Nanoconfined Topochemical Conversion from MXene to Ultrathin Non-Layered TiN Nanomesh toward Superior Electrocatalysts for Lithium-Sulfur Batteries

Multifunctional TiN-MXene-Co@CNTs Networks as Sulfur/Lithium Host for High-Areal-Capacity Lithium-Sulfur Batteries

The inevitable shuttling and slow redox kinetics of lithium polysulfides (LiPSs) as well as the uncontrolled growth of Li dendrites have strongly limited the practical applications of lithium-sulfur batteries (LSBs). To address these issues, we have innovatively constructed the carbon nanotubes (CNTs) encapsulated Co nanoparticles in situ grown on TiN-MXene nanosheets, denoted as TiN-MXene-Co@CNTs, which could serve simultaneously as both sulfur/Li host to kill "three birds with one stone" to (1) efficiently capture soluble LiPSs and expedite their redox conversion, (2) accelerate nucleation/decomposition of solid Li2S, and (3) induce homogeneous Li deposition. Benefiting from the synergistic effects, the TiN-MXene-Co@CNTs/S cathode with a sulfur loading of 2.5 mg cm-2 could show a high reversible specific capacity of 1129.1 mAh g-1 after 100 cycles at 0.1 C, and ultralong cycle life over 1000 cycles at 1.0 C. More importantly, it even achieves a high areal capacity of 6.3 mAh cm-2 after 50 cycles under a sulfur loading as high as 8.9 mg cm-2 and a low E/S ratio of 5.0 μL mg-1. Besides, TiN-MXene-Co@CNTs as Li host could deliver a stable Li plating/striping behavior over 1000 h.

2D Ultrathin Titanium Nitride Nanosheets as Separator Coatings for Li-S Batteries

Transition metal nitrides (TMNs) are affirmed to be an appealing candidate for boosting the performance of lithium-sulfur (Li-S) batteries due to their excellent conductivity, strong interaction with sulfur species, and the effective catalytic ability for conversion of polysulfides. However, the traditional bulk TMNs are difficult to achieve large active surface area and fast transport channels for electrons/ions simultaneously. Here, a 2D ultrathin geometry of titanium nitride (TiN) is realized by a facile topochemical conversion strategy, which can not only serve as an interconnected conductive platform but also expose abundant catalytic active sites. The ultrathin TiN nanosheets are coated on a commercial separator, serving as a multifunctional interlayer in Li-S batteries for hindering the polysulfide shuttle effect by strong capture and fast conversion of polysulfides, achieving a high initial capacity of 1357 mAh g-1 at 0.1 C and demonstrating a low capacity decay of only 0.046% per cycle over 1000 cycles at 1 C.

Ultrathin nitrogen-doped carbon Ti3C2Tx-TiN heterostructure derived from ZIF-8 nanoparticles sandwiched MXene for high-performance capacitive deionization

Rational design of high-performance electrode materials is crucial for enhancing desalination performance of capacitive deionization (CDI). Here, ultrathin nitrogen-doped carbon/Ti3C2Tx-TiN (NC/MX-TiN) heterostructure was developed by pyrolyzing zeolite imidazolate framework-8 (ZIF-8) nanoparticles sandwiched MXene (ZSM), which were formed by assembling ultrafine ZIF-8 nanoparticles with size of 20 nm on both sides of MXene nanosheets. The introduction of ultrasmall ZIF-8 particles allowed for in situ nitridation of the MXene during pyrolysis, forming consecutive TiN layers tightly connected to the internal MXene. The two-dimensional (2D) heterostructure exhibited remarkable properties, including high specific surface area and excellent conductivity. Additionally, the resulting TiN demonstrated exceptional redox capability, which significantly enhanced the performance of CDI and ensured cycling stability. Benefiting from these advantages, the NC/MX-TiN exhibited a maximum adsorption capacity of 45.6 mg g-1 and a steady cycling performance in oxygenated saline water over 50 cycles. This work explores the rational design and construction of MXene-based 2D heterostructure and broadens new horizons for the development of novel CDI electrode materials.

Li+ mobility powered by a crystal compound for fast Li-S chemistry

Placing blocking layers between electrodes has shown paramount prospects in suppressing the shuttle effect of Li-S batteries, but the associated ionic transport would be a concurrent obstacle. Herein, we present a Li-based crystal composited with carbon (LiPN2@C) by a one-step annealing of Li+ absorbed melamine polyphosphate, which simultaneously achieves alleviated polysulfide-shuttling and facilitated Li+ transport. As a homologous crystal, LiPN2 with abundant lithiophilic sites makes Li+ transport more efficient and sustainable. With a LiPN2@C-modified separator, the Li2S cathode exhibits a much-lower activation potential of 2.4 V and a high-rate capacity of 519 mA h g-1 at 2C. Impressively, the battery delivers a capacity of 726 mA h g-1 at 0.5C with a low decay rate of 0.25% per cycle during 100 continuous cycles.

