Ion-Selective Prussian-Blue-Modified Celgard Separator for High-Performance Lithium-Sulfur Battery

Reaction kinetics of lithium-sulfur batteries with a polar Li-ion electrolyte: modeling of liquid phase and solid phase processes

The present investigation fits the reaction kinetics of a lithium-sulfur (Li-S) battery with polar electrolyte employing a novel two-phase continuum multipore model. The continuum two-phase model considers processes in both the liquid electrolyte phase and the solid precipitates phase, where the diffusion coefficients of the Li+ ions in a solvent-softened solid state are determined from molecular dynamics simulations. Solubility experiments yield the saturation concentration of sulfur and lithium sulfides in the polar electrolyte employed in this study. The model describes the transport of dissolved molecular and ion species in pores of different size in solvated or desolvated form, depending on pore size. The Li-S reaction model in this study is validated for electrolyte 1 M LiPF6 in EC/DMC. It includes seven redox reactions and two cyclic non-electrochemical reactions in the cathode, and the lithium redox reaction at the anode. Electrochemical reactions are assumed to take place in the electrolyte solution or the solid state and cyclic reactions are assumed to take place in the liquid electrolyte phase only. The determination of the reaction kinetics parameters takes place via fitting the model predictions with experimental data of a cyclic voltammetry cycle with in operando UV-vis spectroscopy.

Biopolymer separators from polydopamine-functionalized bacterial cellulose for lithium-sulfur batteries

The advancement of the lithium-sulfur (Li-S) batteries is immensely impeded by two main challenges: polysulfide shuttling between the electrodes and Li dendrite formation associated with the Li-metal anode. To tackle these challenges, we synthesized a polydopamine coated bacterial cellulose (PDA@BC) separator in a way to create physical and chemical traps for the shuttling polysulfides and to control the Li+ flux. While nanocellulose offers its dense network as a physical trap, the presence of polydopamine in the separator offers polar functional groups which not only has a high binding energy towards the polysulfides but also helps in redistribution of the Li+ ions across it. The electrochemical and physiochemical results suggest that the synthesized separator can have practical applicability owing to its superior performance compared to a commercial separator. The Li-S batteries assembled with this separator showed a specific discharge capacity of 1449 mAh/g at 0.1C and 877 mAh/g at 1C, and a capacity fade of 0.03 % per cycle over 650 cycles at 1C. Using a PDA@BC separator, a practical Li-S battery cell with S loading of 7.5 mg cm-2 (and E/S ratio of 10 µLmg-1, 82 % S ratio) was also tested at 1C, which delivered a capacity of ∼ 6 mAh cm-2 for 500 cycles.

The role of selenium vacancies functionalized mediator of bimetal (Co, Fe) selenide for high-energy-density lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries are currently only in the basic research stage and have not been commercialized, which is mainly affected by the poor conductivity of sulfur/lithium sulfide (S/Li₂S), volume expansion effect of sulfur and the shuttle effect of lithium polysulfides (LiPSs). Herein, a three dimensional (3D) carbon nanotubes (CNTs) decorated cubic Co₉Se_8-x/FeSe_2-y (0 ＜ x ＜ 8, 0 ＜ y ＜ 2) composite (Co₉Se_8-x/FeSe_2-y@CNTs) is developed, and used as the functionalized mediator on polypropylene (PP) in Li-S batteries. Benefiting from the good electrical conductivity, large number of Se vacancies and the triple block/adsorption/catalytic effects of Co₉Se_8-x/FeSe_2-y@CNTs, the cell with Co₉Se_8-x/FeSe_2-y@CNTs//PP modified separator delivers a high reversible capacity (1103.5 mA h g^-1) at 1C after three cycles activation at 0.5C and remains 446 mA g h^-1 after 750 cycles with a 0.08% capacity decay rate each cycle. Moreover, at 0.2C, a high areal capacity of 3.63 mA h cm^-2 after 100 cycles with a high sulfur loading of 4.1 mg cm^-2 is obtained. The in-situ XRD tests revealing the transition path of α-S₈ → Li₂S → β-S₈ during the first charge-discharge process, then β-S₈ → Li₂S → β-S₈ conversion reaction in the next cycles, and firstly determine the sulfur-selenide active intermediates (Se_1.1S_6.9) during cycles. The work provides a new insight into the development of bimetallic selenide composites by defect engineering with highly adsorptive and catalytic properties for Li-S batteries.

