리튬 이차 전지용 합금계 음극 박막의 전기화학적 특성에 관한 연구

Abstract

이차전지는 휴대전화, 노트북, PDA 등과 같은 이동성 IT 제품의 보급이 확대되면서 리튬계 이차전지를 중심으로 소형 이차전지의 수요가 전세계적으로 급증하고 있으며, 소형 이차전지는 그 중에서도 IT기기의 소형화, 경량화, 고성능화 요구에 따라 리튬 이차전지가 시장을 주도하고 있다. 하지만 현재 상용화 된 리튬 이차전지는 그 용량이 한계에 이르렀다. 상용 리튬 이차전지의 경우 각각 LiCoO2와 graphite를 양극과 음극으로 하여 만들어지는데 음극인 graphite는 용량이 이론적 한계인 372 mAhg-1 에 다다르고 있다. 따라서 고용량화, 고밀도화를 요구하는 차세대 리튬 이차전지에는 적용되기 어렵다. Graphite의 대체 물질로 주목 받는 물질은 리튬과 합금화 하는 III족, IV족, 및 V족 원소(Al, Si, Ge, Sn, Pb, Sb, Bi 등)들이 있으며, 이들 원소들은 리튬과의 합금화 반응을 통해 높은 단위 질량당 용량을 나타내고 있다. 그러나 리튬과 합금화 반응 중 2~3배에 이르는 큰 부피 팽창을 수반하게 되고 계속적인 충?방전 중 부피 변화로 인한 스트레스로 본래의 용량을 잃고 특성이 급격히 저하되는 단점이 있다. 이러한 단점 극복하기 위해 Li+ 이온과 전자에 대한 전도성을 갖는 구조 안에 활물질을 담는 encapsulation 방법이나 이원계 합금 시스템에서 금속간 화합물을 형성 시키는 방법으로 기계적인 유연성을 갖게 하는 방법이 연구되고 있다. 본 연구의 목적은 리튬 이온 이차 전지의 차기 음극 물질로 연구되어 온 실리콘을 활물질로 선정하고 동시 증착법(Co-sputtering)으로 실리콘 전극 내에 실리콘과 비활물질과 금속간 화합물을 형성하여 충?방전 중 급격한 부피변화에 대한 스트레스를 완충 시켜줄 수 있는 전극 구조를 설계하고 이 전극의 전기 화학적 특성을 향상시키는데 있다. 리튬과 금속간 화합물을 형성하는 물질로는 실리콘과의 결합력, 전기전도도 및 녹는점등을 고려하여 몰리브덴과을 선택하였다. 선택된 물질을 실리콘과 함께 동시 증착하여 전극을 형성하였으며, 증착 시 실리콘 타겟의 인가된 전력은 고정 시키고 동시 증착 되는 물질에 인가된 전력은 세 단계로 나누어 증착하여 각기 다른 조성을 갖는 전극을 얻었다. 각 조성에 대한 물리적 특성을 분석하고 전기화학적 테스트를 실시한 후 충?방전 효율이 뛰어나고 우수한 용량 유지율을 보이는 전극을 찾아 내었다. 두께 증가에 따른 특성 악화를 억제하고 사이클 특성을 개선하기 위해 집전체인 전해동박의 표면 처리 및 증착 온도에 따른 특성 분석, 전해액과 정전류 테스트 조건에 따른 전극의 거동에 대해 실험을 진행하였다. 실험 결과, 약 6㎛두께의 Si-Mo 합금 전극의 경우 용량 특성 및 싸이클 특성을 종합해 볼 때 전 조성에서 안정적인 싸이클을 나타내었지만, 특히 SiMo0.19 전극은 첫 번째 방전용량이 2.3mAh/cm2이고 100번째 가역 용량이 1.9mAh/cm2으로 용량유지율이 83%으로 SiMo0.19 전극의 경우 차세대 전극으로서의 가능성을 보여주었다.; Li-ion secondary batteries are the main energy sources used in portable electronic devices such as laptop computers and cell phones. Carbonaceous materials, the most commercial anodes, have a very low Li atomic density at full Li capacity on the carbon intercalation compound (LiC6), resulting in a relatively low volumetric Li capacity. Nowadays, the most important issue in rechargeable Li-ion batteries is how long we can use the batteries without recharging. In other word, there is a growing demand for high specificenergy density, long-life, compact power sources for portable electronic devices. Lithium alloys (e.g., LixM (M = Sn, Si, Ge, Al, etc)) have attracted great attention as alternative anode materials for Li-ion batteries because of their high gravimetric capacities and high packing densities compared with generally used anode materials. Among Lithium alloys, silicon shows the highest gravimetric capacities (up to 4000mAhg-1 for the end of member Li21Si5). Although Silicon is the most noticeable candidate of next generation anodes, Silicon undergoes large volume change during lithium insertion and extraction. It results in pulverization of the Silicon and loss of electrical contact between the Silicon and the current collector during the lithiation and delithiation. Thus, its capacity on cycling fades remarkably rapid. In this work, we focused on multiphase electrode materials. Multiphase electrode materials composed of two metal compounds, in which only one component forms an alloy with lithium, exhibit better cycling performance than a single-phase host. Another inactive metal matrix helps reduce the mechanical disintegration of the multiphase electrode during cycling testing. We investigated the electrochemical properties of SiMo alloy electrodes in an attempt to overcome the problems of single-phase Si electrodes. We attempted to fabricate SiMo alloy electrodes by means of radio-frequency (rf) magnetron sputtering system. In order to examine the intrinsic properties of this electrode versus lithium electrode at 298K, silicon thin films was deposited by radio frequency