Fe-cation Doping in NiSe2 as an Effective Method of Electronic Structure Modulation towards High-Performance Lithium-Sulfur Batteries

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.

Optimized Adsorption-Catalytic Conversion for Lithium Polysulfides by Constructing Bimetallic Compounds for Lithium-Sulfur Batteries

Although lithium-sulfur batteries possess the advantage of high theoretical specific capacity, the inevitable shuttle effect of lithium polysulfides is still a difficult problem restricting its application. The design of highly active catalysts to promote the redox reaction during charge-discharge and thus reduce the existence time of lithium polysulfides in the electrolyte is the mainstream solution at present. In particular, bimetallic compounds can provide more active sites and exhibit better catalytic properties than single-component metal compounds by regulating the electronic structure of the catalysts. In this work, bimetallic compounds-nitrogen-doped carbon nanotubes (NiCo)Se2-NCNT and (CuCo)Se2-NCNT are designed by introducing Ni and Cu into CoSe2, respectively. The (CuCo)Se2-NCNT delivers an optimized adsorption-catalytic conversion for lithium polysulfide, benefitting from adjusted electron structure with downshifted d-band center and increased electron fill number of Co in (CuCo)Se2 compared with that of (NiCo)Se2. This endows (CuCo)Se2 moderate adsorption strength for lithium polysulfides and better catalytic properties for their conversion. As a result, the lithium-sulfur batteries with (CuCo)Se2-NCNT achieve a high specific capacity of 1051.06 mAh g-1 at 1C and an enhanced rate property with a specific capacity of 838.27 mAh g-1 at 4C. The work provides meaningful insights into the design of bimetallic compounds as catalysts for lithium-sulfur batteries.

Hybrid Membrane Composed of Nickel Diselenide Nanosheets with Carbon Nanotubes for Catalytic Conversion of Polysulfides in Lithium-Sulfur Batteries

Lithium-sulfur batteries demonstrate enormous energy density are promising forms of energy storage. Unfortunately, the slow redox kinetics and polysulfides shuttle effect are some of the factors that prevent the its development. To address these issues, the hybrid membrane with combination of nickel diselenide nanosheets modified carbon nanotubes (NSN@CNTs) and utilized Li2 S6 catholyte for lithium sulfur battery. The conductive CNTs facilitates fast electronic/ionic transport, while the polarity of NSN as a strong affinity to lithium polysulfides, effectively anchoring them, facilitating the redox conversion of polysulfide species, and effectively diminishing reaction barriers. The cell with NSN@CNTs delivers the first discharge capacity of 1123.8 mAh g-1 and maintains 786.5 mAh g-1 after 300 cycles (0.2 C) at the sulfur loading 5.4 mg. Its rate capability is commendable, enabling it to sustain a capacity of 559.8 mAh g-1 even at a high discharge rate of 2 C. In addition, its initial discharge capacity can remain 8.33 mAh even at 10.8 mg for duration of 100 cycles. This research indicates the potential application of NSN@CNTs hybrid materials in lithium-sulfur 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.

In Situ Fabrication of Mn-Doped NiMoO4 Rod-like Arrays as High Performance OER Electrocatalyst

The slow kinetics of the oxygen evolution reaction (OER) is one of the significant reasons limiting the development of electrochemical hydrolysis. Doping metallic elements and building layered structures have been considered effective strategies for improving the electrocatalytic performance of the materials. Herein, we report flower-like nanosheet arrays of Mn-doped-NiMoO₄/NF (where NF is nickel foam) on nickel foam by a two-step hydrothermal method and a one-step calcination method. The doping manganese metal ion not only modulated the morphologies of the nickel nanosheet but also altered the electronic structure of the nickel center, which could be the result of superior electrocatalytic performance. The Mn-doped-NiMoO₄/NF electrocatalysts obtained at the optimum reaction time and the optimum Mn doping showed excellent OER activity, requiring overpotentials of 236 mV and 309 mV to drive 10 mA cm^-2 (62 mV lower than the pure NiMoO₄/NF) and 50 mA cm^-2 current densities, respectively. Furthermore, the high catalytic activity was maintained after continuous operation at a current density of 10 mA cm^-2 of 76 h in 1 M KOH. This work provides a new method to construct a high-efficiency, low-cost, stable transition metal electrocatalyst for OER electrocatalysts by using a heteroatom doping strategy.

