Synthesis And Modification Of ZnM₂O₄ （M=Mn,Fe） As Anode Materials For Lithium-ion Batteries

In recent years,the lithium-ion batteries（LIBs）is more and more widely used in various fields,which has penetrated into every corner of human life.LIBs have many advantages,such as superior security and stability,excellent cycle performance,high working voltage and energy density.The selection of anode electrode materials plays a vital role in the cycling performance of LIBs.Nowadays,the commercial LIBs usually use graphite as an anode material,which is plentiful and cheap.However,graphite’s theoretical capacity is very low(only 372 mAh g^-1),which cannot meet the requirements of the next generation of high-performance LIBs with large specific capacity and safety.In order to satisfy these demands on LIBs,massive efforts have been made to develop new anode materials which could replace graphite with higher specific capacity and longer cycle-life.Recently,transition metal oxides as anode materials have attracted extensive attention on account of their high theoretical capacity(600-1200 mAh g^-1),but they still have some disadvantages,such as fast capacity damping upon cycling,therefore,they are not widely used in LIBs.Especially the binary transition metal oxides that have better electrochemical performance than the single transition metal oxides have been used as anode materials in LIBs.Considering the electrochemical synergistic effect between different transition metal atoms,the mesoporous ZnMn₂O₄ and ZnFe₂O₄microspheres were selected as the research object to explore their synthesis methods and structural characteristics.We strive to improve their inherent disadvantages,such as poor conductivity,large volume change,rapid capacity decay from two aspects of preparation and modification.Our aim is to improve the electrochemical performance of ZnMn₂O₄and ZnFe₂O₄,in order for them to be used as a highly effective anode electrode material in LIBs.In this paper,the ZnMn₂O₄ and ZnFe₂O₄mesoporous spheres with small size（nanoscale）were constructed by a novel synthesis method,which could improve the cycle performance of the electrode material.The electrochemical performance of them was assessed by cyclic voltammograms and galvanostatic discharge/charge testing.The experimental data provide the important theoretical and practical basis for the design and development of using transition metal oxides as anode materials in high-performance LIBs.The main research contents are as follows:（1）ZnMn₂O₄ hollow microspheres were prepared via an improved solvothermal method using ZnAc₂·2H₂O and MnAc₂·4H₂O as starting materials,as well as urea and ammonium bicarbonate as complex precipitant,adding citric acid as carbon source.The as-synthesized samples are characterized by X-ray diffraction,scanning electron microscopy,and transmission electron microscopy.The electrochemical properties of the samples are investigated by the LAND Battery-Testing System.In addition,the effect of the water-soluble sodium carboxymethyl cellulose（CMC）binder on the electrochemical performance is also studied in this paper.When evaluated as anode materials for LIBs,these porous ZnMn₂O₄ hollow microspheres can exhibit a stable reversible capability of 1209 mAh g^-1 even after 100 cycles at a current density of 200mA g^-1.It remains a high reversible capability of 648 mAh g^-1 even after 500 cycles at a current density of 500 mAh g^-1.As far as we know,the discharge capacity is the highest value among the currently reported pure phase ZnMn₂O₄ under the same conditions.Compared with the traditional polyvinylidenefluoride（PVDF）binder,the CMC can substantially improve the performance of ZnMn₂O₄ hollow microspheres.The outstanding performance indicates its potential as a promising anode material for LIBs.（2）We prepared ZnFe₂O₄ hollow microspheres by two-step method（hydrothermal synthesis and high temperature calcination）and synthesized ZnFe₂O₄/MWCNTs composite using ultrasonic method to mix ZnFe₂O₄with MWCNTs（which concentrated by nitric acid）.These samples are characterized by X-ray diffraction,Raman Shift,scanning electron microscopy and transmission electron microscopy.The mass fraction of MWCNTs was 6.5 wt%which was proved by TGA.When evaluated as anode materials for LIBs,the ZnFe₂O₄/MWCNTs composite exhibits an excellent electrochemical performance:when the current density is 100 mA g^-1,the specific capacity is 908 mAh g^-1 after 100 cycles,which is much higher than 523mAh g^-1 of ZnFe₂O₄.Under the same conditions,the discharge specific capacity of ZnFe₂O₄and ZnFe₂O₄/MWCNTs are much higher than that of graphite.The addition of MWCNTs could effectively improve the electrochemical performance of ZnFe₂O₄.（3）In order to obtain the ZnFe₂O₄/MWCNTs composite anode material with higher electrochemical performance,we designed to increase the content of MWCNTs.ZnFe₂O₄and ZnFe₂O₄/MWCNTs composites were synthesized by solvothermal method with further heat-treatment.ZnFe₂O₄nanoparticles are firmly anchored to the surface of MWCNTs（TGA data proved that the mass fraction of MWCNTs was up to 23.5 wt%）.ZnFe₂O₄/MWCNTs nanocomposite displays high specific capacity(1278 mAh g^-1 at a current density of 200 mA g^-1 after 200 cycles,and 565 mAh g^-1 at a current density of 1500 mA g^-1 after 500 cycles),and good rate performance(367 mAh g^-1 even at a current density of 6000 mA g^-1 after 80 cycles).The superior electrochemical performance may promote ZnFe₂O₄to be a promising alternative anode in LIBs.