Preparation Of Self-Standing Polyimide-Derived Carbon Anode Materials For Lithium-Ion Batteries

With the growth of flexible wearable devices,there is a corresponding demand for higher performance of flexible lithium-ion batteries（FLIBs）,which are expected to be light,thin,and portable,as well as high energy density.The heavy mass,unbendable and other defects plague the traditional lithium-ion battery,limiting its wide application in this field.The design and development of flexible lithium-ion batteries with excellent flexibility,bending resistance,lightness,thinness,stable charge/discharge cycles and long life are of great significance to the actual utilization of flexible electrical energy storage equipment.Electrode,one of the core components of LIBs,is critical to the performance of LIBs.Therefore,the exploration of high-performance flexible electrode materials has become one of the promising hot spots for study in the area.In this paper,polyamide acid（PAA）,a precursor solution,was synthesized by polymerization reaction between monomers using pyromellitic dianhydride（PMDA）and 4,4’-diaminodiphenyl ether（ODA）as monomers and N,N’-dimethylacetamide（DMAc）as solvent.Self-supported carbon nanofiber anode materials with one-dimensional nanostructures using polyimide as carbon precursors were prepared by electrospinning,imidization and high-temperature carbonization processes and directly applied to the anode of LIBs.An effective method to enhance the electrochemical performance of the self-supported anode was explored.One-dimensional structural materials such as polyimide carbon nanofiber membrane,crosslinking polyimide carbon nanofiber membrane,and nitrogen-doped crosslinking polyimide porous carbon nanofiber membrane were designed sequentially without any adhesive,conductive agent,or fluid collector.The main studies are as follows.（1）Polyimide carbon nanofiber membranes（CNMs）with three-dimensional network structure were prepared by electrospinning technique and imidization and carbonization.A series of CNMs with different carbonization temperatures were prepared by designing different temperature carbonization conditions.The impacts of carbonization temperature on the structure,composition,morphology,wettability,conductivity,and lithium storage properties of CNMs were investigated to reveal the mechanism of the effect of carbonization temperature on the electrochemical properties of CNMs when used as the anode of LIBs.（2）To enhance the electrical conductivity of fibers,a novel crosslinking strategy was proposed for the fluffy structure,and polyimide carbon nanofiber membranes（CNMs-CC）with chemically crosslinking structure were prepared.The effect of the introduction of the crosslinking structure on the lithium storage performance of the battery was investigated.The results show that CNMs-CC provides a reversible specific capacity of 495 m Ah g^-1 at 50 m A g^-1.Especially at high current densities,CNMs-CC shows its superior long-cycle stability.The specific capacity of CNMs-CC reached 290.87 m Ah g^-1 at 1 A g^-1.After 1000 charge/discharge cycles,it still reached201.38 m Ah g^-1.The excellent electrochemical performance of CNMs-CC is attributed to its unique crosslinking structure,which is derived from a novel triple crosslinking strategy.It provides a good conductive network that confers fast electron transfer and shortens the Li⁺transport distance.The stable structure can withstand the repeated impacts of Li⁺de-embedding process and mitigate the volume expansion.（3）Nitrogen-doped crosslinking polyimide porous carbon nanofiber membranes（PNCNMs-CC）were prepared by electrospinning technology combined with crosslinking treatment and nitrogen-doped pore-making process,using melamine as the nitrogen source and ethyl orthosilicate as the pore-making agent.It was shown that the pore-formed carbon fibers had a superior specific surface area(546.78 m² g^-1)compared to that of the non-porous treated carbon fibers(398.43 m² g^-1),which enhanced the specific surface area by 37%and supplied more activity spots for Li⁺.The first discharge specific capacity of PNCNMs-CC showed 710 m Ah g^-1 at 50 m A g^-1,which was much better than that of pure fibers.At a current density of 1 A g^-1,PNCNMs-CC showed 88%capacity retention after 1000 charges and discharges.