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.

Tuning the Metal Ions of Prussian Blue Analogues in Separators to Enable High-Power Lithium Metal Batteries

The Li dendrite issue is the major barrier that limits the implement of Li metal anode practically, especially at high current density. From the perspective of the nucleation and growth mechanism of the Li dendrite, we rationally develop a novel Prussian blue analogues (PBA)-derived separator, where tuning the metal ions bestows the PBAs with open metal site to confine anion movement and thereby afford a high Li⁺ transference number (0.78), and PBA with ordered micropores could act as an ionic sieve to selectively extract Li⁺ and thereby homogenize Li⁺ flux. This demonstrates a highly reversible Li plating/stripping cycling for 3000 h at a practically high current density (5.0 mA cm^-2). Consequently, a high loading Li||LiFeO₄ battery (∼10.0 mg cm^-2) demonstrates ultralong cycling life at high current densities (∼5.1 mA cm^-2). This work highlights the prospect of optimizing PBAs in regulating ion transport behavior to enable high-power Li metal batteries.

Plasma and magnetron sputtering constructed dual-functional polysulfides barrier separator for high-performance lithium-sulfur batteries

In order to fundamentally suppress the shuttle effect, N₂ Plasma & Al₂O₃ magnetron sputtered separators (Al₂O₃@N-PP) are proposed for lithium-sulfur batteries (LSBs). Such a dual-functional polysulfides (LiPSs) barrier separator greatly inhibits the shuttle effect from the perspective of physical and chemical interaction. Physically, the inherently electronegative amorphous Al₂O₃ first achieves the repulsion of LiPSs to the sulfur cathode through the electrostatic repulsive effect, effectively preventing a large amount of soluble LiPSs from accumulating at the separator. At the same time, the Al₂O₃ film seals the shuttle channel of LiPSs to a certain extent. Chemically, N₂ plasma-doped N heteroatoms form a lithium bond with Li⁺ in LiPSs to achieve the first step chemical adsorption and anchoring of LiPSs. When the LiPSs reaches the amorphous Al₂O₃ film, more stable chemical bonds are formed between Al³⁺ and S^2-, Li⁺ and O^2- to achieve more effective adsorption and anchoring of LiPSs. At 1C with a high sulfur loading up to 3-5 mg cm^-2 the LSB contributes a specific charge capacity of 717.4 mAh g^-1, with high retention rate up to 75.49 % after 450 cycles. The U-shaped electrolytic cell experiment and ultraviolet-visible spectrum experiment confirmed the LiPSs barrier function of the functional separator.

Layered Double Hydroxide-Assisted Fabrication of Prussian Blue Membranes for Precise Molecular Sieving

Prussian Blue (PB), which was first discovered as robust blue-colored pigment in the year 1706, has shown promising prospects in disease treatment, energy conversion, water splitting, and sensing. Relying on the uniform 3.2 Å-sized pore channels as well as high stability in aqueous environments, in this study, we pioneered in situ preparation of polycrystalline PB membranes to justify their dye rejection and metal ion discrimination ability in aqueous environments. Among various factors, the introduction of calcined NiFe layered double hydroxide buffer layers on porous α-Al₂ O₃ substrates was found to play a paramount role in the formation of continuous polycrystalline PB membranes, thereby leading to excellent dye rejection efficiency (>99.0 %). Moreover, prepared PB membranes enabled discriminating different monovalent metal ions (e.g., Li⁺ , Na⁺ , and K⁺ ) depending on their discrepancy in Stokes diameters, showing great promise for lithium extraction from smaller-sized metal ions.

