Analysis of Slot-Coating Flows for Thick Battery Electrodes = 후막 전극 생산을 위한 슬롯코팅 유동분석

다국어 초록 (Multilingual Abstract)

The production of film products using liquid coating materials is widely employed across various industries, including displays, solar cells, fuel cells, and secondary batteries. Among the diverse coating methods, slot-coating has attracted significant attention due to its pre-metered nature, which allows for precise control of coating thickness and facilitates large-area coating, making it an economical choice. However, limitations of slot-coating have been reported across these industries, prompting numerous researchers to contribute to its advancement.

For example, previous studies have explored the range of quality production conditions (quality window) for various slot-coating systems. However, they often provide general process condition guidelines for achieving them, while practical solutions to further enhance the production efficiency of slot-coated products have yet to be fully established. Moreover, the reported quality windows are typically narrow, limiting the operational freedom of slot-coating and making it difficult to predict quality windows. As a result, in many industrial settings, process conditions for slot coating are still determined by manually and iteratively adjusting parameters and verifying outcomes to achieve high-quality coated products. The author recognizes this dependence on experiential knowledge and trial-and-error approaches in establishing slot-coating process conditions as a critical issue, which this study aims to address.

Therefore, this study employs both theoretical and experimental methods to address the shortcomings and challenges of existing slot-coating processes. Theoretically, the research reinterprets the components of the visco-capillary model (Higgins and Scriven, 1980) used for predicting the range of stable process conditions (operability window) and proposes a simplified version of the model to improve upon the existing model, which has been difficult for field operators to understand and use. As a result, this proposed simple model, being easily comprehensible for field workers, facilitates the dissemination of methods for establishing stable process conditions. Experimentally, a custom-designed lab-scale roll-to-roll (R2R) slot-coating system allows for the observation of both the coating profile and cross-section of electrode coatings to analyze the quality window.

Consequently, this research successfully systematizes a strategy for establishing mass production conditions for each category of process conditions, with these categories defined by classifying slot-coating process conditions according to criteria such as thick/thin film and the necessity of vacuum facilities. Furthermore, this study proposes a novel thick-film slot-coating method that significantly improves the thick-film electrode process. The concept of a pseudo-multilayer coating method is introduced and successfully realized through the development of a component called a feed-spacer. This coating method was experimentally validated to produce quality electrodes across a wide range of process conditions. Therefore, this new thick-film slot-coating method expands the quality window, reducing the gap with the operability window, thereby increasing the selection options for slot-coating process conditions and making the quality window predictable (by utilizing the visco-capillary model to predict the operability window). Additionally, the coating method demonstrates that the edge elevation phenomenon of conventional electrode coatings can be suppressed, reducing the loss of product near the elevated edges and thus improving production yield. This shows that production efficiency can be improved depending on the coating method, even with the same materials. Therefore, this breakthrough not only addresses the limitations of existing slot-coating but also significantly improves battery electrode processes, paving the way for a new research field in novel slot-coating methods. Finally, as this method is first proposed in this dissertation, there are numerous opportunities for further development and improvement, and the dissertation discusses potential directions for advancing the newly developed slot-coating method.

국문 초록 (Abstract)

액상 코팅 재료를 사용한 필름 제품의 생산 공정은 디스플레이, 태양광 전지, 연료전지, 이차 전지 등 다양한 산업 분야에서 널리 활용되고 있다. 이때, 필름 생산 공정에는 다양한 코팅 방법들이 사용되며, 이들 방법 중 슬롯코팅은 가장 많은 주목을 받고 있다. 슬롯코팅은 코팅 두께를 정밀하게 제어할 수 있는 방식이면서, 대면적 코팅이 용이하여 매우 경제적인 코팅 방법이다. 그만큼, 다양한 산업에서 사용되면서 슬롯코팅의 한계점 역시 여러 산업에서 보고되고 있으며, 이를 개선하기 위해 많은 연구자들이 기여하고 있다.

대표적으로, 선행연구들은 다양한 슬롯코팅 시스템의 양품생산조건 범위(quality window)를 연구했다. 그러나 상당수의 연구에서 보고된 quality window는 매우 좁은 영역으로 보고된다. 이에 따라, quality window는 슬롯코팅의 운영 자유도를 제한하여, 슬롯코팅 공정 수립이 어려운 이유를 명확히 보여준다. 게다가, 이론적으로 예측 가능한 안정공정조건영역(operability window)과 달리 quality window는 예측이 어려운 형태를 보인다. 이에 따라, 현재까지도 많은 산업 현장에서는 슬롯코팅 공정조건 수립을 엔지니어의 경험과 시행착오에 의존하고 있으며, 본 논문에서는 해당 공정 수립과정의 비효율 문제 등을 해결하고자 하였다.

