Unlocking Ultrahigh Initial Coulombic Efficiency of MXene Anode via Presodiation and Electrolyte Optimization.

The low initial Coulombic efficiency (ICE) greatly hinders the practical application of MXenes in sodium-ion batteries. Herein, theoretical calculations confirm that -F and -OH terminations as well as the tetramethylammonium ion (TMA+) intercalator in sediment Ti3C2Tx (s-Ti3C2Tx) MXene possess strong interaction with Na+, which impedes Na+ desorption during the charging process and results in low ICE. Consequently, Na+-intercalated sediment Ti3C2Tx (Na-s-Ti3C2Tx) is constructed through Na2S·9H2O treatment of s-Ti3C2Tx. Specifically, Na+ can first exchange with TMA+ of s-Ti3C2Tx and then combine with -F and -OH terminations, thus leading to the elimination of TMA+ and preshielding of -F and -OH. As expected, the resulting Na-s-Ti3C2Tx anode delivers considerably boosted ICE values of around 71% in carbonate-based electrolytes relative to s-Ti3C2Tx. Furthermore, electrolyte optimization is employed to improve ICE, and the results demonstrate that an ultrahigh ICE value of 94.0% is obtained for Na-s-Ti3C2Tx in the NaPF6-diglyme electrolyte. More importantly, Na-s-Ti3C2Tx exhibits a lower Na+ migration barrier and higher electronic conductivity compared with s-Ti3C2Tx based on theoretical calculations. In addition, the cyclic stability and rate performance of the Na-s-Ti3C2Tx anode in various electrolytes are comprehensively explored. The presented simple strategy in boosting ICE significantly enhances the commercialization prospect of MXenes in advanced batteries.

Freestanding and Flexible Interfacial Layer Enables Bottom-Up Zn Deposition Toward Dendrite-Free Aqueous Zn-Ion Batteries.

Aqueous rechargeable zinc ion batteries are regarded as a competitive alternative to lithium-ion batteries because of their distinct advantages of high security, high energy density, low cost, and environmental friendliness. However, deep-seated problems including Zn dendrite and adverse side reactions severely impede the practical application. In this work, we proposed a freestanding Zn-electrolyte interfacial layer composed of multicapsular carbon fibers (MCFs) to regulate the plating/stripping behavior of Zn anodes. The versatile MCFs protective layer can uniformize the electric field and Zn2+ flux, meanwhile, reduce the deposition overpotentials, leading to high-quality and rapid Zn deposition kinetics. Furthermore, the bottom-up and uniform deposition of Zn on the Zn-MCFs interface endows long-term and high-capacity plating. Accordingly, the Zn@MCFs symmetric batteries can keep working up to 1500 h with 5 mAh cm-2. The feasibility of the MCFs interfacial layer is also convinced in Zn@MCFs||MnO2 batteries. Remarkably, the Zn@MCFs||α-MnO2 batteries deliver a high specific capacity of 236.1 mAh g-1 at 1 A g-1 with excellent stability, and maintain an exhilarating energy density of 154.3 Wh kg-1 at 33% depth of discharge in pouch batteries.

Facile Synthesis of Hybrid Anodes with Enhanced Lithium-Storage Performance Realized by a "Synergistic Effect".

Alloying-type anodes including Si- and Sn-based materials are considered the most exploitable anodes for high-performance lithium-ion batteries. However, problems of poor kinetics properties and structural failures such as grain pulverization and coarsening hinder their large-scale application. Herein, SnO2/Si@graphite hybrid anodes, with nano-SnO2 and nano-Si thoroughly mixed with each other and loaded onto graphite flakes, have been prepared by a facile ball milling method. Attributed to the "synergistic effect" between SnO2 and Si, the mechanical stability and kinetics properties can be remarkably enhanced. Furthermore, graphite substrate supplies a fast electrically conductive path and buffers the volume expansion of active particles. Accordingly, SnO2/Si@graphite delivers 798.9 mAh g-1 at 200 mA g-1 and maintains 550.8 mAh g-1 after 1000 cycles at 1 A g-1 in half cells. Impressively, a high energy density of 431.4 Wh kg-1 (based on the mass of anode and cathode) can be obtained in full cells when paired with the NCM622 cathode. This work presents an effective strategy to exploit high-performance alloying-type anodes for LIBs by designing hybrid materials with multiple active components.

