Protons intercalation induced hydrogen bond network in δ-MnO2 cathode for high-performance aqueous zinc-ion batteries.

Birnessite-type MnO2 (δ-MnO2) exhibits great potential as a cathode material for aqueous zinc-ion batteries (AZIBs). However, the structural instability and sluggish reaction kinetics restrict its further application. Herein, a unique protons intercalation strategy was utilized to simultaneously modify the interlayer environment and transition metal layers of δ-MnO2. The intercalated protons directly form strong O  H bonds with the adjacent oxygens, while the increased H2O molecules also establish a hydrogen bond network (O  H···O) between H2O molecules or bond with adjacent oxygens. Based on the Grotthuss mechanism, these bondings ultimately enhance the stability of layered structures and facilitate the rapid diffusion of protons. Moreover, the introduction of protons induces numerous oxygen vacancies, reduces steric hindrance, and accelerates ion transport kinetics. Consequently, the protons intercalated δ-MnO2 (H-MnO2-x) demonstrates exceptional specific capacity of 401.7 mAh/g at 0.1 A/g and a fast-charging performance over 1000 cycles. Density functional theory analysis confirms the improved electronic conductivity and reduced diffusion energy barrier. Most importantly, electrochemical quartz crystal microbalance tests combining with ex-situ characterizations verify the inhibitory effect of the interlayer proton environment on basic zinc sulfate formation. Protons intercalation behavior provides a promising avenue for the development of MnO2 as well as other cathodes in AZIBs.

Oxygen vacancies in MnOx regulating reaction kinetics for aqueous zinc-ion batteries.

MnO2 cathode materials have presented challenges due to their poor conductivity, unstable structure, and sluggish diffusion kinetics for aqueous zinc-ion batteries (AZIBs). In this study, a nanostructured MnOx cathode material was synthesized using an acid etching method, Which introduced abundant Mn(III) sites, resulting in the formation of numerous oxygen vacancies. Comprehensive characterizations revealed that these oxygen vacancies facilitated the reversible adsorption/desorption of Zn2+ ions and promoted efficient electron transfer. In addition, the designed mesoporous structure offered ample active sites and shortened the diffusion path for Zn2+ and H+ ions. Consequently, the nanosized MnOx cathode exhibited enhanced reaction kinetics, achieving a considerable reversible specific capacity of 388.7 mAh/g at 0.1 A/g and superior durability with 72.0% capacity retention over 2000 cycles at 3.0 A/g. The material delivered a maximum energy density of 639.7 Wh kg-1 at 159.94 W kg-1. Furthermore, a systematic analysis of the zinc storage mechanism was performed. This work demonstrates that engineering oxygen vacancies with nanostructure regulation provides valuable insights into optimizing MnO2 cathode materials for AZIBs.

Engineering p-Band Center of Oxygen Boosting H+ Intercalation in δ-MnO2 for Aqueous Zinc Ion Batteries.

In aqueous zinc ion batteries (ZIBs), the H+ intercalation possesses superior electrochemical kinetics with excellent rate capability, however, precisely modulating H+ intercalation has been still challenging. Herein, a critical modification of pre-intercalating metal ions in the MnO2 interlayer (M-MnO2 ) with controllable p-band center (ϵp ) of O is reported to modulate the H+ intercalation. The modulation of metal-O bond type and covalency degree on the average charge of O atom results in optimized ϵp and H+ adsorption energy for M-MnO2 , thus promoting the balance between H+ adsorption and desorption, which plays a determinant role on H+ intercalation. The optimized Cu-MnO2 delivers superior rate capability with the capacity of 153 mAh g-1 at a high rate of 3 A g-1 after 1000 cycles. This work demonstrates that ϵp could be a significant descriptor for H+ intercalation, and tuning ϵp effectively increases H+ intercalation contribution with excellent rate capability in ZIBs.

Doping-Induced Electronic/Ionic Engineering to Optimize the Redox Kinetics for Potassium Storage: A Case Study of Ni-Doped CoSe2.