Metal-organic frameworks enable broad strategies for lithium-sulfur batteries

The lithium-sulfur (Li-S) battery is considered to be a potential next-generation power battery system, however, it is urgent that suitable materials are found in order to solve a series of challenges, such as the shuttle effect and lithium dendrite growth. As a multifunctional porous material, metal-organic frameworks (MOFs) can be used in different parts of Li-S batteries. In recent years, the application of MOFs in Li-S batteries has been developed rapidly. This review summarizes the milestone works and recent advances of MOFs in various aspects of Li-S batteries, including cathode, separator and electrolyte. The factors affecting the performance of MOFs and the working mechanisms of MOFs in these different parts are also discussed in detail. Finally, the opportunities and challenges for the application of MOFs in Li-S batteries are proposed. We also put forward feasible solutions to related problems. This review will provide better guidance for the rational design of novel MOF-based materials for Li-S batteries.

Defect-Rich Amorphous Iron-Based Oxide/Graphene Hybrid-Modified Separator toward the Efficient Capture and Catalysis of Polysulfides

The sluggish sulfur reduction reaction, severe shuttle effect, and poor conductivity of sulfur species are three main problems in lithium-sulfur (Li-S) batteries. Functional materials with a strong affinity and catalytic effect toward polysulfides play a key role in addressing these issues. Herein, we report a defect-rich amorphous a-Fe₃O_4-x/GO material with a nanocube-interlocked structure as an adsorber as well as an electrocatalyst for the Li-S battery. The composition and defect structure of the material are determined by X-ray diffraction, high-resolution transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy measurements. The distinctive open framework architecture of the as-engineered composite inherited from the metal-organic framework precursor ensures the stability and activity of the catalyst during extended cycles. The oxygen defects in the amorphous structure are capable of absorbing polysulfides and similarly work as catalytic centers to boost polysulfide conversion. Taking advantage of a-Fe₃O_4-x/GO on the separator surface, the Li-S battery shows a capacity over 610 mA h g^-1 at 1 C and a low decay rate of 0.12% per cycle over 500 cycles and superior rate capability. The functional material made via the low-cost synthesis process provides a potential solution for advanced Li-S batteries.

Advances in Lithium-Sulfur Batteries: From Academic Research to Commercial Viability

Lithium-ion batteries, which have revolutionized portable electronics over the past three decades, were eventually recognized with the 2019 Nobel Prize in chemistry. As the energy density of current lithium-ion batteries is approaching its limit, developing new battery technologies beyond lithium-ion chemistry is significant for next-generation high energy storage. Lithium-sulfur (Li-S) batteries, which rely on the reversible redox reactions between lithium and sulfur, appears to be a promising energy storage system to take over from the conventional lithium-ion batteries for next-generation energy storage owing to their overwhelming energy density compared to the existing lithium-ion batteries today. Over the past 60 years, especially the past decade, significant academic and commercial progress has been made on Li-S batteries. From the concept of the sulfur cathode first proposed in the 1960s to the current commercial Li-S batteries used in unmanned aircraft, the story of Li-S batteries is full of breakthroughs and back tracing steps. Herein, the development and advancement of Li-S batteries in terms of sulfur-based composite cathode design, separator modification, binder improvement, electrolyte optimization, and lithium metal protection is summarized. An outlook on the future directions and prospects for Li-S batteries is also offered.

A Review of Functional Separators for Lithium Metal Battery Applications

Lithium metal batteries are considered “rough diamonds” in electrochemical energy storage systems. Li-metal anodes have the versatile advantages of high theoretical capacity, low density, and low reaction potential, making them feasible candidates for next-generation battery applications. However, unsolved problems, such as dendritic growths, high reactivity of Li-metal, low Coulombic efficiency, and safety hazards, still exist and hamper the improvement of cell performance and reliability. The use of functional separators is one of the technologies that can contribute to solving these problems. Recently, functional separators have been actively studied and developed. In this paper, we summarize trends in the research on separators and predict future prospects.

