In Situ Phosphorization for Constructing Ni5P2-Ni Heterostructure Derived from Bimetallic MOF for Li-S Batteries.

Heterostructured materials commonly consist of bifunctions due to the different ingredients. For host material in the sulfur cathode of lithium-sulfur (Li-S) batteries, the chemical adsorption and catalytic activity for lithium polysulfides (LiPS) are important. This work obtains a Ni5P2-Ni nanoparticle (Ni5P2-NiNPs) heterostructure through a confined self-reduction method followed by an in situ phosphorization process using Al/Ni-MOF as precursors. The Ni5P2-Ni heterostructure not only has strong chemical adsorption, but also can effectively catalyze LiPS conversion. Furthermore, the synthetic route can keep Ni5P2-NiNPs inside of the nanocomposites, which have structural stability, high conductivity, and efficient adsorption/catalysis in LiPS conversion. These advantages make the assembled Li-S battery deliver a reversible specific capacity of 619.7 mAh g- 1 at 0.5 C after 200 cycles. The in situ ultraviolet-visible technique proves the catalytic effect of Ni5P2-Ni heterostructure on LiPS conversion during the discharge process.

Triple Effect of "Conductivity-Adsorption-Catalysis" Enables MXene@FeCoNiP to be Sulfur Hosts for Lithium-Sulfur Batteries.

The weak chemical immobilization ability and poor catalytic effect of MXene inhibit its application in lithium-sulfur (Li-S) batteries. Herein, a novel MXene@FeCoNiP composite is rationally developed and utilized as a sulfur host for Li-S batteries. In this well-designed MXene-based nanostructure, the introduction of FeCoNiP in the interlayer of MXene nanosheets can not only effectively inhibit the restacking of MXene nanosheets but also act as an accelerator for the adsorption and catalysis of polysulfides to restrain the shuttling effect and facilitate the transformation of polysulfides. The existence of two-dimensional MXene nanosheets provides more active sites and improves the conductivity, which is beneficial for accelerating the reaction kinetics. Thus, the as-prepared MXene@FeCoNiP composites achieve an outstanding performance for Li-S batteries. This work provides an opportunity to construct an ideal sulfur host with the triple effect of "conductivity-adsorption-catalysis".

Synthesis of Conductive MOFs and Their Electrochemical Application.

Conductive metal-organic frameworks (MOFs) are a type of porous material. It consists of metal ions coordinated with highly conjugated organic ligands. The high density of carriers and orbital overlap contribute to the amazing conductivity. Additionally, conductive MOFs inherit the advantages of large specific surface area, structural diversity, and adjustable pore size from MOFs. These excellent properties have attracted many researchers to explore controllable synthesis and electrochemical applications over the past decade. This work provides an overview of the recent advances in the synthesis strategies of conductive MOFs and highlights their applications in electrocatalysis, supercapacitors, sensors, and batteries. Finally, the challenges faced by the synthesis and application of conductive MOFs are discussed, as well as the views on promising solutions for them are presented.

Synthesis of 2D Co-MOF Nanosheets and Low-Temperature Calcination Activation for Lithium-Ion Batteries.

Metal-organic frameworks (MOFs), a novel type of porous crystalline material, have aroused widespread interest in lithium-ion batteries (LIBs). The design and preparation of MOF electrodes with a stable structure and excellent electrochemical performance are primary concerns for improving the capacity of LIBs. Among them, two-dimensional (2D) materials with larger specific surface areas, richer active sites, and higher aspect ratios have great potential. We adopted a facile approach to synthesize unique Co-MOF nanosheets with a 2D flaky morphology and a mesoporous structure. In addition, low-temperature calcination increases the specific surface area and improves the porosity to achieve mass transfer. Sample M2 delivers high specific capacities and long lives (1402.0 mA h g-1 after 100 cycles at 500 mA g-1 and 462.4 mA h g-1 after 300 cycles at 1.0 A g-1) when the calcination temperature is 200 °C. Significant improvements in the cycle life and stability are attributed to the 2D flaky structure of the M2 sample and the available low-temperature calcination activation, which provide a simple strategy for the fabrication of inexpensive and excellent anodes for LIBs.

