회전킬른반응기를 이용한 리튬이온전지용 Si/C/CNF 음극활물질의 제조 및 전기화학적 특성 조사

Preparation and Electrochemical Characterization of Si/C/CNF Anode Material for Lithium ion Battery Using Rotary Kiln Reactor

Abstract

흑연은 리튬이온전지에 사용 되는 대표적인 음극활물질이다. 그러나 최대 이론 용량이 $372mA\;h\;g^{-1}$으로 제한되기 때문에 고용량의 리튬이온전지 개발을 위해서는 새로운 음극 소재 활물질이 필요하다. 실리콘의 최대 이론 용량은 $4200mA\;h\;g^{-1}$으로 흑연보다 높은 값을 나타내지만 부피 팽창이 400%로 크기 때문에 음극 소재 활물질로 바로 적용하기에는 적합하지 않다. 따라서 부피 팽창으로 인한 방전 용량의 감소를 최소화하기 위해 건식 방법으로 실리콘을 분쇄 하여 기계적 응력 및 반응상의 체적 변화를 감소시키고 입도 제어 된 실리콘 입자에 탄소를 코팅하여 체적의 변화를 억제하였다. 그리고 탄소 섬유를 입자 표면에 실타래처럼 성장시켜 2차적으로 부피 팽창을 제어하고 전기전도성을 개선하였다. 실험 변수에 따른 재료들의 물리화학적 특성을 XRD, SEM 및 TEM을 사용하여 측정하였고 전기화학적 특성을 평가 하였다. 본 연구에서는 실리콘의 수명 특성을 향상시켜 음극 소재 활물질로 사용 할 수 있는 합성 방법에 대하여 알아보았다.

Graphite is used as a sample anode active material. However, since the maximum theoretical capacity is limited to $372mA\;h\;g^{-1}$, a new anode active material is required for the development of a high capacity lithium ion battery. The maximum theoretical capacity of Si is $4200mA\;h\;g^{-1}$, which is higher than that of graphite. However, it is not suitable for direct application to the anode active material because it has a volume expansion of 400%. In order to minimize the decrease of the discharge capacity due to the volume expansion, the Si was pulverized by the dry method to reduce the mechanical stress and the volume change of the reaction phase, and the change of the volume was suppressed by coating the carbon layers to the particle size controlled Si particles. And carbon fiber is grown like a thread on the particle surface to control secondary volume expansion and improve electrical conductivity. The physical and chemical properties of the materials were measured by XRD, SEM and TEM, and their electrochemical properties were evaluated. In this study, we have investigated the synthesis method that can be used as anode active material by improving cycle characteristics of Si.

Fig. 1. Schematic diagram of problem and improvement of Si anode material.

Fig. 2. Rotary kiln reactor.

Fig. 3. PC coating and CNF growth process diagram.

Fig. 4. PC coated Si for TEM analysis: thickness measurement with (a) 5 nm scale, (b) 10 nm scale, (c) 20 nm scale.

Fig. 5. Charge and discharge voltage profile of PC coated Si.

Fig. 6. Charge and discharge characteristics of PC coated Si up to 10 cycles: (a) capacity, (b) efficiency.

Fig. 7. SEM images of CNF: (a) 5 μm scale, (b) 1 μm scale, (c) 500 nm scale, (d) 500 nm scale after 6% hydrochloric acid treatment.

Fig. 8. TEM-EDS analysis: (a) TEM image of Fe catalyst, (b) EDS mapping of Fe catalyst, (c) TEM image for acid treatment, (d) EDS mapping for acid treatment.

Fig. 9. Component analysis of Si/C/CNF material after removal of Fe with acid treatment.

Fig. 10. Evaluation of reversible capacity of Si with CNF growth: (a) CNF 78 wt% growth, (b) CNF 100 wt% growth, (c) CNF 140 wt% growth.

Fig. 11. Charge and discharge characteristics of CNF 100 wt% up to 10 cycles: (a) capacity, (b) efficiency.

Table 1. Comparison of anode electrode properties for particle size controlled Si and coated Si and commercial SiO

Table 2. Charging and discharging voltage profile of Si/C/CNF