Phosphorus-Doped Metal-Organic Framework-Derived CoS2 Nanoboxes with Improved Adsorption-Catalysis Effect for Li-S Batteries

Preparation of a lithium-sulfur battery diaphragm catalyst and its battery performance

Lithium-sulfur batteries (LSBs) with metal lithium as the anode and elemental sulfur as the cathode active materials have attracted extensive attention due to their high theoretical specific capacity (1675 mA h g-1), high theoretical energy density (2600 W h kg-1), low cost, and environmental friendliness. However, the discharge intermediate lithium polysulfide undergoes a shuttle side reaction between the two electrodes, resulting in low utilization of the active substances. This limits the capacity and cycle life of LSBs and further delays their commercial development. However, the number of active sites and electron transport capacity of such catalysts still do not meet the practical development needs of lithium-sulfur batteries. In view of these issues, this paper focuses on a zinc-cobalt compound catalyst, modifying it through heteroatom doping, bimetallic synergistic effect and heterogeneous structure design to enhance the performance of LSBs as a separator modification material. A carbon shell-supported boron-doped ZnS/CoS2 heterojunction catalytic material (B-ZnS/CoS2@CS) was prepared, and its performance in lithium-sulfur batteries was evaluated. A carbon substrate (CS) was prepared by pyrolysis of sodium citrate, and the boron-doped ZnS/CoS2 heterojunction catalyst was formed on the CS using a one-step solvothermal method. The unique heterogeneous interface provides numerous active sites for the adsorption and catalysis of polysulfides. The uniformly doped, electron-deficient boron further enhances the Lewis acidity of the ZnS/CoS2 heterojunction, while also regulating electron transport. The B-ZnS/CoS2@CS catalyst effectively inhibits the diffusion of LiPS anions by utilizing additional lone-pair electrons. The lithium-sulfur battery using the catalyst-modified separator achieves a high specific capacity of 1241 mA h g-1 at a current density of 0.2C and retains a specific capacity of 384.2 mA h g-1 at 6.0C. In summary, B-ZnS/CoS2@CS heterojunction catalysts were prepared through boron doping modification. They can promote the conversion of polysulfides and effectively inhibit the shuttle effect. The findings provide valuable insights for the future modification and preparation of lithium-sulfur battery catalysts.

Progresses and Perspectives of Carbon-Free Metal Compounds-Modified Separators for High-Performance Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) have the advantages of high theoretical specific capacity, excellent energy density, abundant elemental sulfur reserves. However, the LSBs is mainly limited by shuttling of lithium polysulfides (LiPSs), slow reaction kinetics of sulfur cathode. For solving the above problems, by developing high-performance battery separators, the reversible capacity, Coulombic efficiency (CE) and cycle life of LSBs can be effectively enhanced. Carbon-free based metal compounds are expected to be highly efficient separator modifiers for a new generation of high-performance LSBs by virtue of superior chemical adsorption capacity, strong catalytic properties and excellent lithophilicity to a certain extent. They can give play to the synergistic effect of their "adsorption-catalysis" sites to accelerate the redox kinetics of LiPSs, and their good lithophilicity can accelerate the Li+ transport kinetics, thus showing more remarkable electrochemical performances. However, a comprehensive summary of carbon-free metal compounds-modified separators for LSBs is still lacking. Here, this review systematically summarizes the researching progresses and performance characteristics of carbon-free-based metal compounds modified materials for separators of LSBs, and summarizes the corresponding mechanisms of using carbon-based separators to enhance the performance of LSBs. Finally, the review also looks forward to the prospects of LSBs using carbon-free metal compounds separators.

Manipulating Orbital Hybridization of CoSe2 by S Doping for the Highly Active Catalytic Effect of Lithium-Sulfur Batteries

In recent years, various transition metal compounds have been extensively studied to deal with the problems of slow reaction kinetics and the shuttle effect of lithium-sulfur (Li-S) batteries. Nevertheless, their catalytic performance still needs to be further improved by enhancing intrinsic catalytic activity and enriching active sites. Doping is an effective means to boost the catalytic performance through adjusting the electron structure of the catalysts. Herein, the electron structure of CoSe2 is adjusted by doping P, S with different p electron numbers and electronegativity. After S doping (S-CoSe2), the content of Co2+ increases, and charge is redistributed. Furthermore, more electrons are transferred between Li2S4/Li2S and S-CoSe2, and optimal Co-S bonds are formed between them with optimized d-p orbital hybridization, making the bonds of Li2S4/Li2S the longest and easy to break and decompose. Consequently, the Li-S batteries with the S-CoSe2-modified separator achieve improved rate performance and cycling performance, benefiting from the better bidirectional catalytic activity. This work will provide reference for the selection of the anion doping element to enhance the catalytic effect of transition metal compounds.

