Sandwich‐like Catalyst–Carbon–Catalyst Trilayer Structure as a Compact 2D Host for Highly Stable Lithium–Sulfur Batteries

All-in-One Sulfur Host: Smart Controls of Architecture and Composition for Accelerated Liquid-Solid Redox Conversion in Lithium-Sulfur Batteries

The development of Li-S batteries (LSBs) is largely impeded by sluggish redox kinetics and notorious polysulfide shuttling. Herein, hierarchical MoC@Ni-NCNT arrays are reported as a multifunctional sulfur host in Li-S batteries, which comprised a flexible carbon fiber cloth substrate decorated with vertical MoC porous nanorods rooted by interconnected nitrogen-doped carbon nanotubes (NCNTs). In the designed host, the inner MoC porous backbone (composed of nanoparticles) along with the in situ-grafted interwoven NCNT shell can greatly maximize the host-guest interactive surface for homogeneous sulfur dispersion, thus realizing decent high-sulfur-loading performance. Ni nanoparticles, encapsulated within NCNTs in the outer shell, act as strong chemical-anchoring centers effectively trap-escaped polysulfides and propel the bidirectional sulfur transformation kinetics. In merit of sufficient adsorption and catalytic sites, the cell configured with the MoC@Ni-NCNT cathode delivers not only high capacity (1421 mA h g^-1 at 0.1 C) but also superior rate performance and ultralong lifespan. The cell can still achieve a superb areal capacity of 6.1 mA h cm² under an increased sulfur loading up to 6 mg cm^-2. This work could open a new avenue for the construction of a multifunctional cathode for high-performance LSBs.

Rich Heterointerfaces Enabling Rapid Polysulfides Conversion and Regulated Li2S Deposition for High-Performance Lithium-Sulfur Batteries

The practical uses of lithium-sulfur batteries are greatly restricted by the sluggish reaction kinetics of lithium polysulfides (LiPSs), leading to low sulfur utilization and poor cyclic stability. Using the heterostructure catalysts is an effective way to solve the above problems, but how to further enhance the conversion efficiency and avoid the surface passivation by the insulative Li₂S has not been well investigated. Herein, a heterostructure catalyst with rich heterointerfaces was prepared by modifying Mo₂N microbelt with SnO₂ nanodots. The formed rich interfaces with high accessibility act as the profitable nucleation sites guiding the Li₂S 3D growth, which avoids the catalyst surface passivation and facilitates the LiPS conversion. The introduction of SnO₂ nanodots also enhances the LiPS adsorption. Thus, the assembled battery with the above catalyst as the cathode additive shows a high capacity of 738.3 mAh g^-1 after 550 cycles at 0.5 C with an ultralow capacity decay of 0.025% per cycle. Even with high sulfur loading of 9.0 mg cm^-2, good cyclic stability is also achieved at 0.5 C with a low E/S ratio of 5 μL mg_s^-1. This work shows an effective way to enhance the LiPS conversion kinetics and guide Li₂S deposition in Li-S batteries.

Ultrahigh-Volumetric-Energy-Density Lithium-Sulfur Batteries with Lean Electrolyte Enabled by Cobalt-Doped MoSe2/Ti3C2Tx MXene Bifunctional Catalyst

It is a significant challenge to design a dense high-sulfur-loaded cathode and meanwhile to acquire fast sulfur redox kinetics and suppress the heavy shuttling in the lean electrolyte, thus to acquire a high volumetric energy density without sacrificing gravimetric performance for realistic Li-S batteries (LSBs). Herein, we develop a cation-doping strategy to tailor the electronic structure and catalytic activity of MoSe₂ that in situ hybridized with conductive Ti₃C₂T_x MXene, thus obtaining a Co-MoSe₂/MXene bifunctional catalyst as a high-efficient sulfur host. Combining a smart design of the dense sulfur structure, the as-fabricated highly dense S/Co-MoSe₂/MXene monolith cathode (density: 1.88 g cm^-3, conductivity: 230 S m^-1) achieves a high reversible specific capacity of 1454 mAh g^-1 and an ultrahigh volumetric energy density of 3659 Wh L^-1 at a routine electrolyte and a high areal capacity of ∼8.0 mAh cm^-2 under an extremely lean electrolyte of 3.5 μL mg_s^-1 at 0.1 C. Experimental and DFT theoretical results uncover that introducing Co element into the MoSe₂ plane can form a shorter Co-Se bond, impel the Mo 3d band to approach the Fermi level, and provide strong interactions between polysulfides and Co-MoSe₂, thereby enhancing its intrinsic electronic conductivity and catalytic activity for fast redox kinetics and uniform Li₂S nucleation in a dense high-sulfur-loaded cathode. This deep work provides a good strategy for constructing high-volumetric-energy-density, high-areal-capacity LSBs with lean electrolytes.

