Research On The Lithiation/Delithiation Properties Of VO_x Nanoribbon Cathode And SiO_x@C Core-shell Anode

Lithium-ion batteries(LIBs)is one of the common energy storage units in production and life.The electrode materials are the most important components to improve the performance of LIBs.With respect to cathode materials,LiCoO₂,LiNiO₂,LiMn₂O₄,LiFePO₄ and NCM were widely used,because of their excellent stability and mature scale preparation method.However,these traditional cathodes generally have low specific capacity(¹50-170 mAh/g)and actually cannot satisfy the requirements of energy storage batteries with high-performance.Vanadium-based materials(e.g.,V₂O₅,V₃O₇,VO₂,etc.)as cathodes for LIBs cause the attention of researchers due to their high theoretical specific capacity.For example,when one molecule of V₂O₅ was inserted by two Li⁺,it could display the specific capacity of²94 mAh/g,which is much higher than the above mentioned traditional cathodes.Nevertheless,vanadium-based cathodes have some problems such as poor conductivity and unstable structure,which restrict their practical application.In order to improve the lithium storage performance of vanadium-based cathodes,the nanocrystallization and combining with conductive substances are usually introduced.In this work,we prepared one dimensional vanadium oxide nanoribbons(VO_x NBs)with different ratios of V and O including V₃O₇·H₂O NBs,VO₂(B)NBs/MWCNTs and V₂O₅ NBs by hydrothermal method using vanadium oxide sol as precursor and adding different amounts of alcohol and MWCNTs as the reducing agent and conductive compounds.The growth mechanism and lithium storage properties of those VO_x NBs are discussed under the same synthetic system.Among them,although the capacity of VO₂(B)NBs/MWCNTs was low(the capacity just remained at 148.9 mAh/g after 50 cycles at100 mA/g),it showed the lowest charge transfer resistance(Rct)due to the presence of dominant V⁴⁺in bulk phase and the addition of MWCNTs.Besides,only one pair of reversible phase transition voltage platforms appeared during the lithiation/delithiation process,which is beneficial to the output of stable working voltage and power in the practical application.V₂O₅ NBs delivered the highest discharge capacity(240.4 mAh/g)and stable capacity retention(the capacity remained at 213.3 mAh/g after 50 cycles at100 mA/g),which is attributed to its higher V valence state and good crystallinity.In the aspect of anode materials for LIBs,the capacity of commercial graphite is low(372 mAh/g),which also limits the further improvement of LIBs'overall performance.Silicon is an ideal choice to replace the graphite anode on account of its high theoretical capacity,low price and easy access.Among them,silicon dioxide is a common material and has the theoretical capacity of¹965 mAh/g,which can be a promising anode candidate.While the poor conductivity and severe volume change(²00%)of SiO₂ upon lithiation/delithiation hinder its application as anode material.In order to overcome these shortcomings,the nanocrystallization,carbon coating and design of core-shell structure for SiO₂ were proposed,in which,the hollow core-shell structure with silicon-based material inside and carbon layer outside can not only alleviate the volume expansion problem of silicon-based anode by inner void,but also improve the conductivity by outer carbon shell.This design is an effective strategy for performance improvement.In the study,SiO_x@C core-shell nanosphere anode was synthesized via the self-assembly process under wet chemical environment by adopting tetraethyl orthosilicate,resorcinol and formaldehyde as raw materials and combining with subsequent sintering and corrosion steps.The effective size control of inner SiO_x core was realized by strong alkali corrosion.The Li storage characteristics of SiO_x@C core-shell nanospheres with different structures were evaluated through electrochemical tests.The results showed that with the decrease of the size of inner SiO_x core,the Li storage performance was not improved as expected,indicating that the effective active site for Li⁺embedding is the key factor impacting on the final Li storage performance in SiO_x@C core-shell structure.In this work,the synthesis and Li storage properties of VO_x NBs cathode and SiO_x@C anode were investigated.Through the simple hydrothermal reaction,the controllable preparation of VO_x NBs with different V valence states was realized by introduction of different reducing components.Based on the molecular self-assembly process under wet chemical conditions,SiO_x@C core-shell nanosphere with adjustable SiO_x core was obtained by subsequent corrosion control.Various characterization techniques(SEM,TEM,XRD,FTIR,Raman,TG-DSC and BET)were used to explain the formation mechanism and structural characteristics of VO_x NBs and SiO_x@C.The Li storage properties of VO_x NBs and SiO_x@C were systematically evaluated by electrochemical tests such as constant current charge/discharge,cyclic voltammetry and AC impedance.Dependence of the Li storage performance on electrode microstructure was analyzed.Our study provides a useful reference for the rational structure design of future vanadium and silicon-based nano-structured electrode materials.