| The research on lithium-ion batteries has been very active in recent years due to the continuous introduction of new energy electric vehicles by major auto companies.However,it is not the electric vehicle itself but the performance of its power source,that is,the battery life and battery life.The level of capability is related to many factors.The drawback of fast decay of energy density of batteries under high power has always inhibited the breakthrough of lithium-ion battery performance;supercapacitors are also attracting attention due to fast charge and discharge,and long life,but because of its low energy density,it has always been the direction researchers want to improve.“Lithium-ion supercapatteries”is a new type of energy storage device in recent years.It combines the energy storage advantages of lithium-ion batteries and supercapacitors,and its energy storage mechanism is novel and diverse.It has become a research hotspot in recent years.The monoclinic polyanionic compound lithium vanadium phosphate?Li3V2?PO4?3?has many advantages such as three-dimensional network three-dimensional structure,high theoretical specific volume,excellent safety performance and low synthesis cost.However,pure phase lithium vanadium phosphate has lower conductivity(2.4×10-7S·cm-1)and lower diffusion coefficient(10-12-10-1010 cm2·s-1)and its defects such as severe capacity attenuation inhibit its energy storage development at high voltage potential?4.8V?.In this thesis,the modification of the lithium vanadium phosphate by different carbon sources and the improvement of the preparation process of the composite materials were carried out,and finally,the high specific capacity,the excellent rate performance and the large current were prepared,carbon coated lithium vanadium phosphate electrode material which can work stably for a long time under density.Subsequent assembly into“non-consumable electrolyte type”and“internal series type”lithium-ion supercapatteries improves the energy density by improving the matching of the positive and negative materials of the whole device.The main research contents and results of this thesis are as follows:?1?Experiment 1 was based on oxalic acid as a reducing agent and citric acid as a composite carbon source.The sol-gel method with improved?delayed quantitative hydration?is used to prepare nano carbon films.A composite electrode material?LVP-C17?in which a lithium vanadium phosphate nanocrystal cluster is uniformly embedded in a film.The prepared materials were characterized by structural tests,and it was confirmed that LVP-C17 has the above-mentioned special morphology.The specific surface area of the LVP-C17 electrode material is as high as 53.7 m2·g-1,and has a high degree of crystallization.In the subsequent electrochemical tests,LVP-C17 exhibited excellent performance with a first discharge specific capacity of up to 128 mAh·g-1.?2?Subsequently,LVP-C17 was used as the positive electrode,activated carbon?AC?was used as the negative electrode,and LiPF6 was used as the organic electrolyte to assemble the“non-consumable electrolyte type”and“internal series type”lithium-ion supercapattery?LVP-C17//AC?.The new device exhibits an energy density of up to 24Wh·kg-11 over a wide voltage range?0-2.7 V?with a power density of 405 W·kg-11 and a charge and discharge of 2000 cycles.The specific capacity can still maintain a good performance of 74.5%,and even at a higher power density of 2.03 kW·kg-1,the energy density can still reach 12.4 Wh·kg-1.?3?Experiment 2 was based on oxalic acid as a reducing agent,TTAB is a surfactant and a carbon source,and a carbon composite lithium vanadium phosphate mesoporous sheet was prepared by a hydrothermal method assisted modified sol-gel method?hydration delayed evaporation method?.Materials,through a series of process optimization,Li3V2?PO4?3/C mesoporous nanosheet composite electrode material?LVP-T2?with high specific surface area ordered.The SEM can observe that the structure is a nanosheet like ordered arrangement,and the nano carbon film can also be observed from the TEM.It was identified as a mesoporous material by BET test,and its pore volume was 0.095 cm3·g-1,and the specific surface area was as high as 40.9 m2·g-1.Combined with the CV test data,the lithium ion diffusion coefficient can reach 5.0×10-8-1.4×10-77 cm2·s-1.Compared with LVP-C17,the discharge specific capacity and rate performance are obviously improved.The specific discharge capacity is up to 166 mAh·g-1.Even at the higher rate of 20 C,the specific capacity can be maintained at 90 mAh·g-1.?4?Using LVP-T2 as the positive electrode and AC as the negative electrode,the same organic electrolyte is used to assemble the“non-consumable electrolyte type”and“internal series type”lithium-ion supercapattery?LVP-T2//AC?.LVP-T2//AC the discharge time is longer than the LVP-C17//AC 854.5 s extended to 1369.24 s,the maximum energy density and power density can reach 53.22 Wh·kg-11 and 3.01 kW·kg-1.This experiment 2significantly improved the disadvantages of the electrochemical device attenuating the energy density at a relatively high power. |
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