Self-Zincophilic Dual Protection Host of 3D ZnO/Zn⊂CF to Enhance Zn Anode Cyclability.

Zn dendrite growth and side reactions restrict the practical use of Zn anode. Herein, the design of a novel 3D hierarchical structure is demonstrated with self-zincophilic dual-protection constructed by ZnO and Zn nanoparticles immobilized on carbon fibers (ZnO/Zn⊂CF) as a versatile host on the Zn surface. The unique 3D frameworks with abundant zinc nucleation storage sites can alleviate the structural stress during the plating/stripping process and overpower Zn dendrite growth by moderating Zn2+ flux. Moreover, given the dual protection design, it can reduce the contact area between active zinc and electrolyte, inhibiting hydrogen evolution reactions. Importantly, density functional theory calculations and experimental results confirm that the introduced O atoms in ZnO/Zn⊂CF enhance the interaction between Zn2+ and the host and reduce Zn nucleation overpotential. As expected, the ZnO/Zn⊂CF-Zn electrode exhibits stable Zn plating/stripping with low polarization for 4200 h at 0.2 mA cm-2 and 0.2 mAh cm-2 . Furthermore, the symmetrical cell displays a significantly long cycling life of over 1800 h, even at 30 mA cm-2 . The fabricated full cells also show impressive cycling performance when coupled with V2 O3 cathodes.

Molybdenum Oxynitride Atomic Nanoclusters Bonded in Nanosheets of N-Doped Carbon Hierarchical Microspheres for Efficient Sodium Storage.

Transition metal nitrides have attracted considerable attention as great potential anode materials due to their excellent metallic conductivity and high theoretical specific capacity. However, their cycling performance is impeded by their instability caused by the reaction mechanism. Herein, we report the engineering and synthesis of a novel hybrid architecture composed of MoO2.0N0.5 atomic nanoclusters bonded in nanosheets of N-doped carbon hierarchical hollow microspheres (MoO2.0N0.5/NC) as an anode material for sodium-ion batteries. The facile self-templating strategy for the synthesis of MoO2.0N0.5/NC involves chemical polymerization and subsequent one-step calcination treatments. The design is beneficial to improve the electrochemical kinetics, buffer the volume variation of electrodes during cycling, and provide more interfacial active sites for sodium uptake. Due to these unique structural and compositional merits, these MoO2.0N0.5/NC exhibits excellent sodium storage performance in terms of superior rate capability and stable long cycle life. The work shows a feasible and effective way to design novel host candidates and solve the long-term cycling stability issues for sodium-ion batteries.

Two-Dimensional Cr-Doped MoO2.5(OH)0.5 Nanosheets: A Promising Anode Material for Lithium-Ion Batteries.

α-MoO3 has gained growing attention as an anode material of lithium-ion batteries (LIBs) because it has a high theoretical specific capacity of 1111 mA h g-1 and unique layer structure. However, the electrochemical reactions of MoO3 exhibit sluggish kinetics and structural instability caused by pulverization during charge and discharge. Herein, we report new two-dimensional Cr-doped MoO2.5(OH)0.5 (doped MoO2.5(OH)0.5) ultrathin nanosheets prepared by a facile hydrothermal process. The formation of the ultrathin nanosheets was clarified by a "doping-adsorption" model. Compared with doped MoO3, doped MoO2.5(OH)0.5 has larger expanded spacing of the {0 l0} crystal planes for fast Li+ storage. The electrodes after cycling were investigated by ex situ transmission electron microscopy in combination with X-ray photoelectron spectroscopy analysis to reveal the reversible conversion reaction mechanism of doped MoO2.5(OH)0.5 nanosheets. Interestingly, for doped MoO2.5(OH)0.5 nanosheet electrodes, it was found that the as-formed intermediate Li xMoO3 nanodots were well-dispersed in the mesoporous amorphous matrix and had an expanded (040) crystal plane after 10 cycles. These unique structural features increased the effective surface of intermediate products Li xMoO3 to react with Li+ and shortened the diffusion length and thus promoted the electrochemical reactions of doped MoO2.5(OH)0.5. Additionally, the presence of Cr also played a critical role in the reversible decomposition of Li2O and enhanced specific capacity. When employed as an anode in LIBs, doped MoO2.5(OH)0.5 nanosheets show superior reversible capacity (294 mA h g-1 at 10 A g-1 after 2000 cycles). Moreover, the reversible capacity after electrochemical activation is quite stable throughout the cycling, thereby presenting a potential candidate anode material for LIBs.

Unusual Improvement of Pseudocapacitance of Nanocomposite Electrodes: Three-Dimensional Amorphous Carbon Frameworks Triggered by TiO2 Nanocrystals.

Both nanocrystals and carbon materials have attracted considerable attention in lithium-ion batteries (LIBs) because of their fast kinetics for lithium storage or long-life cycles. However, the easy aggregation of nanocrystals and high-temperature doping process of carbon materials seriously hindered their application in LIBs. Here, we report the development of unprecedented TiO2-x@C nanocomposite electrodes through a unique "melting low-temperature pyrolysis" strategy. It is found that the continuous and interconnected three-dimensional amorphous carbon frameworks (3DCFs) in the composites are closely connected with TiO2 nanocrystals by Ti-O-C covalent bonding, forming robust 3D framework architectures. Interestingly, we find that TiO2 nanocrystals can greatly improve the pseudocapacitance of TiO2-x@C nanocomposite electrodes with increasing cycles, which significantly exceeds previously reported TiO2-based anodes and carbon materials. Furthermore, for the first time, the unusual improvement of pseudocapacitance of TiO2-x@C electrodes is carefully investigated by means of dQ/dV curves and electrochemical kinetic analysis to reveal the extra contribution of lithium storage. 3DCF, a "lithium-ion reservoir", possesses an unexpected capacity enhancement behavior that is triggered by TiO2 nanocrystals and exhibits bicontinuous pathways for both rapid ion and electron transport. In this case, TiO2 nanocrystals stabilizing the 3DCF acted as a conductive agent during charge and discharge. Our findings confirm that the 3DCF triggered by TiO2 nanocrystals boosted the electrochemical performance of TiO2-x@C nanocomposite electrodes, especially the pseudocapacitance enhancement. The unique characteristics of ingenious combination of TiO2 nanocrystals and amorphous carbon materials make them attain superior electrochemical properties in all known TiO2- and carbon-based anodes (289 mA h g-1 at 5 A g-1 after 4000 cycles). Above all, our findings reveal previously ignored fundamental aspects of pseudocapacitance improvement of nanocomposite electrodes and offer new hope for structural design and carbon coating process of high-performance anode materials.