Poly(ether imide) Porous Membrane Developed by a Scalable Method for High-Performance Lithium-Sulfur Batteries: Combined Theoretical and Experimental Study

Versatile Separators Toward Advanced Lithium-Sulfur Batteries: Status, Recent Progress, Challenges and Perspective

Lithium-sulfur batteries (LSBs) have recently gained extensive attention due to their high energy density, low cost, and environmental friendliness. However, serious shuttle effect and uncontrolled growth of lithium dendrites restrict them from further commercial applications. As "the third electrode", functional separators are of equal significance as both anodes and cathodes in LSBs. The challenges mentioned above are effectively addressed with rational design and optimization in separators, thereby enhancing their reversible capacities and cycle stability. The review discusses the status/operation mechanism of functional separators, then primarily focuses on recent research progress in versatile separators with purposeful modifications for LSBs, and summarizes the methods and characteristics of separator modification, including heterojunction engineering, single atoms, quantum dots, and defect engineering. From the perspective of the anodes, distinct methods to inhibit the growth of lithium dendrites by modifying the separator are discussed. Modifying the separators with flame retardant materials or choosing a solid electrolyte is expected to improve the safety of LSBs. Besides, in-situ techniques and theoretical simulation calculations are proposed to advance LSBs. Finally, future challenges and prospects of separator modifications for next-generation LSBs are highlighted. We believe that the review will be enormously essential to the practical development of advanced LSBs.

Electrolyte Superwetting and Electrode Friendly of Porous Membrane for Better Cycling Stability of Lithium Metal Batteries

Porous polymer membranes as separator plays important roles in separating cathode and anode, storing electrolytes, and transporting ions in energy storage devices. Here, an effective strategy is reported to prepare an electrolyte superwetting membrane, which shows good Li+ transport rate and uniformity, as well as electrode-friendly properties to afford the reduction and oxidation of electrodes. It thereby improves the cycle stability and safety of Li metal batteries. With the arrayed capillaries technique, a thin layer of polyvinylidene fluoride (PVDF) and polyacrylonitrile (PAN) composite is uniformly coated on the surface and pores of polypropylene (PP) membrane with a total thickness of 30 µm. After treating it with n-butyllithium and LiNO3 in turn, a chemically inert membrane with efficient and uniform ion transport is prepared, and the cycle stability of Li||Li symmetric cells is up to 1500 h, 4 times higher than that of PP membrane. Moreover, the Li||LiFePO4 with as-prepared membranes achieve a higher capacity retention rate of 92% after 190 cycles at a current density of 3.6 mA cm-2 and a capacity of 3.6 mAh cm-2, and the Li||NCM721 batteries achieve a capacity retention rate of 71% after 600 cycles at a current density of 1.8 mA cm-2.

Advanced PEI/PAN Membrane to Suppress Zinc Dendrite Growth in Zinc Metal Batteries

Aqueous zinc-ion batteries (AZIBs) are a potential new technology in energy storage due to their high energy density, affordability, and environmental friendliness. The development of AZIBs is still hampered by unchecked zinc dendrite formation during cycling, which results in an unstable interface, a short cycling life, and a considerable capacity decline with security issues. Herein, we demonstrate a novel nanofiber membrane based on polyetherimide-polyacrylonitrile (PEI/PAN) polymer via electrospinning method with entangled nanofibers for AZIBs applications. The as-fabricated PEI/PAN membrane has a homogeneous, tortuous, and linked porous structure, high porosity, and superior electrolyte wettability. The resulting PEI/PAN membrane exhibits a decent thermal stability of 200 °C and a strong ionic conductivity of up to 5.3×10-4 S cm-1. This membrane gives Zn/Zn symmetric cells an ultralong cycle life of more than 250 hours at 3 mA cm-2. In the meantime, the MnO2/Zn cell outperforms commercial filter paper regarding cycle stability and rate performance. This work demonstrates the design of a straightforward technique to fabricate advanced nanofiber membranes for AZIBs to modify Zn2+ deposition behavior and improve Zn dendrite resistance.

Highly Stretchable Polyurethane Porous Membranes with Adjustable Morphology for Advanced Lithium Metal Batteries

A highly flexible, tunable morphology membrane with excellent thermal stability and ionic conductivity can endow lithium metal batteries with high power density and reduced dendrite growth. Herein, a porous Polyurethane (PU) membrane with an adjustable morphology was prepared by a simple nonsolvent-induced phase separation technique. The precise control of the final morphology of PU membranes can be achieved through appropriate selection of a nonsolvent, resulting a range of pore structures that vary from finger-like voids to sponge-like pores. The implementation of combinatorial DFT and experimental analysis has revealed that spongy PU porous membranes, especially PU-EtOH, show superior electrolyte wettability (472%), high porosity (75%), good mechanical flexibility, robust thermal dimensional stability (above 170 °C), and elevated ionic conductivity (1.38 mS cm-1) in comparison to the polypropylene (PP) separator. The use of PU-EtOH in Li//Li symmetric cell results in a prolonged lifespan of 800 h, surpasing the longevity of PU or PP cells. Moreover, when subjected to a high rate of 5 C, the LiFePO4/Li half-cell with a PU-EtOH porous membrane displayed better cycling performance (115.4 mAh g-1) compared to the PP separator (104.4 mAh g-1). Finally, the prepared PU porous membrane exhibits significant potential for improving the efficiency and safety of LMBs.

Unveiling high-power and high-safety lithium-ion battery separator based on interlayer of ZIF-67/cellulose nanofiber with electrospun poly(vinyl alcohol)/melamine nonwoven membranes

Due to the poor thermal stability of conventional separators, lithium-ion batteries require a suitable separator to maintain system safety for long-term cycling performance. It must have high porosity, superior electrolyte uptake ability, and good ion-conducting properties even at high temperatures. In this work, we demonstrate a novel composite membrane based on sandwiching of zeolitic imidazole frameworks-67 decorated cellulose acetate nanofibers (ZIF-67@CA) with electrospun poly(vinyl alcohol)/melamine (denoted as PVAM) nonwoven membranes. The as-prepared sandwich-type membranes are called PVAM/x%ZIF-67@CA/PVAM. The middle layer of composite membranes is primarily filled with different weight percentages of ZIF-67 nanoparticles (x = 5, 15, and 25 wt%), which both reduces the non-uniform porous structure of CA and increases its thermal stability. Therefore, our sandwich-type PVAM/x%ZIF-67@CA/PVAM membrane exhibits a higher thermal shrinkage effect at 200 °C than the commercial polyethylene (PE) separator. Due to its high electrolyte uptake (646.8%) and porosity (85.2%), PVAM/15%ZIF-67@CA/PVAM membrane achieved high ionic conductivity of 1.46 × 10-3 S cm-1 at 70 °C, as compared to the commercial PE separator (ca. 6.01 × 10-4 S cm-1 at 70 °C). Besides, the cell with PVAM/15%ZIF-67@CA/PVAM membrane shows an excellent discharge capacity of about 167.5 mAh g-1after 100 cycles at a 1C rate with a capacity retention of 90.3%. The ZIF-67 fillers in our sandwich-type composite membrane strongly attract anions (PF6-) through Lewis' acid-base interaction, allowing uniform Li+ ion transport and suppressing Li dendrites. As a result, we found that the PVAM/15%ZIF-67@CA/PVAM composite nonwoven membrane is applicable to high-power, high-safety lithium-ion battery systems that can be used in electric vehicles (EVs).