Silicon Microchannel Plate Based Three-dimensional Energy Storage Devices

Micro Electro Mechanical Systems (MEMS) technology has been highly developed in recent years. Silicon microchannel plate (Si-MCP), fabricated by MEMS technique, is a type of unique three-dimensional (3D) structural material which plays important roles in a wide range of application areas and is drawing much attention nowadays. In this paper, a series of exploration and researches on energy storage materials and devices based on Si-MCPs were carried out and following results were obtained:1. Fabricated Si-MCPs with large aspect ratios and Si-MCP/Ni structures for current collectorsThe chemical etching activity of silicon in hydrofluoric acid was discussed first. The formation mechanism of Si-MCP was studied based on that of porous silicon. The detailed fabrication process of Si-MCP, which includes wafer cleaning, oxidation, lithography, formation of the induction pits and electrochemical etching, was exhibited in this part. The relevant parameters were studied to show their influences on the formation of Si-MCP. Particularly, the electrochemical etching process was carefully investigated, and several factors such as wafer properties, etching solution, bias, current density, impulse current, illumination intensity and system temperature were taken into consideration. A typical as-prepared Si-MCP consists of microchannel arrays. A single microchannel has a side length of5μm and a depth of round100μm giving rise to a large aspect ratio of40. The principle of chemical deposition of nickel was introduced, and the method was carried out to fabricate3D current collectors with Si-MCP/Ni structures. The fabrication process of nickel layer on Si-MCP was detailed. SEM was used to observe the morphology of the Si-MCP/Ni structure, and the results show that the3D nickel layer was smooth and continuous with no material blocking the microchannels.2. Developed Si-MCP/Ni/MoS2and Si-MCP/Ni structures for anode materials for lithium ion batteriesThe Si-MCP/Ni/MoS2structure was fabricated by electro depositing MoS2thin film on Si-MCP/Ni structure. The SEM images show that the3D MoS2thin film was uniform and smooth, and the XRD patterns show that the as prepared MoS2was amorphous. The Si-MCP/Ni/MoS2structure and lithium foil were used respectively as anode and cathode for a full battery. Detailed package process of the button cell was introduced. The formation process of SEI membrane could be studied from the first discharge curve. The cell charge/discharge performances at different rates were investigated. A specific capacity of2.5mAh·cm-2, which is much larger than planar thin film silicon anode, was obtained at0.1C. At1C, a specific capacity of2.5mAh-cm-2was able to be maintained, while at10C the capacitance loss seemed to be significant. The cell showed high coulomb efficiency at each rate indicating excellent charge/discharge reversibility.The Si-MCP/Ni structure was fabricated through chemical deposition of nickel layer on Si-MCP. The reaction time was adjusted to ensure that the nickel grains were fine and the material was suitable for anode of lithium ion batteries. The first discharge curve of Si-MCP/Ni structure is quite similar with that of pure silicon anode material. They have the same discharge voltage values and long discharge platforms, except the voltage values which correspond to the formation of SEI membrane can not be found in first discharge curve of Si-MCP/Ni structure, indicating that the nickel layer prevented the formation process which usually occurs at the interface between silicon and electrolyte. The failure of formation of SEI membrane enabled the increase of coulomb efficiency of first cycle. The first discharge and charge specific capacity of Si-MCP/Ni structure reached5.3mAh·cm-2and5.1mAh·cm-2respectively. The cell became stable with a specific capacity of1.2mAh·cm-2around100cycles. The results indicated that although the nickel layer on the Si-MCP was not able to prevent the volume expansion of Si-MCP, the failure of the active material was restrained and the conductivity of the active material was enhanced, which are mainly responsible for obtainment of considerable specific capacity after many cycles. The coulomb efficiencies were generally above95%demonstrating excellent charge/discharge reversibility.3. Developed a Si-MCP/NiSi2/BST structure for3D double electric layer supercapacitors3D BST thin film was firstly prepared on the Si-MCP/Ni structure by modified sol-gel method which combined with vacuum and spin-coating techniques. The nickel layer reacted with silicon during the thermal treatment of the BST precursor and the NiSi2layer was thus formed. Because of its low resistivity the3D NiSi2layer could serve as a current collector. The SEM images show that the thickness of as-prepared3D BST thin film was about200nm, and the XRD patterns indicate typical perovskite structure of the BST. The cyclic voltammetry results prove that the sample worked under the double electric layer mechanism and exhibited good electrochemical activity at large rate. A specific capacitance of near800F-g"1was yielded in first charge/discharge test. After700cycles, the specific capacitance loss was no more than7%. SEM was carried out to observe the surface morphology of the sample after cycling test. Cracks were found in the BST thin film, and