| The overuse of traditional fossil fuels results in severe environmental iusses,so it is urgent to develop renewable energy sources,such as wind and solar.However,renewable energy sources cannot be the sole provider of energy without an associated energy storage facility.Therefore,it is necessary to develop highly efficienct energy storage devices.Rechargable lithium ion batteries are widely used in electric vehicles and portable electronics due to their characteristics of high energy density,high power density,and relatively long lifespan.Currently,commercial graphite anodes are reaching to their theoretical specific capacity.Consequently,it is urgent to seek alternative anode materials.Transition metal compounds are considered to be one of the most promising anode materials owing to their high reversible capacities.Unfortunately,the inherent low conductivity and the large volume change during charge and discharge give rise to their low rate performance and poor cycle performance.In this thesis,we prepared a new type of transition metal compound,Co(OH)(OCH3),and improved its electrochemical performance by means of regulating the thickness,doping other metal and conbining with carbon materials.The main research contents are shown as follows(1)The thickness of Co(OH)(OCH3)was regulated by the addition of tyrosine,glycine,trpptophan,glutamicacid and methionine during the solvothermal process.Unmodified Co(OH)(OCH3)consists of platelet-like nanosheets with the thickness of 20-50 nm.Among them,when tyrosine was added,Co(OH)(OCH3)-Tyr(Co(OH)(OCH3)-Tyrosine)shows a flower-like architecture assembled by nanosheets with the thickness of 15-40 nm.Benefitting from the unique flower-like structure of Co(OH)(OCH3)-Tyr,it can deliver high specific capacities of 868.6,775.3,731.9,647.0 and 543.4 mAh g-1 at the current densities of 0.2,0.3,0.5,1 and 2 A g-1,respectively,and it can retain 445.4 mAh g-1 at a high current density of 2 A g-1 after 130 cycles.On the contrary,the pristine Co(OH)(OCH3)can only deliver a low capacity of 124.6 mAh g-1 at 2 A g-1 after 130 cycles.The results show that the electrichemical performance of Co(OH)(OCH3)can be enhanced by adjusting its thickness(2)Bimetallic MnxCo1-x(OH)(OCH3)nanoplatelets with different Mn/Co ratios ranging from 0.1 to 10 were synthesized by a one-step solvothermal method.The compounds form a single-phase solid solution and show the two-dimensional nanoplatelet structure.Among them,when used as the electrode of lithium ion battery,Mrn1/11Co10/11(OH)(OCH3)shows the best electrochemical performance,The specific capacities of Mn1/11Co10/11(OH)(OCH3)are 907.4,867.9,750.8,637.8 and 520.3 mAh g-1 at the current densities of 0.2,0.3,0.5,1 and 2 A g-1,respectively.The Mu1/11Co10/11(OH)(OCH3)electrode can still deliver a specific capacity of 864.0 mAh g-1 when the current density returns to 0.2 A g-1;while the specific capacity of Co(OH((OCH3)electrode are 773.6,650,8,520.8,412.5 and 368.3 mAh g-1 at different current densities of0.2,0.3,0.5,1 and 2 A g-1,respectively,and the specific capacity only was 744.3 mAh g-1 when the current density returned to 0.2 A g-1 again.Furthermore,even at a high current density of 1 A g-1,the Mn1/11Co10/11(OH)(OCH3)electrode maintains the discharge capacity of 646.0 mAh g-1 after 200 cycles,which is much better than that of Co(OH)(OCH3)electrode.Compared with Co(OH)(OCH3),the inproved electrochemical properties of Mn1/11Co10/11(OH)(OCH3)can be ascribed to the synergistic effect between Zn and Co.(3)Co(OH)(OCH3)-based compounds can also be used as the precursor for the preparation of the corresponding oxi des with rocksalt structure.Here,using NixCo1-x(OH)(OCH3)@polydopamine(NixCo1-x(OH)(OCH3)@PDA)as the precursor,N-doped carbon-coated NixCo1-x@N-C composites were obtained by calcination in the nitrogen atmosphere.Thanks to the presence of PDA,NixCo1-x(OH)(OCH3)is pyrolyzed and formed a single-phase NixCo1-xO solid solution.At the same time,the PDA coating was carbonized into a continuous and uniform carbon layer.NixCo1-xO@N-C at a Ni/Co ratio of 0.2 shows the best rate performance and cycle performance as the negative electrode of lithium ion battery with a high specific capacity of 949 mAh g-1 at a current density of 0.2 A g-1 and a specific capacity of 530 mAh g-1 at 2 Ag-1.Moreover,the specific capacity can still reach up to 563 mAh g-1 at 2 A g-1 after 500 cycles.The excellent lithium storage performance benefits from the synergistic effect between bimetals,as well as the two-dimensional structure and N-doped carbon coating. |
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