| The effective exploitation and utilization of new clean energy depend on the development of electrochemical energy storage technology,so secondary battery technology represented by lithium ion batteries(LIBs)came into being.As one of the core components,the anode materials are all-important to improve the performance of LIBs.Compared with graphite anode,titanium-based oxides have attracted the attention of many researchers due to the small volume change during the insertion/extraction of Li+.Li2ZnTi3Os(LZTO),as a typical titanium-based oxide,has great advantages when used as an anode material for LIBs,such as low cost,good structural stability,high capacity,high safety and excellent reversibility.However,the poor electronic conductivity of LZTO results in the poor performance at high rates and hinders its industrial application.Additionally,the study about the lithium storage mechanism of LZTO anode is not thorough and sufficient,and the phenomenon that the capacity gradually increases with cycling has not been explained.Therefore,different types of modifiers were employed to modify LZTO,and the electronic and ionic conductivities could be simultaneously improved by adjusting the internal structure and optimizing the surface composition.Moreover,the cyclic voltammetry characteristic for specific cycles,in-situ structure and composition tests with cycling were conducted to clarify the lithium storage mechanism of LZTO.The main content is as follows:(1)The LZTO anode modified by Li2ZrO3(LZO)was acquired by the simple reaction between LiNO3 and Zr(NO3)4 on the surface of the as-prepared LZTO.When the precursor with a LZO/LZTO mass ratio of 0.008 was sintered at 750? for 5 h,the product exhibited greatly enhanced rate capabilities(acquiring reversible capacities of 196.1,175.9,154.9,135.1 and 109.8 mAh g-1 at 0.1,0.2,0.4,0.8 and 1.6 A g-1,respectively),and outstanding long-term cyclability(retaining a capacity of 199.2 mAh g-1 after 600 cycles at 0.5 A g-1).On the basis of detailed characterizations on structure and composition as well as DFT calculations,the dispersed LZO nanoparticles and partial coating bridge the LZTO particles to facilitate Li+migration among the LZTO particles,while the superficial Zr4+doping in LZTO promotes electron transfer,resulting in the simultaneously ameliorated electronic and ionic conductivities of LZTO,as well as alleviated polarization(2)In view of the water solubility,low melting point,good electrical conductivity,and wettability to LZTO,Na2MoO4(NMO)was firstly selected as the surface modifier to prepare the LZTO anode modified by NMO.The electrochemical performance of LZTO could be enhanced by adjusting the percent of NMO and sintering temperature,The LZTO modified by 2 wt%NMO at 750? exhibited the most excellent rate capabilities(achieving lithiation capacities of 253.0,225.1,207.2,187.1 and 161.3 mAh g-1 at 0.1,0.2,0.4,0.8 and 1.6 A g-1,respectively)as well as outstanding long-term cycling stability(delivering the lithiation capacities of 229.0 and 135.6 mAh g-1 for 400 cycles at 0.5 and 1.0 A g-1,respectively).Structure and composition characterizations together with electrochemical impedance spectra analysis demonstrate that the molten NMO at 750? is beneficial to diffuse into the LZTO lattice near the surface of LZTO particles to yield uniform modification layer,simultaneously ameliorating the electronic and ionic conductivities of LZTO,and thus is responsible for the enhanced electrochemical performance of LZTO.First-principles calculations further verify the substitution of Mo6+for Zn2+to realize doping in LZTO.Additionally,a small amount of Na+on the surface of LZTO could reduce the irreversible lithium loss in the first cycle,lower the interface impedance and stabilize the structure of SEI film(3)Commercial lithium magnesium silicate(LMS)exhibits strong adsorbability,cohesiveness,suspensibility,chemical stability and good cation exchangeability,which was chosen in this work to modify LZTO by simply mixing LZTO in LMS colloidal solution.The uniform LMS coating on LZTO particles gives rise to the marked enhancement in electrochemical performance of LZTO anode.The product with a LMS/LZTO mass ratio of 0.015 and calcined at 750? achieved a high reversible capacity of 236.3 mAh g-1 at 0-1 A g-1,applicable as capacity-type anode materials.The LZTO/LMS with a LMS/LZTO mass ratio of 0.02 demonstrated excellent rate capabilities(attaining capacities of215.6,176.8,166.8,158.9 and 144.9 mAh g-1 at the current rates of 0.1,0.2,0.4,0.8 and 1.6 A g-1),capable of using as power-type anode materials.The appreciable performance is attributable to the uniform LMS coating for protectling LZTO against the direct contact with the electrolyte to refrain from the side reactions,enhancing the Coulombic efficiency in the initial cycle,improving electronic and ionic conductivities to weaken polarization by the interaction between LMS and electrolyte.Additionally,a small amount of Na+in the commercial LMS could reduce the irreversible lithium loss in the first cycle,lower the interface impedance and stabilize the structure of SEI film.Meanwhile,a small amount of F-could improve the cycling stability of LZTO.(4)It is commonly deemed that LZTO is an intercalation-type of Li+-storage anode based on limited lithiation/delithiation cycles,and the phenomenon that the capacity gradually increases with cycling has not yet been explained.Therefore,LZTO/LMS half-cells underwent more than 300 lithiation/de-lithiation cycles at 0.2 and 0.5 A g-1,and the changes in cyclic voltammogram,structure and composition with cycling were probed.It was found,besides the redox reaction between Ti4+ and Ti3+,other reactions of Zn2+(?)Zn0(?)LixZn also occur to provide extra Li+storage with cycling,i.e.LZTO is actually an anode material with three ordinal Li-ion storage behaviors:intercalation,conversion and alloying,and thus reveals a specific capacity of about 454 mAh g-1.The new Li+storage mechanism will provide an alternative strategy for designing anode material with high capacity. |
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