따라서, 본 연구는 기존 슬롯코팅 공정의 단점과 문제점들을 해결하기 위해 이론적 및 실험적 방법을 사용하였다. 이론적으로는, 기존의 안정공정조건을 예측하는 비스코-캐필러리 모델(Higgins and Scriven, 1980)의 구성 요소를 재해석하고, 간결한 버전의 모델을 제안하였다. 제안하는 간결한 모델은 현장 근로자들이 쉽게 이해할 수 있는 형태로, 현장에서 안정공정 조건을 체계적으로 수립하는 데 도움을 줄 수 있다. 실험적으로는, 자체 설계한 랩 스케일 롤투롤(R2R) 슬롯코팅 시스템을 통해, 다양한 공정조건에서의 전극의 외관 코팅 프로파일과 내부 단면을 모두 관찰하였다.

결과적으로, 본 연구는 박막/후막 및 감압장치 필수 여부의 네 가지 기준에 따라 슬롯코팅 공정 조건을 분류하고, 각 부류 별로 안정공정조건을 수립하는 전략을 체계화하는데 성공하였다. 아울러, 후막 전극 공정을 획기적으로 개선할 신규 후막 슬롯코팅 방법을 제안하고자 한다. 신규 후막 슬롯코팅 방법을 위해, 적층형 다중층 코팅 방법의 개념을 최초로 제안하고, 이를 실현할 피드-스페이서 부품을 개발하였다. 그 결과, 해당 코팅 방법이 넓은 공정조건 범위에서 양품 전극을 생산하는 것을 실험적으로 검증할 수 있었다.

무엇보다, 신규 후막 슬롯코팅 방법은 quality window를 확장하여 operability window와의 차이를 줄여 슬롯코팅 공정조건 선택지를 증가시키며, quality window를 예측 가능하게 함을 확인하였다. 아울러, 동일한 소재로도 코팅 방법에 따라 전극 생산 효율의 차이를 만들 수 있음을 보일 수 있었다. 결과적으로, 기존 슬롯코팅의 한계점을 해결할 뿐만 아니라 신규 후막 슬롯코팅이라는 새로운 공정의 연구 분야를 개척할 수 있었다. 다만, 신규 후막 슬롯코팅 방법은 본 논문에서 처음 제안한 만큼, 추가로 개선될 여지가 많다. 이에 따라, 신규 후막 슬롯코팅 방법을 발전시키기 위한 방향을 논의함으로써 본 학위논문을 마무리하고자 한다.

목차 (Table of Contents)

• 1. Introduction
• 2. Operability Window: Stable Process Conditions of Slot-Coating From Thin-Film Displays to Thick-Film Battery Electrodes
• 2.1 Introduction
• 2.2 Methods
• 2.2.1 Standard Visco-Capillary Model (Standard Model)
• 2.2.2 Simplified Model for Operability Window of Slot-Coating without Vacuum (SMOW)
• 2.2.3 Comparisons
• 2.3 Experimental
• 2.4 Results and Discussion
• 2.4.1 70 wt% GW Solution (Low NCa)
• 2.4.2 95 wt% GW Solution (High NCa)
• 2.4.3 Application of SMOW for Inverse Problem
• 2.5 Final Remarks
• 3. Quality Window within Operability Window: High Quality Coating Conditions with Controlling Slot-Coating Edge Profiles of Thick-Film Battery Electrodes
• 3.1 Introduction
• 3.2 Methods
• 3.2.1 Coating Edge Profile Analysis Methods
• 3.2.2 Data Acquisition and Experimental Design
• 3.3 Visco-Capillary Model for Slot-Coating Flows with Power-Law Fluids
• 3.4 Results and Discussion
• 3.4.1 Results
• 3.4.2 Discussion
• 3.4.3 Conclusion: Strategies to Obtain Ideal Edge Profiles
• 3.5 Final Remarks
• 4. Expanding Quality Window: Pseudo-Multilayer Slot-Coating and the Concept of Effective Feed-Slot Height for Thick-Film Battery Electrodes
• 4.1 Introduction
• 4.2 Novel Thick-Film Electrode Coating Method
• 4.2.1 Issues with Conventional Thick-Film Coating Methods
• 4.2.2 Pseudo-Multilayer Slot-Coating Method and Its Slot-Die
• 4.3 Methods
• 4.3.1 Coating System for Lab-Scale Electrode Production
• 4.3.2 Post-Coating System for Lab-Scale Electrode Production
• 4.4 Results and Discussion
• 4.4.1 Results
• 4.4.2 Discussion
• 4.5 Final Remarks
• 5. Concluding Remarks
• A. Appendix
• A.1 Standard Visco-Capillary Model with Dimensionless Numbers
• A.2 Two NCa,BB for R*gt < Rgt < 2 in Table 2.3
• A.3 Dimensional Operability Window of SMOW
• A.3.1 Derivation for Dimensional Operability Window of SMOW
• A.3.2 Further Simplification of SMOW for High Viscosity Liquid
• A.4 Material (Anode Slurry) Preparation in Chapter 3
• A.5 Nomenclature
• A.6 Flows Induced by Marangoni Effect and Gravity: Systematic and Random Errors in Edge Profile Measurement
• A.7 Supplementary for Fig. 3.9
• A.8 Supplementary for Algorithm of Edge Width Detection
• A.9 Supplementary for Algorithm of kedge Parameter Detection
• A.10 Preparation of Slurry in Chapter 4
• Abstract (In Korean)
• Acknowledgements