Multifunctional Heterostructured Fe3O4-FeTe@MCM Electrocatalyst Enabling High-Performance Practical Lithium-Sulfur Batteries Via Built-in Electric Field

The development of capable of simultaneously modulating the sluggish electrochemical kinetics, shuttle effect, and lithium dendrite growth is a promising strategy for the commercialization of lithium-sulfur batteries. Consequently, an elaborate preparation method is employed to create a host material consisting of multi-channel carbon microspheres (MCM) containing highly dispersed heterostructure Fe3O4-FeTe nanoparticles. The Fe3O4-FeTe@MCM exhibits a spontaneous built-in electric field (BIEF) and possesses both lithophilic and sulfophilic sites, rendering it an appropriate host material for both positive and negative electrodes. Experimental and theoretical results reveal that the existence of spontaneous BIEF leads to interfacial charge redistribution, resulting in moderate polysulfide adsorption which facilitates the transfer of polysulfides and diffusion of electrons at heterogeneous interfaces. Furthermore, the reduced conversion energy barriers enhanced the catalytic activity of Fe3O4-FeTe@MCM for expediting the bidirectional sulfur conversion. Moreover, regulated Li deposition behavior is realized because of its high conductivity and remarkable lithiophilicity. Consequently, the battery exhibited long-term stability for 500 cycles with 0.06% capacity decay per cycle at 5 C, and a large areal capacity of 7.3 mAh cm-2 (sulfur loading: 9.73 mg cm-2) at 0.1 C. This study provides a novel strategy for the rational fabrication of heterostructure hosts for practical Li-S batteries.

Applications, prospects and challenges of metal borides in lithium sulfur batteries

Although the lithium-sulfur (Li-S) battery has a theoretical capacity of up to 1675 mA h g-1, its practical application is limited owing to some problems, such as the shuttle effect of soluble lithium polysulfides (LiPSs) and the growth of Li dendrites. It has been verified that some transition metal compounds exhibit strong polarity, good chemical adsorption and high electrocatalytic activities, which are beneficial for the rapid conversion of intermediate product in order to effectively inhibit the "shuttle effect". Remarkably, being different from other metal compounds, it is a significant characteristic that both metal and boron atoms of transition metal borides (TMBs) can bind to LiPSs, which have shown great potential in recent years. Here, for the first time, almost all existing studies on TMBs employed in Li-S cells are comprehensively summarized. We firstly clarify special structures and electronic features of metal borides to show their great potential, and then existing strategies to improve the electrochemical properties of TMBs are summarized and discussed in the focus sections, such as carbon-matrix construction, morphology control, heteroatomic doping, heterostructure formation, phase engineering, preparation techniques. Finally, the remaining challenges and perspectives are proposed to point out a direction for realizing high-energy and long-life Li-S batteries.

A Review on Engineering Transition Metal Compound Catalysts to Accelerate the Redox Kinetics of Sulfur Cathodes for Lithium-Sulfur Batteries

Engineering transition metal compounds (TMCs) catalysts with excellent adsorption-catalytic ability has been one of the most effective strategies to accelerate the redox kinetics of sulfur cathodes. Herein, this review focuses on engineering TMCs catalysts by cation doping/anion doping/dual doping, bimetallic/bi-anionic TMCs, and TMCs-based heterostructure composites. It is obvious that introducing cations/anions to TMCs or constructing heterostructure can boost adsorption-catalytic capacity by regulating the electronic structure including energy band, d/p-band center, electron filling, and valence state. Moreover, the electronic structure of doped/dual-ionic TMCs are adjusted by inducing ions with different electronegativity, electron filling, and ion radius, resulting in electron redistribution, bonds reconstruction, induced vacancies due to the electronic interaction and changed crystal structure such as lattice spacing and lattice distortion. Different from the aforementioned two strategies, heterostructures are constructed by two types of TMCs with different Fermi energy levels, which causes built-in electric field and electrons transfer through the interface, and induces electron redistribution and arranged local atoms to regulate the electronic structure. Additionally, the lacking studies of the three strategies to comprehensively regulate electronic structure for improving catalytic performance are pointed out. It is believed that this review can guide the design of advanced TMCs catalysts for boosting redox of lithium sulfur batteries.

