In Situ Polymerization in COF Boosts Li-Ion Conduction in Solid Polymer Electrolytes for Li Metal Batteries

Abstract

Solid polymer electrolytes (SPEs) have garnered considerable interest in the field of lithium metal batteries (LMBs) owing to their exceptional mechanical strength, excellent designability, and heightened safety characteristics. However, their inherently low ion transport efficiency poses a major challenge for their application in LMBs. To address this issue, covalent organic framework (COF) with their ordered ion transport channels, chemical stability, large specific surface area, and designable multifunctional sites has shown promising potential to enhance lithium-ion conduction. Here, we prepared an anionic COF, TpPa-COOLi, which can catalyze the ring-opening copolymerization of cyclic lactone monomers for the in situ fabrication of SPEs. The design leverages the high specific surface area of COF to facilitate the absorption of polymerization precursor and catalyze the polymerization within the pores, forming additional COF-polymer junctions that enhance ion transport pathways. The partial exfoliation of COF achieved through these junctions improved its dispersion within the polymer matrix, preserving ion transport channels and facilitating ion transport across COF grain boundaries. By controlling variables to alter the crystallinity of TpPa-COOLi and the presence of –COOLi substituents, TpPa-COOLi with partial long-range order and –COOLi substituents exhibited superior electrochemical performance. This research demonstrates the potential in constructing high-performance SPEs for LMBs.

1. Introduction

As superior ion carriers, solid polymer electrolytes (SPEs) typically necessitate good lithium salt solubility, high lithium-ion transport efficiency, low electrode–electrolyte interface impedance, and an electrochemical stability window that is compatible with the positive electrode material [1, 2]. Polyester-based SPEs are extensively studied due to their stable electrochemical performance and higher lithium-ion transference numbers compared to polyether-based polymer electrolytes [3–6]. The consensus within the researchers is the primary challenge that constrains the utilization of SPEs in all-solid-state batteries stemming from their inherently low ambient ionic conductivity (<10–7 S cm−1) and low lithium-ion transference number (<0.5) in room temperature [7, 8]. Incorporating functional fillers within polymer electrolyte as a compensatory agent is a charming modification way to improve the low ion transport efficiency of SPEs [9]. The conventional ex situ composite method, which involves dissolving the polymer with a suitable solvent, dispersing the fillers in the polymer solution, and subsequently fabricating solid electrolyte membranes through pouring, hot pressing, coating, or other techniques, often fails to address the issue of uneven filler dispersion and extremely high interface impedance [10, 11]. The reason is that the inherent defects of the fillers, such as high zeta potential and low specific surface area, can lead to aggregation and reduced utilization rates, particularly at high fillers concentrations, resulting in poor tensile strength of the electrolyte membrane, sluggish ion transport kinetics, and uneven ion flux [8, 12]. Furthermore, the ex situ assembly battery method employed in the preparation of solid-state electrolyte membranes still exhibits high interface impedance between the electrode, which hinders their application in solid-state batteries [13, 14].

Long-range ordered porous materials, such as covalent organic frameworks (COFs) [15], metal–organic frameworks (MOFs) [16–18], and zeolites [19], have been widely used in energy storage materials owning to their unique channels for orderly ion transport, good chemical structural stability, large specific surface area, and strong designability of multifunctional sites. COFs are a class of porous crystalline polymer materials assembled into 2D or 3D long-range ordered periodic structures through highly designable building blocks of organic monomers linked by covalent bonds [20, 21]. The stable chemical structure and high modulus of COFs ensure good dendrite suppression, and their defined nanoscale channels endow them with high ambient ionic conductivity (>10–5 S cm−1) and lithium-ion transference number (>0.6) at room temperature [20–25]. However, due to the inherent disadvantage of powder materials as solid electrolytes, lithium ions are difficult to transport across grain boundaries, leading to high interfacial resistance and limited performance. Therefore, developing strategies to integrate COFs into polymer matrices while maintaining their ion-conducting properties is crucial for advancing SPEs.