Electrochemical Lithium Storage Performance of Molten Salt Derived V2SnC MAX Phase

Universal Intercalation/Alloying Hybrid Mechanism with -ICOHP Criterion in MAX Toward Steadily Ascending Lithium-Ion Batteries

Profiting from the unique atomic laminated structure, metallic conductivity, and superior mechanical properties, transition metal carbides and nitrides named MAX phases have shown great potential as anodes in lithium-ion batteries. However, the complexity of MAX configurations poses a challenge. To accelerate such application, a minus integrated crystal orbital Hamilton populations descriptor is innovatively proposed to rapidly evaluate the lithium storage potential of various MAX, along with density functional theory computations. It confirms that surface A-element atoms bound to lithium ions have odds of escaping from MAX. Interestingly, the activated A-element atoms enhance the reversible uptake of lithium ions by MAX anodes through an efficient alloying reaction. As an experimental verification, the charge compensation and SnxLiy phase evolution of designed Zr2SnC MAX with optimized structure is visualized via in situ synchrotron radiation XRD and XAFS technique, which further clarifies the theoretically expected intercalation/alloying hybrid storage mechanism. Notably, Zr2SnC electrodes achieve remarkably 219.8% negative capacity attenuation over 3200 cycles at 1 A g-1. In principle, this work provides a reference for the design and development of advanced MAX electrodes, which is essential to explore diversified applications of the MAX family in specific energy fields.

A Hybrid Structure to Improve Electrochemical Performance of SiO Anode Materials in Lithium-Ion Battery

The wide utilization of lithium-ion batteries (LIBs) prompts extensive research on the anode materials with large capacity and excellent stability. Despite the attractive electrochemical properties of pure Si anodes outperforming other Si-based materials, its unsafety caused by huge volumetric expansion is commonly admitted. Silicon monoxide (SiO) anode is advantageous in mild volume fluctuation, and would be a proper alternative if the low initial columbic efficiency and conductivity can be ameliorated. Herein, a hybrid structure composed of active material SiO particles and carbon nanofibers (SiO/CNFs) is proposed as a solution. CNFs, through electrospun processes, serve as a conductive skeleton for SiO nanoparticles and enable SiO nanoparticles to be uniformly embedded in. As a result, the SiO/CNF electrochemical performance reaches a peak at 20% the mass ratio of SiO, where the retention rate reaches 73.9% after 400 cycles at a current density of 100 mA g-1, and the discharge capacity after stabilization and 100 cycles are 1.47 and 1.84 times higher than that of pure SiO, respectively. A fast lithium-ion transport rate during cycling is also demonstrated as the corresponding diffusion coefficient of the SiO/CNF reaches ~8 × 10-15 cm2 s-1. This SiO/CNF hybrid structure provides a flexible and cost-effective solution for LIBs and sheds light on alternative anode choices for industrial battery assembly.

Defect Engineering of Hexagonal MAB Phase Ti2InB2 as Anode of Lithium-Ion Battery with Excellent Cycling Stability

Hexagonal MAB phases （h-MAB） have attracted attention due to their potential to exfoliate into MBenes, similar to MXenes, which are predicted to be promising for Li-ion battery applications. However, the high cost of synthesizing MBenes poses challenges for their use in batteries. This study presents a novel approach where a simple ball-milling treatment is employed to enhance the purity of the h-MAB phase Ti2InB2 and introduce significant indium defects, resulting in improved conductivity and the creation of abundant active sites. The synthesized Ti2InB2 with indium defects (VIn-Ti2InB2) exhibits excellent electrochemical properties, particularly exceptional long-cycle stability at current densities of 5 A g-1 (5000 cycles, average capacity decay of 0.0018%) and 10 A g-1 (15 000 cycles, average capacity decay of 0.093%). The charge storage mechanism of VIn-Ti2InB2, involving a dual redox reaction, is proposed, where defects promote the In-Li alloy reaction and a redox reaction with Li in the TiB layer. Finally, a Li-ion full cell demonstrates cycling stability at 0.5 A g-1 after 350 cycles. This work presents the first accessible and scalable application of VIn-Ti2InB2 as a Li-ion anode, unlocking a wealth of possibilities for sustainable electrochemical applications of h-MAB phases.

