental analysis, X-ray powder diffraction, X-ray photoelectron spectroscopy, and measurement of specific surface area. Results show that this kind of carbon is amorphous. Nitrogen atoms exist in the prepared polymeric carbon mainly as two states, that is, graphene nitrogen and conjugated nitrogen, and favor the enhancement of reversible lithium capacity. All the prepared polymeric carbon has a reversible capacity higher than that of the theoretic value of graphite, 372 mAh/g, and the highest reversible capacity can be up to 536 mAh/g.

Poly(acrylonitrile) encapsulated graphite as anode materials for lithium ion batteries

Novel surface modification approach for graphite anode of lithium ion batteries was developed in this study. Poly(acrylonitrile) was in situ encapsulated on the surface of natural graphite (N-graphite) particles via radiation-initiated polymerization. The graphite obtained shows a large improvement in electrochemical performance such as initial Coulombic efficiency and cycleability compared with the original N-graphite. The structural stability of graphite surface is enhanced due to the fact that encapsulated poly(acrylonitrile) can depress the co-intercalation of solvated lithium ion.

Carbonaceous Anodes for Lithium‐Ion Batteries Prepared from Phenolic Resins with Different Cross‐linking Densities

The effect of cross-link density of phenolic resin precursor on the capacity of as-prepared carbon was investigated. The results show that a higher cross-link density of phenolic resin is beneficial to the charge capacity of the carbon, and the charge capacity of carbon prepared from phenolic resin with the highest cross-link density is greater than the theoretical capacity of carbonaceous materials. Moreover, the charge curves exhibit a potential plateau at approximately 1 V vs. Li/Li + , and the increase of charge capacity results mainly from the lengthening of this plateau. To explain this phenomenon, we proposed a mechanism that, except for the lithium intercalated between graphene layers, lithium is doped mainly at the edges of graphene layers rather than the surfaces of graphene layers, and it is the interaction between lithium and atoms at the edges that leads to the plateau at about 1 V.

Novel modified graphite as anode material for lithium ion batteries

A novel graphite material for lithium ion batteries was prepared by encapsulation of an ionic conductive polymer on the surface of natural graphite particles via radiation-initiated polymerization. The graphite obtained shows great improvement in electrochemical performance such as initial coulombic efficiency and cycleability compared with the original natural graphite. Raman spectroscopy indicates that the structural stability of the graphite surface is enhanced due to the fact that encapsulated polymers can depress the exfoliation of graphite layers caused by co-intercalation of solvent molecules. The solid electrolyte interface (SEI) film formed on the encapsulated graphite electrode retains a stable morphology during repeated cycling, and thus avoids an increase in the electrodes impedance.