리튬 이차 전지용 저마늄계 음극박막의 전기화학적 특성

Abstract

현재 전자산업과 정보통신산업의 급격한 발전으로 초소형 mobile IT기기들의 발전을 가져오면서 언제 어디서나 정보에 접근할 수 있는 계기를 마련해 주었다. 이러한 mobile 기기들은 휴대성과 이동성이 필수적이라 할 수 있으며 이에 소형화, 경량화 추세를 보이고 있다. 이에 부피가 작고 무게가 가벼우면서도 용량이 높은 에너지원의 개발이 시급하다고 할 수 있다. 현재 에너지원으로 사용되고 있는 리튬 이차전지에는 음극 재료로 카본재료가 사용되고 있다. 카본재료는 낮은 가격과 안정성이 우수하며 뛰어난 전기화학적 특성 등 많은 장점을 가지고 있어 널리 사용되고 있다. 하지만 이론적 용량이 372 mAh/g 이기 때문에 고용량화를 요구하는 현재의 추세에 한계를 드러내고 있다. 따라서 현재 여러 대안 물질이 연구가 진행되고 있으며 그 중 리튬과 합금화 반응을 이루는 소재들이 관심을 받고 있다. 이 소재들은 카본재료에 비해서 높은 에너지 밀도를 가지고 있어 고용량화에 적합한 물질이라 할 수 있다. 하지만 충전과 방전과정에서 리튬과 합금을 이룰 때 큰 부피 변화를 수반하여 전극의 전기화학적 특성이 급격히 열화되는 현상이 발생하기 때문에 리튬 이차 전지의 음극으로서 널리 사용되기 위해서는 이러한 열화 문제를 해결해야 한다. 따라서 이러한 열화문제를 해결하기 위하여 많은 연구가 진행되고 있으며, 특히 복합 소재를 이용한 연구가 많이 진행되고 있다. 본 연구에서는 합금계 중 이론적 한계 용량이 높은 비정질 저마늄을 주된 활물질로 이용하고, 비정질 저마늄이 가진 가장 큰 문제점인 부피 팽창에 대한 완충재로 카본을 선택하여 교대 증착을 통하여 전극의 특성을 향상시키고자 한다. 전극은 물리적 기상 증착법을 통하여 형성되었고, 저마늄과 카본을 각각 교대로 증착하여 다층구조(multilayer) 전극을 형성하여 물리적, 화학적 특성을 분석하였다. 이러한 연구를 통해 저마늄-카본 다층 구조 전극이 리튬 이차전지용 음극물질로써 사용 될 가능성을 모색하였다.; Lithium ion batteries have been used as the power source for portable electronic device such as cellular phone, MP3 player and digital camera. Graphitic carbon is currently used as anode materials and commercially-used graphite has theoretical lithium storage capacity of 372 mAhg^(-1) to form LiC_(6). Recently, lithium ion batteries with high energy densities and high rate capabilities are demanded due to the development of portable electric devices and hybrid electric cars. Thus lithium alloy-based anode materials have attracted great attention as an alternative material for the anode. Among lithium alloy anodes, Si have a high gravimetric capacity. However, the capacity of Silicon fades rapidly like other lithium alloy materials due to a significant volume change during the lithium alloying and dealloying processes. In this study, Germanium and Germanium/Carbon multilayer were deposited by sputtering. Thin films of amorphous germanium exhibited capacities of 1.7 Ah/g with no capacity loss over 62 cycles. The diffusivity of lithium in germanium may be over 400 times greater than that of lithium in silicon at room temperature through the empirical equation for the diffusion coefficient. In addition to the benefit from diffusivity of lithium, the Li-Ge system is a model alloy for investigating the properties of nanostructured electrodes of lithium with group IV elements due to minimal native oxide on germanium. Carbon was used to reduce the volume change of Germanium and to enhance electric conductivity. Germanium thin films were deposited by radio-frequency (rf) magnetron sputtering . Carbon thin films were deposited by direct-current (dc) magnetron sputtering. And Sequential deposited Ge/C multilayer anodes were fabricated by rf/dc magnetron sputtering using Ar as plasma gas. The electrochemical measurements were conducted with a typical coin-type cell. The deposited Ge, C and multilayer electrodes were identified as an amorphous Ge and C component by XRD and Raman analysis. The cycle performance of Ge electrode and multilayer electrode generally had a good capacity retention and coulombic efficiency. A Ge and Ge/C thin film was deposited on a electrolytic Cu foil to analyze electrochemical properties by using rf and dc magnetron sputtering, respectively, The morphology of the Cu foil has a rough surface and Ge and Ge/C thin film deposited on the RT Cu foil have also a rough morphology like Cu foil substrate. This morphology helps to improve the cyclability because surface of the Cu substrate increases the adhesion and decreases the electrical resistance between substrate and Ge electrode. The XRD pattern of the as-deposited Germanium thin film contains no diffractions except for those of the Cu substrate. Thus, the as-deposited Germanium thin film is amorphous. The electrochemically discharged electrode crystallizes to form a crystalline Li_(15)Ge₄ phase and a crystalline Li_(15)Ge₄ phase converts to amorphous phase after electrochemical delithiation. This result shows that the Germanium thin film electrode is reversible during the cycle. This XRD pattern also shows that specific capacity was lower than the theoretical capacity of 1600mAh/g for Germanium because Germanium thin film electrode doesn’t convert to Li_(22)Ge₄. However, the cyclability is improvement that the volume of Li_(15)Ge₄ phase is fewer than Li_(22)Ge₄ . In the first cycle of the Ge thin film electrodes, the discharge capacity was over 1000 mAhg-1. In the case of the 5-㎛ Ge thin film electrode, although the 1,3-㎛ Ge thin film showed the good capacity retention, the capacity retention after 50 cycles (R_(50/1)) was above 72 %. The Ge/C 180-multilayer (Ge:C=26nm:2nm) delivers an initial capacity of 860 mAhg^(-1). The capacity retention (R_(1/50)) of the Ge/C multilayer was 86 % although Ge thin film was 72 %. This can be explained as follows. The deterioration of electrochemical performance due to the increase in the thickness of the two-dimensional film is largely related to the increase of the volume change and internal resistance and residual stress in the film. It is guessed Carbon operated with the buffer layer of the volume expansion of the electrode. According to this work, Ge/C multilayer electrodes deposited on rough Cu were strong candidates as the anode of Li rechargeable batteries.