Design and synthesis of novel pomegranate-like TiN@MXene microspheres as efficient sulfur hosts for advanced lithium sulfur batteries

Lithium-sulfur (Li-S) batteries have the characteristics of low cost, environmental protection, and high theoretical energy density, and have broad application prospects in the new generation of electronic products. However, there are some problems that seriously hinder the Li-S batteries from going from the laboratory to the factory, such as poor stability caused by the large volume expansion of sulfur during charging and discharging, sluggish kinetics of the electrochemical reaction resulting from the low conductivity of the active materials, and loss of active materials arising from the dissolution and diffusion of the intermediate product lithium polysulfides (LiPSs). In this paper, the two-dimensional layered material MXene and TiN are firstly combined by spray drying method to prepare pomegranate-like TiN@MXene microspheres with both adsorption capacity and catalytic effect on LiPSs conversion. The interconnected skeleton composed of MXene not only solves the problem of easy stacking of MXene sheets but also ensures the uniform distribution of sulfur. Without affecting the excellent characteristics of MXene itself, the overall conductivity of the composite electrode material is improved. The TiN hollow nanospheres are coated with MXene layers to form a shell, catalyzing the adsorption of LiPSs and accelerating the transformation of high-order LiPSs to Li2S2/Li2S. As a result, the TiN@MXene cathode delivers a high initial discharge capacity of 1436 mA h g-1 at 0.1C, excellent rate performance of 636 mA h g-1 up to 3C, and an ultralong lifespan over 1000 cycles with a small capacity decay of 0.048% per cycle at the current density of 1.0C.

Self-Supporting Multicomponent Hierarchical Network Aerogel as Sulfur Anchoring-Catalytic Medium for Highly Stable Lithium-Sulfur Battery

The low utilization rate of active materials, shuttle effect of lithium polysulfides (LiPSs), and slow reaction kinetics lead to the extremely low efficiency and poor high current cycle stability of lithium sulfur batteries (Li-S batteries). In this paper, a self-supporting multicomponent hierarchical network aerogel is proposed as the modified cathode (S/GO@MX@VS₄ ). It consists of graphene (GO) and MXene nanosheets (MX) loaded with VS₄ nanoparticles. The experimental results and first-principles calculations show that the GO@MX@VS₄ aerogel has strong adsorption and reversible conversion effects on LiPSs. It can not only inhibit the shuttle effect and improve the utilization rate of active substances by keeping the chain crystal structure of VS₄ , but also promote the reversibility and kinetics of the reaction by accelerating the liquid-solid transformation in the reduction process and the decomposition of insoluble Li₂ S in the oxidation process. The GO@MX@VS₄ aerogel modified cathode with a multicomponent synergy exhibits the capacity ratios (Q₁ /Q₂ ) at different discharge stages is close to the theoretical value (1:2.8), and the capacity decay per cycle is 0.019% in 1200 cycles at 5C. Also, a high areal capacity of 6.90 mAh cm^-2 is provided even at high sulfur loading (7.39 mg cm^-2 ) and low electrolyte/sulfur ratio (E/S, 8.0 µL mg^-1 ).

Atomic Imaging and Thermally Induced Dynamic Structural Evolution of Two-Dimensional Cr2S3

In this study, facile salt-assisted chemical vapor deposition (CVD) was used to synthesize ultrathin non-van der Waals chromium sulfide (Cr₂S₃) with a thickness of ∼1.9 nm. The structural transformation of as-grown Cr₂S₃ was studied using advanced in situ heating techniques combined with transmission electron microscopy (TEM). Two-dimensional (2D) and quasi-one-dimensional (1D) samples were fabricated to investigate the connection between specific planes and the dynamic behavior of the structural variation. The rearrangement of atoms during the phase transition was driven by the loss of sulfur atoms at elevated temperatures, resulting in increased free energy. A decrease in the ratio of the (001) plane led to an overall increase in surface energy, thus lowering the critical phase transition temperature. Our study provides detailed insight into the mechanism of structural transformation and the critical factors governing transition temperature, thus paving the way for future studies on intriguing Cr-S compounds.

