High-Quality Carbon-based Composite Materials for High-Performance Lithium-ion Batteries
With the continuous increase of global population and consumption of resources, the dire need for an efficient and reliable energy system is becoming progressively prominent. The current energy system is still heavily dependent on fossil fuel, which is limited and harmful to the environment. In recent years, many countries have taken the initiative to transition into a phase where renewable and clean energy sources are gradually replacing fossil fuels and applied in various scenarios from residential buildings to power grids. However, renewable energies has their intrinsic drawbacks because of their intermittent and fluctuating nature. Consequently, it is crucial for the energy storage system to be highly capable in terms of storage capacity, instant supply, and durability. Among various energy storage systems, lithium-ion batteries have emerged as a preferable choice because of its high capacity, chemical and thermal stability, and gradually decreasing cost. In this dissertation, we developed a series of composite electrode materials for lithium-ion batteries of high performance and low cost.
Carbonaceous materials have been reported to improve the battery performance to a certain extent. However, the lack of focus in the designing of the architecture of the carbon materials could limit the effect that they might bring. We thus developed an intercalation method that allows FeCl3 to be directly embedded into the graphite matrix to synthesize LFP/Graphite as a cathode material. This method significantly increased electronic and ionic conductivity through a robust and highly conductive graphite matrix, tremendously improving the performance of commercial LFP to a reversible capacity of 160 mA h g-1, a rate performance of 107 mA h g-1 at 60 C, and an outstanding cycling ability of > 95% reversible capacity retention over 2000 cycles.
The intercalation method is then combined with Fe2O3 to improve the performance of graphite, which is the most prevalent material of the anode market. Graphite scaffold made via the intercalation process was able to provide a stable supporting structure to prevent structural failure due to large volumetric expansion and a highly electroconductive network. We synthesized a high-performance anode material with a specific capacity of 391 mAh/g after 350 cycles of charging/ discharging @ 500 mA/g, which improved the capacity of graphite by 50%.
With rapidly growing world population and economy growth, the need for high-energy batteries with fast-charging capability is surging. Thus, it is essential to strategically combine materials so that while maintaining a high capacity and energy density, they could also exhibit an ability to accept fast charging. Graphene has been numerously studied and applied in composite materials in recent years, but its performance in terms of fast-charging has always been less than satisfactory because of both the poor quality of graphene, and the irreversible stacking of 2D structure within graphene. With this beard in mind, we designed synthesis processes for a hierarchical flower-like nitrogen-doped graphene-based LiFePO4 composite material for cathodes, and high-quality mesoporous graphene particles for high-energy and fast-charging anodes.
In a hierarchical flower-like nitrogen-doped graphene-based LiFePO4 composite, we used a template-based process to obtain a CVD-grown nitrogen-doped graphene; the structure was able to withstand acid etching and subsequent charging/discharging processes, remaining at a ~100% coulombic efficiency at a high rate of 20C.
In high-quality mesoporous graphene particles, we strategically combined a robust yet flexible graphene network with LFP nanoparticles that are closely packed. Followed by a microwaving process to largely increase the quality of the graphene to better provide an excellent electronic and ionic conducting network. The HNMG electrode provides a reversible capacity of 448 mA h g-1 even at a high charge-discharge rate of 60 C, 3 times the capacity of the NMG electrode (163 mA h g-1) and 70 times the capacity of the graphite electrode (6 mA h g-1). HNMG electrode also shows an excellent reversibility. Besides, due to the high tap density (0.63 g cm-3) of HNMG particles, the volumetric capacity of 334 mA h mL-1 at a high rate of 60C.
These methods provided potential solutions to the current issues of electrode materials of LIBs by synthesizing a series of carbon-based composite materials with unique designs targeting the conductivity issue of high-performance materials.