MXene and Carbon-Based Electrodes of Thermocells for Continuous Thermal Energy Harvest

Low-grade heat represents a significant form of energy loss; thermocells (TECs) utilizing the thermogalvanic effect can convert thermal energy into electricity without generating vibrations, noise, or waste emissions, making them a promising energy conversion technology for efficiently harvesting low-grade heat. Despite recent advancements, the reliance on high-cost platinum electrodes in TECs has considerably hindered their widespread adoption. Developing cost-effective electrodes that maintain the same thermoelectrochemical performance is crucial for the successful application of TECs. In this review article, the exploration of MXene materials as TEC electrodes is discussed first, emphasizing the immense potential of the MXene family for low-grade heat harvesting applications. Next, recent research on carbon-based electrodes is summarized, and morphological and structural optimizations are comprehensively discussed aiming at enhancing the thermoelectrochemical performance of TECs. In the concluding section, the challenges are outlined and future perspectives are offered, which provide valuable insights into the ongoing development of high-performance TEC electrodes using MXene and carbon-based materials.

Two-Dimensional MXenes Derived from Medium/High-Entropy MAX Phases M2 GaC (M = Ti/V/Nb/Ta/Mo) and their Electrochemical Performance

Two-dimensional (2D) transition metal carbides and/or nitrides, MXenes, are prepared by selective etching of the A-site atomically thin metal layers from their MAX phase precursors. High entropy MXenes, the most recent subfamily of MXenes, are in their infancy and have attracted great interest recently. They are currently synthesized mainly through wet chemical etching of Al-containing MAX phases, while various MAX phases with A-sites elements other than Al have not been explored. It is important to embody non-Al MAX phases as precursors for the high entropy MXenes synthesis to allow for new compositions. In this work, it is reported on the design and synthesis of Ga-containing medium/high entropy MAX phases and then their corresponding medium/high entropy MXenes. Gallium atomic layer etching is carried out using a Lewis acid molten salt (CuCl2). The as-prepared (Ti1/4 V1/4 Nb1/4 Ta1/4 )2 CTx exhibits a Li+ specific capacity of ≈400 mAh g-1 . For (Ti1/5 V1/5 Nb1/5 Ta1/5 Mo1/5 )2 CTx a specific capacity of 302 mAh g-1 is achieved after 300 cycles, and high cycling stability is observed at high current densities. This work is of great significance for expanding the family members of MXenes with tunable chemistries and structures.

Lithium storage performance and mechanism of nano-sized Ti2InC MAX phase

Fine powders of MAX phases (a family of layered carbides/nitrides) have been showing great promise in energy storage applications. A feasible method of obtaining nano-sized MAX phase particles is critical to realizing the practical application of the vast MAX phase family in more technologically important fields. Herein, ball milling, a commercial and feasible method, is employed to prepare nano-sized Ti₂InC, which delivers a high specific capacity of 590 mA h g^-1 after 500 cycles and maintains 574.4 mA h g^-1 after 600 cycles at 0.1 A g^-1 when used as a lithium storage anode. Compared with other methods (e.g., partial etching), decreasing the size of Ti₂InC particles by ball milling can preserve the exfoliated indium (In) atoms, which have great volumetric and gravimetric capacities. In situ XRD analysis indicates that the capacity of the nano-sized Ti₂InC primarily comes from the lithiation of elemental In exfoliated from Ti₂InC, and in particular, the exfoliated In atoms by ball milling can increase the initial capacity. The lithiation/delithiation cycle can effectively activate and even exfoliate the Ti₂InC grains, which accounts for the increasing capacity upon cycling.

Molten salt electrosynthesis of Cr2GeC nanoparticles as anode materials for lithium-ion batteries

The two-dimensional MAX phases with compositional diversity are promising functional materials for electrochemical energy storage. Herein, we report the facile preparation of the Cr₂GeC MAX phase from oxides/C precursors by the molten salt electrolysis method at a moderate temperature of 700°C. The electrosynthesis mechanism has been systematically investigated, and the results show that the synthesis of the Cr₂GeC MAX phase involves electro-separation and in situ alloying processes. The as-prepared Cr₂GeC MAX phase with a typical layered structure shows the uniform morphology of nanoparticles. As a proof of concept, Cr₂GeC nanoparticles are investigated as anode materials for lithium-ion batteries, which deliver a good capacity of 177.4 mAh g^-1 at 0.2 C and excellent cycling performance. The lithium-storage mechanism of the Cr₂GeC MAX phase has been discussed based on density functional theory (DFT) calculations. This study may provide important support and complement to the tailored electrosynthesis of MAX phases toward high-performance energy storage applications.

