Heat-Treatment Process for Enhancing the Performance of Naphthalenediimide (NDI)-Based Polymer Cathodes in Li-Ion Batteries

국문 초록 (Abstract)

본 연구에서는 n-type 공액 고분자인 P(NDI2OD-T2)의 낮은 이론용량 한계를 극복하기 위해 간단하면서도 실용적인 열처리 기반 공정을 제안하였다. 전기화학적으로 비활성인 알킬 사이드체인을 420 °C에서 30분간 선택적으로 제거함으로써 활성 질량을 증가시켜 전극의 방전 용량을 56.3 mAh g-1에서 117.0 mAh g-1로 향상시켰다. 열처리는 고분자의 결정성을 감소시켜 전자전도도를 저하시켰으나, 동시에 다공성 구조를 형성하여 Li+ 확산성을 크게 개선하였다. 그 결과, 실온에서는 열처리 전(fresh) 셀이 우수한 속도 성능을 보였고, -10 °C에서는 열처리 후(heat-treated) 셀이 더 뛰어난 속도 성능과 사이클 안정성을 보였다. 단순한 열처리만으로 P(NDI2OD-T2) 기반의 고분자 전극의 용량 한계를 효과적으로 개선할 수 있음을 보여주었으며, 향후 부분적 사이드체인 제거, 백본 구조 변경, 가교 네트워크 도입 등을 통해 전자전도도와 이온 확산 거동의 균형을 고려한 고분자 전극 설계 방향을 제시한다.

다국어 초록 (Multilingual Abstract)

A simple heat-treatment process is presented to address the intrinsically low theoretical capacity of the n-type conjugated polymer P(NDI2OD-T2). By selectively removing electrochemically inactive alkyl side chains through controlled thermal cleavage (420 °C, 30 min), the active mass fraction of the polymer electrode was significantly increased, resulting in a nearly twofold enhancement in discharge capacity from 56.3 mAh g-1 to 117.0 mAh g-1. Although heat-treatment reduced polymer ordering and electronic conductivity, it simultaneously generated a porous framework that substantially improved Li+ diffusivity, leading to a clear trade-off between structural order and ion transport. At room temperature, the fresh cell exhibited superior rate capability due to higher crystallinity, whereas under diffusion-limited sub-zero conditions (–10 °C), the heat-treated cell demonstrated enhanced rate performance and cycling stability. Overall, these results demonstrate that a straightforward heat-treatment can effectively address capacity limitations of conventional P(NDI2OD-T2)-based cathodes while providing insights into structure–property relationships governing organic polymer electrodes. Furthermore, the findings highlight future design opportunities—including partial side-chain removal, backbone engineering, and incorporation of crosslinked networks—to achieve a balanced optimization of electronic conductivity, ion transport, and specific capacity in next-generation organic Li-ion batteries.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• Chapter 2. Experimental 16
• 2.1 Materials and polymer synthesis 16
• 2.2 Characterization 17
• 2.3 Coin cell fabrication 18
• 2.4 Electrochemical measurement 19
• Chapter 3. Results and Discussion 20
• 3.1 Design of heat-treatment process 20
• 3.1.1 Overview of the heat-treatment approach 20
• 3.1.2 Optimization of treatment conditions 21
• 3.2 Analysis of alkyl chain cleavage 29
• 3.2.1 Characterization of physical properties 29
• 3.2.2 Investigation of chemical structure and solubility 32
• 3.3 Effects of heat-treatment 39
• 3.3.1 Structural and morphological changes 39
• 3.3.2 Electrochemical reaction mechanism 47
• 3.4 Electrochemical performance in Li half-cells 55
• Chapter 4. Conclusion 62
• [References] 64