Stepwise Electrocatalysis as a Strategy against Polysulfide Shuttling in Li-S Batteries

Most issues with Li-S batteries are caused by the slowness of the multielectron sulfur electrochemical reaction resulting in the loss of sulfur as soluble polysulfides to the electrolyte and the redox shuttling of polysulfides between the cathode and anode during battery charge and discharge. The acceleration of the polysulfide conversion reaction to their end products via electrocatalysis has the appeal of a root-cause solution. However, the polysulfide electrocatalysts developed to date have rarely considered polysulfide conversion as a multistep reaction and, as such, were not optimized to target specific steps in the overall S₈ ↔ Li₂S_n ↔ Li₂S conversion. The targeting approach is however beneficial, as it can be used to design multicatalyst systems to reduce as many rate-limiting steps in the overall catalysis as effectively possible. This article demonstrates the concept and implementation of stepwise electrocatalysis in polysulfide conversion, using Fe-N and Co-N co-doped carbons to selectively catalyze the long-chain polysulfide conversion (S₈ ↔ Li₂S₄) and the short-chain polysulfide conversion reactions (Li₂S₄ ↔ Li₂S), respectively. The two electrocatalysts were deployed in the sulfur cathode as a dual layer, using an ordered spatial separation to synergize their catalytic effects. A sulfur electrode designed as such could utilize ∼90% of the sulfur theoretical specific capacity and support a high areal capacity of ∼8.3 mAh cm^-2 and a low electrolyte/sulfur ratio of 5 μL mg^-1.

Compactly Coupled Nitrogen-Doped Carbon Nanosheets/Molybdenum Phosphide Nanocrystal Hollow Nanospheres as Polysulfide Reservoirs for High-Performance Lithium-Sulfur Chemistry

Lithium-sulfur (Li-S) batteries have been disclosed as one of the most promising energy storage systems. However, the low utilization of sulfur, the detrimental shuttling behavior of polysulfides, and the sluggish kinetics in electrochemical processes, severely impede their application. Herein, 3D hierarchical nitrogen-doped carbon nanosheets/molybdenum phosphide nanocrystal hollow nanospheres (MoP@C/N HCSs) are introduced to Li-S batteries via decorating commercial separators to inhibit polysulfides diffusion. It acts not only as a polysulfides immobilizer to provide strong physical trapping and chemical anchoring toward polysulfides, but also as an electrocatalyst to accelerate the kinetics of the polysulfides redox reaction, and to lower the Li₂ S nucleation/dissolution interfacial energy barrier and self-discharge capacity loss in working Li-S batteries, simultaneously. As a result, the Li-S batteries with MoP@C/N HCS-modified separators show superior rate capability (920 mAh g^-1 at 2 C) and stable cycling life with only 0.04% capacity decay per cycle over 500 cycles at 1 C with nearly 100% Coulombic efficiency. Furthermore, the Li-S battery can achieve a high area capacity of 5.1 mAh cm^-2 with satisfied capacity retention when the cathode loading reaches 5.5 mg cm^-2 . This work offers a brand new guidance for rational separator design into the energy chemistry of high-stable Li-S batteries.

A Novel Hierarchically Porous Polypyrrole Sphere Modified Separator for Lithium-Sulfur Batteries

The commercialization of Lithium-sulfur batteries was limited by the polysulfide shuttle effect, and modifying the routine separator was an effective method to solve this problem. In this work, a novel hierarchically porous polypyrrole sphere (PPS) was successfully prepared by using silica as hard-templates. As-prepared PPS was slurry-coated on the separator, which could reduce the polarization phenomenon of the sulfur cathode, and efficiently immobilize polysulfides. As expected, high sulfur utilization was achieved by suppressing the shuttle effect. When tested in the lithium-sulfur battery, it exhibited a high capacity of 855 mAh·g^-1 after 100 cycles at 0.2 C, and delivered a reversible capacity of 507 mAh·g^-1 at 3 C, showing excellent electrochemical performance.