Heteroatom doping effectively tunes the electronic conductivity of transition metal selenides (TMSs) with rapid K+ accessibility in potassium ion batteries (PIBs). Although considerable efforts are dedicated to investigating the relationship between the doping strategy and the resulting electrochemistry, the doping mechanisms, especially in view of the ion and electronic diffusion kinetics upon cycling, are seldom elucidated systematically. Herein, the crystal structure stability, charge/ion state, and bandgap of the active materials are found to be precisely modulated by favorable heteroatom doping, resulting in intrinsically fast kinetics of the electrode materials. Based on the combined mechanisms of intercalation and conversion reactions, electron and K+ ion transfer in Ni-doped CoSe2 embedded in carbon nanocomposites (Ni-CoSe2 @NC) can be significantly enhanced via electronic engineering. Benefiting from the synthetic controlled Ni grains, the heterointerface formed by the intermediate products of electrochemical reactions in Ni-CoSe2 @NC strengthens the conversion kinetics and interdiffusion process, developing a low-barrier mesophase with optimized potassium storage. Overall, an electronic tuning strategy can offer deeper atomic insights into the conversion reaction of TMSs in PIBs.

Flexible core/shelled PPy@PANI nanotube porous films for hybrid supercapacitors.

Flexibility of the films and the limited ion transport in the vertical direction of film highly restrict the development of flexible supercapacitors. Herein, we have developed hybrid porous films consisting of N-doped holey graphene nanosheets (NHGR) with abundant in-plane nanopores and the vertically aligned polyaniline nanowires arrays on polypyrrole nanotubes (PPy@PANI) via a two-step oxidative polymerization strategy and vacuum filtration. The rational design can efficiently shorten the diffusion path of electrons/ions, alleviate volume variation of electrodes during cycling, enhance electric conductivity of the hybrids, and while offer abundant active interfacial sites for electrochemical reaction. Benefiting from the distinctive structural and compositional merits, the obtained PPy@PANI/NHGR film electrode manifests an excellent electrochemical properties in terms of specific capacity (1348 mF cm-2at a current density of 1 mA cm-2), rate capability (81.2% capacitance retention from 1 to 30 mA cm-2), and cycling stability (capacitance retention of 73.7% at 20 mA cm-2after 7000 cycles). Matched with NHGR negative electrode, the assembled flexible all-solid-state asymmetric supercapacitor displays a remarkable areal capacitance of 359 mF cm-2at 5 mA cm-2, maximum areal energy density of 112.2µWh cm-2at 3.747 mW cm-2, and good flexibility at various bending angles while preserving stable cycling performance. The result shows the PPy@PANI/NHGR film with high flexibility and 3D ions transport channels is highly attractive for flexible energy storage devices.

The slow magnetic relaxation regulated by the coordination, configuration and intermolecular dipolar field in two mononuclear DyIII single-molecule magnets (SMMs).

Tuning the magnetic dynamics of single-molecule magnets (SMMs) is a crucial challenge for chemists. Some feasible approaches have been developed to understand parts of the magneto-structural correlations and regulate the relaxation behaviors via rational design. Herein, the syntheses, structures and magnetic properties of two mononuclear DyIII SMMs are reported. The first structural motif reveals a trigonal dodecahedron (D2d) N2O6 coordination environment in 1, while the second one displays a square antiprismatic configuration (D4d). A DyDy distance of 8.589 Å in 1 is clearly shorter than that of 2 (10.433 Å) because of the existence of ππ stacking between benzene rings from two adjacent dbpy molecules in 1. The temperature and frequency-dependent out-of-phase ac susceptibility peaks were observed in the absence of a static dc field for 1 and 2. Two distinct thermal relaxation processes were observed in 1, while 2 exhibits one thermal relaxation process. It is interesting that the quantum tunneling of magnetization (QTM) was suppressed when optimum dc fields (1000 Oe) were applied. From ab initio calculations, the energies of the first excited state (KD1) are indeed close to the experimental relaxation energy barrier (Ueff) under zero dc field, which also reveals the typical features associated with the SMM behavior. In detail, the Ueff values are 103.62 cm-1 (149.87 K) as well as 55.10 cm-1 (79.69 K) for 1 and 116.07 cm-1 (167.87 K) for 2. The KD1 of 1 (133.82 cm-1) is slightly higher than that of 2 (129.31 cm-1). Comparing 1 and 2, this discrepancy from KD1 and the experimental Ueff might come from the apparent difference in the magnitude of tunneling probability between the two compounds. In other words, the intermolecular dipolar field plays an important role in their magnetic properties.