magnetron sputtering system and characterized. The as-deposited silicon alloy films were amorphous. It was indicated by Xray diffraction analysis, Raman spectrum, and SAED pattern. To evaluate the electrochemical properties of electrodes, 1M LiPF6/EC:DEC as 1:1 in volume (or 1M LiPF6/EC:DMC as 1:1 in volume), and lithium metal was utilized as electrolyte and the counter electrode, respectively. All the three layers, including silicon electrode, separator and lithium metal, were stacked in a 2023 coin type cell in argon filled glove box. Unless stated elsewhere, cycling was carried out at a constant current density of 0.2 mAcm-1 to 1.6 mAcm- 1 and a voltage cutoff at 1.0/0.01 V versus Li/Li+. Discharge and charge of the cell refer, respectively, to lithium extraction from and insertion into active hosts. The electrode capacity was calculated according to the weight of active material. The alloy films induced huge stress by volume changes during cycling. After a few cycles, silicon films lost their own capacities, whereas Si-Mo alloy films maintained their capacities because the films could endure the stress in lithiation and delithiation. Capacities of over 1700 mAhg-1 have been measured with composition of SiMo0.06, SiMo0.19, SiMo0.79. Rough surface of the copper foil increases the adhesion force of deopsited film and the copper foil. From SEM observation, The surface morphology of the as-deposited film has also a rough morphology like its subtrate. The thickness of the as-deposted film is about 6㎛ for 4 hours of deposition corresponding to a growth rate of about 25nm/min. And, The SiMo electrodes had cracks along ravines because of volume changes after Li intercalation/deintercalation. Although cracks were observed, the cracked film still had good adhesion with the Cu foil and no pulverized particles were. And, each column was not peeled off from the copper foil. Those films had good cycle retention and high capacities in spite of increasing film thickness. Even the very small amounts molybdenum in Si films help the films endure the volume changes. Molybdenum makes Si-Mo bonding in films, respectively. These bonding makes inactive matrix partially shown by XPS spectra. These inactive sites reduce the mechanical stress during the cycles. Also, we attempted many experiments about improvement of the cycle performances. In this study, rf-sputtered Si-Mo alloy films showed excellent electrochemical properties. These films resemble closely to next generation anode for LIBs. The initial discharge capacity of 3.1 mAhcm-2 for Si electrode can be obtained. However, Reversible capacity of a Si electrode is degraded continuously with increasing cycle number up to 50 cycles. The Si anode has only about 1.2 mAhcm-2 capacity after cycle test. In the case of the SiMo0.19 electrode, the initial discharge capacity of 2.3 mAhcm-2 and, its capacity could be maintained over 1.9 mAhcm-2 for 100 cycles. In accordance with results, SiMo0.19 electrode has much higher Coulombic efficiency and better cycle retention than the Si film. Consequently, that mainly two major parameters seem to contribute to the electric performance of Si based alloy electrodes. First, the dispersed Mo in the films is essential to increase the cycle performance of electrode because Mo plays an important role as an electrical connector. Secondary, mechanical buffers against electrode pulverization due to large volume expansion contribute to maintain good cycleability. In this study, co-sputtered Si Mo alloy films showed excellent electrochemical performance. These films resemble closely next-generation anodes for Lithium rechargeable batteries.