Ion-Selective Polyamide Acid Nanofiber Separators for High-Rate and Stable Lithium-Sulfur Batteries

In-situ constructing porous N-doped carbon skeleton with rich defects from modified polyamide acid to boost the high performance of Na3V2(PO4)3 cathode for full sodium-ion batteries

Generally, the transport of electrons and Na+ is seriously constrained in Na3V2(PO4)3 (NVP) due to intense interactions of V-O and PO bonds. Besides, polyamide acid (PAA) is hardly used in the sol-gel route due to insolubility. This work develops a facile liquid synthesis strategy based on modified PAA, achieving in-situ construction of a porous N-doped carbon framework with rich defects to improve the kinetics of NVP. The addition of triethylamine (TEA) reacts with carboxyls in PAA to achieve acid-base neutralization, turning PAA into polyamide salts with good solubility. The special morphology construction mechanism of this unique system was observed by ex-situ scanning electron microscopy (SEM) and Transmission electron microscopy (TEM). Specifically, PAA undergoes in-situ conversion into chain-like polyimide (PI) through a thermal polymerization mechanism during the pre-sintering process. Meanwhile, NVP precursors are evenly dispersed in the PI fibers, efficiently reducing the particle size. After the final treatment, the favorable porous carbon skeleton could be generated derived from the partial decomposition of PI, on which small active grains are in situ grown. The resulting N-doped carbon substrate contains rich defects, benefiting from the migration of Na+. Furthermore, the porous construction is conducive to alleviating the stress and strain generated by the high current impact, increasing the contact area between electrodes/electrolytes to improve the utilization efficiency of active substances. Comprehensively, the optimized samples exhibit a capacity of 82.1 mAh g-1 at 15C with a retention rate of 95.45 % after 350 cycles. It submits a capacity of 67.6 mAh g-1 at 90C and remains 52.2 mAh g-1 after 1500 cycles. Even in full cells, it reveals a value of 110.6 mAh g-1. This work guides the application of in-situ multiple modifications of polymers in electrode materials.

Grafting Polyethyleneimine-Poly(ethylene glycol) Gel onto a Heat-Resistant Polyimide Nanofiber Separator for Improving Lithium-Ion Transporting Ability in Lithium-Ion Batteries

To improve the lithium-ion transporting ability in lithium-ion batteries, a high-performance polyimide-based lithium-ion battery separator (PI-mod) was prepared by chemically grafting poly(ethylene glycol) (PEG) onto the surface of a heat-resistant polyimide nanofiber matrix with the assistance of amino-rich polyethyleneimine (PEI). The resulted PEI-PEG polymer coating exhibited unique gel-like properties with an electrolyte uptake rate of 168%, an area resistance as low as 2.60 Ω·cm², and an ionic conductivity up to 2.33 mS·cm^-1, which are 3.5, 0.10, and 12.3 times that of the commercial separator Celgard 2320, respectively. Meanwhile, the heat-resistant polyimide skeleton can effectively avoid thermal shrinkage of the modified separator even after 200 °C treatment for 0.5 h, which ensures the safety of the battery working under extreme conditions. The modified PI separator possessed a high electrochemical stability window of 4.5 V. Compared with the batteries from the commercial separator Celgard 2320 and the pure polyimide matrix, the assembled coin cell with the PI-mod separator showed much better rate capabilities and capacity retention due to the high electrolyte affinity of the PEI-PEG polymer coating. The developed strategy of using the electrolyte-swollen polymer to modify the thermal-resistant separator network provides an efficient way for establishing high-power lithium-ion batteries with good safety performance.

Watermelon Flesh-Like Ni3 S2 @C Composite Separator with Polysulfide Shuttle Inhibition for High-Performance Lithium-Sulfur Batteries

The shuttle effect limits the practical application of lithium-sulfur (Li-S) batteries with high specific capacity and cheap price. Herein, a three-dimensional carbon substrate containing Ni₃ S₂ nanoparticles is created to modify the separator. The in situ optical visualization battery proves that the material can realize the rapid conversion of Li₂ S₆ . Moreover, the impact of lithium-ion diffusion on the reactions in the cell is investigated, and the mechanism of Ni₃ S₂ @C in the cell is proposed based on the "adsorption-diffusion-conversion" mechanism. The "adsorption-diffusion-conversion" process of polysulfide is carried out on the surface of the composite separator, showing positive effects on the inhibition of polysulfide shuttle and the promotion of conversion. The separator is modified to improve sulfur utilization and reduce dead sulfur accumulation through a strategy of chemical immobilization and physical blocking. This helps to bridge the existing gaps of Li-S batteries.

