Design of novel nanocrack-structured separators and their application in lithium-ion batteries

The separator, which provides an ion transport pathway while preventing short circuits between electrodes, is an essential component of secondary metal batteries and plays a pivotal role in the stable operation of energy storage systems. Currently, commercial lithium-ion battery separators are primarily prepared using dry and wet methods. Despite rapid advancements in various secondary energy storage systems, the domestic high-end separator market remains heavily reliant on imports, posing a bottleneck to the development of the energy storage industry. This dependence is due to not only the quality of raw materials but also the lack of precision separator production equipment. Consequently, developing high-performance separators based on new materials and preparation technologies has become a top priority in current research. Although traditional wet-process separator fabrication offers controllable pore size distribution and porosity, it requires extensive use of extraction solvents, leading to high energy consumption, complex procedures, and elevated costs. The dry process demands high-quality raw materials and involves complex production steps, increasing costs and production difficulty. According to the principle of similar compatibility, polymers with different polarities generally exhibit poor compatibility. Phase separation can form an interphase between polymers with significant polarity differences. Inevitably, weak interactions near this interphase result in a loose structure that allows ion transport. Abundant interphases can form sufficient ion channels for migration between the cathode and anode. Herein, owing to the insufficient compatibility of poly(butylene adipate-co-terephthalate) (PBAT) and isostatic polypropylene (IPP), we developed a novel nanocrack-structured separator with uniform microphase separation by blending via the blow molding process, enabling efficient ion transport with an ionic conductivity of 0.25 mS·cm⁻¹. In traditional wet processes, pore-making involves the elution of pore-forming agents with bulk organic solvents and drying steps. The heat drawing and cooling parameters play key roles in forming long, narrow pore morphologies in the dry process. In comparison, the new preparation technology for nanocrack-structured separators is simple and mature, omitting the pore-making step. Furthermore, this low-equipment-cost process is suitable for large-scale production. The abundant nanocracks at the polymer interfaces maintain sufficient ion transport flux for normal charge-discharge processes. This method significantly enhances production efficiency while reducing energy consumption. Moreover, the prepared separator demonstrates superior mechanical properties, with transverse and longitudinal ultimate tensile strengths of 7.52 MPa and 21.74 MPa, respectively, and strains reaching 392.41% and 597.58%. The excellent ion conductivity and mechanical properties of the nanocrack-structured separator satisfy battery separator requirements. When assembled into Li//LiFePO₄ batteries, the battery delivers an initial discharge capacity of 155.99 mA·h·g⁻¹ at a current density of 0.1 C, with 92.9% capacity retention after 100 cycles, validating the application feasibility of this nanocrack-structured separator and preparation process. This study successfully eliminates the pore-forming step typical in traditional separator preparation, achieving dual benefits of reduced energy consumption and environmental protection. Furthermore, it innovatively utilizes nanocrack structures for ion transport, expanding applications in electrochemical energy storage and providing novel insights for advancing battery separator fabrication technologies.