Preparation Of Electrospun PAN-based Activated Carbon Fibers And The Performance In Lithium-air Batteries

Lithium-air batteries,using lithium metal and air as anode and cathode active materials,have received extensive attention in recent years.Due to their high theoretical energy density up to 11,680 Wh/kg,Lithium-air battery has been considered to be one of the strongest competitors for the next generation of batteries for powering automobiles.So far,the technical difficulties such as low capacity density and short cycle life in lithium-air batteries make it unable to be commercialized.The solution depends on the cathode materials and the design of cathode structures.Lithium peroxide(Li2O2)is generated in oxygen reduction reaction(ORR)on the cathode,which is hardly soluble in the electrolyte.The large bandgap of Li2O2 leads to the limited electron transfer capability,which reduces the effective reaction interfaces in the cathode and discharge capacity and creates a high overpotential.In addition,the cathode material is usually made of porous materials and/or coated by catalysts to give sufficient space for Li2O2 deposition and reduce the overpotential,which helps to reduce side reactions.The electrode is usually consists of a carbon paper substrate and a reaction layer with carbon-based materials..In order to maintain the structural stability of the reaction layer,binders are usually added.But binders such as polyvinylidene fluoride(PVDF),polytetrafluoroethylene(PTFE)have been confirmed to react with the electrolyte,limiting the battery performance.The formation of Li2O2 on the electrode will also destroy the electrode structure and affect the battery performance.Therefore,the cathode material and its structural design are one of the important research directions of lithium-air batteries.Carbon materials have been widely used in energy storage.Literature review indicates that carbon materials such carbon black,carbon nanotubes,graphene have been widely used in the lithium air battery.But for carbon nanofibers(CNF)with high specific surface and controllable morphology,there are only a few studies have been taken in lithium-air batteries system.Therefore,in this thesis,polyacrylonitrile(PAN)is used as a precursor to prepare various kinds of carbon nanofibers by using electrospinning,high-temperature heat treatment and surface activation techniques.The physical and chemical characterizations have been carried out for both carbon paper-based and free-standing CNF electrodes to analysis the performance of the CNF electrodes in lithium air batteries.The main research content and results include:(1)Polyacrylonitrile/N,N-dimethylformamide(PAN/DMF)was used as the spinning solution.Through the adjustment of the mass fraction of PAN,different morphologies of CNF were prepared by electrospinning and heat treatment.Based on CO2 or KOH activation,activated CNF was obtained.Compared with the unactivated CNF,the specific surface area and pore volume of the activated CNF have greatly increased.(2)Carbon paper-based cathode for lithium-air batteries was prepared by ultrasonic spraying with binder-free CNF slurry.Electrochemical test results show that the activated CNF has a better ORR activity due to the increase of specific surface area.And the battery capacity and cycle life are improved.The 6%PAN-based CO2-activated CNF has a specific surface area of 919 m~2/g,enabling the battery to yield a specific capacity of 4580 mAh/g at a current density of 50 mA/g and 25 cycles at a current density of 200 mA/g.(3)In order to avoid the use of carbon paper with low ORR activity,the performance of binder-free free-standing CNF cathodes was also studied in this thesis.Compared with carbon paper-based activated CNF electrodes,the specific capacity of the free-standting activated CNF electrode battery is much larger,reaching 8291 mAh/g at a current density of 50 mA/g.It is speculated that the free-standing electrode may have more active surfaces in the cathode.In addition,the activated free-standing electrode is superior to the untreated electrode in terms of specific capacity and cycle life.It is also found that the performance of the CO2-activated electrode is better than the one with KOH activation,which may be related to the fact that CO2-activated CNF has more mesopores(>3 nm).