Active Sulfur-Host Material VS4 with Surface Defect Engineering: Intercalation-Conversion Hybrid Cathode Boosting Electrochemical Performance of Li-S Batteries

Transition-metal sulfides as late-model electrocatalysts usually remain inactive in lithium-sulfur (Li-S) batteries in spite of their advantages to accelerate the rapid conversion of lithium polysulfides (LiPSs). Herein, a series of cobalt-doped vanadium tetrasulfide/reduced graphene oxide (x%Co-VS₄/rGO) composites with an ultrathin layered structure as an active sulfur-host material are prepared by a one-pot hydrothermal method. The well-designed two-dimensional ultrathin 3%Co-VS₄/rGO with heteroatom architecture defects (defect of Co-doping and defect of S-vacancies) can significantly improve the adsorption ability on LiPSs, the electrocatalytic activity in the Li₂S potentiostatic deposition, and the active sulfur reduction/oxidation conversion reactions and greatly boost the electrochemical performances of Li-S batteries. On the one hand, the ultrathin 3%Co-VS₄/rGO possesses good conductivity inheriting from rGO which contributes to the capacity of internal redox reactions on lithiation from VS₄. On the other hand, the hybrid architectures provide strong adsorption and excellent electrocatalytic ability on LiPSs, which benefit from the surface defects caused by heteroatom doping. The S@3%Co-VS₄/rGO cathode displays a high specific capacity of 1332.6 mA h g^-1 at 0.2 C and a low-capacity decay of only 0.05% per cycle over 1000 cycles at 3 C with a primary capacity of 633.1 mA h g^-1. Furthermore, when the sulfur loading (single-side coating) reaches 4.48 mg cm^-2, it still can deliver 756.2 mA h g^-1 after the 100th cycle at 0.2 C with 89.5% capacity retention. In addition, the in situ X-ray diffraction test reveals that the sulfur conversion mechanism is the processes of α-S₈ → Li₂S → β-S₈ (first cycle) and then β-S₈ ↔ Li₂S during the subsequent cycles. The designing strategy with heteroatom doping and self-intercalation capacity adopted in this work would provide novel inspiration for fabricating advanced sulfur-host materials to achieve excellent electrochemical capability in Li-S batteries.

Multisize CoS2 Particles Intercalated/Coated-Montmorillonite as Efficient Sulfur Host for High-Performance Lithium-Sulfur Batteries

The chemisorption and catalysis of lithium polysulfides (LiPSs) are effective strategies to suppress the shuttle effect in lithium-sulfur (Li-S) batteries. Herein, multisize CoS₂ particles intercalated/coated-montmorillonite (MMT) as an efficient sulfur host is synthesized. As expected, the obtained S/CoS₂ @MMT cathode achieves an absorption-catalysis synergistic effect through the polar MMT aluminosilicate sheets and the well-dispersed nano-micron CoS₂ particles. Furthermore, efficient interlamellar ion pathways and interconnected conductive network are constructed within the composite host due to the intercalation/coating of CoS₂ in/on MMT. Therefore, the S/CoS₂ @MMT cathode achieves an outstanding rate performance up to 5C (∼548 mAh g^-1 ) and a high cycling stability with low capacity decay of 0.063 and 0.067 % per cycle for 500 cycles at 1C and 2C, respectively. With a higher sulfur loading of 4.0 mg cm^-2 , the cathode still delivers satisfactory rate and cycling performance. It shows that the CoS₂ @MMT host has great application prospects in Li-S batteries.

CoS2@montmorillonite as an efficient separator coating for high-performance lithium-sulfur batteries

The shuttle effect and sluggish redox kinetic of polysulfides still hinder the large-scale application of lithium-sulfur (Li-S) batteries. Herein, we adopt a CoS2-intercalated/coated-montmorillonite (CoS2@montmorillonite) composite to work as an efficient...