Blocking Polysulfide with Co2B@CNT via "Synergetic Adsorptive Effect" toward Ultrahigh-Rate Capability and Robust Lithium-Sulfur Battery

Li-S batteries have attracted great interest as the next-generation secondary batteries due to their high energy density, being environmentally friendly, and low price. However, the road to commercialization of lithium-sulfur batteries remains limited owing to the "shuttle effect" of soluble polysulfides, which results in the inferior cycle stability. Herein, a potent functional separator is developed to restrain the "shuttle effect" by coating Co₂B@carbon nanotube layer on the commercialized polypropylene separator. In merits of the coadsorption of Co sites and B sites, such Co₂B shows highly efficient polysulfides block (11.67 mg/m² for Li₂S₆). Besides, the composite also exhibits obviously catalysis from Li₂S₈ to Li₂S. By combining the fast electron transportation along the carbon nanotube, a superior rate performance is achieved with the modified separator and common carbon-sulfur cathode. Typically, the cell with Co₂B@CNT shows prominent cycling life with a capacity degradation of 0.0072% per cycle (3000 cycles) and ultrahigh-rate capability at 5 C current (1172.8 mAh/g), which outstands the previously reported polysulfides barrier layer. The cell with Co₂B@CNT can exhibit electrochemical performance at areal capacity of 5.5 mAh/cm² (0.5 C) when the sulfur loading increased to 5.8 mg/cm². This work defines an efficacious strategy to restrain the "shuttle effect" of polysulfides and shed light on the great potential of borides in Li-S battery.

Polysulfide Trapping in Carbon Nanofiber Cloth/S Cathode with a Bifunctional Separator for High-Performance Li-S Batteries

The development of carbon nanofiber (CNF) cloth-based cathodes is essential for the fabrication of high energy density and flexible Li-S batteries. Surface modification is generally used to improve the electrochemical performance of CNF cloth/S cathodes. However, this strategy creates some problems such as structure collapse, complex fabrication steps, and poor consistency. Herein, a β-MnO₂ nanowire/graphene-modified separator is used to improve the performance of CNF cloth/S cathodes without changing their structure. β-MnO₂ can facilitate chemical bonding with polysulfides, whereas graphene can decrease the inner resistance and trap polysulfide by physical shielding. The cathode with the bifunctional separator displayed a high discharge capacity of 529.9 mAh g^-1 with a low capacity decay of 0.051 % per cycle after 500 cycles at 1 C, which is 3 times higher compared with a bare separator. Even with a high sulfur loading of 9.0 mg cm^-2 , a high areal capacity of 3.8 mAh cm^-2 was delivered over 100 cycles.

A Bioinspired Functionalization of Polypropylene Separator for Lithium-Sulfur Battery

Lithium-sulfur batteries have received intensive attention, due to their high specific capacity, but the shuttle effect of soluble polysulfide results in a decrease in capacity. In response to this issue, we develop a novel tannic acid and Au nanoparticle functionalized separator. The tannic acid and gold nanoparticles were modified onto commercial polypropylene separator through a two-step solution process. Due to a large number of phenolic hydroxyl groups contained in the modified layer and the strong polarity of the gold nanoparticles, the soluble polysulfide generated during battery cycling is well stabilized on the cathode side, slowing down the capacity fade brought by the shuttle effect. In addition, the modification effectively improves the electrolyte affinity of the separator. As a result of these benefits, the novel separator exhibits improved battery performance compared to the pristine polypropylene separator.

Inhibiting polysulfide shuttling using dual-functional nanowire/nanotube modified layers for highly stable lithium–sulfur batteries

Dual-functional MnO₂ nanowire/CNT modified layers were prepared to inhibit the polysulfide shuttle effect utilizing their strong adsorption capability and high conductivity.