Precisely manipulated π-π stacking of catalytic exfoliated iron polyphthalocyanine/reduced graphene oxide hybrid via pyrolysis-free path.

Metal macrocycles with well-defined molecular structures are ideal platforms for the in-depth study of electrochemical oxygen reduction reaction (ORR). Structural integrity of metal macrocycles is vital but remain challenging since the commonly used high-temperature pyrolysis would cause severe structure damage and unidentifiable active sites. Herein, we propose a pyrolysis-free strategy to precisely manipulate the exfoliated 2D iron polyphthalocyanine (FePPc) anchored on reduced graphene oxide (rGO) via π-π stacking using facile high-energy ball milling. A delocalized electron shift caused by π-π interaction is firstly found to be the mechanism of facilitating the remarkable ORR activity of this hybrid catalyst. The optimal FePPc@rGO-HE achieves superior half-wave potential (0.90 V) than 20 % Pt/C. This study offers a new insight in designing stable and high-performance metal macrocycle catalysts with well-defined active sites.

Extra Sodiation Sites in Hard Carbon for High Performance Sodium Ion Batteries.

Hard carbons are promising anodes for sodium-ion batteries (SIBs). However, the low practical capacity from limited sodiation sites impedes their applications. Herein, ultrahigh concentration of pyridine N (≈7.9%) is introduced inside hard carbon, considering that pyridine N may provide extra sodium storage sites with stable CN• and CC• radicals during cycling. To expose more radical sites for sodium storage, a 3D structure with a multistage pore structure is constructed through NH3 release during the pyrolyzation process. As expected, the hard carbon with extra sodiation sites exhibits an impressively high capacity of 434 mA h g-1 at 20 mA g-1 , superior rate performance of 238 mA h g-1 at a current density of 5 A g-1 and a high-capacity retention of 98.7% after 5000 cycles. The radicals induced Na-adsorption mechanism was further explored through ex situ electron paramagnetic resonance technology, in situ Raman technology and density functional theory calculations. The results reveal that the extra sodiation sites come from the electrostatic interaction at low potentials. This work constructs a sodium ions storage model of extra radicals and provides an extended strategy to improve the electrochemical performance of SIBs anode materials.

Coupling a Three-Dimensional Nanopillar and Robust Film to Guide Li-Ion Flux for Dendrite-Free Lithium Metal Anodes.

Lithium metal batteries with high theoretical capacity critically suffer from low cycling stability and safety issues mostly due to lithium dendrites. Regulating the Li-ion conduction and Li deposition is essential to achieve dendritic-free Li metal anodes. Herein, a synergistic strategy that combines a 3D nanocopper layer and a robust polymer protective layer is proposed. The 3D nanocopper layer in situ formed on the Li surface could achieve a uniform electric field distribution and contribute to reducing the nucleation barrier for Li deposition and refining the grain size of Li crystallites. Meanwhile, the Li-Nafion film with high Li-ion conductivity and good flexibility was used as a protective layer to provide homogeneous ion distribution and adapt to the volume change during the Li deposition. Consequently, the NCuLi∥LiCoO2 full cells exhibited outstanding cycling stability (a capacity retention of 90% over 500 cycles). Our results indicate that the synergistic control of Li-ion conduction and Li deposition is a promising method to achieve dendritic-free Li metal anodes.

Redox of naphthalenediimide radicals in a 3D polyimide for stable Li-ion batteries.

A 3D polyimide is designed as an organic cathode for Li-ion batteries. Detailed characterization and DFT simulations demonstrate that the 3D polyimide undergoes the redox of naphthalenediimide radicals and the rigidity effect of the 3D structure contributes to the stability of the radical intermediates for high-performance organic batteries.

Single copper sites dispersed on defective TiO2-x as a synergistic oxygen reduction reaction catalyst.

Catalysts containing isolated single atoms have attracted much interest due to their good catalytic behavior, bridging the gap between homogeneous and heterogeneous catalysts. Here, we report an efficient oxygen reduction reaction (ORR) catalyst that consists of atomically dispersed single copper sites confined by defective mixed-phased TiO2-x. This synergistic catalyst was produced by introducing Cu2+ to a metal organic framework (MOF) using the Mannich reaction, occurring between the carbonyl group in Cu(acac)2 and the amino group on the skeleton of the MOF. The embedding of single copper atoms was confirmed by atomic-resolution high-angle annular dark-field scanning transmission electron microscopy and x-ray absorption fine structure spectroscopy. Electronic structure modulation of the single copper sites coupling with oxygen vacancies was further established by electron paramagnetic resonance spectroscopy and first-principles calculations. Significantly enhanced ORR activity and stability were achieved on this special Cu single site. The promising application of this novel electrocatalyst was demonstrated in a prototype Zn-air battery. This strategy of the stabilization of single-atom active sites by optimization of the atomic and electronic structure on a mixed matrix support sheds light on the development of highly efficient electrocatalysts.

