Preparation And Electrochemical Properties Of Novel Electrode Materials For Magnesium Batteries

Rechargeable magnesium batteries have been promising energy storage and conversion technologies.Metal Mg is one of the advantageous anode for high energy density batteries.The reduction potential(-2.37 V vs.SHE)of Mg is low.Magnesium metal has a higher volume specific capacity(3833 mAh cm-3)than lithium metal(2046 mAh cm-3)due to the bivalent nature of Mg2+.Mg metal possesses several other advantages,such as mild chemical properties,rich natural abundance,as the fifth element in the earth's crust.More importantly,unlike lithium metal,magnesium metal doesn't form dendrites in reversible electrochemical deposition/dissolution processes,which are of high safety as anode material for magnesium batteries.Therefore,the development of rechargeable magnesium batteries has potential advantages for large-scale power battery systems.Until now,the study of rechargeable magnesium batteries is still in its infancy,and Mg batteries are mainly confronted with two major challenges:One challenge is the lack of suitable cathode materials in which Mg2+ can diffuse with fast kinetics.The other is the lack of appropriate electrolytes allowing the reversible deposition/dissolution for magnesium and compatible with electrodes.In this paper,we focus on the design and synthesis of the novel cathode and anode materials for rechargeable Mg batteries.These materials show high capacity,long cycling stability,and suitable voltage platform.The storage mechanisms of Mg2+ in electrode materials were scrutinized by means of various characterization methods.Our research progresses are summarzized as follows.1.We are the first to report that vanadium tetrasulfide(VS4)with special one-dimensional atomic-chain structure can serve as a favorable cathode material for the construction of high-performance magnesium batteries.Through a surfactant-assisted solution-phase process,seaurchin-like VS4 nanodendrites with multi-branched nanoarchitecture were controllably prepared.Benefited from the one-dimensional chain-like crystalline structure of VS4,the S22-dimers in VS4 nanodendrites provide abundant void sites for Mg2+ intercalation.Moreover,the VS4 atomic-chains are only bonded by weak van der Waals forces,therefore the transport and diffusion of Mg2+ions inside the open channels of VS4 are very smooth and accompanied by rapid kinetics.By a series of systematic ex-situ characterizations and density functional theory calculations,the magnesiation/demagnesiation mechanism of VS4 were elucidated,revealing a unique reversible internal redox behavior,whereby during the discharge processes,the S22-ions in VS4 were partially reduced to S2-and the V4+ species were partially oxidized to V5+.The VS4 nanodendrites presented remarkable performance for Mg2+ storage among existing cathode materials,exhibiting a remarkable initial discharge capacity of 251 mAh g-1 at 100 mA g-1 and an impressive long-term cyclability at large current density of 500 mA g-1(74 mAh g-after 800 cycles).2.The hybrid Mg2+/Li+ batteries(MLIBs)have attracted recent research attentio as promising energy storage technologies that combine the advantages of the Li and Mg electrochemistry.However,the battery performances of many previous researches are hindered due to the fact that only Li+ participate in reaction on the cathode.The obstacle can be overcome by significantly improving the diffusion/transfer kinetics of highly-polarized divalent Mg2+ in the cathode,so that both Li+ and Mg2+ can be intercalated into the cathode.Our previous work have shown that vanadium tetrasulfide(VS4)with special one-dimensional atomic-chain structure can serve as a promising cathode material and demonstrates fast diffusion kinetics for Mg2+.Herein,we report a facile approach to further improve the electrochemical energy storage capability of VS4 by the efficient intercalation of both Mg2+ and Li+ in MLIBs.The VS4 cathode displays a reversible capacity of-300 mAh g-1 at 500 mA g-1 in MLIBs,and an impressive long-term cyclability at large current density of 1000 mA g-1,the capacity still retains 110 mAh g-1 after 1500 cycles.3.Organic materials,in particular organic carbonyl compounds,are considered to be promising electrode materials due to their numerous advantages,including lightweight,redox stability,and low cost.Herein,two dianhydride-based polyimides(PI1 and PI2)are presented as the cathode materials for Mg battery.Their electrochemical storage performance of Mg2+ ions are further improved by combination with multi-walled carbon nanotubes(MWCNT).PI2/MWCNT composites show batter electrochemical performances than PI1/MWCNT,which may be attribute to the relatively lower solubility of PI2 in electrolyte than PI1.PI2/MWCNT displays an initial discharge capacity of 140 mAh g'1 at 1 C(158 mAg-1)with coulombic efficiency of 88%,and the discharge capacity retains 161 mAh g-1 after 150 cycles due to the activation of cathode materials after the initial few cycles.PI2/MWCNT composites also exhibit an impressive long-term cyclability at large current rates of 20 C,and the discharge capacity still retains 42 mAh g-1 after 8000 cycles.62%of the capacity(68 mAh g-1)at the 10th cycle.This work may arouse much more interest for the development of more organic electrode materials for long cycling rechargeable Mg battery.4.The oxygen vacancies dominate the intrinsic physical and chemical properties of transition metal oxides,which play a vital role in many application areas.The introduction of oxygen vacancies is expected to enhance the performance of batteries due to better electrical conductivity.Herein,we demonstrate an atomic substitution strategy for the controlled preparation of ultrathin black TiO2-x(B-TiO2-x)nanoflakes with rich oxygen vacancies(OVs)and porosity by utilizing ultrathin 2D TiS2 nanoflakes as precursors.And for the first time,we find out that the presence of OVs in electrode material can greatly contribute to the electrochemical performances of rechargeable Mg batteries.Both experimental results and density functional theory(DFT)calculations confirm that the introduction of OVs can significantly improve the electrical conductivity and increase the number of active sites for Mg2+ ion storage.The vacancy-Rich B-TiO2-x nanoflakes exhibit high reversible capacity and good capacity retention after long-term cycling at large current densities.We expect this work can provide good insights and inspirations on the defect engineering of electrode materials for rechargeable magnesium batteries.