In situ crafting of a 3D N-doped carbon/defect-rich V2O5-x·nH2O nanosheet composite for high performance fibrous flexible Zn-ion batteries

Rational electrolyte design and electrode regulation for boosting high-capacity Zn-iodine fiber-shaped batteries with four-electron redox reactions

Aqueous Zn ion-based fiber-shaped batteries (AZFBs) with the merits of high flexibility and safety have received much attention for powering wearable electronic devices. However, the relatively low specific capacity provided by cathode materials limits their practical application. Herein, we first propose a simple strategy for fabricating high-capacity Zn-iodine fiber-shaped batteries with a high concentration electrolyte and a reduced graphene oxide fiber (GF) cathode. It was found that oxygen functional groups in the graphene sheet demonstrate strong interaction with polyiodides but hinder electron conductivity; thus, the optimal balance between the specific capacity and coulombic efficiency of the GF electrode can be a function of the surface properties at different hydrothermal temperatures. Besides, the regulated high concentration electrolyte effectively suppresses the diffusion of polyiodides, which is attributed to the constrained freedom of water. More importantly, a four-electron redox mechanism was experimentally revealed through in situ Raman spectra. As a result, this fiber-shaped battery delivers a superior high reversible capacity of 390 mA h cm-3 at 1 A cm-3, an excellent rate performance of 125.7 mA h cm-3 at a high current density of 8 A cm-3 and outstanding cycling life with 82% capacitance retention after 2500 cycles.

Synthesis of MnOOH and its application in a supporting hexagonal Pd/C catalyst for the oxygen reduction reaction

With the expansion of global energy problems and the deepening of research on oxygen reduction reaction (ORR) in alkaline media, the development of low cost and high electrocatalytic performance catalysts has become a research hotspot. In this study, a hexagonal Pd-C-MnOOH composite catalyst was prepared by using the triblock copolymer P123 as the reducing agent and protective agent, sucrose as the carbon source and self-made MnOOH as the carrier under hydrothermal conditions. When the Pd load is 20% and the C/MnOOH ratio is 1 : 1, the 20% Pd-C-MnOOH-1 : 1 catalyst obtained by the one-step method has the highest ORR activity and stability in the alkaline system. At 1600 rpm, the limiting diffusion current density and half-wave potential of the 20% Pd-C-MnOOH-1 : 1 electrocatalyst are -4.78 mA cm-2 and 0.84 V, respectively, which are better than those of the commercial 20%Pd/C catalyst. According to the Koutecky-Levich (K-L) equation and the linear fitting results, the electron transfer number of the 20%Pd-C-MnOOH-1 : 1 electrocatalyst for the oxygen reduction reaction is 3.8, which is similar to that of a 4-electron process. After 1000 cycles, the limiting diffusion current density of the 20%Pd-C-MnOOH-1 : 1 catalyst is -4.61 mA cm-2, which only decreases by 3.7%, indicating that the 20%Pd-C-MnOOH-1 : 1 catalyst has good stability. The reason for the improvement of the ORR performance of the Pd-C-MnOOH composite catalyst is the improvement of the conductivity of the carbon layer formed by original carbonization, the regular hexagonal highly active Pd particles and the synergistic catalytic effect between Pd and MnOOH. The method of introducing triblock copolymers in the synthesis of oxides and metal-oxide composite catalysts is expected to be extended to other electrocatalysis fields.

Emerging Amorphous to Crystalline Conversion Chemistry in Ca-Doped VO2 Cathodes for High-Capacity and Long-Term Wearable Aqueous Zinc-Ion Batteries

Amorphous transition metal oxides have attracted significant attention in energy storage devices owing to their potentially desirable electrochemical properties caused by abundant unsaturated dangling bonds. However, the amorphization further amplifies the shortcoming of the poor intrinsic electronic conductivity of the metal oxides, resulting in unsatisfying rate capability and power density. Herein, freestanding amorphous Ca-doped V2 O5 (a-Ca-V2 O5 ) cathodes are successfully prepared via in situ electrochemical oxidation of Ca-doped VO2 nanoarrays for wearable aqueous zinc-ion batteries. The doping of Ca and construction of freestanding structure effectively uncover the potential of amorphous V2 O5 , which can make full use of the abundant active sites for high volumetric capacity and simultaneously achieve fast reaction kinetics for excellent rate performance. More importantly, the introduction of Ca can notably reduce the formation energy of VO2 according to theoretical calculation results and realizes amorphous to crystalline reversible conversion chemistry in the charge/discharge procedure, thereby facilitating the reversible capacity of the newly developed a-Ca-V2 O5 . This work provides an innovative design strategy to construct high-rate capacity amorphous metal oxides as freestanding electrodes for low-cost and high-safe wearable energy-storage technology.