| The rechargeable lithium-ion batteries（LIBs）have become one of the ideal and reliable energy storage system over the last 20 years due to the advantages of high energy density,no memory effect and long cycling life.In order to meet the needs of high-capacity and low-cost energy storage systems,the exploration of electrode materials is considered to be an important part for improving the performance of LIBs.Polyimide materials based on pyromellitic dianhydride（PMDA）have the advantages of high theoretical specific capacity,easy-controlled chain structure,and abundant resources,which have become the reliable candidates of LIBs anode material.However,limited by molecular weight,structural stability,and electronic conductivity,it still faces great challenges in practical applications.Therefore,considering the lithium storage sites,molecular weight,structural stability and electrochemical activity,improving the materials from the perspective of molecular structure design and preparation of composite materials with high conductivity is the key to improve the electrochemical performance.In this paper PMDA-based polyimides are took as the research object,different numbers of aromatic rings are introduced to adjust the molecular structure of polyimides,and the electrochemical performance of polyimides are improved by preparing composite materials in different ways.The specific research contents are as follows:The pyromellitic acid diimide polymer material（PI）with pyromellitic dianhydride（PMDA）as matrix and aromatic ring-free ethylenediamine（EDA）as linker is prepared via a condensation reflux method.The optimal reaction conditions are obtained by adjusting the reaction concentration to 0.1 mmol mL-1,and pure PI-0.1 hierarchical microspheres with uniform size are obtained.Under this concentration,PI@CNTs is also prepared through the in-situ polymerization method.The PI nanoflakes on PI@CNTs become larger in spacing and shrink in size,exposing more surface area and shortening the Li-ion diffusion distance.Compared with the poor cycling performance of pure PI,PI@CNTs show excellent long-cycle stability,and the capacity remains stable after the highest point.A discharge specific capacity of 493 mAh g-1 still maintains after 2000 cycles at a current density of 1 A g-1,Coulombic efficiency is up to 99.8%,and the rate performance has also been improved.The XPS results show that the reversible lithium storage sites of PI@CNTs are C=O groups and C6 rings on the PI structure.The pyromellitic acid aniline polymer material（PPI）with pyromellitic dianhydride（PMDA）as matrix and p-phenylenediamine（PPD）which contains one aromatic ring as linker is prepared through solvothermal method.The grain-like PPI-2.5 particles with uniform morphology and better lithium storage performance are obtained by adjusting the concentration of raw materials.The core-shell structure PANI-PPI composite materials with PPI-2.5 active material core and the PANI conductive shell are designed and constructed.By adjusting the PANI treatment time,a tremella-like PANI-PPI-9 with uniform morphology and better lithium storage performance was obtained.This method effectively prevents the stacking of PPI-2.5 particles,increases the contact between the material and the electrolyte,and improves the electronic conductivity of the PPI-2.5 core,thereby enhancing the electrochemical performance.The rate and long-terms cycling performance of PANI-PPI-9 are greatly improved compared with PPI-2.5.The discharge specific capacity of PANI-PPI-9 is 309 mAh g-1 at 50 mA g-1,when the current increases to 2000 mA g-1,it still shows a discharge specific capacity of 121 mAh g-1.The PANI-PPI-9 anode exhibits a discharge specific capacity of 503 mAh g-1 after 2400 cycles at a current density of 1.5 A g-1.Meanwhile,a PANI-PPI-9 ‖ PANI-PPI-9（Li）full cell and a PANI-PPI-9 ‖ LiFePO4 full cell are assembled,both full cells show good cycling performance at 100 mA g-1,specific capacities of 95 and 101 mAh g-1 were obtained after 50 cycles,respectively.The matching of cathode and anode in PANI-PPI-9 ‖ LiFePO4 full cell is better.Based on the consideration of improving the lithium storage capability and stability of polyimide molecules,more aromatic rings were introduced into the molecular structure.4,4’-diaminodiphenyl ether（ODA）which containing two aromatic rings is used as the linker,and the pyromellitic acid diphenyl ether imide polymer material（OPI）is prepared.Benefiting from the increased aromatic ring in the molecular structure,the long-terms cycling performance of pure OPI is significantly improved than that of PI and PPI,indicating that OPI has a more stable molecular structure.Then,the PVP layer is coated on the OPI via hydrothermal method,and the C@OPI materials with carbon coating layer was obtained by adjusting the carbonization temperature.The dense carbon coating layer of C@OPI can effectively reduce the irreversible capacity and improve the initial Coulombic efficiency of the material.At the same time,the electrical conductivity of the material is enhanced and the structure is maintained after cycling.Compared with pure OPI,the cycling life of C@OPI mateials at different carbonization temperatures are all improved.Among them,C@OPI-300 has the most outstanding performance,which obtains a specific capacity of 220 mAh g-1 after 5000 cycles at the current density of 2 A g-1,and the Coulombic efficiency is up to 100%.XPS and FT-IR results show that the C=O and C6 rings are still the reversible lithium storage sites of the composites.A C@OPI-300 ‖ LiFePO4 full cell is assembled,and a charge capacity of 103 mAh g-1 is maintained after 300 cycles.The pyromellitic dianhydride anhydride anthraquinone polymer material（PMAQ）with 2,6-diaminoanthraquinone（2,6-AQ）which containing directly connected aromatic rings and more stable structure as the linker is prepared through condensation reflux method.Compared with PI,PPI,and OPI,the lithium storage performance of PAMQ is improved,proving that introducing more aromatic rings into the structure is an effective strategy.Furthermore,rGO was introduced into the material,and the content of rGO is adjusted to obtain PMAQ/rGO composite materials with cellular network structure.Among them,PMAQ/rGO-40 has better morphology and lithium storage performance.The PMAQ particles were dispersed in the composites uniformly and attached to the rGO flakes tightly.The rGO flakes with large-area and the abundant pores in the structure enable more contact between the material and electrolyte.In addition,the preparation of electrodes with this composite material can save a large amount of additional conductive agent.Compared with PMAQ,the rate performance and cycling life of PMAQ/rGO-40 are significantly improved.The discharge specifc capacity of 253 mAh g-1 is still maintained after 3000 cycles at 2 A g-1,the Coulombic efficiency is up to 100%,and a specific capacity of 196 mAh g-1 is obtained at 5 A g-1.A PMAQ/rGO-40 ‖ LiFePO4 full cell is assembled and a charge specific capacity of 154 mAh g-1 is obtained after 150 cycles at 100 mA g-1. |
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