Co/CoS2 Heterojunction Embedded in N, S-Doped Hollow Nanocage for Enhanced Polysulfides Conversion in High-Performance Lithium-Sulfur Batteries

Modulating the electronic configuration of the substrate to achieve the optimal chemisorption toward polysulfides (LiPSs) for boosting polysulfide conversion is a promising way to the efficient Li-S batteries but filled with challenges. Herein, a Co/CoS2 heterostructure is elaborately built to tuning d-orbital electronic structure of CoS2 for a high-performance electrocatalyst. Theoretical simulations first evidence that Co metal as the electron donator can form a built-in electric field with CoS2 and downshift the d-band center, leading to the well-optimized adsorption strength for lithium polysulfides on CoS2 , thus contributing a favorable way for expediting the redox reaction kinetics of LiPSs. As verification of prediction, a Co/CoS2 heterostructure implanted in porous hollow N, S co-doped carbon nanocage (Co/CoS2 @NSC) is designed to realize the electronic configuration regulation and promote the electrochemical performance. Consequently, the batteries assembled with Co/CoS2 @NSC cathode display an outstanding specific capacity and an admirable cycling property as well as a salient property of 8.25 mAh cm-2 under 8.18 mg cm-2 . The DFT calculation also reveals the synergistic effect of N, S co-doping for enhancing polysulfide adsorption as well as the detriment of excessive sulfur doping.

Nitrogen/sulfur dual-doped micro-mesoporous hierarchical porous carbon as host for Li-S batteries

A simple hydrothermal process employing sucrose and glutathione as the source of carbon and nitrogen-sulfur, respectively, a porous carbon/sulfur composite material doped with nitrogen and sulfur (NSPCS) was synthesized. The detailed structure information of the material was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The morphology information was investigated through Scanning Electron Microscope (SEM) methods. Structure of the pores and pore size distribution were investigated employing N₂ adsorption-desorption isotherm. The material was treated Thermogravimetric analysis (TGA) in order to know the weight ratio of sulfur. The synthesized NSPCS composite produced high specific capacity, excellent rate performance and exceptionally good cycle stability when used as the positive electrode in Li-S batteries.

The Catalyst Design for Lithium-Sulfur Batteries: Roles and Routes

Lithium-sulfur battery is a promising candidate for next-generation high energy density batteries due to its ultrahigh theoretical energy density. However, it suffers from low sulfur utilization, fast capacity decay, and the notorious "shuttle effect" of lithium polysulfides (LiPSs) due to the sluggish reaction kinetics, which severely restrict its practical applications. Using the electrocatalyst can accelerate the redox reactions between sulfur, LiPSs and Li₂ S and suppress the shuttling of LiPSs, and thus, it is a promising strategy to solve the above problems, enabling the battery with high energy density and long cycling stability. In this personal account, we discuss the catalyst design for lithium-sulfur batteries according to the sulfur reduction reaction (SRR) and sulfur evolution reaction (SER) in the discharging and charging processes. The catalytic effects for each step in SRR and SER are highlighted and the homogenous catalysts, the selective catalysts, and the bidirectional catalysts are discussed, which can help guide the rational design of the catalysts and practical applications of lithium-sulfur batteries.

Manipulating the Li-S reaction kinetics via the V8C7/phosphorus defect-integrated carbon promoter

V₈C₇/phosphorus defect-integrated carbon (VPC) is proposed as a dual-function promoter for Li-S chemistry. The well-dispersed V₈C₇ and phosphorus defects exhibit ample polar sites and remarkable electron conductivity. Such rational integration of dual active centers simultaneously suppresses the shuttle effect and propels the Li-S redox reaction kinetics. Therefore, the S/VPC cathode shows an initial capacity of 1090.0 mA h g^-1 and a high retention of 83.5% at 0.2C after 100 cycles and a low decay rate of 0.076% at 2C over 600 cycles.