Electrochemical Performance Enhancement of Micro-Sized Porous Si by Integrating with Nano-Sn and Carbonaceous Materials.

Silicon is investigated as one of the most prospective anode materials for next generation lithium ion batteries due to its superior theoretical capacity (3580 mAh g-1), but its commercial application is hindered by its inferior dynamic property and poor cyclic performance. Herein, we presented a facile method for preparing silicon/tin@graphite-amorphous carbon (Si/Sn@G-C) composite through hydrolyzing of SnCl2 on etched Fe-Si alloys, followed by ball milling mixture and carbon pyrolysis reduction processes. Structural characterization indicates that the nano-Sn decorated porous Si particles are coated by graphite and amorphous carbon. The addition of nano-Sn and carbonaceous materials can effectively improve the dynamic performance and the structure stability of the composite. As a result, it exhibits an initial columbic efficiency of 79% and a stable specific capacity of 825.5 mAh g-1 after 300 cycles at a current density of 1 A g-1. Besides, the Si/Sn@G-C composite exerts enhanced rate performance with 445 mAh g-1 retention at 5 A g-1. This work provides an approach to improve the electrochemical performance of Si anode materials through reasonable compositing with elements from the same family.

Dual Immobilization of SnOx Nanoparticles by N-Doped Carbon and TiO2 for High-Performance Lithium-Ion Battery Anodes.

The grain aggregation engendered kinetics failure is regarded as the main reason for the electrochemical decay of nanosized anode materials. Herein, we proposed a dual immobilization strategy to suppress the migration and aggregation of SnOx nanoparticles and corresponding lithiation products through constructing SnOx/TiO2@PC composites. The N-doped carbon could anchor the tin oxide particles and inhibit their aggregation during the preparation process, leading to a uniform distribution of ultrafine SnOx nanoparticles in the matrix. Meanwhile, the incorporated TiO2 component works as parclose to suppress the migration and coarsening of SnOx and corresponding lithiation products. In addition, the N-doped carbon and TiO2/LixTiO2 can significantly improve the electrical and ionic conductivities of the composites, enabling a good diffusion and charge-transfer dynamics. Owing to the dual immobilization from the "synergistic effect" of N-doped carbon and the "parclose effect" of TiO2, the conversion reaction of SnOx remains fully reversible throughout the cycling. Thereby, the composites exhibit excellent cycling performance in half cells and can be fully utilized in full cells. This work may provide an inspiration for the rational design of tin-based anodes for high-performance lithium-ion batteries.

Rational Design of Pillared SnS/Ti3C2Tx MXene for Superior Lithium-Ion Storage.

MXenes have been widely explored in energy storage because of their extraordinary properties; however, the majority of research on their application was staged at multilayered MXenes or assisted by carbon materials. Scientifically speaking, the two most distinctive properties of MXenes are usually neglected, composed of large interlayer spacing and abundant surface chemistry, which distinguish MXenes from other two-dimensional materials. Herein, few-layered MXene (f-MXene) nanosheet powders can be easily prepared according to the modified solution-phase flocculation method, completely avoiding the restacking phenomenon of f-MXene nanosheets in preparation and oxidation issues during the storage process. Via further employing the solvothermal reaction and annealing treatment, we successfully constructed pillared SnS/Ti3C2Tx composites decorated with in situ formed TiO2 nanoparticles. In the composites, MXenes can play the role of a conductive network, a buffer matrix for volume expansion of SnS, while the active SnS nanoplates can fully deliver the advantage of high capacity and further induce interlayer engineering of Ti3C2Tx during cycling. As a result, the pillared SnS/Ti3C2Tx MXene composites exhibit obvious improvement in electrochemical performance. Interestingly, there is an apparent enhancement of capacity at succedent cycling, which can be ascribed to the "pillar effect" of Ti3C2Tx MXenes. The efforts and attempts made in this work can further broaden the development of pillared MXene composites.

Recent advances in MXenes and their composites in lithium/sodium batteries from the viewpoints of components and interlayer engineering.