Sulfuric acid etching for fabrication of porous MnO2 for high-performance supercapacitor.

We developed a facile and efficient route to prepare highly porous nanostructure MnO2 by etching of proton-type layered manganese oxide (H-MnO2) with sulfuric acid (H2SO4). Results from TEM images and N2 adsorption showed that H2SO4 etching created porous MnO2 with average pore size of about 4 nm and high specific surface area (315 m2 g-1). With such porous structure, the obtained MnO2 exhibits a high specific capacitance of 253 F g-1 and enhanced rate capability (62.1% capacitance retention from 0.5 to 10 A g-1) when comparing with the H-MnO2 precursor (154 F g-1, 45.5%) and annealed H-MnO2 in the absence of H2SO4 (134 F g-1, 43.3%). The excellent capacitive properties demonstrate that creation of porous structure on H-MnO2 not only provides large ion-accessible surface area for efficient charge storage, but also to some extent promotes the kinetics of electrochemical reactions.

Novel layered polyaniline-poly(hydroquinone)/graphene film as supercapacitor electrode with enhanced rate performance and cycling stability.

It is a challenge to fabricate polyaniline (PANI) materials with high rate performance and excellent stability. Herein a new special supercapacitor electrode material of polyaniline-poly(hydroquinone)/graphene (PANI-PHQ/RGO) film with layered structure was prepared by chemical oxidative polymerization of aniline and hydroquinone (H2Q) in the presence of RGO hydrogel film. The synergistic effect and loose layered structure of the composite film facilitate fast diffusion and transportation of electrolyte ions through unimpeded channels during rapid charge-discharge process, resulting in high rate capability and stable cycling performance. As a result, the PANI-PHQ/RGO-61 film electrode exhibited 356 F g-1 at a current density of 0.5 A g-1 and high capacitance retention of 83% from 0.5 to 30 A g-1. Moreover, it presented an excellent cycling stability with 94% of capacitance retention in comparison with 60% of pure PANI electrode and an outstanding Coulombic efficiency of 99% after 1000 cycles of galvanostatic charge-discharge. These superior electrocapacitive properties make it one of promising candidates for electrochemical energy storage.

Rational design and controllable preparation of holey MnO2 nanosheets.

A novel technique for the design and controllable preparation of holey MnO2 nanosheets was first developed by an in situ redox reaction between the MnO2 nanosheets and adsorbed Fe2+ ions at room temperature, in which Fe2+ ions originated from a redox reaction between Cu wire and Fe3+ ions. The formation of in-plane nanopores on the MnO2 nanosheets obviously improved the rate capability when they were used as promising supercapacitor electrodes.

Three-Dimensional Tubular MoS2/PANI Hybrid Electrode for High Rate Performance Supercapacitor.

By using three-dimensional (3D) tubular molybdenum disulfide (MoS2) as both an active material in electrochemical reaction and a framework to provide more paths for insertion and extraction of ions, PANI nanowire arrays with a diameter of 10-20 nm can be controllably grown on both the external and internal surface of 3D tubular MoS2 by in situ oxidative polymerization of aniline monomers and 3D tubular MoS2/PANI hybrid materials with different amounts of PANI are prepared. A controllable growth of PANI nanowire arrays on the tubular MoS2 surface provides an opportunity to optimize the capacitive performance of the obtained electrodes. When the loading amount of PANI is 60%, the obtained MoS2/PANI-60 hybrid electrode not only shows a high specific capacitance of 552 F/g at a current density of 0.5 A/g, but also gives excellent rate capability of 82% from 0.5 to 30 A/g. The remarkable rate performance can be mainly attributed to the architecture with synergistic effect between 3D tubular MoS2 and PANI nanowire arrays. Moreover, the MoS2/PANI-60 based symmetric supercapacitor also exhibits the excellent rate performance and good cycling stability. The specific capacitance based on the total mass of the two electrodes is 124 F/g at a current density of 1 A/g and 79% of its initial capacitance is remained after 6000 cycles. The 3D tubular structure provides a good and favorable method for improving the capacitance retention of PANI electrode.