A thin and multifunctional CoS@g-C3N4/Ketjen black interlayer deposited on polypropylene separator for boosting the performance of lithium-sulfur batteries

The sluggish redox kinetic and shuttle effect of polysulfides still obstruct the commercial application of lithium-sulfur (Li-S) batteries. Herein, a nanocomposite consisting of well-dispersed and lamellar-like shape CoS anchored on g-C₃N₄ nanosheets (CoS@g-C₃N₄) is prepared firstly, and then it is integrated on a polypropylene membrane combined with little conductive Ketjen black (KB) to fabricate a multifunctional and quite thin interlayer, with a thickness of only ∼ 2.1 um and areal mass loading of ∼ 0.07 mg·cm^-2. The as-prepared interlayer firstly can capture polysulfides by Li-N bond as well as Lewis acid-base interaction between CoS and polysulfide anions (S_n^2-), and more importantly, it also displays a positive effect on catalyzing the redox conversion of intermediate polysulfides. As expected, a Li-S cell assembled with this modified separator and high sulfur content cathode displays an excellent electrochemical performance, with specific capacity of ∼ 1290 mAh g^-1 at 0.2C and a low fading rate of 0.03% per cycle after 500 cycles at 1.0C. Furthermore, a high sulfur mass loading of ∼ 4.0 mg·cm^-2 electrode paired with this multifunctional separator exhibits a stable specific capacity of ∼ 600 mAh g^-1 after 250 cycles under 0.1C. This work can give some guides to rational design a quite thin and light interlayer for improving the utilization of sulfur species, with little damage to the energy density and Li ion transportation in Li-S batteries.

Conductive Al-Doped ZnO Framework Embedded with Catalytic Nanocages as a Multistage-Porous Sulfur Host in Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries possess many practical challenges including the lithium polysulfide (LiPS) "shuttle effect" and their sluggish conversion kinetics. To address these issues, a unique hierarchical porous architecture, combining highly conductive ordered macroporous skeleton and embedded microporous particles is rationally designed as a dual-effective polysulfide immobilizer and conversion promoter. In this nanoporous architecture, Al-doped ZnO (AZO) acts as a conductive macroporous framework, profiting chemical anchoring of LiPS as well as facilitating electrolyte infiltration and ion diffusion; Co nanoparticle-anchored N-doped carbon (Co-NC) derived from CoZn-metal-organic framework is embedded in the macropores to further strengthen the LiPS adsorption, catalytically accelerating conversion kinetics of LiPS simultaneously. Consequently, the Co-NC@AZO/S cathode delivers a notable rate capability of 635.5 mA h g^-1 at 5 C. A high area capacity of about 5.8 mA h cm^-2 with a mass loading of 6.8 mg cm^-2 is also achieved under a lean electrolyte (E/S = 5.7). Additionally, the Li-S pouch cells equipped with Co-NC@AZO can be extended to sulfur loading as high as 4.0 mg cm^-2, delivering a superb capability of 897.5 mA h g^-1 after 100 cycles. This work puts forward a design for stably cycled and practically viable Li-S batteries.

Tg-C3N4-coated functional separator as polysulfide barrier of high-performance lithium-sulfur batteries

Lithium sulfur (Li-S) battery is considered as a promising alternative for the development of battery technologies. However, the shuttle effect seriously limits its practical use. Herein, hollow tubular graphene-like carbon nitride (Tg-C₃N₄) is synthesized and utilized as a functional interlayer to inhibit shuttling effect and promote catalytic kinetics. Both experiments and DFT calculations together suggest that N-doping enhances the electron transfers between Tg-C₃N₄and LiPSs, leading to improved chemical adsorptions and catalytic effects towards the redox conversions of the active sulfur species. Besides, Tg-C₃N₄delivers a unique hollow tubular architecture with massive ion transfer pathways and fully exposed active interfaces. In addition, the abundant C-N heteroatomic structures also impose strong chemical immobilization toward lithium polysulfides. Benefiting from these unique superiorities, the cell with the Tg-C₃N₄-modified separator exhibits a reversible capacity of 494 mAh g^-1after 500 cycles at 1 C with a negligible capacity decay of 0.085% per cycle, indicating an efficient strategy toward high-performance modified separators.

A multi-responsive MOF-based fluorescent probe for detecting Fe3+, Cr2O72− and acetylacetone

A novel ZnII-based MOF can selectively and sensitively recognize Fe3+, Cr2O72− and acetylacetone, simultaneously.