Recently, lithium ion batteries have attracted considerable attention due to increasing demands for portable electronic devices. Graphite has been extensively used as the commercial anode material for Li-ion secondary batteries due to its excellent cycling behavior upon repeated charge and discharge cycles. However, the theoretical capacity of graphite is limited to 372 mAh/g. Therefore, many new anode materials with a high specific capacity have been investigated. Among these new anode materials, especially Si has been reported to be one of the most promising material which can be substituted for graphite because of its much higher theoretical capacity [~4200 mAh/g]. Unfortunately, large volume changes of Si occur upon alloying and de-alloying processes which lead to a rapid capacity fade during cycles. Therefore, this work was focused on the improvement of the electrochemical properties of the Si-based material.
The Si-Ni-Carbon composite was prepared by two-step high energy ball-milling processes. First, the Si-Ni composite was made by high energy ball-milling process. When the ball-milling time and the ratio of Si and Ni were 4 hours and 4 : 1 (weight ratio), respectively, the Si-Ni composite showed the best performance as an anode materials. The obtained Si-Ni composite has a mixture structure, where Ni is homogeneously dispersed in Si. Also, NiSi or NiSi2 phases were not formed by high energy ball-milling process in the Si-Ni-C composite. Next, the Si-Ni-Carbon composite was formed by high energy ball-milling process for 30 minutes with the ratio of Si-Ni composite and Carbon as 1 : 1 (weight ratio). The Si-Ni-Carbon composite has a core-shell structure where Si-Ni composite is homogeneously covered with Carbon. Besides SiC phase was not formed in the Si-Ni-Carbon composite.
The charge-discharge test of the Si-Ni-Carbon composite anode material showed the drastically improved cycle life as well as high reversible capacity. The charge-discharge curve of the 1st cycle has a different behavior from the other cycles. XRD analyses were performed to know the reasons for different charge-discharge behavior of the Si-Ni-Carbon composite during 1st cycle. During 1st discharge, the peak of the Carbon did not change while the intensity of Si peak decreased and during 1st charge, the intensity of Si peak did not recover owing to the amorphization of Si. From XRD analyses, it was found that crystalline Si transforms into amorphous Si completely during 1st discharge and only Si reacts with Li ion in the Si-Ni-Carbon composite.
In case of the Si-Carbon composite, the capacity was ~680 mAh/g [Si utilization : 32%] with a good cycle life, but the Si utilization was very low. In case of the Si/Ni alloy-Carbon composite, its capacity was ~780 mAh/g [Si utilization : 54%] with a good cycle life, but inactive secondary phases of Si and Ni such as NiSi, NiSi2 decreased the useful portion of Si. On the other hand, the Si-Ni-Carbon composite showed much improved capacity. The capacity of the Si-Ni-Carbon composite was ~960 mAh/g [Si utilization : 58% ] with a good cycle life. Further, the electrochemical properties of the Si-Ni-Carbon composite were greatly improved.
BJH, FTIR, HRTEM, XRD and resistivity analyses were performed to clarify the reasons why the Si-Ni-Carbon composite showed good electrochemical properties. BJH analyses showed that the Si-Ni-Carbon composite has large amount of pores formed by high energy ball-milling. Through FT-IR and HR-TEM analyses, the disordered carbon layer attributed to Si-C bonding was observed in the Si-Ni-Carbon composite very similar to Si-Carbon composite. Also, It was found that dispersed Ni increased electronic conductivity of the Si-Ni-Carbon composite from resistivity analyses and inert secondary phases of Si and Ni were prevented effectively by high energy ball-milling method.
In conclusion, Si-C bonding, pores and dispersed Ni in the Si-Ni-Carbon composite accommodate the volume change of Si during cycling efficiently. Therefore, the cycle life of the Si-Ni-Carbon composite was much improved. In addition, the Si-Ni-Carbon composite showed much high Si utilization up to 58 % of its theoretical capacity because dispersed Ni induced fast Li ion transport property and increased electronic conductivity of the Si-Ni-Carbon composite. Also, Si-C bonding and dispersed Ni improved electronic contact between active materials. Moreover, inactive secondary phases were prevented effectively by high energy ball-milling process.
최근 첨단 전자기기의 소형화, 경량화 추세에 따라 그 전력원으로 리튬 이온 이차전지에 대한 관심이 증대되고 있다. 현재, 리튬 이차전지의 상용 음극재료로 사용되고 있는 탄소계 물질은 전극수명특성이 매우 우수해 널리 사용되고 있지만 이론용량이 372 mAh/g에 불과하다. 따라서 새로운 고용량 음극재료로서 Si합금에 대한 연구가 활발히 진행되고 있다. Si은 이론용량이 4200 mAh/g으로 매우 우수하지만 충/방전 과정에서 400%에 이르는 부피팽창에 의해 급격한 용량의 퇴화가 일어나는 단점을 가지고 있다.
이러한 문제를 해결하기 위하여 본 연구에서는 2단계의 고에너지볼밀링 과정을 통해 Si-Ni-C 복합물을 제조하였다. Si-Ni-C 복합물은 Si과 Ni을 무게비 4:1로 칭량하여 4시간의 밀링과정을 거친 후 다시 C과 1:1의 비율로 30분간 밀링을 통해서 제조되었으며 Si내에 Ni가 균일하게 분포되어있고 표면에 조각형태의 C이 붙어있는 중심-외곽 구조를 가지고 있음을 알 수 있었다.
Si-Ni-C을 활물질로 전극을 제조한 후 충/방전 실험을 실시한 결과 960 mAh/g이라는 향상된 용량에서도 우수한 전극 수명특성을 보였다. Li 이온과의 반응기구를 살펴보고자 XRD, CV 분석을 실시한 결과 Si-Ni-C 복합물은 첫 충/방전과정 동안 Si은 모두 비정질화되며 C은 Li 이온과 반응하지 않고 단지 Si만 반응에 기여함을 알 수 있었다. 680 mAh/g의 용량을 갖는 Si-C 복합물과 780 mAh/g의 용량을 보인 Si/Ni합금-C 복합물에 비해 향상된 전기화학적 특성을 보이는 원인을 살펴보고자 EDS 분석과 전기전도도측정을 실시한 결과 밀링과정 동안 Si과 Ni 사이의 불활성 이차상 형성 제어와 Si내부의 Ni에 의한 전기전도도향상이 우수한 용량 특성의 원인임을 확인 할 수 있었다. 한편, BJH 분석을 통해 밀링과정에서 3nm의 다량의 기공이 형성되었다. 이 기공들이 Si의 부피 팽창을 수용해주며 HR-TEM과 FTIR 결과로부터 확인한 Si과 C 사이의 비정질 층이 Si의 부피 팽창을 억제해 주고 전기적 접촉을 증가시켜 우수한 전극 수명특성을 보이는 것으로 사료된다. 따라서 Si-Ni-C 복합물은 탐소계 물질을 대체할 새로운 고용량 음극물질로서 매우 유망함을 알 수 있었다.