Nanostructured electrodes with high surface area and porosity for energy storage applications

Although the highest combined energy and power density of rechargeable lithium-ion (Li-ion) batteries should make them an optimal energy storage solution for miniaturized devices, small Li-ion batteries exhibit only a fraction of the energy density of their bigger counterparts. To increase the energy density of small batteries, the superficial loading of energy-storing components needs to be increased by, for example, coating active materials on three-dimensional (3D) nanostructured substrates. Using the advantages of fast electronic and ionic transport in nanofilms, the energy and power density of the 3D electrodes could be maximized if they utilized high loadings of sub-50 nm thick active material coatings. To realize such electrodes, they must be based on nanostructured 3D current collectors that combine high porosity, large internal surface area, sufficiently large pores and mechanical stability - the target combination for not only the batteries, but virtually any electrochemical device. Besides using adequate substrates, harnessing the potential of nanofilm-based battery electrodes requires finding new methods for conformal coating of functional materials within the inherently small pores of nanostructured current collectors. Importantly, the entire fabrication process needs to be cheap and fast to bring the new batteries from the laboratory to the market. In this work, nanostructuring has been exploited to introduce high energy and power density to the electrodes for small Li-ion batteries. First, new 3D-intreconnected nanowire meshes (nanomeshes) were developed to act as free-standing structural current collectors. To assess the potential of the material for various electrochemical applications, its surface area and porosity were accurately determined using three newly-established electrochemical methodologies. Based on the results, the unique combination of high surface area and porosity in the nanomesh was revealed with respect to the properties of over 70 porous metals reported in the literature. The electrochemical performance of the thin nanomesh was demonstrated during electrolytic hydrogen generation and compared to that of 300-times thicker commercial electrodes. To functionalize the nanomesh with a Li-ion cathode precursor, a new method was developed for fast coating complex 3D nanostructures with thin layers of MnO2. Through a combination of the new method and electrodeposition, the nanomeshes were conformally coated with nanofilms of MnO2 using industrially-relevant processes. With the help of thermodynamic simulations, a low temperature conversion was developed to transform the MnO2-coated nanomeshes into electroactive Li-ion cathodes. Finally, the energy and power density of the new nanomesh-base Li-ion electrodes were assessed and compared to these of the known 3D Li-ion cathodes.

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