Recently, lithium ion secondary battery has been widely used as energy storage device for portable electronic goods, such as cellular phone, notebook computer, camcorder, and so on. As anode materials for lithium ion cells, various carbonaceous materials have been widely commercialized. In addition to carbonaceous materials, attention has been focused on alloys because of their high specific gravitational and volumetric capacity (e.g. Li/Sn: 994mAh/g, Li/Sb: 660mAh/g, Li/graphite: 372mAh/g). Understanding reaction mechanism of metal and alloy electrodes with Li is very important for development of an effective anode material. Many research group studied Li reaction mechanisms of alloy electrodes by electrochemical experiments, but the microstructure of lithiated metal and alloy electrodes was not identified clearly. In this study, we have identified the intermediate phase of metal and alloy electrode during lithiation reaction, especially Sn-base alloy, and specified modification of Li reaction mechanism of them.
Microstructure of nanometer-scale tin powder synthesized by WEE (wire electric explosion) method was examined by transmission electron microscopy (TEM) at various Li insertion states, and electrochemical properties of the tin powder electrode were characterized by galvanostatic charge/discharge experiment. It is shown that several Li/Sn inter-metallic compounds formed during lithium insertion were Lix′Sn, $L_{13}Sn_5$ and $Li_7Sn_2$ by electron diffraction patterns and high-resolution TEM images. Thus, we concluded that fully lithiated Li-Sn alloy produced electrochemically at room temperature was $Li_{3.5}Sn$ ($Li_7Ge_2 structure$) instead of $Li_{4.4}Sn$. The passivation layer (or solid electrolyte interface, SEI) on the surface of particles cycled in an organic electrolyte cell was characterized as $Li_2CO_3$ and $ROCO_2Li$ by FT-IR spectroscopy. The major part of the passivation layer was amorphous, while the minor part of it showed poor crystallinity.
Electrochemical lithiation reactions of a thin-film $Cu_6Sn_5$ alloy electrode were studied in an organic carbonate electrolyte containing $LiPF_6$ salt using conventional charge-discharge techniques and analyzing the reaction products by TEM. A partially lithiated phase, $Li_xCu_6Sn_5$ (where x < 3), that appears to be a solid solution of Li having the basic crystal structure of $Cu_6Sn_5$, was identified by TEM in a $Cu_6Sn_5$ -alloy electrode discharged at 0.5 V. At a further cathodic potential of 0.3 V the formation of $Li_2CuSn$ was observed in agreement with earlier reports. At the potential close to that of the Li-metal plating (~ 0.01 V discharge cut-off) $Li_7Sn_2$ phase was identified as the main composition without observation of a further lithiated phase beyond $Li_7Sn_2$. The fully lithiated product was the same structure formula as those from Sn electrode. Slightly de-lithiated structure of $Li_7Sn_2$ showed many defects in their structure, such as APB, stacking faults. This modified $Li_7Sn_2$ is intermediate phase between $Li_7Sn_2$ and $Li_2CuSn$ during Li reaction of $Cu_6Sn_5$ electrode.
Intermediate phases formed by electrochemical lithiation of CuTiSn and CuZrSn alloy anode was examined by TEM. Their electrochemical behavior was studied by charge/discharge experiments. The CuTiSn alloy was made of nanometer-sized $Cu_6Sn_5$ and $Ti_6Sn_5$ grains. $Cu_6Sn_5$ grains act as reaction host and $Ti_6Sn_5$ act as inactive matrix during electrochemical lithiation and de-lithiation reaction. CuZrSn was also made of $Cu_6Sn_5$ and Cu-Zr alloy ($Cu10Zr_7$, $Cu_{1.74}Zr_{2.26}$ and $CuZr_2$) grains. $Cu_6Sn_5$ and Cu-Zr alloy play similar roles to the case of CuTiSn alloy during electrochemical Li reaction. But the lithiation reaction of $Cu_6Sn_5$ in CuTiSn and CuZrSn alloy electrode was different from that of $Cu_6Sn_5$ electrode. $Li_2CuSn$ phase that was observed from lithiation of $Cu_6Sn_5$ was not observed from CuTiSn and CuZrSn alloy electrodes, whereas the lithiated product of $Cu_6Sn_5$ in CuTiSn and CuZrSn alloy electrode was amorphous. Inactive matrix, $Ti_6Sn_5$ and Cu-Zr ($Cu_10Zr_7$, $Cu_{1.74}Zr_{2.26}$ and $CuZr_2$) act role like as a frame in network to support and hold $Cu_6Sn_5$ tightly. Thus, this composite structure hindered formation of crystalline $Li_2CuSn$ structure. The CuTiSn composite electrode showed large volumetric Li reaction capacity than the graphite material used in commercial electrodes. Charge/discharge capacity of CuTiSn electrode decreased sharply in the first a few cycles, but further capacity decrease during subsequent 100 cycles was minimal. The atomic composition and morphology of CuTiSn alloy particles in electrode was not changed after the 100-cycle experiment. This means that the pulverization of alloy electrode with cycling, that was reported previously, did not occurred.
