Nanomaterial design and fabrication for energy storage

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Clancy, Tomás M.
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University College Cork
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The ‘Internet of Things’ scenario envisions billions of wireless sensors acting as the environmental interface to provide data that will, amongst other benefits reduce analysis costs, improve safety and predict future trends. Non-rechargeable batteries are the predominant energy source for today’s commercial wireless sensors and both the energy and power demands dramatically reduce the lifetime of the primary batteries. The value of the useful data gathered is offset by the frequent battery replacement necessitated by their short lifetimes. The ultimate challenge facing the mass distribution of wireless sensors is meeting the energy and power requirements to match the lifetime of the microdevices. Hybrid systems comprising a significantly smaller and rechargeable energy storage elements coupled to energy harvesters are of interest to enable wireless operation over the lifetime of the device. Li-ion rechargeable batteries provides the highest energy density (~270 Wh/kg) but the limitations of a typical organic solvent–based Li-ion batteries include a modest cycle life (<1,000) and low power density (<1,000 W/kg) which can hamper device operation particularly during the energy intensive periods of sensor measurement and wireless communication. Microbatteries, such as solid-state Li-ion batteries have a larger potential energy density due to the removal of inactive binder and conductive additive materials in the electrodes. They also offer the potential for Li metal anodes and a cycle life (≥ 5,000). The drawbacks which have limited their use in commercial systems include the need to maintain thin electrodes (at the micron level) particularly for the low electronic conductivity oxide cathodes typically utilised. Based on the time (τ) it takes to diffuse in a material of dimension L (τ = L 2 /D), where D is the diffusion coefficient, it can be estimated that the time taken for Li+ to diffuse in typical battery materials of micron dimension will be two to three orders of magnitude slower with a corresponding lower power capability than for a nanoscale (≤ 100 nm) material. A cathode with limited thickness and conductivity in combination with a low ionic conductivity solid-state electrolyte results in poor power capabilities and a significant potential drop can occur during high current operation. A small form factor capable of high current operation is critical in the development of the next-generation hybrid systems. Changing the geometry, size and thickness of the electrodes will have a direct effect on the battery capabilities. In this work, we focused on electrode design, electrode nanoscale electrochemical properties, fabrication of new materials and substrates. We have optimised the 3D structuring, the fabrication of 3D nanoarchitectures, the electrochemical performance of advanced electrode materials and nanoscale thin-films. COMSOL Multiphysics simulations demonstrated the advantages of 3D and 3D core-shell nanoarchitectures when used with the appropriate electrolyte characteristics. It is shown that the planar thin-film architecture gave better cell performance when used with the solid-state electrolyte. The 3D and 3D core-shell nanoarchitectures show better battery performance for the polymer electrolyte than the planar thin film, with 3D being the best. The 3D core-shell nanoarchitecture shows a significant improvement in performance by comparison with the thin-film and 3D nanoarchitecture when a polymer-gel or a liquid electrolyte are used. The 3D nanoarchitecture shows a slight decline in performance when going from a polymer-gel electrolyte to a liquid electrolyte with faster Li-ion transport. The 3D core-shell shows improved cell performance with faster Li-ion transport. The adoption of nanoarchitectures with suitable electrolytes can have a significant improvement in battery areal energy and power performance. A 3D core-shell nanoarchitecture electrode with Cu nanotubes as current collector with a Ge thin-film, achieved a capacity increase of 153 % in comparison to a planar Ge electrode. The 3D core-shell nanoarchitecture gave mechanical stability to the active Ge electrode as it underwent volume expansion during lithiation which enhanced cycle life and allowed overlithiation of the crystalline Li15Ge4 phase to increase the capacity capabilities of the active Ge. The utilisation of DC sputtering for the deposition of LiCoO2 and the optimisation of rapid thermal annealing as an annealing technique is described. The electrochemical performance of a nanoscale thin-film of LiCoO2 was studied and revealed a hybrid Li+ ion storage system of intercalation and intercalation pseudocapacitance in an aqueous system. At extremely high scan rates and galvanostatic current densities of up to 100 mV/s and 200 C respectively, a capacity retention equivalent to 97 mAh/g (4.8 µAh/cm2 , 48.3 µAh/cm2µm) is obtained. A significant contribution of non-diffusion controlled kinetics (intercalation pseudocapacitance) at high scan rates is shown. Electrodeposited thin-film V2O5 exhibited high power capabilities due to intercalation pseudocapacitance electrochemical kinetics when used in an aqueous electrolyte. However, V2O5 suffers from dissolution in aqueous electrolytes which results in severe capacity fade and ultimately a loss of capacity after 100 cycles. A TiO2 coating and a vinylene carbonate electrolyte additive were used to enhance cycle stability and improve electrochemical kinetics. Capacity retention was increased to 59 % after 200 cycles for V2O5 in aqueous electrolyte with 10 wt. % vinylene carbonate additive and a 100 nm TiO2 coated V2O5 in aqueous electrolyte with 5 wt. % vinylene carbonate additive.
Microbattery , 3D nanoarchitectures , Finite element simulations , Ultra-fast Li-cycling , Li-ion
Clancy, T. 2019. Nanomaterial design and fabrication for energy storage. PhD Thesis, University College Cork.