Functionalization of nanostructured silicon electrodes by nano-metric dielectric layers deposited by ALD: an effective and adaptable protection for ultra-stable micro-supercapacitors in aqueous media

In recent years, significant attention has been paid to the development of micro-devices as innovative energy storage solutions. For instance micro-sensor networks such as sensors actuators or implantable medical devices require power densities and cyclability that are several orders of magnitude higher than those of conventional Lithium-Ion batteries. For such applications, Micro-supercapacitors (MSCs), a developing novel class of micro/nanoscale power source are rising alternatives, and their integration 'on-chip' could allow significant innovations to emerge.1 Therefore, a great deal of attention has been focused on MSCs, for which large series of nanostructured active materials have been developed. Following this trend, we have demonstrated through comprehensive investigations the interest of silicon nanostructures grown by Chemical Vapor Deposition (CVD) as electrodes materials for MSCs using ionic liquid electrolytes. The fine morphological tuning of the nanostructure allowed by the bottom-up approach enables specific designs of electrode architectures, with a considerable leeway compared to other techniques. Such latitude allows optimising porosity and ionic and electronic pathways while keeping robust mechanical and thermal performances, depending on the target application. Nanostructures such as SiNWs and SiNTrs have displayed excellent electrochemical performances being stable over more than 1 million cycles of galvanostatic charge/discharge under a 4 V wide electrochemical windows in EMITFSI ionic liquid, with large power densities of 10 mW.cm^-2 and good capacitance values of 0.5 mF.cm^-2 at high current density of 0.5 mA.cm^-2. However a major silicon weakness which was still hindering its use with aqueous electrolytes is the native uncontrolled growth of silica when subjected to ambient atmosphere. In this thesis we have developed and investigated a highly conformal passivation coating of a nanometric high-k dielectric layer of Al₂O₃ based on the rising Atomic Layer Deposition (ALD) technique. ALD has proven to allow a nanometric thickness control of the deposited layer while being highly conformal and covering. Moreover, as discusses in this manuscript the protective alumina layer enables the use of aqueous electrolytes for nanostructured Si based MSCs, which significantly increases the specific power of the devices up to 200 mW.cm^-2 at 0.5 mA.cm^-2 while keeping the capacitance performances at 0.5 mF.cm^-2. Furthermore the system is remarkably able to retain 99% of its initial capacitance after 2 billion galvanostatic charge/discharge cycles at high current density of 0.5 mA.cm^-2 in an aqueous electrolyte of Na₂SO₄. In this manuscript we have also performed a comprehensive electrical study of the alumina/silicon interface which demonstrates that such nanometric layer of dielectric is not fully resistive as assumed by most the electrochemist but rather able to conduct electricity through tunnelling effect dependant on the thickness. Eventually we have used this conductive and protective layer to strengthen a pseudocapacitive conductive polymer which is electrochemically active in aqueous electrolytes. A promising composite material is described and realised by a simple drop-cast method of a PEDOT-PSS film onto silicon nanowires. The device exhibited promising performances with a specific energy of 2 Wh.kg^-1 and a power density of 300 W.kg-1 at a current density of 1 A.g^-1. The MSCs was able to retain 80% its initial capacitance after 500,000 galvanostatic charge-discharge cycles at 0.5 A.g^-1. The last part of the thesis describes the collaboration sets with a Norwegian company, ELKEM Silicon Materials, which has lead to the rethinking of our silicon nanostructure growing process and the large increase of the production capacity. (author)