Anion Vacancies Regulating Endows MoSSe with Fast and Stable Potassium Ion Storage.

Vacancy engineering is a promising approach for optimizing the energy storage performance of transition metal dichalcogenides (TMDs) due to the unique properties of vacancies in manipulating the electronic structure and active sites. Nevertheless, achieving effective introduction of anion vacancies with adjustable vacancy concentration on a large scale is still a big challenge. Herein, MoS2(1-x)Se2x alloys with anion vacancies introduced in situ have been achieved by a simple alloying reaction, and the vacancy concentration has been optimized through adjusting the chemical composition. Experimental and density functional theory calculation results suggest that the anion vacancies in MoS2(1-x)Se2x alloys could enhance the electronic conductivity, induce more active sites, and alleviate structural variation in the alloys during the potassium storage process. When applied as potassium ion battery anodes, the most optimized vacancy-rich MoSSe alloy delivered high reversible capacities of 517.4 and 362.4 mAh g-1 at 100 and 1000 mA g-1, respectively. Moreover, a reversible capacity of 220.5 mAh g-1 could be maintained at 2000 mA g-1 after 1000 cycles. This work demonstrates a practical approach to modifying the electronic and defect properties of TMDs, providing an effective strategy for constructing advanced electrode materials for battery systems.

Defect-Assisted Selective Surface Phosphorus Doping to Enhance Rate Capability of Titanium Dioxide for Sodium Ion Batteries.

Phosphorus doping is an effective strategy to simultaneously improve the electronic conductivity and regulate the ionic diffusion kinetics of TiO2 being considered as anode materials for sodium ion batteries. However, efficient phosphorus doping at high concentration in well-crystallized TiO2 nanoparticles is still a big challenge. Herein, we propose a defect-assisted phosphorus doping strategy to selectively engineer the surface structure of TiO2 nanoparticles. The reduced TiO2-x shell layer that is rich in oxygen defects and Ti3+ species precisely triggered a high concentration of phosphorus doping (∼7.8 at. %), and consequently a TiO2@TiO2-x-P core@shell architecture was produced. Comprehensive characterizations and first-principle calculations proved that the surface-functionalized TiO2-x-P thin layer endowed the TiO2@TiO2-x-P with substantially enhanced electronic conductivity and accelerated Na ion transportation, resulting in great rate capability (167 mA h g-1 at 10 000 mA g-1) and stable cycling (99% after 5000 cycles at 10 A g-1). Combining in/ex situ X-ray diffraction with ex situ electron spin resonance clearly demonstrated the high reversibility and robust mechanical behavior of TiO2@TiO2-x-P upon long-term cycling. This work provides an interesting and effective strategy for precise heteroatoms doping to improve the electrochemical performance of nanoparticles.

Polyvinylpyrrolidone-Induced Uniform Surface-Conductive Polymer Coating Endows Ni-Rich LiNi0.8Co0.1Mn0.1O2 with Enhanced Cyclability for Lithium-Ion Batteries.

The Ni-rich LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode has attracted great interest owing to its low cost, high capacity, and energy density. Nevertheless, rapid capacity fading is a critical problem because of direct contact of NCM811 with electrolytes and hence restrains its wide applications. To prevent the direct contact, the surface inert layer coating becomes a feasible strategy to tackle this problem. However, to achieve a homogeneous surface coating is very challenging. Considering the bonding effect between NCM811, polyvinylpyrrolidone (PVP), and polyaniline (PANI), in this work, we use PVP as an inductive agent to controllably coat a uniform conductive PANI layer on NCM811 (NCM811@PANI-PVP). The coated PANI layer not only serves as a rapid channel for electron conduction, but also prohibits direct contact of the electrode with the electrolyte to effectively hinder side reaction. NCM811@PANI-PVP thus exhibits excellent cyclability (88.7% after 100 cycles at 200 mA g-1) and great rate performance (152 mA h g-1 at 1000 mA g-1). In situ X-ray diffraction and in situ Raman are performed to investigate the charge-discharge mechanism and the cyclability of NCM811@PANI-PVP upon electrochemical reaction. This surfactant-modulated surface uniform coating strategy offers a new modification approach to stabilize Ni-rich cathode materials for lithium-ion batteries.

