Controllable Preparation Of 2D Metal-organic Frameworks And Their Electrochemical Lithium Ion Storage Performance

Lithium-ion batteries（LIBs）have the advantages of high specific capacity,high voltage,good safety performance,no memory effect and fast charging or discharging properties.Anode materials play an important role in improving the performance of batteries.Therefore,the investigation of anode materials with high specific capacity and long cycling life becomes a hotspot in the field of energy storage.Metal-organic frameworks（MOFs）with high specific area,high porosity and easy modification,especially for two-dimensional MOF can expose more active sites on surface and effectively shorten ion diffusion path used as anode materials for LIBs.In this dissertation,ultrathin 2D MOF nanosheets with suitable metal-ions and organic ligands were synthesized by liquid-phase ultrasonication-assistance method and used as anode materials for LIBs.The morphology,composition and microstructure of ultrathin 2D MOF nanosheets were characterized systematically.Lithium ion storage performance and mechanism of different MOF materials were deeply discussed.The main research contents and results are as follows:（1）Ultrathin Co-BDC nanosheets were successfully prepared and used as anode materials of LIBs,exhibiting activation phenomenon to some extent and high reversible specific capacity.After activation,ultrathin Co-BDC nanosheets’specific capacity reached up to 1071.6 m Ah g^-1 at 0.1 A g^-1 after 10 cycles;the capacity retention rate was60.5%after 500 cycles at 0.5 A g^-1 and 75.8%after 1000 cycles at 2 A g^-1,showing high specific capacity and good cycling performance.（2）Bimetallic Zn/Co-BDC nanosheets were successfully prepared and investigated the influence of Zn content on the morphology and microstructure of Zn/Co-BDC nanosheets,the electrochemical lithium ion storage performance of Zn/Co-BDC nanosheets was studied.After activation,ultrathin 10Zn90Co-BDC nanosheets achieved a specific capacity of 997.1 m Ah g^-1 at 0.1 A g^-1 after 10 cycles;the capacity retention rate was 89.5%at 0.5A g^-1 after 500 cycles and 80.5%after 1000 cycles at 2 A g^-1.Compared to ultrathin Co-BDC nanosheets,the specific capacity of ultrathin 10Zn90Co-BDC nanosheets was slightly inferior,however it exhibited better cycling stability.（3）Furthermore,the ultrathin bimetallic Ni/Co-BDC nanosheets were obtained,and their composition and microstructure on the performance of LIBs were systematically studied and optimized;the ultrathin bimetallic Ni/Co-BDC nanosheets showed significant activation phenomenon as anode materials.After optimization,the ultrathin 10Ni90Co-BDC nanosheets exhibited high reversible capacity.The specific capacity 1628.4 m Ah g^-1 after 10 cycles at 0.1 A g^-1was achieved.The capacity retention rate was about 100%after 500 cycles at 0.5 A g^-1 and 74.5% after 1000 cycles at 2 A g^-1,exhibiting high specific capacity and excellent cycling stability.Furthermore,ultrathin 10Ni90Co-BDC nanosheets were used as anode materials to explore the Li-ion storage mechanism.During the discharging process,Li-ions were firstly intercalated into nanosheets and enlarged the interplanar spacing of ultrathin10Ni90Co-BDC nanosheets.Subsequently,excessive Li-ions were inserted into anode materials,replaced the position of Ni/Co and part of MOFs were transformed into lithium terephthalate and metal-oxide nanocrystalline.On the one hand,MOFs nanosheets can store Li-ions,on the other hand,metal-oxide nanocrystallines form in the process of SEI film formation exhibit extra Li-ion storage performance.Lithium terephalate was coated on the surface of metal-oxide nanocrystallines,which could effectively inhibit the volume expansion of metal-oxide nanocrystallines during cycling.The ultrathin 10Ni90Co-BDC nanosheets exhibited high specific capacity and excellent cycling stability.