Single-walled carbon nanotube gutter layer supported ultrathin zwitterionic microporous polymer membrane for high-performance lithium-sulfur battery

Development of efficient lithium-sulfur (Li-S) battery requires the need to develop an appropriate functional separator that allows strong facilitation and transport of lithium ions together with limited passage of polysulfides. In this work, a multifunctional separator (TB-BAA/SWCNT/PP) is developed through spin coating of a novel zwitterionic microporous polymer (TB-BAA) on the gutter layer constructed from single-walled carbon nanotubes (SWCNT), where commercially available polypropylene (PP) separator is used to act as the mechanical support. SWCNT in this study serves as the first modification layer to decrease the size of the macropores in the PP separator, while the ultrathin TB-BAA top barrier layer with the presence of zwitterionic side chains allows the creation of confined ionic channels with both lithiophilic and sulfophilic groups. Due to the presence of available chemical interactions with lithium polysulfides, selective ion transport can be foreseen through such separator. In this regard, shuttle effect that is frequently encountered in Li-S battery can be suppressed effectively via implementing the as-obtained functional separator, resulting in the creation of credible and stable sulfur electrochemistry. The TB-BAA/SWCNT/PP-based Li-S battery has been investigated to possess high cycling ability (capacity fading per cycle of 0.055% over 500 cycles at 1 C) together with decent rate capability (736.6 mAh g^-1 at 3 C). In addition, a high areal capacity retention of 5.03 mAh cm^-2 after 50 cycles can be also obtained under raised sulfur loading (5.4 mg cm^-2).

Improved performance of lithium-sulfur batteries by employing a sulfonated carbon nanoparticle-modified glass fiber separator

Some of the most promising alternatives in the energy storage sector are lithium-sulfur batteries, which have a high energy density and theoretical capacity. However, the low electrical conductivity of sulfur and the shuttle effect of polysulfides remain important technical obstacles in the practical use of lithium-sulfur batteries (LSBs). This work employed a glass fiber separator with sulfonated carbon nanoparticles (SCNPs) to reduce the shuttle effect. The negatively charged sulfonic groups in SCNPs might prevent polysulfide migration and anchor lithium polysulfides. By using carbon-based interlayers, this method improves ion conductivity. Furthermore, the equally scattered sulfonic groups serve as active sites, causing sulfur to be distributed consistently and limiting sulfur growth while enhancing active sulfur utilization. After 200 cycles at 1C, the SCNP separator-containing cell showed a specific capacity of 1080 mA h g^-1. After 200 cycles, the cell with a CNP separator only showed a specific capacity of 854 mA h g^-1, demonstrating that CNPs' polysulfide diffusion suppression was ineffective. The cell with the SCNP separator still showed a high capacity of 901 mA h g^-1 after 500 cycles, with an average coulombic efficiency of almost 98%.

Ion Selective Covalent Organic Framework Enabling Enhanced Electrochemical Performance of Lithium-Sulfur Batteries

Ion selective separators with the capability of conducting lithium ion and blocking polysulfides are critical and highly desired for high-performance lithium-sulfur (Li-S) batteries. Herein, we fabricate an ion selective film of covalent organic framework (denoted as TpPa-SO₃Li) onto the commercial Celgard separator. The aligned nanochannels and continuous negatively charged sites in the TpPa-SO₃Li layer can effectively facilitate the lithium ion conduction and meanwhile significantly suppress the diffusion of polysulfides via the electrostatic interaction. Consequently, the TpPa-SO₃Li layer exhibits excellent ion selectivity with an extremely high lithium ion transference number of 0.88. When using this novel functional layer, the Li-S batteries with a high sulfur loading of 5.4 mg cm^-2 can acquire a high initial capacity of 822.9 mA h g^-1 and high retention rate of 78% after 100 cycles at 0.2 C. This work provides new insights into developing high-performance Li-S batteries via ion selective separator strategy.

