Development of hybrid Si/graphene/CNT nanocomposite lithium ion battery anodes with yolk-shell morphology

Lithium ion batteries are the most commonly used electrochemical energy storage systems in today's market. Graphite is used as anode material in current lithium ion batteries, can store 372 mAh of energy per gram, which is an obstacle to the development of higher energy density lithium ion batteries. Silicon is considered as the best alternative anode material in order to produce high energy density lithium ion batteries with its theoretical storage capacity of 4200 mAh energy per gram. Despite of its high energy storage capability, severe volumetric variations during electrochemical processes avoid silicon to be exploited as anode practically. Volumetric variations, approximately at the rate of 300% cause mechanic failure of silicon anodes and leads cells to become dysfunctional at very low cycle numbers. In this thesis study, it is aimed develop composite silicon lithium ion battery anodes, which possess high cycle life and high specific capacity by means of elimination of aforementioned problems of silicon anodes. For that purpose at first stage silicon particles have been modified as yolk-shell morphology particles that would prevent mechanical damages induced by volumetric expansions. Yolk-shell morphology contains several advantages with amorphous carbon shell and trapped silicon particle inside which provides sufficient free space to accommodate volumetric expansions. At subsequent stage, in order to obtain free-standing electrodes yolk-shell particles have been decorated into skeleton structures which are consist of entangled graphene and carbon nanotubes. In order to determine optimum anode composition which leads to maximum electrode performance, different anodes have been produced with certain amounts of graphene, carbon nano tube and yolk-shell silicon particles. Accordingly, 952 mAhg-1 of specific capacity has been exhibited after 500 cycles by the anode with optimum composition. Detailed phase analysis, morphology and electrochemical tests have been conducted to be able to understand and emphasize the correlation between anode composition and electrode performance.