Sub-20 nm Carbon Nanoparticles with Expanded Interlayer Spacing for High-Performance Potassium Storage.

Carbon materials are most promising candidates for potassium-ion battery (PIB) anodes because of their high electrical conductivities, rational potassium storage capabilities, and low costs. However, the large volume change during the K-ion insertion/extraction and the sluggish kinetics of K-ion diffusion inhibit the development of carbon-based materials for PIBs. Here, under the guidance of density functional theory, N/P-codoped ultrafine (≤20 nm) carbon nanoparticles (NP-CNPs) with an expanded interlayer distance, improved electrical conductivity, shortened diffusion distance of K ions, and promoted adsorption capability toward K ions are synthesized through a facile solvent-free method as a high-performance anode material for PIBs. The NP-CNPs show a high capacity of 270 mA h g-1 at 0.2 A g-1, a remarkable rate capability of 157 mA h g-1 at an extremely high rate of 5.0 A g-1, and an ultralong cycle life with a high capacity of 190 mA h g-1 and a retention of 86.4% at 1.0 A g-1 after 4000 cycles. The potassium storage mechanism and low volume expansion for NP-CNPs are revealed through cyclic voltammetry, in situ Raman, and ex situ XRD. This work paves a new way to design and fabricate carbon-based nanostructures with high reversible capacity, great rate capability, and stable long-term performance.

Morphology-dependent electrochemical performance of Ni-1,3,5-benzenetricarboxylate metal-organic frameworks as an anode material for Li-ion batteries.

The performance of energy storage materials is substantially dependent on their nanostructures. Herein, Ni-1,3,5-benzenetricarboxylate metal-organic frameworks (Ni-BTC MOFs) with different morphologies are controllably synthesized using a facile solvothermal method by simply adjusting the solvent and their electrochemical performance as an anode material for lithium-ion batteries is thoroughly investigated. Among the synthesized Ni-BTC MOFs with different morphologies, a hierarchical mesoporous flower-like Ni-BTC MOF (Ni-BTCEtOH) assembled from two-dimensional nanosheets shows the best electrochemical properties including a high capacity of 1085 mA h g-1 at 100 mA g-1 (358 mA h g-1 at 5000 mA g-1), excellent cycling stability at 1000 mA g-1 for 1000 cycles, and great rate performance, which is superior to most of the reported MOF-based anode materials for lithium-ion batteries. The outstanding electrochemical performance of Ni-BTCEtOH is originated from its unique and stable hierarchical mesoporous morphology with a high specific surface area and improved electrical/ionic conductivity. Moreover, our study demonstrates that the charge-discharge mechanism of the Ni-BTCEtOH electrode involves the insertion/extraction of Li ions to/from the organic moieties in Ni-BTCEtOH during the charge-discharge process without the direct engagement of Ni2+. This work highlights that the nanostructure design is an effective strategy to obtain promising energy storage materials.

Plasma-Induced Oxygen Vacancies in Urchin-Like Anatase Titania Coated by Carbon for Excellent Sodium-Ion Battery Anodes.

The incorporation of oxygen vacancies in anatase TiO2 has been studied as a promising way to accelerate the transport of electrons and Na+ ions, which is important for achieving excellent electrochemical properties for anatase TiO2. However, wittingly introducing oxygen vacancies in anatase TiO2 for sodium-ion anodes by a facile and effective method is still a challenge. In this work, we report an innovative method to introduce oxygen vacancies into the urchin-like N-doped carbon coated anatase TiO2 (NC-DTO) by a facile plasma treatment. The superiorities of the oxygen vacancies combined with the conductive N-doped carbon coating enable the obtained NC-DTO of greatly improved sodium storage performance. When served as the anode for sodium-ion batteries, the NC-DTO electrode shows superior electrochemical performance (capacity: 272 mA h g-1 at 0.25 C, capacity retention: 98.8% after 5000 cycles at 10 C, as well as ultrahigh capacity: 150 mA h g-1 at 15 C). Density functional theory calculations combined with experimental results suggest that considerably improved sodium storage performance of NC-DTO is due to the enhanced electronic conductivity from the N-doped carbon layer as well as narrowed band gap and lowered sodiation energy barrier from the introduction of oxygen vacancies. This work highlights that introducing oxygen vacancies into TiO2 by plasma is a promising method to enhance the electrochemical property of TiO2, which also can be applied to different metal oxides for energy storage devices.