Preparation And Electrochemical Properties Of Naphthalene-based Polyimide As Cathode Materials

With the development of society and the emergences of problems for energy and environments,traditional inorganic secondary battery can not meet the new requirements for noval materials which are presented by contemporary society,and then scientists are inspired to develop and study clean and efficient energy materials.As one of clean and efficient energy storage devices,lithium ion battery has been given high expectations.But there are still some problems for traditional lithium-ion batteries with conventional inorganic materials as primary electrodes.For example,traditional inorganic materials usually have limited storage in nature,which makes them hard to achieve large-scale industrialization.Even if,their environmental problems are still big troubles.Given these reasons,people are aroused to pursue green and clean organic materials.As an important class of organic cathode materials,conjugated carbonyl materials with electrochemical activities have been reported a lot in recent years,including small molecules,polymers and their based composites.And the high theoretical specific capacity and structural stability of conjugated carbonyl material have attracted us to explore the studies and developments of organic electrode materials for lithium-ion battery.Polyimide is an important class of organic conjugated carbonyl materials.The redox behaviors of polyimide are mainly based on the reduction of carbonyl groups derived from dianhydrides.It is expected to show excellent electrochemical properties as the cathode materials of lithium-ion batteries due to its diverse structures,high theoretical specific capacity,chemical structure stability and thermal stability,etc.Therefore,our work is mainly focused on the preparation and electrochemical properties of polyimide cathode materials.The research is mainly to study the synthesis,structures,morphologies and electrochemical performances of naphthalene-based polyimide from the aspects of structural design of polymers,adjustment of polymerization conditions and the fabrication of their composites.?1?Structural design of polymers: two polyimides are polymerized with naphthalene anhydride and ethylenediamine or naphthalene anhydrideandurea?The two polymers were designated as NEP and NOP,respectively?.The chemical structures of polymers are verified by elemental analysis,Fourier transform infrared?FTIR?spectra,C¹³-NMR and the solubility experiments.According to the electrochemical tests,the polyimide exhibit better structural stability than that of dianhydride monomer,and NOP displays better rate performance and cycling stability,with a high specific capacity of 175 m Ah/g at current density of 20 m A/g and 133 m Ah/g at current density of 500 m A/g.After 60 cycles of cycling,the value can still maintain at 153 m Ah/g,and NOP shows a better electrochemical performance than that of NEP.At the same time,the mechanisms of the charge-discharge reaction of NOP are also discussed by theoretical calculations and analysis.?2?Adjustment of polymerization conditions: based on the NOP system,the influences of polymerization conditions on the morphologies and electrochemical properties of NOP are investigated by controlling the reaction times and monomer concentrations.The results of different reaction times show that the increase of polymerization time is beneficial to the enhancement of specific capacity for NOP,and when the polymerization time is higher than 8 hours,the performance increases slowly and gradually remains stable.The product with the polymerization time of 8 hours has the highest specific capacity of 154 m Ah/g and the capacity could remain around 151 m Ah/g.it also shows the smallest charge transfer resistance.The results of different polymerization concentrations show that the specific capacity of NOP decreased gradually with the increase of polymerization concentrations,and the specific capacity of 4 mmol group is the highest.The capacity of NOP could maintain 98% after 60 cycles.Therefore,the optimized conditions of NOP system with good electrochemical performances are obtained with the reaction time of 8hours and the concentration of the monomer of 4 mmol.?3?Preparation of composite electrode materials: NOP/CNT flexible electrode films are prepared by in-situ polymerization and blending methods,respectively.The structures,morphologies and electrochemical properties of the films were investigated.The results show that the NOP/CNT composite electrode film has a feature of flexibility,and can withstand a certain degree of bending.The blocks of NOP are exposed outside obviously with a low content of CNT,while the groups with high loadings of CNT display an improved dispersion.For the electrochemical performance,the specific capacity of the composite film with high concentration of CNT group is good,and NOP-A2 group could show a specific capacity of about 135 m Ah/g at the current density of 500 m A/g,which indicates that the composites fabricated by in-situ polymerization have better rate performance then those by blending at high current densities.