Preparation And Lithium Ion Batteries Anode Applications Of New Graphene Based Nanomaterials

Lithium ion batteries (LIBs) as the second batteries have drawn great attentions of various electrochemical energy storage communities such as portable computers, electronic vehicles, medical equipments. However, its application in high performance LIBs has been limited by the low capacity of current anode electrode materials graphite. Transition metal oxides (MOx) display high theoretical capacities (>600 mAh g-1) and are alternative anode candidates for high performance LIBs. However, electrodes made of MOx nanoparticles have not been used in industry because of their low electrical conductivities and the large volume expansion/contraction associated with Li ion insertion/extraction, leading to irreversible capacity loss and poor cycling stability.Recently, two dimensional graphene with its large theoretical specific-surface area, electrical conductivity, good mechanical properties and chemical stability has attracted considerable interest. It can been combined with MOx to improve the electrochemical performance of composites by increasing the conductivity of MOx and its excellent mechanical flexible can buffer the volume expansion of MOx during the electrochemical process to improve the cycle stability. Moreover, the special porous 3D graphene framework can enhance the ion transport and electron transfer.In this thesis, graphene have been used as conductive substrate to fabricate composites used as LIBs anodes through various methods, including (1) fabricating binder-free graphene-Co3O4 electrodes using direct electrostatic spray deposition (ESD) on conductive substrates; (2) Preparing graphene-TiO2 flexible paper electrode through hydrothermal reaction; (3) heat treatment ammonia and graphene oxide to synthesis of nitrogen doped graphene (NGr); (4) fabricating the composite of porous Fe2O3 nanorods on NGr and coating Al2O3 nanolayer on electrodes via atomic layer deposition (ALD) to improve the electrochemical performance.Direct ESD the solution of Graphene Oxide and Co3O4 nanoparticles on current collector copper foils to fabricate binder-free flexible electrode with a special 3D porous framework. The highly-interconnected mesoporous structure enables fast ion and electron transport and accommodates the volume expansion of Co3O4 upon Li+ insertion/extraction. Electrochemical testing results show that a high reversible capacity of 631 mAhg-1 can be obtained after 58 cycles under current density of 1000 mA g-1, for the binder-free Graphene-Co3O4 electrode, which was much better than the pure Co3O4 electrode, and the Columbic efficiency can be reached to 97%.Flexible wet graphene paper with porous 3D nanostructure can be used as metal oxides growing block. Pillared TiO2 nanomaterials can be grown between the graphene nanosheets in the favor of surfactant, an open and porous graphene with TiO2 nanostructure can be obtained. This novel 3D paper electrode pillared by high density TiO2 as LIB anodes significantly enhances the Li-ion insertion/extraction rate, and a specific capacity of 122 mAh g-1 can be achieved after 100 charge/discharge cycles at a current rate of 2 Ag-1. More importantly, the flexible binder-free paper electrode shows an excellent stability when the rates decrease from 4 Ag-1 back to 200 mAg-1 with the retaining capacity of 175 mAh g-1.To further investigate the lithium ion storage of graphene, a novel and rapid (-30 seconds) process to fabricate nitrogen-doped graphene (NGr) by simultaneous thermal reduction of graphene oxide with ammonium hydroxide. The porous NGr with dominant pyridinic N atoms displays greatly enhanced reversible capacities, rate performance and exceptional cyclic stability as compared with pristine graphene. The reversible discharge capacity of the NGr electrode cycled between 0.01-3V can reach 453 mAh g-1 after 550 cycles at a charge rate of 2 A g-1 (-5.4 C), and 180 mAhg-1 after 2000 cycles at a high charge rate of 10 A g-1 (-27 C) without any capacity fading. When charged within 0.01-1.5 V, the NGr anode still exhibits high reversible capacities of 224 mAh g-1 and 169 mAhg-1 after 700 cycles and 800 cycles at a charge rate of 1 Ag-1 and 5 Ag-1, respectively. Ex situ X-ray photoelectron spectroscopy (XPS) analysis of the NGr electrode upon lithiation and delithiation indicated that the pyridinic-N dominates the capacity enhancement at 3V, while the pyrrolic-N contributes primarily to Li ion storage below 1.5 V.Porous iron oxide (Fe2O3) nanorods with a diameter of 20-30 nm, length of 70-80 nm, anchored on nitrogen-doped graphene sheets (NGr) were synthesized by a one-step hydrothermal route. After a simple microwave treatment, the iron oxide and graphene composite (NGr-I-M) exhibits excellent electrochemical performances as an anode for LIBs. A high reversible capacity of 1016 mAh g-1 can be reached at the current rate of 0.1 A g-1. When the current density increased to 2 and 3 A g-1, discharge capacities of 651 and 522 mAh g-1 still can be obtained for NGr-I-M electrodes, respectively.To avoid the large capacity decay, the NGr-I-M electrode was further coated by 2 ALD cycles of ultrathin Al2O3 film (-5 A), the initial cycling coulombic efficiency, rate performance and cycling stability of the coated electrode can be greatly improved, which can be attributed to the active materials from electrodes are able to maintain their structural integrity and their electrical contact with 2 cycles Al2O3 ALD coating. Particularly, for the NGr-I-M-2ALD, the Coulombic efficiency of the first cycle is 89%, which is much higher than uncoated electrodes NGr-I-M (65%). Moreover, for both ALD coating electrodes, the Coulombic efficiency steadily reaches around 99% after three cycles under 2 A g-1. A stable capacity of 508 mAh g-1 can be achieved at 2A g-1 for 200 cycles, and an impressive capacity of 249 mAh g-1 at 20 A g-1 can be maintained without capacity fading for 2000 cycles. The excellent electrochemical performance can be attributed to the synergy of porous iron oxide structures, nitrogen-doped graphene framework, and ultrathin Al2O3 film coating. These results highlight the importance of a rational design of electrode materials improving ionic and electron transports, and potential of using ALD ultrathin coatings to mitigate capacity fading for ultrafast and long-life battery electrodes.