Since their discovery in 2011, MXenes have attracted considerable interest in the fields of energy storage due to their unique combination of properties, such as metallic conductivity, hydrophilic nature, large interlayer spacing, and rich surface chemistries. Although there have been extensive reports on MXenes, the most distinguishing features of MXenes have been ignored, namely larger, adjustable interlayer spacing and rich surface chemistry (adsorbed functional groups on the surface and surface electronegativity), which make MXenes different from other two-dimensional materials. By changing the type of active materials, substituting graphene with MXenes to prepare MXene-based composites in a sequential manner is easy, which does not make much sense for the development of MXenes. Based on this view as well as considering the distinguishing nature of MXenes, we mainly discuss the various preparation methods of MXenes and their stability, and we summarize their applications in high-capacity anodes, metal anodes, and lithium-sulfur batteries, particularly pillared MXenes. We highlight the viewpoints on the basis of their components (individual MXenes or MXene-based composites) and adjustable interlayer engineering by using the pillaring technology, which are based on the distinguishing properties of MXenes. Finally, conclusions and perspectives, together with potential proposals for MXenes and their composites in the energy storage field, are also outlined.

Flowerlike Ti-Doped MoO3 Conductive Anode Fabricated by a Novel NiTi Dealloying Method: Greatly Enhanced Reversibility of the Conversion and Intercalation Reaction.

Anodes made of molybdenum trioxide (MoO3) suffer from insufficient conductivity and low catalytic reactivity. Here, we demonstrate that by using a dealloying method, we were able to fabricate anode of Ti-doped MoO3 (Ti-MoO3), which exhibits high catalytic reactivity, along with enhanced rate performance and cycling stability. We found that after doping, interestingly, the Ti-MoO3 forms nanosheets and assembles into a micrometer-sized flowerlike morphology with enhanced interlayer distance. The density functional theory result has further concluded that the band gap of the Ti-doped anode has been reduced significantly, thus greatly enhancing the electronic conductivity. As a result, the structure maintains stability during the Li+ intercalation/deintercalation processes, which enhances the cycling stability and rate capability. This engineering strategy and one-step synthesis route opens up a new pathway in the design of anode materials.

Fast and Universal Solution-Phase Flocculation Strategy for Scalable Synthesis of Various Few-Layered MXene Powders.

MXenes have gained great attention in various fields because of their fascinating properties; however, the preparation of few-layered MXene powders is still limited by serious restacking of MXene nanosheets. Herein, for the first time, we have demonstrated an effective ammonium ion route to fundamentally address restacking and aggregation of the MXene nanosheets, using a solution-phase flocculation method (NH4+ method and modified NH4+ method) for large-scale preparation of few-layered Ti3C2Tx MXene powders in large quantities. The as-prepared few-layered MXene nanosheet powders show large size in the ab plane without the restacking phenomenon even at scanning electron microscopy measurements of 400× magnification, demonstrating the effectiveness of the proposed method. The method is also suitable for large-scale synthesis of other few-layered MXene powders, including Nb4C3Tx, V2CTx, Nb2CTx, etc., providing a general approach for the preparation of various few-layered MXene nanosheet powders, which represents a significant result for the development of MXenes.

Partial Atomic Tin Nanocomplex Pillared Few-Layered Ti3C2Tx MXenes for Superior Lithium-Ion Storage.

MXenes have attracted great interest in various fields, and pillared MXenes open a new path with larger interlayer spacing. However, the further study of pillared MXenes is blocked at multilayered state due to serious restacking phenomenon of few-layered MXene nanosheets. In this work, for the first time, we designed a facile NH4+ method to fundamentally solve the restacking issues of MXene nanosheets and succeeded in achieving pillared few-layered MXene. Sn nanocomplex pillared few-layered Ti3C2Tx (STCT) composites were synthesized by introducing atomic Sn nanocomplex into interlayer of pillared few-layered Ti3C2Tx MXenes via pillaring technique. The MXene matrix can inhibit Sn nanocomplex particles agglomeration and serve as conductive network. Meanwhile, the Sn nanocomplex particles can further open the interlayer spacing of Ti3C2Tx during lithiation/delithiation processes and therefore generate extra capacity. Benefiting from the "pillar effect," the STCT composites can maintain 1016 mAh g-1 after 1200 cycles at 2000 mA g-1 and deliver a stable capacity of 680 mAh g-1 at 5 A g-1, showing one of the best performances among MXene-based composites. This work will provide a new way for the development of pillared MXenes and their energy storage due to significant breakthrough from multilayered state to few-layered one.

One-Pot Synthesis of a Copolymer Micelle Crosslinked Binder with Multiple Lithium-Ion Diffusion Pathways for Lithium-Sulfur Batteries.

