High-rate electrochemical energy storage for increased penetration levels of photovoltaics in electricity grids

Increases in intermittent photovoltaic (PV) generation and reductions in system inertia can destabilise electricity networks. This thesis explored electrochemical energy storage solutions for fast-response grid stabilisation services that could facilitate increased penetration levels of PV in future electricity grids. Of particular interest, was the development of high-rate Li-ion batteries (LIBs) and an increased understanding for these devices. An initial assessment of the technological suitability of different state-of-the-art electrochemical energy storage technologies for PV power ramp-rate control suggested that LIBs are the most promising technology and highlighted the need for not only high energy density but also high power capability and long cycle lives for this grid service. These findings motivated further research in engineering advanced battery materials and electrodes that could be utilised to improve the rate performance and cycle life in LIBs, with a particular focus being directed at Ti oxide materials for alternative high rate anodes. Self-supported TiO2-x nanotube (NT) electrodes, fabricated by anodising three-dimensional porous Ti scaffolds, have demonstrated superior areal capacities and rate capability (507 µAh cm-2 at 50 µA cm-2 and 347 µAh cm-2 at 5 mA cm-2) and prolonged cycle life (over 2000 cycles) over previously-reported TiO2-x NT electrodes using foil current collectors. The former was attributed to the greater surface area of the foams that facilitate higher per-electrode-area mass loadings of TiO2-x without unfavourable increases in NT length whereas the longer cycle life was assumed to be due to the enhanced adhesion of NTs to the rough surface of the foam. It is shown that amorphous TiO2-x NT electrodes exhibit not only superior capacity and rate capability over their anatase counterparts but also different Li ion storage mechanisms. A kinetic analysis revealed that the enhanced rate performance in amorphous TiO2-x was due to both more rapid Li ion diffusion (D of 2 × 10-14 – 3 × 10-13 cm2 s-1, almost two orders of magnitude larger than that for an anatase electrode) and interfacial charge transfer kinetics (RCT of 29 Ω cf 78 Ω for an anatase electrode) in the amorphous oxide that arise from its highly defective structure and loose atomic packing