Synthesis And Electrochemical Performance Of Electrode Materials For High-power Flexible Lithium Ion Betteries

Currently, lithium ion batteries (LIBs) are widely used in various portable electronic devices, because they have the merits of high voltage, large energy density, long-life performance, low self-discharge rate, and no memory effect. With in the development of next generation flexible/bendable electronic devices, flexible LIBs have attracted extensive attention in past years. Developing better flexible electronic devices with long life and fast charging performance has triggered a gold rush for exploiting flexible LIBs with high energy density and power density (fast charging performance). However, the electrochemical performance of the reported flexible LIBs, especially the fast charging performance, is far from the level of conventional LIBs and the requirements of practical applications. The power capability of LIBs depends critically on the rate at which lithium ions and electrons can migrate through the surface and bulk of the active electrode materials. Strategies to increase the rate performance have focused on improving electron transport in the bulk or at the surface of the materials, or on reducing the path length over which electrons and Li ions have to move by using nano-sized materials, which can be also used in flexible LIBs. In this dissertation, we focus on fabricating flexible LIBs with ultrahigh charge rates, by using nanostructured materials and design of new electrode structure. The main results are as follows:1. A free-standing flexible Li4TieO12(LTO) electrode that does not require a current collector:We proposed a free-standing flexible LTO electrode with the nanosheets of active material coatd by a N-doped carbon layer as building blocks. The flexible electrode showed an excellent rate performance and a good cycling stability due to the carbon coating, nanostructure and monolithic block, producing a highly conductive pathway for electrons and fast transport channels for lithium ions. The free-standing flexible LTO electrodes without any ancillary materials could open up a possibility for high power flexible LIBs.The flexible LTO electrodes were obtained by assembling the N-doped carbon coated LTO (C-LTO) nanosheets by a vacuum filtration process. The rate performance and cycling performance of the flexible C-LTO nanosheet electrode are much better than that of other LTO materials. A rate capability equivalent to discharge in30s can be achieved. The performance of the C-LTO nanosheet electrode could be attributed to the following factors. First, the porous nanosheet structure possesses a large specific volume that facilitates the fast transfer of Li+. Second, the electronic conductivity of the electrode is significantly improved by the thin N-doped carbon layer on the nanosheet surface, which ensures all the nanosheets have an ultrafast rate of electrochemical reaction. Moreover, this unique electrode is a monolithic block.2. The anisotropic TiO2/graphene flexible electrode:Based onthe understanding that Li ion transport through the surface and bulk of active electrode material is anisotropic, we proposed a new concept of an anisotropic electrode. We designed and fabricated an anisotropic TiO2/graphene flexible electrode and found that, in this anisotropic electrode, Li ions can easily intercalate into the (001) facet of TiO2, manifesting an excellent rate capability and a significantly improved cycling performance. The design of anisotropic electrodes could open up a possibility for high power flexible LIBs.The (001) facets of the nanosheets were parallel to the surface of the graphene and forming an anisotropic electrode, and the graphene was used as a current collector. Li ions could easily intercalate into the typical reactive surface of a TiO2nanocrystal when the reactive facet is parallel to the surface of the current collector due to the presence of the electric field in the cell. The anisotropic TiO2/graphene flexible electrode delivers111mAh/g at a rate of100C (equal to finishing a charge/discharge cycle within36s) and the capacity decreases less than2%of the initial value after100cycles, demonstrating the excellent rate performance and electrochemical stability of the electrode. The excellent performance of the anisotropic TiO2/graphene flexible electrode could be attributed to the following factors. First, Li ion can easily move along the [001] axis, which is the fast Li ion diffusion direction in anatase TiO2Second, the unique structure and inherently short transport path length of TiO2nanosheets improve Li ion diffusion kinetics. TiO2nanosheets grown directly on graphene also provide favorable transport kinetics for electrons.3. Flexible graphene-based lithium ion cells:We proposed a flexible lithium ion cell consisting of graphene foam (GF), a three-dimensional flexible and conductive interconnected network. No metal current collectors, conducting additives and binders are used. The excellent electrical conductivity and pore structure of the hybrid electrodes enable rapid electron and ion transport. And we obtained a flexible full cell with ultrafast charging performance both under flat and bent states. The LTO/GF and LiFePO4(LFP)/GF hybrid materials were fabricated by in situ hydrothermal deposition of active materials on GF followed by heating in an argon atmosphere. Using the flexible LFP/GF cathode and LTO/GF anode, we further built a thin, lightweight and flexible LTO/GF//LFP/GFfull cell. The free-standing LTO/GF and LFP/GF electrodes were first laminated onto both sides of a polypropylene separator, and then sealed with poly (dimethyl siloxane)(PDMS) in Ar-filled glovebox. The flexible cell shows excellent rate performance and superior cycling stability, which surpasses most batteries reported. Even at a high rate of IOC,90%of the initial capacity is provided. Our strategy is versatile and can be used to fabricate a’ broad class of anode and cathode materials with greatly improved rate performance. Both the fabrication of GF and subsequent filling and coating of active materials can be easily scaled up, which opens up the possibility for the large-scale fabrication of flexible cells for powering next-generation flexible electronic devices that can be operated at a high power rate and fully charged in a very short time.