Fast lithium-ion diffusion is very important to obtain high capacity and excellent cycling stability of lithium-sulfur batteries. In this study, a copolymer micelle crosslinked binder (FNA) for lithium-sulfur batteries was successfully synthesized through a one-pot environmentally friendly approach. The micelles were used as crosslinkers and carriers for the electrolyte. The FNA binder provided multiple lithium-ion diffusion pathways to increase the lithium-ion diffusion, which reduced the polarization of the sulfur cathode during the cycling process. The lithium-ion diffusion pathways of the FNA were provided by the electrolyte hosted in the micelles, the polyethylene oxide and polypropylene oxide segments, and the carboxylate and sulfonate groups in the FNA. In addition, FNA possesses strong lithium polysulfides adsorption and high adhesion properties. Therefore, the electrode with the FNA binder presented a reversible capacity of 571 mAh g-1 with a capacity fade of 0.032 % after 1000 cycles at a cycling rate of 0.5 C, which is much higher than those of the polyvinylidene fluoride (PVDF) sulfur cathode.

Novel Synthesis of Red Phosphorus Nanodot/Ti3C2Tx MXenes from Low-Cost Ti3SiC2 MAX Phases for Superior Lithium- and Sodium-Ion Batteries.

MXenes, synthesized from MAX, have emerged as new energy-storage materials for a good combination of metallic conductivity and rich surface chemistry. The reported MXenes are synthesized mostly from Al-based MAX. It is still a big challenge to synthesize MXenes from abundant Si-based MAX because of its strong Ti-Si bonds. Here, we report for the first time a high-energy ultrasonic cell-crushing extraction method to successfully prepare Ti3C2Tx MXenes from Si-based MAX using a single low-concentration etchant. This novel strategy for preparing MXenes has a high extraction efficiency and is a fast preparation process of less than 2 h for selective etching of Si. Furthermore, through the high-energy ball-milling technology, unique P-O-Ti bonded red phosphorus nanodot/Ti3C2Tx (PTCT) composites were successfully prepared, which enable superior electrochemical performance in lithium- and sodium-ion batteries because of the double-morphology structure, where the amorphous nano red phosphorus particles were strongly absorbed to Ti3C2Tx MXene sheets, facilitating the transport of alkali ions during cycling processes. This novel synthesis method of Ti3C2Tx MXenes from Si-based MAX and unique P-O-Ti bonded PTCT composites opens a new door for preparing high-performance MXene-based materials and facilitating the development of low-cost MXenes and other two-dimensional materials for next-generation energy storage.

Vapor Deposition Red Phosphorus to Prepare Nitrogen-Doped Ti3C2Tx MXenes Composites for Lithium-Ion Batteries.

MXenes have great application prospect in energy storage fields due to a series of special physicochemical properties. However, the application of MXenes is greatly limited due to low intrinsic capacity. Here, through spray drying and vapor deposition methods, N-doped Ti3C2Tx and phosphorus composites (N-Ti3C2Tx/P) were prepared for the first time. The red phosphorus particles were absorbed to a walnut-like N-Ti3C2Tx matrix, facilitating the transport of Li+ and electrons. When used as anodes for lithium-ion batteries, the battery can cycle up to 1040 cycles with a high stable capacity of 801 mAh/g at 500 mA/g. Impressively, there is an obvious increase of capacity in the subsequent cycles at higher current density due to the increment of interlayer spacing of Ti3C2Tx nanosheets. XPS measurements confirm that the Ti-O-P bond was formed in the composites, granting the robust structure of the composites and leading to superior performances during cycling. The facile synthesis method of red phosphorus by vapor deposition will facilitate the development of other 2D materials combined with high-capacity red phosphorus for energy storage.

Preparation of an Amorphous Cross-Linked Binder for Silicon Anodes.

An amorphous cross-linked binder is prepared from abundant and low-cost sodium alginate and carboxymethyl cellulose by protonation and mixing and is used to improve the electrochemical performance of silicon anodes in lithium-ion batteries. The amorphous cross-linked structure, formed by intermolecular hydrogen bonding between the functional groups in the two polymers, effectively enhances the flexibility and strength of the binder, resulting in strong adhesion between the binder and other components in the silicon anodes. Furthermore, the binder tolerates large volume changes and reduces the pulverization of silicon during the charge-discharge process. The hydrogen bonding in the binder helps to maintain the anode integrity during the volume change, leading to an excellent cycling stability and superior rate capability with a capacity of 1863 mAh g-1 at 500 mA g-1 after 150 cycles.

