Hierarchically-structured electrodes for Li-air batteries

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Li-air batteries are secondary batteries with the highest theoretical energy density of any battery system. In order to achieve this in practice, many technical problems must first be resolved. One key issue limiting the discharge capacity at higher rates is that of oxygen diffusion through the cathode material. At higher discharge rates, oxygen from the air side of the cathode does not have time to diffuse throughout the whole electrode before being consumed, resulting in a build up of discharge product at the air side. This results in eventual pore blockage and the underutilisation of the cathode volume. Another issue affecting the energy efficiency of these batteries is the high overpotentials caused by the slow kinetics of the oxygen reduction and evolution reactions on discharge and charge. One of the main goals of this project is to develop an understanding of the relationship between oxygen diffusivity and discharge capacity of Li-air batteries at high discharge rates. In this project, the oxygen diffusivity through the cathode will initially be controlled via laser processing of carbon nanotube (CNT) mats. Building from this, more advanced material synthesis methods can be employed, such as nano-lithography to define a patterned catalyst followed by CNT forest growth. The material properties and electrochemical performance of this cathode material will be characterised, and correlated with the effective oxygen diffusivity through the electrode. Using this system as an experimental basis, operando XRD and gas pressure monitoring will be used to study the effects of varying oxygen diffusivity and cycling rate on cell kinetics in detail. Following this, cathodes functionalised with catalytic nanoparticles will be synthesised and have their electrochemical performance characterised. The operando techniques developed earlier in the project will be used to study the effects of heterogeneous catalysis on discharge product formation in Li-air batteries. These studies will help improve the cell capacity and efficiency of Li-air batteries, particularly athigh discharge rates, relevant for commercial applications.