Interfacial Interaction And Lithium Storage Performance Of Graphene/Metal Oxide Composites

With the development of electric vehicle and energy-storage station, lithium ion batteries (LIBs) with higher energy density and power density had attracted much attention, which raised higher demands on electrode materials for LIBs. Metal oxides had attracted much attention because of their ecofriendliness, low cost, and high theoretical specific capacity. However, some of their drawbacks, such as low conductivity, pulverization caused by large volume expansion, and aggregation, led to their rapid capacity attenuation during charge/discharge process and hindered severely their practical application in future. Graphene was used to improve the lithium storage performance of metal oxide due to its own excellent performance. It had been widely reported about graphene/metal oxide (G/MO) anodes with high lithium storage performance. In general, improvement of electrochemical performance of metal oxide was attributed to the interfacial interaction (or synergistic effect) between metal oxide and graphene. However, up to now, the interfacial interaction was still a very vague conception, and there was no clear understanding to what extent of the interfacial interaction affect the electronic structure of composites as well as their electrochemical performance. Therefore, there were limited studies about how to design G/MO anodes with high-performance based on interfacial interaction.Herein, we investigated the interfacial interaction between graphene and metal oxide and verified the influence of interfacial nature on the electrochemical performance of composites for directing the design of graphene-based composites. Firstly, we revealed the forming rules of strong interfacial interaction between metal oxide and graphene through investigating the reaction of functional groups (FGs) on graphene oxide (GO) and metal compounds. Based on above results, we designed G/MO with strong interfacial interaction and investigated the influence of interfacial changes on the electronic structure and electrochemical performance of G/MO. Finally, we prepared graphene-pore-confined hollow metal oxide films and porous graphene films through carbothermal reaction (stronger interfacial interaction of graphene and metal oxide), and investigated their electrochemical performance. The results were as follows:Reversible chemical reaction of FGs on GO and metal compounds was found firstly. FGs disappeared with the loading of metal compounds (CuO, MnO2, and Ni(OH)2) on GO. After removal of metal compunds, FGs could recover dramatically. We inferred the reactive modes of different FGs with metal compunds, including reversible ring opening/closing of epoxy, bidentate coupled coordination of carboxyl, and reaction of hydroxy with metal salts. Though different reactive modes, metal-oxygen-carbon covalent bonds (M-O-C, such as Cu-O-C, Mn-O-C, and Ni-O-C) were formd through the reaction of various FGs and metal compunds. Thus we revealed the intrinsic nature of interfacial interaction between metal oxide and graphene and forming process of M-O-C.We designed CuO/GNSs and Co3O4/GNSs composites with strong interfacial interaction through carbonizing treatment based on the above results. CuO/GNSs was obtained through carbonization of CuO/GO at 300 ℃ and Cu-O-C bonds between CuO and GNSs reserved well. The CuO/GNSs exhibited a reversible capacities of 586 mAhg-1 for the first charge process and had no any decay after 120 cycles at 50 mAg-1. At 1000 mAg-1, the reversible capacity could retain 272 mAhg-1, which was 46% of that at 50 mAg-1. Subsequently, Co3O4/GNSs hybrid material was prepared by in situ pyrolysis and Co-O-C was detected between Co3O4 and GNSs. The Co3O4/GNSs exhibited a reversible capacities of 911 mAhg-1 for the first charge process and 1060 mAhg-1 after 50 cycles at 50 mAg-1, respectively. At 500 and 1000 mAg-1, the reversible capacities could reach 763 and 561 mAhg-1, which were 84% and 62% of that at 50 mAg-1. The high capacity and excellent rate performance could be attibuted to the synergistic effect between metal oxide and graphene, i.e. the formation of M-O-C covalent bonds.Subsequently, we utilized CuO/GNSs with Cu2O (4 wt%) in the interface as a model to illustrate directly the influence of interfacial changes on the electronic structure and electrochemical performance of G/MO through removing of Cu2O by using NH3·H2O. The small interfacial alteration resulted in the obvious changes in electronic structure, such as removal of 31% covalent Cu-O-C bonds and partial recovery of π bonds in graphene, and simultaneously led to variations in electrochemical performance of composites, including a 21% increase of reversible capacity due to the increase of lithium storage sites by removal of Cu2O, degradation of cyclic stability and rate-performance and obvious increase of charge-transfer resistance due to weakening of Cu-O-C. We revealed interface was the critical factor of affecting electronic structure and electrochemical performance, and pointed that it was an effective method to enhance interfacial interaction for improving the electrochemical performance nanocarbon/metal oxide composites.Finally, graphene-pore-confined hollow metal oxide films (GPCH-Fe2O3 and GPCH-CuO) were synthesized through stronger interfacial interaction-carbothermal reaction. Also, new covalent bonds Fe-O-C and Cu-O-C were formed in GPCH-Fe2O3 and GPCH-CuO, respectively. When used as anode for LIBs, the graphene pore confined hollow metal oxide films showed high reversible capacities, improved cycling stability, and unusual high rate performance (GPCH-Fe2O3:741,249, and 141 mAhg-1 at the rates of 10,30, and 50 Ag-1 after 10000 cycles, respectively; GPCH-CuO:980,314, and 168 mAhg-1 at the rates of 10,50, and 100 Ag-1 after 10000 cycles, respectively.). In addition, it was interesting to find that the specific capacities of these anode materials (GPCH-Fe2O3 and GPCH-CuO) at 500 mAg-1 were higher than those at 50 mAg-1. The amazing performance should be attributed to the special structure. The Fe2O3 and CuO were located just in the position of graphene pores. Therefore, metal oxide would be in adequater contact and closer bonding with the graphene pores, which endowed fast diffusion and transfer of lithium ions and electrons throughout the electrodes.Porous graphene films (PGF) were obtained after removal of metal component from graphene-pore-confined metal oxide films by using HCl. PGF with different pore sizes and pore densities could be prepared through adjusting the percentage of metal salts and GO. When used as anode for lithium ion battery, the PGF-1 showed a high reversible capacity, improved cycling stability, and ultra-high rate performance (971,298, and 163 mAhg-1 at the rates of 10,30, and 50 Ag-1 after 10000 cycles, respectively). When used as anode in sodium ion battery, the PGF-1 showed a reversible capacity of 111 mAhg-1 at 1000 mAg-1 after 1000 cycles. The excellent performance of PGF should be attributed to the special porous structure. On one hand, plenty of defects within the PGF provided extra reaction sites for lithium and sodium ions storage. On the other hand, the porous structure of PGF endowed fast diffusion of lithium and sodium ions throughout the electrodes.