New, Effective, and Low-Cost Dual-Functional Binder for Porous Silicon Anodes in Lithium-Ion Batteries.

In this work, a new effective and low-cost binder applied in porous silicon anode is designed through blending of low-cost poly(acrylic acid) (PAA) and poly(ethylene- co-vinyl acetate) (EVA) latex (PAA/EVA) to avoid pulverization of electrodes and loss of electronic contact because of huge volume changes during repeated charge/discharge cycles. PAA with a large number of carboxyl groups offers strong binding strength among porous silicon particles. EVA with high elastic property enhances the ductility of the PAA/EVA binder. The high-ductility PAA/EVA binder tolerates the huge silicon volume variations and keeps the electrode integrity during the charge/discharge cycle process. EVA colloids acting as host materials for electrolytes increase the electrolyte uptake of electrodes. The porous silicon electrode with the PAA/EVA binder exhibits a reversible capacity of 2120 mA h g-1 at 500 mA g-1 after 140 cycles because of the excellent ductility and lithium-ion transport properties of the PAA/EVA binder.

Rational Design of Porous N-Ti3C2 MXene@CNT Microspheres for High Cycling Stability in Li-S Battery.

Herein, N-Ti3C2@CNT microspheres are successfully synthesized by the simple spray drying method. In the preparation process, HCl-treated melamine (HTM) is selected as the sources of carbon and nitrogen. It not only realizes in situ growth of CNTs on the surface of MXene nanosheets with the catalysis of Ni, but also introduces efficient N-doping in both MXene and CNTs. Within the microsphere, MXene nanosheets interconnect with CNTs to form porous and conductive network. In addition, N-doped MXene and CNTs can provide strong chemical immobilization for polysulfides and effectively entrap them within the porous microspheres. Above-mentioned merits enable N-Ti3C2@CNT microspheres to be ideal sulfur host. When used in lithium-sulfur (Li-S) battery, the N-Ti3C2@CNT microspheres/S cathode delivers initial specific capacity of 927 mAh g-1 at 1 C and retains high capacity of 775 mAh g-1 after 1000 cycles with extremely low fading rate (FR) of 0.016% per cycle. Furthermore, the cathode still shows high cycling stability at high C-rate of 4 C (capacity of 647 mAh g-1 after 650 cycles, FR 0.027%) and high sulfur loading of 3 and 6 mg cm-2 for Li-S batteries.

Ultra-stable binder-free rechargeable Li/I2 batteries enabled by "Betadine" chemical interaction.

An activated carbon cloth/polymer-iodine (ACC/PVP-I2) composite was prepared by the "Betadine" method and employed as a high-performance cathode for rechargeable Li/I2 batteries. Due to the synergistic effect of ACC and PVP-I2, Li/I2 cells with ACC/PVP-I2 as the cathode exhibited superior electrochemical performance.

Polyiodide-Shuttle Restricting Polymer Cathode for Rechargeable Lithium/Iodine Battery with Ultralong Cycle Life.

Rechargeable lithium/iodine (Li/I2) batteries have attracted much attention because of their high gravimetric/volumetric energy densities, natural abundance and low cost. However, problems of the system, such as highly unstable iodine species under high temperature, their subsequent dissolution in electrolyte and continually reacting with lithium anode prevent the practical use of rechargeable Li/I2 cells. A polymer-iodine composite (polyvinylpyrrolidone-iodine) with high thermostability is employed as cathode material in rechargeable Li/I2 battery with an organic electrolyte. Because of the chemical interaction between polyvinylpyrrolidone (PVP) and polyiodide, most of the polyiodide in the cathode could be effectively trapped during charging/discharging. In-situ Raman observation revealed the evolution of iodine species in this system could be controlled during the process of I5- ↔ I3- ↔ I-. Herein, the Li/I2 battery delivered a high discharge capacity of 278 mAh g-1 at 0.2 C and exhibited a very low capacity decay rate of 0.019% per cycle for prolonged 1100 charge/discharge cycles at 2 C. More importantly, a high areal capacity of 4.1 mAh cm-2 was achieved for the electrode with high iodine loading of 21.2 mg cm-2. This work may inspire new approach to design the Li/I2 (or Li/polyiodide) system with long cycle life.