Stable Electrochemical Li Plating/Stripping Behavior by Anchoring MXene Layers on Three-Dimensional Conductive Skeletons.

The ultrahigh specific capacity of lithium (Li) metal makes it possible to serve as the ultimate candidate for an anode in high-energy density secondary batteries, whereas the safety hazards caused by Li dendrite growth severely hamper the commercialization process of a lithium metal anode. Here, we propose a 3D conductive skeleton by anchoring MXene on Cu foam (MXene@CF) to significantly improve the electrochemical Li plating/stripping behavior. Li metal tends to nucleate uniformly and grow horizontally along the MXene nanosheets under the strong Coulomb interaction between adsorbed Li and MXene. Moreover, the abundant fluorine termination groups in MXene contribute to forming a stable fluorinated solid electrolyte interphase (SEI) and thus effectively regulating the Li deposition behaviors and prolonging the stability of the Li metal anode. Therefore, the MXene@CF skeleton maintains a high Coulombic efficiency (CE) of 98.5% after 200 cycles at 1 mA cm-2. The MXene@CF-based symmetric cells can run for more than 1000 h without intense voltage fluctuation and demonstrates remarkable deep charge/discharge abilities. The MXene@CF-Li|LiFePO4 full cell exhibits outstanding long-term cycling stability (95% capacity retention after 300 cycles). Our research suggests that MXene could effectively regulate the Li plating behavior that might provide a feasible solution for a dendrite-free Li anode.

Core-Shell-Structured Sulfur Cathode: Ultrathin δ-MnO2 Nanosheets as the Catalytic Conversion Shell for Lithium Polysulfides in High Sulfur Content Lithium-Sulfur Batteries.

Owing to their low cost and high theoretical energy density, lithium-sulfur (Li-S) batteries are highly promising as a contender for the post-lithium-ion battery era. However, the intrinsic low reversible conversion ability of lithium polysulfides to sulfur/Li2S during charging/discharging seriously hinders the sulfur utilization, resulting in poor cycling life of batteries. Herein, we report an improvement of core-shell-structured sulfur nanospheres@ultrathin δ-MnO2 nanosheet electrode materials prepared by a simple precipitation reaction method, in which the ultrathin δ-MnO2 nanosheets as a catalytic layer can promote the chemical adsorption of lithium polysulfides and their conversion rates to sulfur/Li2S. Using a combination of the UV-visible adsorption spectra and first-principles calculation, the results indicate that the Mn-O coordination center on the surface of the MnO2 structure plays an efficient catalytic role in the conversion reaction of lithium polysulfides to insoluble S/Li2S. The sulfur nanospheres@ultrathin δ-MnO2 nanosheet composite with a high S mass ratio of 82 wt % reveals a high specific capacity of 846 mA h g-1 at 1 C rate and good cycling stability. Moreover, the areal capacity of the electrode with a high sulfur loading mass of 10 mg cm-2 is 5.2 mA h cm-2, approaching the practical application standard at a current density of 1 mA cm-2.

High-Stability MnOx Nanowires@C@MnOx Nanosheet Core-Shell Heterostructure Pseudocapacitance Electrode Based on Reversible Phase Transition Mechanism.

A stable MnOx @C@MnOx core-shell heterostructure consisting of vertical MnOx nanosheets grown evenly on the surface of the MnOx @carbon nanowires are obtained by simple liquid phase method combined with thermal treatment. The hierarchical MnOx @C@MnOx heterostructure electrode possesses a high specific capacitance of 350 F g-1 and an excellent cycle performance owing to the existence of the pore structure among the ultrasmall MnOx nanoparticles and the rapid transmission of electrons between the active material and carbon coating layer. Particularly, according to the in situ Raman spectra analysis, no characteristic peaks corresponding to MnOOH are found during charging/discharging, indicating that pseudocapacitive behavior of the MnOx electrode have no relevance to the intercalation/deintercalation of protons (H+ ) in the electrolyte. Further combining in situ X-ray powder diffraction analysis, the diffraction peak of α-MnO2 can be detected in the process of charging, while Mn3 O4 phase is found in discharge products. Therefore, these results demonstrate that the MnOx undergoes a reversible phase transformation reaction of Mn3 O4 ↔α-MnO2 . Moreover, the assembled all-solid-state asymmetric supercapacitor with a MnOx @C@MnOx electrode delivers a high energy density of 23 Wh kg-1 , an acceptable power density of 2500 W kg-1 , and an excellent cyclic stability performance of 94% after 2000 cycles, showing the potential for practical application.