High-Entropy Prussian Blue Analogues and Their Oxide Family as Sulfur Hosts for Lithium-Sulfur Batteries.

The introduction of high-entropy into Prussian blue analogues (PBAs) has yet to attract attention in the field of lithium-sulfur battery materials. Herein, we systematically synthesize a library of PBAs from binary to high-entropy by a facile coprecipitation method. The coordination environment in PBAs is explored by X-ray absorption fine structure spectroscopy, which together with elemental mapping confirm the successful introduction of all metals. Importantly, electrochemical tests demonstrate that high-entropy PBA can serve as polysulfide immobilizer to inhibit shuttle effect and as catalyst to promote polysulfides conversion, thereby boosting its outstanding performance. Additionally, a variety of nanocubic metal oxides from binary to senary are fabricated by using PBAs as sacrificial precursors. We believe that a wide range of new materials obtained from our coprecipitation and pyrolysis methodology can promote further developments in research on PBA systems and sulfur hosts.

CoO/MoO3@Nitrogen-Doped carbon hollow heterostructures for efficient polysulfide immobilization and enhanced ion transport in Lithium-Sulfur batteries.

Lithium-sulfur batteries (LSBs) have emerged as a promising energy storage system, but their practical application is hindered by the polysulfide shuttle effect and sluggish redox kinetics. To address these challenges, we have developed CoO/MoO3@nitrogen-doped carbon (CoO/MoO3@NC) hollow heterostructures based on porous ZIF-67 as separators in LSBs. CoO has a strong anchoring effect on polysulfides. The heterostructure formed after the introduction of MoO3 increases the adsorption of polysulfides. The carbon coating outside the heterostructure improves the ion transmission efficiency of the battery, leading to enhanced electrochemical performance. The modified LSB demonstrates a low-capacity decay rate of 0.092% over 500 cycles at 0.5C, with a high discharge capacity of 613 mAh g-1 at 1C. This work presents a novel approach for the preparation of hollow heterostructure materials, aiming for high-performance LSBs.

Effects of O, S, and P in transition-metal compounds on the adsorption and catalytic ability of sulfur cathodes in lithium-sulfur batteries.

Transition-metal compounds (TMCs) have recently become promising candidates as lithium-sulfur (Li-S) battery cathode materials because they have unique adsorption and catalytic properties. However, the relationship between the anionic species and performance has not been sufficiently revealed. Herein, using FeCoNiX (X = O, S, and P) compounds as examples, we systematically studied the effects of the anion composition of FeCoNiX compounds on the adsorption and catalytic abilities of sulfur cathodes in Li-S batteries. Adsorption tests and density functional theory calculations showed that the adsorption ability toward lithium polysulfides follows the order: FeCoNiP > FeCoNiO > FeCoNiS, while in situ ultraviolet-visible spectroscopy and cyclic voltammetry revealed that the catalytic ability for lithium polysulfide conversion follows the order: FeCoNiP > FeCoNiS > FeCoNiO. These results indicate that FeCoNiP is an excellent polysulfide immobilizer and catalyst that restricts shuttling and improves reaction kinetics. Electrochemical tests further demonstrated that the FeCoNiP cathode delivered superior cycling performance to FeCoNiO or FeCoNiS. In addition, the battery performance order is consistent with that of catalytic ability, which suggests that catalytic ability plays a key influencing role in batteries. This study provides new insight into the use of O-, S-, and P-doped TMCs as functional sulfur carriers.

Confined Synthesis of Amorphous Al2 O3 Framework Nanocomposites Based on the Oxygen-Potential Diagram as Sulfur Hosts for Catalytic Conversion.

Sulfur cathodes in Li-S batteries suffer significant volumetric expansion and lack of catalytic activity for polysulfide conversion. In this study, a confined self-reduction synthetic route is developed for preparing nanocomposites using diverse metal ions (Mn2+ , Co2+ , Ni2+ , and Zn2+ )-introduced Al-MIL-96 as precursors. The Ni2+ -introduced Al-MIL-96-derived nanocomposite contains a "hardness unit", amorphous aluminum oxide framework, to restrain the volumetric expansion, and a "softness unit", Ni nanocrystals, to improve the catalytic activity. The oxygen-potential diagram theoretically explains why Ni2+ is preferentially reduced. Postmortem microstructure characterization confirms the suppressive volume expansion. The in situ ultraviolet-visible measurements are performed to probe the catalytic activity of polysulfide conversion. This study provides a new perspective for designing nanocomposites with "hardness units" and "softness units" as sulfur or other catalyst hosts.

