Graphene-Based Energy Storage Materials as Anodes for Lithium-Ion Batteries

Abstract

Graphene, a one-atom layer and two-dimensional (2D) structure of sp2-bonded carbon, has been considered as an ideal candidate for energy storage, especially lithium ion batteries (LIBs) because it possesses high conductivity, large specific surface area, great mechanical strength, low weight, chemically inert and low price. Furthermore, graphene is facile to be chemically functionalized, called graphene oxide (GO) which is typically prepared from graphite to graphite oxide by oxidization and to GO by subsequent exfoliation. For those reasons, various graphene-based materials have been developed for and applied in LIBs in order to ameliorate the rate capability, overall capacity and so on over the last decade.<br/> Nanomaterial consists of particles or constituents with nanoscale dimensions, usually 1 to 100 nm, or is produced by nanotechnology. The nanomaterial has a significant potential to make a huge impact on the performance of LIBs. The advantages of nanomaterials are better accommodation for the structural strain caused by lithium insertion/extraction, high charge/discharge rates by the extensive electrode/electrolyte interface and short distances for Li+ and electron transport. On the other hand, nanomaterials have also disadvantages such as undesirable side reactions on the electrode/electrolyte interface, resulting in self-discharge, poor cyclability and low volumetric energy density for the same mass of micrometer-sized particles and complex synthesis of nanoparticles with dimension control. To complement these drawbacks, graphene is regarded as an effective host material to enhance the electrochemical capability. In LIBs, graphene-based nanomaterials are expected to be alternative and promising anode materials.<br/> This dissertation is the introduction to graphene-based anode nanomaterials relating to graphene paper, metal oxide/graphene composite and metal oxide on graphene. First of all, graphene paper which is hierarchically intercalated with Mn3O4 nanorods (Mn3O4 NRs) is fabricated and applied to an anode material in LIBs. In an electrode, graphene functions as a buffer layer for volume expansion of Mn3O4 NR during Lithium insertion/extraction and the pathway for Li ion and electron transport. In addition, Mn3O4 NR also plays a crucial role in delivering electrons and Li ions within a paper. A Mn3O4 NR/reduced graphene oxide paper (Mn3O4 NR/rGO paper) has an out-of-plane porous structures through reduction process, leading to facile lithium transfer without loss. The porous Mn3O4 NR/rGO paper (pMn3O4 NR/rGO) shows the first discharge and charge capacities of 943 and 627 mA·h∙g-1, respectively and Coulombic efficiency of 66.5%. The irreversible capacity loss is related to the formation of the solid electrolyte interphase (SEI) layer and electrolyte decomposition. After 100 cycles, the pMn3O4 NR/rGO paper compared to an rGO paper maintains a high specific capacity of 573 mA·h∙g-1, indicating that the pMn3O4 NR and rGO contribute substantially to the capacity retention. The pMn3O4 NR/rGO at various current densities, 50, 100, 500, 1000, and 2000 mA g-1, delivers a high capacity of 692, 618, 411, 313, and 196 mA h g-1, respectively.<br/> Secondly, GO is treated by acid, HNO3, in order to make in-plane pores on the graphene surface. The acid-treated reduced graphene oxide (ArGO) provides Li ion transport pathway through pores and electron transport pathway over the entire graphene surface. Mn3O4 NR, which is a one-dimensional (1D) nanomaterial, akin to ArGO has a large surface area and provides efficient 1D electron transport and short Li ion diffusion distance to be capable of improving the electrochemical performance in LIBs. The acid-treated reduced graphene oxide/Mn3O4 nanorod (ArGO/Mn3O4 NR) is simply prepared by mixing acid-treated graphene oxides (AGO) with MnOOH nanorods (MnOOH NRs) and reduction. The ArGO/Mn3O4 NR electrode exhibits the first discharge and charge capacities of 1130 and 778 mA·h∙g-1 at 200 mA∙g-1, respectively and a low initial irreversible capacity of 32%. Coulombic efficiency is recovered to 98% after 3 cycles. After 100 cycles, the overall capacity reaches to 749 mA·h∙g-1.<br/> Lastly, SnO2 nanoparticles are hydrothermally synthesized onto the graphene surface. In this study, graphite is oxidized and then activated by nitric acid in order to introduce in-plane pores into graphite oxide. After reduction, an ArGO is used as a host material and buffer layer for SnO2 to avoid suffering from pulverization while Li ion is inserted into and extracted from SnO2. Additionally, Li ion diffusion and electron transfer can go through or on ArGO resulting in enhancing the rate capability of the electrode. The ArGO/SnO2 (AGS) electrode displays the first charge and discharge capacity of 979 and 2030 mA h g-1. The Coulombic efficiency of the AGS electrode corresponds to 48% with respect to the first cycle. Moreover, the AGS electrode maintains 720 and 569 mA h g-1 at 200 and 500 mA g-1 without considerable capacity loss when it reaches 200 cycles.<br/> The research implies that such graphene-based nanocomposites enable electrode materials to achieve high capacity, high rate capability and stable cyclability in LIBs. Therefore, it is expected that these nanomaterials have a great potential to sufficiently surpass the performance of existing LIBs.<br/>