Impedance spectroscopic analysis of Sn, $Cu_6Sn_5$, CuTiSn and CuZrSn electrode was carried out in the frequency range, 5 mHz ~ 100 kHz. Results of the impedance spectroscopic analysis agreed well with calculated values using a generic power model - a porous Li ion battery electrode model. Rate determining processes during electrochemical Li reaction with metal and alloy electrode were the diffusion of Li into electrode and the second phase formation reaction. Polarization resistance induced by Li diffusion was increased as Li inserted into electrode in all kind of electrode, but resistance induced by the second phase formation was various with the type of electrode. In case of Sn and $Cu_6Sn_5$ electrode, resistance induced by the second phase formation increased as Li inserted, but the result was vice versa, in case of CuTiSn and CuZrSn electrode. This result agreed well with TEM results that showed multi-step Li-Sn alloy formations in the Sn and $Cu_6Sn_5$ electrode and a single step Li alloy formation in CuTiSn and CuZrSn electrodes.
최근 리튬 이온 전지는 휴대폰, 켐코더, 노트북 등의 휴대형 소형 가전제품의 에너지 원으로 널리 사용되어지고 있다. 리튬 이온 전지의 애노드 재료로는 탄소재료가 널리 상용화되고 있지만 보다 큰 리튬 저장 능력을 보여주는 금속계 전극에 대한 연구도 활발하게 이루어 지고 있다. 보다 큰 리튬 저장 능력을 갖는 전극 재료의 개발을 위해서는 전극과 리튬 이온의 반응 기구를 명확하게 규명하는 것이 필요하다. 때문에 많은 연구자 그룹에서 리튬 반응 기구에 대한 연구를 하였다. 그러나 리튬 반응에 따른 중간상의 생성과 미세구조의 변화는 아직 명확하게 밝혀지지 않은 부분이 많다.
본 연구에서는 금속계 전극 재료의 하나인 Sn 합금의 전기화학 특성 분석과 함께 투과전자현미경 분석을 실시하여 Sn 합금 전극의 리튬 반응기구와 미세구조의 변화를 규명하였다. 그리고 리튬 이온 전지의 고성능 음극 재료로서의 가능성을 고찰하였다.
WEE(wire electric explosion) 방법으로 제조한 나노미터 크기의 Sn 분말 입자로 제조한 전극과 박막 형태의 $Cu_6Sn_5$ 전극이 리튬 이온과 반응에서 보여주는 전기화학적 특성과 미세구조의 변화를 고찰하였다. 상온에서 Sn 전극이 리튬과 반응하여 형성되는 중간상의 조성은 Lix’Sn (x’=0.714), $Li_{13}Sn_5$, $Li_7Sn_2$ 임을 확인할 수 있었다. 아울러 리튬과 Sn이 전기화학적으로 상온에서 형성하는 합금의 형태는 $Li_{4.4}Sn$ 조성의 결정구조가 아닌 $Li_7Ge_2$ 구조를 갖는 $Li_7Sn_2$ 임을 규명하였다. 전기화학 반응과정에서 $Cu_6Sn_5$ 전극으로 리튬 이온이 소량 insertion 되면, $Cu_6Sn_5$ 격자의 공극(vacancy)으로 들어가 $LixCu_6Sn_5$ 를 형성하고, 이후 리튬 이온의 insertion 양이 증가함에 따라서 $Li_2CuSn$ 구조로 변화됨을 보였다. 금속 리튬의 석출 전위에 가까운 0.01 V 까지 discharge된 $Cu_6Sn_5$ 전극은 $Li_7Sn_2$ 합금을 형성하였으며, Sn 분말 전극에서와 같이 상온에서 리튬 이온과 반응하여 형성하는 합금의 형태는 $Li_{4.4}Sn$ 조성의 결정구조가 아닌 $Li_7Sn_2$ 임을 확인하였다.
리튬과 반응하는 합금의 active matrix와 반응을 하지않는 inactive matrix의 복합구조를 갖는 CuTiSn, CuZrSn 합금 전극은 리튬 이온과의 전기화학 반응과정에서 상기의 금속 전극과는 달리 결정화된 중간상을 형성하지 못하고 비정질 형태의 리튬 합금을 형성하였다. 그리고 이러한 비정질 상의 형성은 inactive matrix에 의하여 견고하게 고정되고 부피팽창이 억제되어 나타나는 결과이다. 사이클 특성 실험에서 CuTiSn, CuZrSn 합금 전극은 단일 금속전극에 비하여 개선된 수명특성을 보여주었고, 100 사이클 이후에도 탄소전극에 비하여 높은 체적 에너지 밀도를 보였다.
Impedance spectroscopy 분석을 통하여 금속계 합금 전극의 리튬 이온과의 반응은 SEI(solid electrolyte interface) 형성, 전하 이동 (charge transfer) 반응, 리튬 이온의 확산 및 이차상 형성 (secondary phase formation)의 단계를 거쳐서 일어남을 규명하였고, 합금 전극에서의 율속 반응은 리튬 이온의 확산과 이차상의 형성 반응임을 규명하였다.