Construction of SiOx/nitrogen-doped carbon superstructures derived from rice husks for boosted lithium storage.

Silicon sub-oxides (SiOx) are increasingly becoming a prospective anode material for lithium-ion batteries (LIBs). Nevertheless, inferior electrical conductivity and drastic volume fluctuation upon cycling significantly hamper the electrochemical performance of SiOx. In this work, rice husks (RHs)-derived pitaya-like SiOx/nitrogen-doped carbon (SNC) superstructures have been prepared by a simple electrospray-carbonization approach. SiOx nanoparticles (NPs) are well-dispersed in a spherical nitrogen-doped carbon (NC) matrix. The carbon frameworks discourage the aggregation of SiOx NPs, facilitating the kinetics for ion diffusion and charge transfer, and maintaining structural stability upon cycling, thus bringing about improved electrochemical performance. When the optimized SNC superstructures with SiOx content of 64.3% are utilized as LIBs anodes, a stable specific capacity of 622.8 mA h g-1 after 100 cycles at 0.1 A g-1, and an excellent long cycle performance of 190.1 mA h g-1 after 5000 cycles at 5 A g-1 are obtained. This effective and universal synthetic strategy for fabricating controllable superstructures offers insights into the development of high-performance LIBs.

MIL-96-Al for Li-S Batteries: Shape or Size?

Metal-organic frameworks (MOFs) with controllable shapes and sizes show a great potential in Li-S batteries. However, neither the relationship between shape and specific capacity nor the influence of MOF particle size on cyclic stability have been fully established yet. Herein, MIL-96-Al with various shapes, forming hexagonal platelet crystals (HPC), hexagonal bipyramidal crystals (HBC), and hexagonal prismatic bipyramidal crystals (HPBC) are successfully prepared via cosolvent methods. Density functional theory (DFT) calculations demonstrate that the HBC shape with highly exposed (101) planes can effectively adsorb lithium polysulfides (LPS) during the charge/discharge process. By changing the relative proportion of the cosolvents, HBC samples with different particle sizes are prepared. When these MIL-96-Al crystals are used as sulfur host materials, it is found that those with a smaller size of the HBC shape deliver higher initial capacity. These investigations establish that different crystal planes have different adsorption abilities for LPS, and that the MOF particle size should be considered for a suitable sulfur host. More broadly, this work provides a strategy for designing sulfur hosts in Li-S batteries.

Rational Design and General Synthesis of Multimetallic Metal-Organic Framework Nano-Octahedra for Enhanced Li-S Battery.

Metal-organic frameworks (MOFs), which consist of central metal nodes and organic linkers, constitute a fast growing class of crystalline porous materials with excellent application potential. Herein, a series of Mn-based multimetallic MOF (bimetallic and trimetallic MIL-100) nano-octahedra are prepared by a facile one-pot synthetic strategy. The types and proportions of the incorporated elements can be tuned while retaining the original topological structure. The introduction of other metal ions is verified at the atomic level by combining X-ray absorption fine structure experiments and theoretical calculations. Furthermore, these multimetallic Mn-based MIL-100 nano-octahedra are utilized as sulfur hosts to prepare cathodes for Li-S batteries. The MnNi-MIL-100@S cathode exhibits the best Li-S battery performance among all reported MIL-100@S composite cathode materials, with a reversible capacity of ≈708.8 mAh g-1 after 200 cycles. The synthetic strategy described herein is utilized to incorporate metal ions into the MOF architecture, of which the parent monometallic MOF nano-octahedra cannot be prepared directly, thus rationally generating novel multimetallic MOFs. Importantly, the strategy also allows for the general synthesis and study of various micro-/nanoscale MOFs in the energy storage field.