Electrochemical and structural evolution of structured V_(2)O_(5) microspheres during Li-ion intercalation

With the development of stable alkali metal anodes,V_(2)O_(5) is gaining traction as a cathode material due to its high theoretical capacity and the ability to intercalate Li,Na and K ions.Herein,we report a method for synthesizing structured orthorhombic V_(2)O_(5) microspheres and investigate Li intercalation/deintercalation into this material.For industry adoption,the electrochemical behavior of V_(2)O_(5) as well as structural and phase transformation attributing to Li intercalation reaction must be further investigated.Our synthesized V_(2)O_(5) microspheres consisted of small primary particles that were strongly joined together and exhibited good cycle stability and rate capability,triggered by reversible volume change and rapid Li ion diffusion.In addition,the reversibility of phase transformation(a,e,d,c and xLixV_(2)O_(5))and valence state evolution(5+,4+,and 3.5+)during intercalation/de-intercalation were studied via in-situ X-ray powder diffraction and X-ray absorption near edge structure analyses.

Redox Charge Transfer Kinetics and Reversibility of VO_(2) in Aqueous and Non-Aqueous Electrolytes of Na-Ion Storage

The deep understanding about the electrochemical behavior of the nanostructured electrode in electrolytes provides crucial insights for the rational design of electrode for sodium(Na)-ion storage system(NIS).Here,we report redox charge transfer kinetics and reversibility of VO_(2)(B) nanorod electrodes in both aqueous and organic electrolytes for NIS.The assynthesized VO_(2)(B) nanorods show the reversible redox reaction with the higher specific and rate capacitances at high current density in aqueous electrolytes than in organic electrolytes.Temperature-dependent impedance measurements demonstrate the more facile interfacial charge transfer of Na ions into VO_(2)(B) nanorods in aqueous electrolytes.The reversible evolution in oxidation state and chemical composition of VO_(2)(B) nanorods is observed in aqueous electrolytes,as confirmed by ex situ XRD and ex situ X-ray photoelectron spectroscopy analyses.Given by the facile and reversible pseudocapacitive feature,the electrochemical performances of VO_(2)(B) nanorods are further improved by constructing the hierarchical structure of the reduced graphene oxide-VO_(2) composite for aqueous Na+ion storage.

Two-Dimensional Pseudocapacitive Nanomaterials for High-Energy-and High-Power-Oriented Applications of Supercapacitors

CONSPECTUS:Supercapacitors(SCs)are electrochemical energy storage devices that can fill the gap between batteries and electrolytic capacitors.However,the widespread applications of commercialized carbon-based SCs are limited by their energy density,arising from their physical charge storage mechanism,which is by far lower than that of batteries.Moreover,the highpowered applications of SCs are also limited by their kinetics,which are slower than those of electrolytic capacitor due to the diffusion and distribution of ions onto the tortuous porous surface.Therefore,the energy and power performance of SCs need to be improved to open or further extend their practical applications.Since all atoms of two-dimensional(2D)nanomaterials are located on the surface,the design of surface structure is critical to determining the bulk electrochemical properties.Such a surface-oriented property of 2D nanomaterials is well fitted to control the surface charge storage mechanism of SCs,thereby discovering emerging capacitive materials through the rational design of surface chemistry and multiscale structures.This Account discusses our recent progress on 2D pseudocapacitive materials for high-energyand high-power-oriented SCs applications and provides our perspective into the rational design of the microstructure,multiscale architecture,and surface chemistry.Examples of 2D nanomaterials include heteroatom-doped graphene,black phosphorus,transition-metal dichalcogenides,and transition-metal carbide/nitrides(MXene).We also highlight the in-depth spectroelectrochemical and computational analyses that can correlate the structures and chemistries of 2D nanomaterials with their charge storage/transport/transfer behaviors.In this Account,our design concept of 2D nanomaterials is based on two aspects of charge storage capability and kinetics that can determine the thermodynamic(capacitance)and kinetic(rate)performances.First,chemical strategies,such as atomic incorporation,surface functionalization/coordination,and hybridization of 2D nanomaterials,will be provided and correlated with the population of redox storage sites and interaction between sites and ions.The charge storage capacitance can be improved by controlling these factors for high-energy-oriented applications.Second,we will address key factors such as charge-transfer kinetics,ion-transporting pathways,and percolated electron transport for high-power-oriented applications.Several approaches such as multiscale architecture,hybridization with electronically conductive materials,pore orientation,and an expanded interlayer space will be introduced to improve the kinetic performance of 2D nanomaterials.Finally,we will provide our perspective on technical impediments and future research directions of 2D nanomaterials for practical energy-and power-oriented applications of SCs.