the electric drain caused by exposure of the current collector and loss of active materials were believed to be responsible for the capacitance decrease. To prove that the large capacitance yield was mainly due to the3D structure, a planar capacitor with Si/Ni/BST structure was also fabricated and evaluated. The planar structure yielded much smaller specific capacitance under same fabrication and test conditions than the Si-MCP based one because of limited surface area and large weight of planar sandwiched structure. Trough the comparison it is clearly showed that the3D supercapacitor with Si-MCP/NiSi2/BST structure owns apparent advantages in terms of specific capacitance, weight and volume.4. Developed Si-MCP/Ni/Ni(OH)2and Si-MCP/Ni/Co(OH)2structures for3D faradic supercapacitorsIn this part, chemical liquid deposition was carried out to fabricate nano-sized Ni(OH)2or Co(OH)2on the surface of3D current collector. The as-prepared supercapacitors have Si-MCP/Ni/Ni(OH)2or Si-MCP/Ni/Co(OH)2structures. SEM was carried out to study the morphology of the Ni(OH)2and Co(OH)2nano materials, and the images demonstrate that on the surface of side walls of the microchannels, the Ni(OH)2and Co(OH)2both consist of nano-flakes. At the out surface of the microchannels, Ni(OH)2nano-flakes and Co(OH)2nano-rods were found. The mechanism of the chemical liquid deposition, the location of reaction and the concentration of the reaction ions were all taken into consideration to investigate the difference of the morphology of Ni(OH)2and Co(OH)2. The XRD patterns indicate that the Ni(OH)2and Co(OH)2are both a-phase crystals. The reason of the formation of the a-phase crystals was discussed. In the cyclic voltammetry test, both the two samples showed faradic electrochemical activities. The highest specific capacitance of3.75F·cm-2and1.46F·cm-2were obtained for Si-MCP/Ni/Ni(OH)2and Si-MCP/Ni/Co(OH)2structure respectively in the cyclic voltammetry and potentiometry tests at different scan rates and discharge current densities. In the SEM images of the two samples after cycling test, no obvious active materials loss was observed giving rise to a slight irreversible capacitance loss. The capacitance loss of the two samples were both less than20%after1000cycles, and the samples became quite stable after2000cycles exhibiting their excellent cycling performances. A theoretical study on the surface area of the3D structure was carried out to prove that the large surface area of the active materials was the main reason of the large specific capacitance yield. The potential of the Si-MCP based structure for device miniaturization was also demonstrated.In summary, first, this dissertation used MEMS fabrication technique to fabricated Si-MCPs from pure silicon wafers. The Si-MCP consists of many ordered microchannels with large aspect ratio and large surface areas. The Si-MCP/Ni structure was formed by depositing a nickel layer on the Si-MCP by chemical deposition method, and it could be used as a unique current collector and template for further device construction. Second,3D MoS2thin film was fabricated on the Si-MCP/Ni structure to form the Si-MCP/Ni/MoS2structure, which could be used as a novel anode material for lithium ion batteries. The Si-MCP/Ni/MoS2structure yielded large specific capacitance and showed excellent cycling performance and reversibility. The Si-MCP/Ni structure prepared by chemical depositing nickel layer on Si-MCP and adjusting the deposition parameter was also found to be suitable for anode material for lithium ion batteries. The nickel layer on top of the Si-MCP prohibited the failure of the active materials after cycling and enhanced the conductivity of the fractured Si-MCP, thus the cycling performance of the Si-MCP/Ni structure was much better than planer silicon thin film as anode material and a relatively large specific capacitance was maintained. Third, a3D BST thin film was produced by modified sol-gel method on the Si-MCP/Ni structure to fabricate a double electric layer supercapacitor with Si-MCP/NiSi2/BST structure. According to the testing results, the large surface area of the active material was believed to be mainly responsible for large specific capacitance and excellent cycling property. Finally, chemical liquid deposition was carried out to prepare Ni(OH)2and Co(OH)2nano-crystals on Si-MCP/Ni structure to make Si-MCP/Ni/Ni(OH)2and Si-MCP/Ni/Co(OH)2faradic supercapacitors. The supercapacitors were found to be excellent at cycling performances with very large specific capacitances. Besides, the volume of the Si-MCP/Ni/Ni(OH)2and Si-MCP/Ni/Co(OH)2structures are quite small and thus making them ideal for miniature3D supercapacitors.The research topics in this dissertation are based on Si-MCP and combined with technology of MEMS, semiconductor materials, novel micro-structures and energy-storage devices, providing a brand new idea and research routine for development of novel energy-storage devices. The fabrication processes of Si-MCP in this dissertation are generally compatible with the current IC fabrication techniques, so it is expected that the fabrication of Si-MCP can reach the stage of mass-production. This dissertation will also lend an impetus to reciprocal development among energy, MEMS and semiconductor areas, which will definitely promote the economy and benefits the society greatly in the future.