Cobalt and Molybdenum Carbide Nanoparticles Grafted on Nitrogen‐Doped Carbon Nanotubes as Efficient Chemical Anchors and Polysulfide Conversion Catalysts for Lithium‐Sulfur Batteries

Mo3P/Mo heterojunction for efficient conversion of lithium polysulfides in high-performance lithium-sulfur batteries

Realizing efficient immobilization of lithium polysulfides (LiPSs) as well as reversible catalytic conversion between LiPSs and the insoluble Li2S is vital to restrain the shuttle effect, which requires highly reactive catalysts for high-performance Li-S batteries. Here, three-dimensional ordered porous Mo-based metal phosphides (3DOP Mo3P/Mo) with heterogeneous structures were fabricated and utilized as separator-modified coatings for Li-S batteries to catalyze the conversion of LiPSs. The adsorption, catalytic and electrochemical performance of the corresponding cells were compared among 3DOP Mo3P/Mo and 3DOP Mo, by kinetic and electrochemical performance measurements. It was found that the cell with 3DOP Mo3P/Mo modified separator deliver better electrochemical performance, with a high specific capacity of 469.66 mAh g-1 after 500 cycles at a high current density of 1°C. This work provides an idea and a guideline for the design of the separator modification for high-performance 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.

Hierarchical carbon hollow nanospheres coupled with ultra-small molybdenum carbide as sulfiphilic sulfur hosts for lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries are an attractive candidate to replace the current state-of-the-art lithium-ion batteries due to their promising theoretical capacity of 1675 mA h g^-1 and energy density of 2500 W h kg^-1. However, the lithium polysulfide (LiPS) shuttle effect and the slow sulfur redox kinetics seriously decrease the utilization of sulfur and deteriorate battery performance. Here, hierarchical carbon hollow nanospheres containing intimately coupled molybdenum carbide nanocrystals were synthesized as a sulfiphilic sulfur host. The sufficient interior void space accommodates the sulfur and physically confines LiPSs, while the in situ introduced molybdenum carbide nanoparticles can chemically immobilize LiPSs and catalytically accelerate their redox transformations. As a result, the Li-S batteries with this synergistic effect achieve an excellent rate capability of 566 mA h g^-1 at 2C and a long cycle stability over 300 cycles at 1C.

Enhanced Electrochemical Kinetics with Highly Dispersed Conductive and Electrocatalytic Mediators for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are promising energy-storage devices because of their high theoretical energy densities. However, the practical application of Li-S batteries is still impeded by the poor cycling performance and rate capability at practical conditions. In order to improve the performance of practical Li-S batteries, a hierarchical Mo₂ C nanocluster/carbon nanosheets hybrid based hollow spherical material (Mo₂ C/CHS) is designed and prepared. The hollow spheres composed of stacked carbon nanosheets can facilitate the infiltration of electrolyte. The ultrasmall and highly conductive Mo₂ C nanocrystals are confined in the carbon nanosheets and expose more active sites for anchoring and conversion of lithium polysulfides and increase the number of the nuclei for Li₂ S₂ /Li₂ S precipitation. Benefitting from the synergistic effects, Mo₂ C/CHS greatly promotes electrochemical kinetics in Li-S batteries with high sulfur loading (5 mg cm^-2 ). Even under lean electrolyte conditions (E/S = 7 μL mg_sulfur ^-1 ), the Li-S batteries with Mo₂ C/CHS added exhibit a discharge capacity of 904 mAh g^-1 at the high current rate of 0.5 C, and with 894 mAh g^-1 maintained after 200 cycles. This work provides a fundamental understanding of the electrochemical processes and guides the rational design of host and additive materials for practical Li-S batteries.

Recent Advances in Molybdenum-Based Materials for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries as power supply systems possessing a theoretical energy density of as high as 2600 Wh kg-1 are considered promising alternatives toward the currently used lithium-ion batteries (LIBs). However, the insulation characteristic and huge volume change of sulfur, the generation of dissolvable lithium polysulfides (LiPSs) during charge/discharge, and the uncontrollable dendrite formation of Li metal anodes render Li-S batteries serious cycling issues with rapid capacity decay. To address these challenges, extensive efforts are devoted to designing cathode/anode hosts and/or modifying separators by incorporating functional materials with the features of improved conductivity, lithiophilic, physical/chemical capture ability toward LiPSs, and/or efficient catalytic conversion of LiPSs. Among all candidates, molybdenum-based (Mo-based) materials are highly preferred for their tunable crystal structure, adjustable composition, variable valence of Mo centers, and strong interactions with soluble LiPSs. Herein, the latest advances in design and application of Mo-based materials for Li-S batteries are comprehensively reviewed, covering molybdenum oxides, molybdenum dichalcogenides, molybdenum nitrides, molybdenum carbides, molybdenum phosphides, and molybdenum metal. In the end, the existing challenges in this research field are elaborately discussed.