hydrofluoric acid (HF) at different current densities and for different durations. ...... Effect of Fluoroethylene Carbonate (FEC) on the Performance and Surface.
Porous Silicon as Anode Material for Li-ion Batteries Structure and Performance
Asbjørn Ulvestad
Materials Science and Engineering Submission date: June 2013 Supervisor: Volodymyr Yartys, IMTE Co-supervisor: Jan Ketil Solberg, IMT Martin Kirkengen, Institutt for Energiteknikk Norwegian University of Science and Technology Department of Materials Science and Engineering
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Abstract Silicon has proven to have a great potential as anode material in lithium-ion batteries due to its high theoretical electrochemical capacity. However, silicon anodes deteriorate quickly during cyclic charging and discharging, rendering them useless in only a few cycles. This has been attributed to the stresses induced by the large volume change of the material during cycling. By using finely structured silicon, these stresses can be effectively reduced, in what is aptly called dimensional stabilization. Numerous attempts have been made to exploit this effect, e.g. by using nanoparticles, nanorods and nanowires, with a varying degree of success. This study is an investigation into the use of porous silicon for anodes with reduced level of stress. The work consists of two main phases; the formation and characterization of porous silicon, and the testing of the resulting porous structures in electrochemical cells. In addition, an investigation into the use of prelithiated silicon as base material for composite electrodes has been conducted. Porous silicon was obtained by electrochemical etching of n- and p-type silicon wafers in hydrofluoric acid (HF) at different current densities and for different durations. The resulting material was analyzed by SEM to characterize its morphology and pore size. One n-type and one p-type structure were selected for electrochemical testing, involving cyclic charge and discharge in half cells with lithium metal as a counter electrode. The chosen samples were tested both asetched and with a carbon coating made by a pyrolization procedure with polyacrylonitrile (PAN) as precursor. In a different part of the work, composite electrodes were made from an alloy consisting of the intermetallic phase Li7Si3, carbon black and PAN. These were tested using the same conditions as the porous electrodes. From the SEM analysis, it was determined that porous silicon structures were indeed formed on the wafer surfaces. The p-type wafers are believed to form structures with pores