Towards Greener and More Sustainable Synthesis of MXenes: A Review

The unique properties of MXenes have been deemed to be of significant interest in various emerging applications. However, MXenes provide a major drawback involving environmentally harmful and toxic substances for its general fabrication in large-scale production and employing a high-temperature solid-state reaction followed by selective etching. Meanwhile, how MXenes are synthesized is essential in directing their end uses. Therefore, making strategic approaches to synthesize greener, safer, more sustainable, and more environmentally friendly MXenes is imperative to commercialize at a competitive price. With increasing reports of green synthesis that promote advanced technologies and non-toxic agents, it is critical to compile, summarize, and synthesize the latest development of the green-related technology of MXenes. We review the recent progress of greener, safer, and more sustainable MXene synthesis with a focus on the fundamental synthetic process, the mechanism, and the general advantages, and the emphasis on the MXene properties inherited from such green synthesis techniques. The emerging use of the so-called green MXenes in energy conversion and storage, environmental remediation, and biomedical applications is presented. Finally, the remaining challenges and prospects of greener MXene synthesis are discussed.

Facet-Controlled LiMn2O4/C as Deionization Electrode with Enhanced Stability and High Desalination Performance

HIGHLIGHTS

First report of a lithium-ion battery cathode as a deionization electrode for desalination. A novel approach to suppress manganese dissolution by exposing the (111) facet is proposed. Excellent desalination performance by the LiMn₂O₄/C cathode. The material achieves an ultrahigh desalination capacity of 117.3 mg g⁻¹ at 1.0 V and a longer cycle life (200 cycles without capacity decay) with minor manganese dissolution during the cycling test in 10 mM aqueous LiCl solution.

ABSTRACT

Battery materials as emerging capacitive deionization electrodes for desalination have better salt removal capacities than traditional carbon-based materials. LiMn₂O₄, a widely used cathode material, is difficult to utilize as a deionization electrode due to its structural instability upon cycling and Mn dissolution in aqueous-based electrolytes. Herein, a facile and low-cost ball-milling routine was proposed to prepare a LiMn₂O₄ material with highly exposed (111) facets. The prepared electrode exhibited relatively low dissolution of Mn during cycling, which shows its long cycle stability. In the hybrid capacitive deionization system, the LiMn₂O₄/C electrode delivered a high desalination capacity of 117.3 mg g⁻¹ without obvious capacity decay at a voltage of 1.0 V with a 20 mM initial salt concentration. In addition, the exposed (111) facets significantly alleviated Mn ion dissolution, which also enhanced the structural steadiness. [Image: see text]

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s40820-022-00897-3.

Ti3Si0.75Al0.25C2 Nanosheets as Promising Anode Material for Li-Ion Batteries

Herein we report that novel two-dimensional (2D) Ti₃Si_0.75Al_0.25C₂ (TSAC) nanosheets, obtained by sonically exfoliating their bulk counterpart in alcohol, performs promising electrochemical activities in a reversible lithiation and delithiation procedure. The as-exfoliated 2D TSAC nanosheets show significantly enhanced lithium-ion uptake capability in comparison with their bulk counterpart, with a high capacity of ≈350 mAh g^-1 at 200 mA g^-1, high cycling stability and excellent rate performance (150 mAh g^-1 after 200 cycles at 8000 mA g^-1). The enhanced electrochemical performance of TSAC nanosheets is mainly a result of their fast Li-ion transport, large surface area and small charge transfer resistance. The discovery in this work highlights the uniqueness of a family of 2D layered MAX materials, such as Ti₃GeC₂, Ti₃SnC₂ and Ti₂SC, which will likely be the promising choices as anode materials for lithium-ion batteries (LIBs).

MOF-Derived ZnS Nanodots/Ti3C2Tx MXene Hybrids Boosting Superior Lithium Storage Performance

ZnS has great potentials as an anode for lithium storage because of its high theoretical capacity and resource abundance; however, the large volume expansion accompanied with structural collapse and low conductivity of ZnS cause severe capacity fading and inferior rate capability during lithium storage. Herein, 0D-2D ZnS nanodots/Ti₃C₂T_x MXene hybrids are prepared by anchoring ZnS nanodots on Ti₃C₂T_x MXene nanosheets through coordination modulation between MXene and MOF precursor (ZIF-8) followed with sulfidation. The MXene substrate coupled with the ZnS nanodots can synergistically accommodate volume variation of ZnS over charge-discharge to realize stable cyclability. As revealed by XPS characterizations and DFT calculations, the strong interfacial interaction between ZnS nanodots and MXene nanosheets can boost fast electron/lithium-ion transfer to achieve excellent electrochemical activity and kinetics for lithium storage. Thereby, the as-prepared ZnS nanodots/MXene hybrid exhibits a high capacity of 726.8 mAh g^-1 at 30 mA g^-1, superior cyclic stability (462.8 mAh g^-1 after 1000 cycles at 0.5 A g^-1), and excellent rate performance. The present results provide new insights into the understanding of the lithium storage mechanism of ZnS and the revealing of the effects of interfacial interaction on lithium storage performance enhancement.