Encapsulation of sulfur in MoS2-modified metal-organic framework-derived N, O-codoped carbon host for sodium-sulfur batteries

Alkalized MXene/carbon nanotube composite for stable Na metal anodes

Ti3C2 MXenes are emerging 2D materials and have attracted increasing attention in sodium metal anode fabrication because of their high conductivity, multifunctional groups and excellent mechanical performances. However, the severe self-restacking of Ti3C2 MXenes is not conducive to dispersing Na+ and limits the function of regulating sodium deposition. Herein, an alkalized MXene/carbon nanotube (CNT) composite (named A-M-C) is introduced to regulate Na deposition behavior, which consists of Na3Ti5O12 microspheres, Ti3C2 MXene nanosheets and CNTs. Ti3C2 MXene nanosheets with large interlayer spaces and "sodiophilic" functional groups can provide abundant active sites for uniform nucleation and deposition of Na. Plenty of nanosheets are grown on the surface of the microsphere, thereby reducing the local current density, which can guide initial Na nucleation and promote Na dendrite-free growth. Furthermore, CNTs increase the electrical conductivity of the composite and achieve fast Na+ transport, improving the cycling stability of Na metal batteries. As a result, at a capacity of 1 mA h cm-2, the A-M-C electrode achieves a high average coulombic efficiency (CE) of 99.9% after 300 cycles at 2 mA cm-2. The symmetric cells of A-M-C/Na provide a long cycling life of more than 1400 h at 1 mA cm-2 with a minimal overpotential of 19 mV at an areal capacity of 1 mA h cm-2. The A-M-C/Na//NVP@C full cell presents a high coulombic efficiency of 98% with 100 mA g-1 in the first cycle. The strategy in this work provides new insights into fabricating novel MXene-based anode materials for dendrite-free sodium deposition.

Regulating Chemisorption and Electrosorption Activity for Efficient Uptake of Rare Earth Elements in Low Concentration on Oxygen-Doped Molybdenum Disulfide

Recovery of rare earth elements (REEs) with trace amount in environmental applications and nuclear energy is becoming an increasingly urgent issue due to their genotoxicity and important role in society. Here, highly efficient recovery of low-concentration REEs from aqueous solutions by an enhanced chemisorption and electrosorption process of oxygen-doped molybdenum disulfide (O-doped MoS2) electrodes is performed. All REEs could be extremely recovered through a chemisorption and electrosorption coupling (CEC) method, and sorption behaviors were related with their outer-shell electrons. Light, medium, and heavy ((La(III), Gd(III), and Y(III)) rare earth elements were chosen for further investigating the adsorption and recovery performances under low-concentration conditions. Recovery of REEs could approach 100% under a low initial concentration condition where different recovery behaviors occurred with variable chemisorption interactions between REEs and O-doped MoS2. Experimental and theoretical results proved that doping O in MoS2 not only reduced the transfer resistance and improved the electrical double layer thickness of ion storage but also enhanced the chemical interaction of REEs and MoS2. Various outer-shell electrons of REEs performed different surficial chemisorption interactions with exposed sulfur and oxygen atoms of O-doped MoS2. Effects of variants including environmental conditions and operating parameters, such as applied voltage, initial concentration, pH condition, and electrode distance on adsorption capacity and recovery of REEs were examined to optimize the recovery process in order to achieve an ideal selective recovery of REEs. The total desorption of REEs from the O-doped MoS2 electrode was realized within 120 min while the electrode demonstrated a good cycling performance. This work presented a prospective way in establishing a CEC process with a two-dimensional metal sulfide electrode through structure engineering for efficient recovery of REEs within a low concentration range.