Density modulated multilayer silicon thin films as li-ion battery anodes
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DENSITY MODULATED MULTILAYER SILICON THIN FILMS AS LI-ION BATTERY ANODES 1
M. Taha Demirkan1, Xin Li2, Bingqing Wei2, Tansel Karabacak1
Department of Applied Science, University of Arkansas at Little Rock, Little Rock, AR 72204 2 Department of Mechanical Engineering, University of Delaware, Newark, DE 19716
ABSTRACT In this work, we demonstrate a new density modulated multilayered silicon thin film anode approach that can provide a robust high capacity electrode for Li-ion batteries. These films have the ability to tolerate large volume changes due to their controlled microstructure. Silicon films with alternating layers of high/low material density were deposited using a DC sputtering system. Density of the individual layers was controlled by simply changing the working gas pressure during sputtering. Samples of Si films having thicknesses of 460 nm with different number of high/low density layers have been deposited on Cu current collectors. The electrochemical performance of the multilayered anode material was evaluated using a galvanostatic battery testing system at C/10 rate. After reaching a stabilized phase the battery cell showed a high coulombic efficiency of 96% to 99% and reversible specific capacity of 666 mAh g-1 (after 100 cycles). Low-density layers are believed to be acting as compliant sheets during volume expansion making the films more durable compared to conventional Si film anodes. The results indicate that density modulated multilayer Si thin films can be used to improve the mechanical properties of Li-ion battery anodes leading to high reversible capacity values even after high number of cycles. 1. INTRODUCTION Demand for portable electronics and new technologies such as electrical cars has increased, and therefore, new necessities of finding more stable and long-life secondary batteries have been brought up in the last years. Among several battery types, lithium-ion batteries has superior electrochemical characteristics such as high energy density, high voltage and low selfdischarge.1 Although important developments have been made recently, current commercial lithium-ion battery systems don’t meet the requirements of new technologies. For instance, graphite as an anode material has the theoretical capacity value of 372 mAh g-1. Even though more than 90% of its theoretical value is achieved in current batteries, this value is not sufficient, and replacement of carbon with new materials in anode is necessary.2 Silicon (Si) has become one of the most prominent candidates for lithium-ion battery anodes since it was found to be a reversible host material for lithium intercalation.3 Structural properties of Li-Si alloying has been studied, and it was shown that each atom of silicon can host 4.4 atoms of lithium in the structure of Li22Si5 which results in a very high theoretical capacity approaching ~4200 mAh/g.4 Therefore, silicon’s high specific capacity makes it very advantageous among the other candidates for anode materials. Moreover, being the second most abundant element on earth, silicon
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