Multi-Dimensional Composite Frame as Bifunctional Catalytic Medium for Ultra-Fast Charging Lithium-Sulfur Battery

The shuttle effect of soluble lithium polysulfides (LiPSs) between electrodes and slow reaction kinetics lead to extreme inefficiency and poor high current cycling stability, which limits the commercial application of Li-S batteries. Herein, the multi-dimensional composite frame has been proposed as the modified separator (MCCoS/PP) of Li-S battery, which is composed of CoS₂ nanoparticles on alkali-treated MXene nanosheets and carbon nanotubes. Both experiments and theoretical calculations show that bifunctional catalytic activity can be achieved on the MCCoS/PP separator. It can not only promote the liquid-solid conversion in the reduction process, but also accelerate the decomposition of insoluble Li₂S in the oxidation process. In addition, LiPSs shuttle effect has been inhibited without a decrease in lithium-ion transference numbers. Simultaneously, the MCCoS/PP separator with good LiPSs adsorption capability arouses redistribution and fixing of active substances, which is also beneficial to the rate performance and cycling stability. The Li-S batteries with the MCCoS/PP separator have a specific capacity of 368.6 mAh g^-1 at 20C, and the capacity decay per cycle is only 0.033% in 1000 cycles at 7C. Also, high area capacity (6.34 mAh cm^-2) with a high sulfur loading (7.7 mg cm^-2) and a low electrolyte/sulfur ratio (7.5 μL mg^-1) is achieved.

Titanium nitride as a promising sodium-ion battery anode: interface-confined preparation and electrochemical investigation

The search for new electrode materials for sodium-ion batteries (SIBs), especially for enhancing the specific capacity and cycling stability of anodes, is of great significance for the development of new energy conversion and storage materials. Here, a new type of titanium nitride composite anode (TiN@C) coated with 2D carbon nanosheets was prepared for the first time using a rationally designed topochemical conversion approach of interface-confinement. Subsequently, the electrochemical performance and Na⁺ storage mechanism of TiN@C as an anode for SIBs was investigated. The quantum-dot-sized TiN anodes exhibited shorter ionic transport pathways, while the 2D ultrathin carbon nanosheets reinforced the structural stability of the composite and provided a high electron transformation rate. As a result, the TiN/C composite anode can deliver a high reversible capacity of 170 mA h g^-1 and 149 mA h g^-1 after 5000 cycles at a current density of 0.5 A g^-1 and 1 A g^-1, indicating excellent electrochemical properties. This work provides new opportunities to explore the convenient and controllable preparation of metal nitride anodes for other energy conversion and storage applications.

Using sp2 N atom anchoring effect to prepare ultrafine vanadium nitride particles on porous nitrogen-doped carbon as cathode for lithium-sulfur battery

Porous carbon-supported transition metals and their compounds have attracted much attention as sulfur host materials for cathodes of lithium-sulfur batteries, due to their high chemisorption capacity and ability to catalyze the conversion of polysulfides. However, actual activity of these materials is not very high because of low specific surface areas of transition metal compounds synthesized at high temperatures. In this study, ultra-fine vanadium nitride particles with an average particle size of ca. 4 nm (VN/M/NC) are successfully grown on the surface of nitrogen-doped three-dimensional carbon using sp² nitrogen atoms, resulting from melamine pyrolysis in the presence of ammonium metavanadate, as anchor points to lock vanadium atoms in the VN/M/NC material. When used as a cathode for lithium-sulfur battery, VN/M/NC demonstrates initial discharge specific capacity of 1080 mAh g^-1 at 0.2 C, and retains a discharge capacity of 475 mAh g^-1 at a high rate of 2 C. With capacity attenuation of only 0.037% per cycle after 500 cycles at 1 C, the newly obtained VN/M/NC can be a promising cathode material for lithium-sulfur batteries.

Rational construction of a CNTs@VO2 nanosheets modified separator for enhancing the performance of lithium-sulfur batteries

Although lithium-sulfur (Li-S) batteries possess great potential to become the next generation of energy storage technology due to their fivefold higher energy density than commercial lithium-ion batteries, their practical application is still hindered by their poor cycling stability, especially resulting from the disturbing shuttle effect of soluble intermediates. In this study, vanadium dioxide (VO₂) nanosheets were successfully grown onto CNTs to form CNTs@VO₂ through hydrothermal and calcining processes. The hollow structure of the high conductive CNTs offers internal space and mesopores to accommodate the electrolyte combined with the polar metal oxide VO₂ nanosheets providing the chemical anchoring. The hollow binary core-shell host acting as the nanoreactor that serves as the modifier of the separator results in the intensive physical and chemical dual adsorption of lithium polysulfide species (LiPSs), promoting the conversion of long-chain LiPSs to alleviate the shuttle effect significantly and boosting the performance. In addition, the CNTs enhance the electronic conductivity and the electrolyte infiltration of the separator. Notably, the modified separator demonstrates a high initial discharge capacity of 1397 mA h g^-1 at 0.2C and retains a stable cycling ability with a reversible capacity of 965 mA h g^-1 over 200 cycles at 1C. Even for the high sulfur loading of 7.4 mg cm^-2, it can deliver a high areal capacity of 5.4 mA h cm^-2 at 0.5C.