Cation-Selective Separators for Addressing the Lithium-Sulfur Battery Challenges

Lithium-sulfur batteries (LSBs) have become one of the most promising candidates for next-generation energy storage systems owing to their high theoretical energy density, environmental friendliness, and cost effectiveness. However, real-word applications are seriously restricted by an undesirable shuttle effect and Li dendrite formation. In essence, uncontrollable anion transport is a key factor that causes both polysulfide shuttling and dendrite formation, which creates the possibility of simultaneously addressing the two critical issues in LSBs. An effective strategy to control anion transport is the construction of cation-selective separators. Significant progress has been achieved in the inhibition of the shuttle effect, whereas addressing the problem of Li dendrite formation by utilizing a cation-selective separator is still under way. From this viewpoint, this Review analyzes critical issues with regard to the shuttle effect and Li dendrite formation caused by uncontrollable anion transport, based on which roles and advantages of cation-selective separators toward high-performance LSBs are presented. According to the separator-construction principle, the latest advances and progress in cation-selective separators in inhibiting the shuttle effect and Li dendrite formation are reviewed in detail. Finally, some challenges and prospects are proposed for the future development of cation-selective separators. This Review is anticipated to provide a new perspective for simultaneously addressing the two critical issues in LSBs.

Balancing the Seesaw: Investigation of a Separator to Grasp Polysulfides with Diatomic Chemisorption

Lithium-sulfur (Li-S) batteries are promising next-generation high-density energy storage systems due to their advantages of high theoretical specific capacity, environmental compatibility, and low cost. However, high-order polysulfides dissolve in the electrolyte and subsequently lead to the undesired polysulfide shuttle effect, which hinders the commercialization of Li-S batteries. To tackle this issue, morpholine molecules were successfully grafted onto a commercial polypropylene separator. Density functional theory (DFT) calculations were performed and revealed that morpholine side chains could equally and reversibly grasp all the high-order polysulfides. This diatomic chemisorption adjusted the transformation process among the sulfur-related compounds. The modified separator battery possessed a discharge capacity as high as 827.8 mAh·g^-1 after 500 cycles at 0.5 C. The low capacity fading rate, symmetrical cyclic voltammogram, and retention of the electrode morphology all suggest that the diatomic equal adsorption approach can successfully suppress the polysulfide shuttle effect while maintaining excellent battery performance.

Designing of a Phosphorus, Nitrogen, and Sulfur Three-Flame Retardant Applied in a Gel Poly-m-phenyleneisophthalamide Nanofiber Membrane for Advanced Safety Lithium-Sulfur Batteries

Based on the urgent demand of non-flammable electrospun nanofiber separators and the strong adsorption to polysulfides through chemical doping in separators for Li-S cell, in this study, a phosphorus, nitrogen, and sulfur three-flame retardant (di-(2-(5,5-dimethyl-2-sulfido-1,3,2-dioxaphosphinan-2-yl)hydrazineyl)-P-ethylphosphinic) was synthesized and a high-performance flame-retarding poly-m-phenyleneisophthalamide (PMIA) membrane was successfully prepared through blend electrospinning with the flame retardant, it is regarded as a promising gel nanofiber membrane with advanced safety for the lithium-sulfur (Li-S) cell, and it was systematically explored and analyzed. It was presented that the modified PMIA electrospun membrane with the synthesized flame retardant possessed excellent flame retardation, outstanding thermal stability, and good mechanical strength. Meanwhile, the prepared membrane showed extraordinarily high uptake and preserving retention of the liquid electrolyte and enhanced ionic conductivity. More importantly, the assembled Li-S cells using the obtained membrane exhibited excellent cycling retention and outstanding rate capability because of its fast ion transportation and good interfacial compatibility. The assembled batteries with the novel membrane exhibited a high first-cycle discharge capacity of 1121.50 mA h g^-1, superior discharge capacity retention of 713.41 mA h g^-1, and high Coulombic efficiency of 98.46% after 600 cycles at the 0.5 C rate. In addition, the limiting oxygen index of the obtained nanofiber membrane with flame retardancy was as high as ∼30.0%, which could greatly enhance the safety of the electrospun nanofiber separator. The excellent electrochemical performances and safety for the battery assembled with the prepared gel PMIA nanofiber membrane were attributed to the significantly prevented "shuttle effect" of lithium polysulfides based on the physical capturing of lithium polysulfides through the obtained jelly-like gel state and chemical binding of polysulfide intermediates through the tridoped phosphorus, nitrogen, and sulfur elements in the PMIA and the flame retardant. All of these excellent properties will promote the great development of the Li-S battery with high performance and satisfactory safety.