Phase-separated bimetal enhanced sodium storage:Dimer-like Sn-Bi@C heterostructures with high capacity and long cycle life

Phase boundaries facilitate the charge transportation and alleviate the intrinsic stress upon cycles.Therefore,how to achieve regular phase boundaries is very attractive.Herein,dimer-like Sn-Bi@C nanostructures,where is a well-defined phase boundary between Sn and Bi,have been prepared by a two-step process for the first time.The phase boundary not only provides additional and fast transportation for Na+,but also mitigates the structure stress/strain upon cycling.Therefore,Sn-Bi@C exhibits a high capacity(472.1 m A h g^(-1)at 2 A g^(-1)for 200 cycles),an ultra-long cyclic life(355.6 mA h g^(-1)at 5 A g^(-1)for 4500cycles)and an excellent rate performance(372 mA h g^(-1)at 10 A g^(-1))for sodium storage,much higher than those of Sn@C,Bi@C,and Sn@C+Bi@C.Notably,the full cells of Sn-Bi@C//Na_(3)V_(2)(PO_(4))_(3)/rGO(SnBi@C//NVP/rGO)demonstrate impressive performance(323 mA h g^(-1)at 2 A g^(-1)for 300 cycles).The underlying mechanism for such an excellent performance is elucidated by in-situ X-ray diffraction,exsitu scanning electron microscopy/high-resolution transmission electron microscopy and atomic force microscopy,revealing the good electrode stability and improved mechanical properties of Sn-Bi@C.The synthetic method is extended to dimer-like Sn-Pb@C and Sn-Ag@C heterostructures,which also exhibit the good cycle stability for sodium storage.

Nanocarbon-based metal-free and non-precious metal bifunctional electrocatalysts for oxygen reduction and oxygen evolution reactions

The oxygen reduction/evolution reactions(ORR/OER) are a key electrode process in the development of electrochemical energy conversion and storage devices,such as metal-air batteries and reversible fuel cells.The search for low-cost high-performance nanocarbon-based metal-free and non-precious metal bifunctional electrocatalysts for ORR/OER alternatives to the widely-used noble metal-based catalysts is a research focus.This review aims to outline the opportunities and available options for these nanocarbon-based bifunctional electrocatalysts.Through discussion of some current scientific issues,we summarize the development and breakthroughs of these electrocatalysts.Then we provide our perspectives on these issues and suggestions for some areas in the further work.We hope that this review can improve the interest in nanocarbon-based metal-free and non-precious metal bifunctional electrocatalysts for ORR/OER.

Nitrogen and fluorine co-doped TiO_(2)/carbon microspheres for advanced anodes in sodium-ion batteries: High volumetric capacity, superior power density and large areal capacity

Fast charging and high volumetric capacity are two of the critical demands for sodium-ion batteries(SIBs).Although nanostructured materials achieve outstanding rate performance,they suffer from low tap density and small volumetric capacity.Therefore,how to realize large volumetric capacity and high tap density simultaneously is very challenging.Here,N/F co-doped TiO_(2)/carbon microspheres(NF- TiO_(2)/C)are synthesized to achieve both of them.Theoretical calculations reveal that N and F co-doping increases the contents of oxygen vacancies and narrows the bandgaps of TiO_(2) and C,improving the electronic conductivity of NF- TiO_(2)/C.Furthermore,NF- TiO_(2)/C exhibits the high binding energy and low diffusion energy barrier of Na+,significantly facilitating Na+storage and Na+diffusion.Therefore,NF- TiO_(2)/C offers a high tap density(1.51 g cm^(-3)),an outstanding rate performance(125.9 mAh g^(-1) at 100 C),a large volumetric capacity(190 mAh cm^(-3) at 100 C),a high areal capacity(4.8 mAh cm^(-2))and an ultra-long cycling performance(80.2%after 10,000 cycles at 10 C)simultaneously.In addition,NF- TiO_(2)/C||Na_(3)V_(2)(PO_(4))_(3) full cells achieve an ultrahigh power density of 25.2 kW kg^(-1).These results indicate the great promise of NF- TiO_(2)/C as a high-volumetric-capacity and high-power-density anode material of SIBs.

Unraveling the electrocatalytically active sites and stability of Co&Co oxides on nanocarbon for oxygen evolution reaction in acid solution

The oxygen evolution reaction(OER)in acid solution is a significant challenge for non-precious metal electrocatalysts based on the transition metals although they have shown good OER performance in alkaline solution.In this study,we synthesized the electrocatalysts containing two or three Co species(Co,CoO and Co3O4)nanoparticles on porous graphitic carbon(PGC)nanosheets which were prepared by a facile and low-cost synthesis where Co(NO3)2•6H2O and glucose were pyrolyzed in the presence of sodium chloride template.The Co3O4-dominated catalyst as-prepared,Co3O4/PGC,is OER active in acid solution(1.74 V at a current density of 10 mA cm^−2).We identified the OER active sites in the catalyst to be the Co3O4 nanoparticles rather than carbon-coated Co.Through comparative studies of the varied catalysts,we also proved that Co3O4 is catalytically more active than Co and CoO.The Co3O4/PGC catalyst,however,lost almost of all its activity after 100 voltammetric cycles in the 1.2-1.8 V voltage window.When the catalyst stability was examined potentiostatically at different potentials,the catalyst showed good stability at 1.4 V.The stability study also revealed the mechanism of the catalyst instability in acid was caused by Co3O4 reduction below 1.4 V and by Co3O4 oxidation above 1.4 V.1.4 V is therefore a unique potential where Co3O4 nanoparticles are neither oxidized nor reduced to be susceptible to acid dissolution.

Carbon-coated mesoporous Co9S8 nanoparticles on reduced graphene oxide as a long-life and high-rate anode material for potassium-ion batteries

Carbon-coated mesoporous Co9S8 nanoparticles supported on reduced graphene oxide(rGO)are successfully synthesized by a simple process.This composite makes full use of the protection of the carbon layer on the surface,the good conductivity and three-dimensional(3D)structure of rGO,the mesoporous structure and nanoscale size of Co9S8,thereby presenting the excellent electrochemical performances in potassium-ion batteries,407.9 mAh·g^−1 after 100 cycles at 0.2 A·g^−1 and 215.1 mAh·g^−1 at 5 A·g^−1 in rate performances.After 1,200 cycles at 1.0 A·g^−1,this composite still remains a capacity of 210.8 mAh·g^−1.The redox reactions for potassium storage are revealed by ex-situ transmission electron microscope(TEM)/high-resolution TEM(HRTEM)images,selected area electron diffraction(SAED)patterns and X-ray photoelectron spectroscopy(XPS)spectra.The application of this composite as the host of sulfur for Li-S batteries is also explored.It sustains a capacity of 431.8 mAh·g^−1 after 800 cycles at 3 C,leading to a degradation of 0.052%per cycle.These results confirm the wide applications of this composite for electrochemical energy storage.