The evolution of human civilization is accompanied with ever increasing energy consumption. Hydro-electricity, wind, solar, ocean, biomass and geothermal energies are considered as clean, sustainable and renewable energy resources to replace fossil fuels (coal, gas, and oil) and reduce CO2 emission. However, access to the abovementioned energy is limited by geolocation accessibility, mobility and continuous basis. As electricity is the dominant form of energy used, electrochemical energy storage (EES), which reversibly stores and converts energy between the form of chemical energy and electrical energy, is particularly useful towards effective utilization of renewable energy.
Rechargeable batteries (e.g., lithium-ion batteries) and electrochemical capacitors (ECs) are the two-main electrochemical energy storage devices. Bulk redox reaction enables batteries with relatively high energy densities. ECs, which rely on physical adsorption of ions and/or surface (including near surface) redox reaction, could achieve much higher power densities and longer cycle life. Such difference in storage mechanism has been used to guide the selection of electrode materials, as well as the design of electrode architecture.
The redox reactions for “rocking chair” type of batteries (such as lithium/sodium ion batteries) could be characterized based on either an intercalation, alloy or conversion mechanism.1 Alloy and conversion types may provide higher theoretical energy density. However, they are suffered from many disadvantages, such as huge volume change (~150 to 400%) associated with (de)lithiation; relatively low initial columbic efficiency; breakdown, reformation and thickening of solid electrolyte interface (SEI) during (dis)charge; detachment of active materials from conductive networks; and large voltage hysteresis that results in very low round-trip charging efficiency (especially for conversion type). ECs exhibiting high power capability and long cycle life despite with low energy density still holds great promise many energy-storage applications. In this context, the goal of this dissertation is to utilize architectural design to enable high-rate and long-life energy storage.
Commercial anodes based on graphite show limited rate applications. Transition metal oxides, such as titanium dioxide, is considered as a promising material as it is environmentally benign, highly abundant, non-toxic, and low cost. However, titanium dioxide with a bandgap over 3.2 eV (semiconductor) shows poor ionic and electronic conductivity. Extensive research has been conducted in engineering the structure and composition of titanium dioxide aiming for better conductivity and higher energy density.
In this dissertation, begining with anatase TiO2, a novel architecture based on mesoporous single-crystal-like TiO2 particles (mesocages) threaded with carbon nanotubes (CNTs) was successfully synthesized via facile solvothermal method at 110 oC. No subsequent high temperature sintering process is need. Such unique architecture offers micrometer size secondary particle assembled by iso-oriented primary nanocrystals (building blocks with sub-10 nm crystal size). The voids between the primary nanocrystals together with the mesoporous feature ensured sufficient electrolyte penetration, meanwhile shortened the ion diffusion pathway. The threaded CNTs (adjustable ratio with TiO2) only taken some part of the mesoporous sites. Intimate contact between such conductive networks with the secondary particles ensured long-range electron conductivity. More importantly, such design the fabrication of electrodes with high tapping density of 1.12g cm-3, which is significantly higher than conventional nanoscale electrode materials. High rate performance was achieved in a voltage range 1 to 3 V vs Li/Li+ (260 mAh g-1 and 20 C) and extended voltage window from 0.01 to 2.7 V, with improved reversible specific capacity (440 mAh g-1), rate and cycling performance.
Due to the abandon and low-cost of sodium ions, TiO2-graphene based nanocomposites were also investigated in this dissertation as sodium host for sodium-ion supercapacitors. The synthesis of the composites was conducted using microwave as the heating source. Uniform and small-size anatase particles (with primary nanocrystals) on were synthesized on graphene, showing unique electrochemical behaviors. A linear and sloppy profile under a constant-current test indicated a pseudocapacitive behavior suitable as a host for sodium ions. These composites show ultra-long cycling life (~18,000 cycles) and high rate performance (20 C). Electrochemical kinetic analysis confirmed the majority of the capacity provided comes from reversible surface solid and redox reactions. Such strategies can be extended as a general strategy towards the design of high-rate energy-storage materials.