A Novel Synthesizing Strategy of 3D Cose2 Porous Hollow Flowers for High Performance Lithium–Sulfur Batteries

Redox kinetics of lithium polysulfides (LiPSs) conversion and poor electrical conductivity of sulfur during the charge-discharge process greatly inhibit the commercialization of high-performance lithium–sulfur (Li–S) batteries. Herein, we synthesized CoSe2 porous hollow flowers (CoSe2-PHF) by etching and further selenizing layered double hydroxide, which combined the high catalytic activity of transition metal compound and high electrical conductivity of selenium. The obtained CoSe2-PHF can efficiently accelerate the catalytic conversion of LiPSs, expedite the electron transport, and improve utilization of active sulfur during the charge-discharge process. As a result, with CoSe2-PHF/S-based cathodes, the Li–S batteries exhibited a reversible specific capacity of 955.8 mAh g−1 at 0.1 C and 766.0 mAh g−1 at 0.5 C, along with a relatively small capacity decay rate of 0.070% per cycle within 400 cycles at 1 C. Even at the high rate of 3 C, the specific capacity of 542.9 mAh g−1can be maintained. This work enriches the way to prepare porous composites with high catalytic activity and electrical conductivity as sulfur hosts for high-rate, long-cycle rechargeable Li–S batteries.

Exploring the Efficient Na/K Storage Mechanism and Vacancy Defect-Boosted Li+ Diffusion Based on VSe2/MoSe2 Heterostructure Engineering

As typical 2D materials, VSe₂ and MoSe₂ both play a complementary role in Li/Na/K storage. Therefore, we designed and optimized the VSe₂/MoSe₂ heterostructure to gain highly efficient Li/Na/K-ion batteries. Most importantly, achieving fast Li/Na/K-ion diffusion kinetics in the interlayer of VSe₂/MoSe₂ is a key point. First of all, first-principles calculations were carried out to systematically investigate the packing structure, mechanical properties, band structure, and Li/Na/K storage mechanism. Our calculated results suggest that a large interlayer spacing (3.80 Å), robust structure, and metallic character pave the way for achieving excellent charge-discharge performance for the VSe₂/MoSe₂ heterostructure. Moreover, V and Mo ions both suffer a very mild redox reaction even if Li/Na/K ions fill the interlayer space. These structures were all further verified to show thermal stability (300 K) by means of the AIMD method. By analyzing the Li/Na/K diffusion behavior and the effect of vacancy defect on the structural stability and energy barrier for Li interlayer diffusion, it is found that the VSe₂/MoSe₂ heterostructure exhibits very low-energy barriers for Na/K interlayer diffusion (0.21 eV for Na and 0.11 eV for K). Compared with the VSe₂/MoSe₂ heterostructure, the V_0.92Se_1.84/MoSe₂ heterostructure not only can still maintain a stable structure and metallic character but also has much lower energy barrier for Li interlayer diffusion (0.07 vs 0.48 eV). These discoveries also break new ground to eliminate the obstacles preventing Li⁺ diffusion in the interlayer of other heterostructure materials. Besides, both VSe₂/MoSe₂ and V_0.92Se_1.84/MoSe₂ heterostructures have low average open-circuit voltage (OCV) values during Li/Na/K interlayer diffusion (1.07 V for V_0.92Se_1.84/MoSe₂ vs Li⁺, 0.86 V for VSe₂/MoSe₂ vs Na⁺, and 0.54 V for VSe₂/MoSe₂ vs K⁺), such low OCV values are beneficial for anode materials with excellent electrochemical properties. The above findings offer a new route to design anode materials for Li/Na/K-ion batteries.

MoS2 nanosheets grown on hollow carbon spheres as a strong polysulfide anchor for high performance lithium sulfur batteries

Lithium sulfur batteries are expected to be one of the most promising energy storage systems due to their high energy density, low cost and environmental friendliness. However, the shuttle effect of lithium polysulfides severely hampers their practical application. The design of the sulfur cathode is one of the most important approaches to overcome the problem. In this work, MoS2 nanosheets have been successfully grown on the surface of hollow carbon spheres (HCS) to obtain MoS2@HCS nanocomposites with uniform morphology. The growth behavior of MoS2 nanosheets was also proved by adjusting the pore structure of HCS. With a sulfur loading of 74%, the MoS2@HCS/S cathode exhibits a high initial reversible capacity of 1419 mA h g-1 at a current density of 0.1 C and remains at 1010 mA h g-1 after 100 cycles. Even at 0.5 C, a capacity of 795 mA h g-1 can be retained after 600 cycles, corresponding to a capacity retention rate of 63.1%. By adjusting the concentration of the sulfur source, the relationship between different growth quantities of MoS2 and the cycling performance of the battery was also investigated. The excellent electrochemical performance of the MoS2@HCS/S cathode can be fully attributed to its physical and chemical double adsorption effect on lithium polysulfides, which has been confirmed through the visible adsorption and X-ray Photoelectron Spectroscopy (XPS) experiments. This work provides a simple design concept and method to synthesize a nanocomposite-based sulfur host for high performance lithium sulfur batteries.