Achieving a Uniform Solid Structure for Poly-1,3-Dioxolane-Based in-Situ Polymer Electrolytes Via Proton Scavenging
Authors: Yoon Youngbin , Song Hyun-Kon 
Published: December 2024

Recent research has focused on polymer electrolytes derived from the ring-opening polymerization of 1,3-dioxolane(DoL) due to their potential to enhance the ionic conductivity. Poly-1,3-dioxolane(PDoL) has a higher density of ether-like oxygen bonds per unit mass compared to PEO, improving ionic conductivity. Conducting in-situ ring-opening polymerization within the cell can address interfacial contact issues, leading to PDoL-based solid polymer electrolytes with superior interfacial compatibility and increased ionic conductivity. This advancement supports the development of all-solid electrolytes for commercial use. Various initiators like Al(otf)3, LiFSI, LiDFOB, and LiBF4 have been tested, with ring-opening polymerization(ROP) typically initiated by protonic or Lewis acids. However, the in-situ polymerization process requires strict control over the manufacturing environment and does not allow for control over the initiation time or rate. The fast ROP speed of PDoL (~5 to 30 sec) with different initiators leads to non-uniform electrolyte composition and challenges in cell assembly, affecting the reproducibility of studies and hindering commercialization.

To overcome this, we suggest the use of a strong base proton scavenger(PS) to regulate the ring-opening rate of DoL, thereby achieving uniform electrolyte fabrication and enhancing battery performance. NMR analysis has confirmed that an electrolyte precursor solution containing the PS and initiator remains stable at room temperature for over a month. The PS allows for temporal regulation of the ROP reaction, enabling precise control over the initiation and polymerization rates. Systematic optimization of the electrolyte fabrication process through regulated in-situ ROP could lead to reproducible, scalable, and commercially viable solid-state lithium metal batteries featuring PDoL-based electrolytes.

In this study, we address the issue by employing a strong base PS to regulate the ring-opening rate of DoL, thus ensuring uniform electrolyte fabrication and improved battery performance. NMR analysis has verified that the electrolyte precursor solution, containing both the PS and initiator, remains stable at room temperature for more than a month.

To ensure the usability of the PS, two conditions must be met. Firstly, any side reactions caused by the PS should not degrade battery performance. This was verified through LSV and NMR tests. Secondly, initiation must be feasible under polymerization conditions (approximately 60℃). It cannot simply act as a material for in-situ polymerization control by inhibiting ring opening. This was confirmed through a pH test, which showed that under polymerization conditions, the solution containing the PS returned to a pH level similar to that of the solution without it. Furthermore, an in-situ ionic conductivity test using an ionic conductivity probe revealed that ionic conductivity was maintained until the condition for ROP was met, thereby suppressing polymerization. This confirms that polymerization can commence under anticipated future polymerization conditions.

In the case of DoL-based electrolytes, where ring opening is suppressed by PS, it is now possible to produce high-concentration electrolytes that were previously unattainable. For DoL electrolytes employing LiFSI as the salt, the highest concentration previously researched was 5–6 M. However, with the addition of a proton scavenger, LiFSI-DoL electrolyte can be stably produced up to 14M (EO:Li = 2:1).

Thanks to the high concentration and uniform production of the electrolyte, a very high Li transference number is achieved. While previously studied PDoL electrolytes reported high values of 0.76 (2023) and 0.72 (2022), the PDoL electrolytes produced using the proposed method achieved an even higher value of 0.85.

To enhance the ionic conductivity of the PDoL-based electrolyte, a PVA-CN crosslinker was utilized. The use of the crosslinker lowers the Tg value, which improves the segmental motion of the polymer and aids in enhancing ion hopping. Additionally, the Li symmetric cell test confirmed that the crosslinker improved high-temperature stability.

The Li|SPE|LiFePO4 cell fabricated using the PDoL-based SPE exhibits a specific discharge capacity with 80% retention after 250 cycles at a 0.5 C-rate at 25°C (LFP: 1 mAh/cm2). This cell demonstrates a wide range of electrochemical stability at 4.5V (vs. Li/Li+).

This research proposes the strategic inhibition of ring-opening polymerization using PS as an effective solution to the challenges associated with controlling temperature- and time-dependent polymerization kinetics inherent to in-situ polymerizations. Through regulated polymerizations achieved by the judicious addition of PS, this study demonstrates a method to ensure the necessary wetting times crucial for the scalable manufacturing of batteries employing in-situ polymer electrolytes across various battery form factors, including cylindrical cells. This study is promising for the practical commercialization of solid polymer electrolytes synthesized in-situ, addressing the longstanding issue of inadequate wetting times that has previously hindered the adoption of PDoL-based polymer electrolyte.