title: Controlling Transition Metal Oxides Nanostructures for Energy Storage Systems
Creators: name: Adomkevicius, A
Publisher: name: University of Liverpool Repository
Year: 2017

description: This thesis focuses on several approaches for the development of high charge/discharge rate lithium-ion batteries and electrochemical capacitors. The general background of lithium-ion batteries and electrochemical capacitors, and experimental techniques and methods are presented in the Chapter 1 and Chapter 2. The Chapter 3 in this thesis discusses the development of nanostructured cathodes for lithium-ion battery by introducing graphene or base washed graphene oxide as conductive additive. The carbon black is the most used conductivity enhancing additive in today’s lithium-ion batteries, nevertheless, makes it difficult for carbon black particles to form a wide-ranging “point-to-point” conductive network. Essentially, higher amounts of carbon black should be added in the electrodes to achieve percolation, leading to a lowering of the gravimetric capacity of the battery. Chapter 3 will detail the conditions used to investigate the optimal electrode composition containing different ratios and particle sizes of intercalation material and conductive additives (graphene and carbon black) in order to improve connectivity and conductivity leading to superior performance of a lithium-ion cathode material, Li(Ni1/3Mn1/3Co1/3)O2 at high charge/discharge rates. The second part of this work is focuses on electrochemical capacitors based on transition metal oxide. Manganese oxide (MnO2) is recognised as promising pseudocapacitive material, however poor ionic and electronic conductivity is the major limiting factor for its practical application. Chapter 4 discuss that through a straightforward and scalable synthesis it is possible to develop a bulk MnO2 material with randomly isolated layers. The synthesis conditions promote the formation of disordered material that allow ion transfer throughout the material that is not limited by solid state diffusion. Relatively low temperatures and inclusion of Na+ disrupt the formation of a highly crystalline material with a large domains size leading to a capacitance of ~200 F g-1 which was maintained at extremely high rates (1000 mV s-1 and 200 A g-1) for disordered Na0.35MnO2 nanosheets. In Chapter 5, an aqueous asymmetric electrochemical capacitor was assembled with Na0.35MnO2 pseudocapacitive electrode material as the positive electrode and activated carbon (AC) as the negative electrode. The optimisation charge balance between positive and negative electrodes, and modification of 0.5 M Na2SO4 aqueous electrolyte with addition of small amount of NaHCO3, is possible to suppress manganese oxide dissolution and hydrogen evolution, leading to long-term cycling at extended cell voltage of 2.4 V. In Chapter 6 following the promising results with Na0.35MnO2 within Chapter 4, other alkali metal intercalated MnO2 were investigated. Results show that the inclusion of the larger non-hydrated K+ (ionic radius = 1.52 Å) is key in the phase-controlled synthesis, where alkaline ion can serve as a template in the formation of layered structures of MnO2. Moreover, inclusion of small non-hydrated Li+ (ionic radius = 0.9 Å) was unable to prevent from forming α-MnO2 phase leading to relatively poor electrochemical performance.