Yingyan Jiang

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.

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.

Improving the electrochemical properties of carbon anodes in lithium secondary batteries

Abstract The electrochemical properties of carbon anodes in lithium secondary batteries are improved by the addition of vanadium as V2O5. The action of the added V2O5 is different from that obtained by incorporating a nonmetallic element such as nitrogen, boron, phosphorous or silicon. Because it can increase the distance between the 002 planes of the carbon and act as nucleating agent that promotes the formation of a layer-like carbon structure, V2O5, not only enlarges the carbon anodes reversible capacity of lithium storage but also improves the cycling behavior.

Carbons prepared from boron-containing polymers as host materials for lithium insertion

Electrochemical lithium insertion behavior of boron-containing carbons was studied by constant-current potentiometry. The boron-containing carbons were prepared by pyrolyzing a phenolic resin chemically bonded with boron atoms, which was synthesized via an esterification reaction of the phenol hydroxyl groups by boric acid. It was found that the as-prepared boron-containing carbons at pyrolysis temperatures higher than 700 °C could accommodate more lithium species than the corresponding boron-free carbon, yet those prepared at pyrolysis temperatures lower than 700 °C accommodated less lithium than the boron-free control sample. In particular, the boron-containing carbon prepared at 900 °C exhibited a capacity higher than the theoretical value of graphite and reasonable charge/discharge voltage curves. The elemental, X-ray diffractometric and X-ray photoelectron spectroscopic analysis results indicated that at the pyrolysis temperature of 500 °C, the lithium accommodation capacity of the pyrolytic carbon was mainly dependent on its residual hydrogen content, rather than the boron content. However, when pyrolyzed at 900 °C, more boron atoms were bonded with carbon atoms and introduced to the graphene microcrystallite structure. Therefore, boron atoms exerted a considerable effect on the lithium insertion behavior and more lithium species were reversibly inserted into the carbon matrix due to the electron-deficient nature of boron atoms.

Investigation of the effects of V2O5 addition on the electrochemical properties of carbon anodes

Abstract Through observation of the binding energy spectra of V 2p ( S =3/2) and C 1s , and ESR measurements, we suggest that the nucleation agent formed during heat-treatment of a mixture of V 2 O 5 and melamine-formaldehyde resin (polymeric carbon) is the complex VO(graphene) 2 . First, this complex enables the carbon structure to become more ordered and increases its d 002 value. Second, the VO displays electron-absorbing ability. Third, the content of the imperfect carbon structure is decreased. All of these effects result in increased capacity and improved cycling behaviour of lithium secondary batteries using such material as the anode.

Carbon anode materials based on copolymers of nitrogen-containing monomers with DVB

Through the measurement of electrochemical properties of carbon anodes based on copolymers of 4-VP and AN with crosslinker DVB, it is found that the introduction of crosslinker DVB can favor the enhancement of reversible capacity, and the highest can be up to 600 mAh g− 1. Measurements of Thermal gravity analysis, elemental analysis, X-ray powder diffraction, scanning electron microscopy and specific surface area indicate that the incorporation of DVB into polymer PAN can not only affect the contents of nitrogen and hydrogen, but also favor the carbonization process. In addition, it can result in improvement of the regularity of the obtained carbon structure, i.e. the formation of graphite structure, and the number of micropores is increased. By virtue of all these factors, charge capacity below and above 0.9 V enhances and charging voltages decrease with the addition amount of DVB.

Lithium insertion in carbons prepared from phosphorus-containing polymers

Phosphorus-containing carbon has been prepared by the pyrolysis of phenolic resin containing chemically bonded phosphorus atoms, which is synthesized through the esterification of phenolic hydroxyl group by phosphoric acid. The electrochemical insertion of lithium in as-prepared carbon is also investigated. It is found that, at a pyrolysis temperature of 500°C, the addition of phosphoric acid in the precursor lowers the reversible capacity of the resultant carbon, but at 900°C, the addition of phosphoric acid in the precursor resin increases the reversible capacity of resultant carbon. Especially, the phosphorus-containing carbon prepared by the above method at 900°C gives a capacity beyond the theoretical value of graphite and reasonable discharge/charge curves. The phosphorus-containing carbon prepared by the pyrolysis at 500°C shows a similar electrochemical behavior to that of pure carbon and the effect of phosphorus atoms is little. Its reversible capacity is mainly determined by the content of hydrogen atoms rather than phosphorus atoms. However, at a relatively high pyrolysis temperature, 900°C, more phosphorus atoms are bonded with carbon atoms and are introduced to the microcrystallite structure of carbon. Hence, phosphorus element exerts a strike effect on the electrochemical behavior of carbon and, because of its different electronic effect from carbon element, more lithium species are inserted into carbon matrix.