리튬이차전지의 음극바인더인 폴리비닐리덴 플로라이드를 대체할 새로운 유기계 바인더에 대한 연구 : Study on novel organic solvent-based binder instead of Poly(vinylidene fluoride) for anodes in Lithium-ion batteries

다국어 초록 (Multilingual Abstract)

In this study, new solvent-based binders for anode materials such Li4Ti5O12 and Silicon in lithium ion batteries were fabricated.

1. As an appropriate eco-friendly binder additive, rosin ingredients which are extracts of pine trees were synthesized with a conventional solvent-based binder, polyvinylidene fluoride(PVdF) at Various weight percentages of rosin below 50 wt.% using sonication. The addition of rosin decreases the crystallinity of PVdF and disperses conducting agent and binder well through DSC and SEM with EDS mapping. When the binder system was used to evaluate the electrochemical performance of the Li4Ti5O12(LTO) electrodes, the addition of rosin ultimately leads to the increase in the diffusivity and cyclic capacity of lithium ions. When the amount of rosin additive to PVdF binder was 40 wt.%, the LTO electrode exhibited the highest cyclic capacity and diffusivity as 23 mAh g-1 higher than LTO electrode using PVdF. Therefore, the optimum ratio between PVdF and rosin in the binder is 6 to 4 and it can be applied in other solvent-based binders to improve their performance.

2. As a promising anode material, silicon draws a lot of attention due to its surpassing theoretical capacity, 4200 mAh g-1. Notwithstanding the advantage, silicon suffers from a severe loss of capacity because of volume change during lithium insertion/extraction, resulting in electrode pulverization. So far, some solutions for the fatal obstacle have been proposed. One of them is to modify design of silicon using nanowire, nanotube and nanoparticle. An external medium such as graphite has been also employed to suppress the volume expansion of silicon. Unlike these, some of researches have focused on compatible binders improving cycling stability.
In this study, we will compare P(AN-co-AA) with conventional organic binder polyvinylidene fluoride for anode material mixed with graphite and silicon. Fabrication of copolymers will be checked by 13C-NMR, and the distribution of binder on electrode surface will be also indicated through various characterization techniques. The ability of each binder will be analysed using adhesion test and electrochemical characteristics.

국문 초록 (Abstract)

본 실험에서는 유기계 바인더를 제조하여 리튬 이차전지용 음극에 적용하였으며, 이를 통해 리튬 이차전지 성능 향상에 대한 연구를 수행하였다.

1. 친환경 바인더 첨가제로서 소나무 추출물 송진을 기존 유기 바인더 Polyvinylidene Fluoride (PVdF)에 소니케이션을 이용하여 5, 20, 40 그리고 50 wt.%로 합성시켰다. 송진을 첨가할수록 PVdF의 결정성이 감소함을 DSC를 통해 알 수 있었고 그로 인해 도전재의 뭉침이 저하됨을 SEM과 그에 따른 EDS mapping 분석을 통해 알 수 있었다. 제조한 합성 바인더를 Li4Ti5O12에 적용하였을 때 리튬 이온의 확산 속도를 증가시켜 용량 등의 전기 화학적 특성을 향상시켰다. 송진을 40 wt.% 첨가하였을 때 가장 성능이 우수함을 알 수 있었고 이 때에는 PVdF만을 사용하였을 때 보다 23 mAh g-1 높은 용량을 구현하였다. 이로써 최적비는 40 wt.% 첨가한 바인더임을 알 수 있었고, 다른 다양한 바인더에도 적용시켜 친환경 바인더를 제조할 수 있을 것이다.

2. 비탄소계 고용량 활물질 실리콘에 대해 우수한 작용기를 포함하는 Polyacrylonitrile (PAN)과 Polyacrylic acid (PAA)를 80: 20, 50: 50 그리고 20: 80의 비로 공중합을 시도한 Poly(acrylonitrilie-co-acrylic acid) (P(AN-co-AA))에 대해 연구하였다. 13C-NMR을 통해 공중합체 제조를 확인하였고, PVdF, PAA, PAN 그리고 P(AN-AA)에 따른 물리적 및 전기 화학적 특성을 비교 분석하였다. SEM과 EDS mapping을 통해 충?방전 전, 후 전극 표면 상태를 확인하였고 또한 접착력 테스트로 각각 바인더의 성능을 비교하였다. 또한 CV, EIS 그리고 Cycle test의 전기화학적 특성 분석을 통해 AN과 AA의 최적의 비율을 알아 보았다. 이는 고용량 전지, Si 계 활물질에 적합한 바인더 연구에 큰 기여를 할 것이다.

목차 (Table of Contents)

• Acknowledgement
• Abstract in Korean
• Abstract in English
• Contents
• Figure list
• Table list
• 1. Introduction
• 1.1. Lithium ion batteries 1
• 1.2. The principle and components of lithium ion batteries 1
• 1.3. The characteristics of attractive anode materials 4
• 1.3.1. Li4Ti5O12 4
• 1.3.2. Silicon 4
• 1.4. The binders for lithium ion batteries 5
• 2. Experimental
• 2.1. PVdF-Rosin hybrid binder 8
• 2.1.1. Fabrication of hybrid binder 8
• 2.1.2. Preparation of electrode 8
• 2.1.3. Fabrication of half coin cell 8
• 2.2. P(AN-AA) copolymer binder 11
• 2.2.1. Polymerization of PAN, PAA and P(AN-co-AA) 11
• 2.2.2. Preparation of electrode 11
• 2.2.3. Fabrication of half coin cell 12
• 2.3. Physical characteristics of binders 14
• 2.3.1. Differential scanning calorimeter (DSC) 14
• 2.3.2. Lateral force microscopy (LFM) 14
• 2.3.3. Fourier transform infrared spectroscopy (FT-IR) 14
• 2.3.4. 13C-Nuclear magnetic resonance (13C-NMR) 14
• 2.3.5. Electrolyte uptake 15
• 2.4. Physical characteristics of electrodes 15
• 2.4.1. Field emission scanning electron microscopy (FE-SEM) 15
• 2.4.2. Brunauer-Emmett-Teller (BET) 15
• 2.4.3. X-ray photoelectron spectra (XPS) 15
• 2.4.4. Adhesion strength 16
• 2.4.5. Contact angle 16
• 2.5. Electrochemical characteristics 16
• 2.5.1. Galvanostatic charge and discharge test 16
• 2.5.2. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) 16
• 3. Results and discussion
• 3.1. PVdF-Rosin hybrid binder 17
• 3.1.1. Physical characteristics 17
• 3.1.2. Electrochemical characteristics 31
• 3.2. P(AN-AA) copolymer binder 38
• 3.2.1. Physical characteristics 38
• 3.2.2. Electrochemical characteristics 49
• 4. Conclusion
• 4.1. PVdF-Rosin hybrid binder 53
• 4.2. P(AN-AA) copolymer binder 53
• 5. Reference 55