Research On The Design Of Electrolyte And Cathode Structure For Lithium Air Batteries

Energy crisis and environmental issue are two major problems restricting the sustainable development of the world.Pursuing the revolution of the modes for energy supply and storage is the key to solve these problems radically.Lithium sencondary batteries have become the main energy storage techniques due to their high energy density,low self discharge and none memory effect features.Amonge all lithium sencondary batteries,lithium air battery is regarded as the ultimate solution for energy storage because of its highest theoretical energy density(3,505 Wh kg-1),and received world-wide attention.Although lithium air battery has made great progress in the last decade,it is still in its babyhood and far from the practical application.There are many scientific problems in the research of lithium air batteries including the large overpotential during discharge and charge processes,the poor stability of battery components,the low energy conversion efficiency and practical energy density etc.,which are resulted from the unclear discharge/charge mechanism,serious side reactions and lack of objective performance evaluation criterion.In this dissertation,we aimed to solve these problems and proposed some solutions to settled them through controlling electrolyte components,introducing redox mediator catalysts,designing novel cathode structure and constructing hierarchical porous cathode structure.The main research contents and results are as follows:(1)The kinetics of the chemical reaction between redox mediator and Li2O2 is the key factor determining the charge efficiency of lithium air battery.However,there are little researches focusing on this issue.Taking LiI/LiI3 as the model redox mediator,we systematically studied the reaction kinetics between redox mediator and Li2O2 in conventional solvents as well as in protic solvents,and innovatively proposed the use of protic solvents with weak acidity as the electrolyte solvent to activate Li2O2,and thoroughly analyzed the reaction mechanism.In conventional solvents,such as TEGDME,DME and DMSO,the reaction rate between I3-and Li2O2 is extremely slow that severely limited the charge efficiency in lithium air battery.We improved it by introducing protic solvent n-butanol which increased the solubility of Li2O2 in electrolyte while without trigger the disproportionation reaction of Li2O2.The reaction rate between I3-and Li2O2 is greatly increased and the charge overpotential is greatly decreased.Meanwhile,we also designed a graphite-Li2O2 cell structure and studied the electrochemical performance in n-butanol electrolyte containing Lil as redox mediator.The charge potential is only 3.3 V and the charge effieiency of the first 10 cycles is above 94%compared with the battery in conventional electrolyte.The capacity fading is originated from the side reaction of carbon electrode.The investigation of reaction kinetics between redox mediator and Li2O2 and the discovery of protic solvent provide new approaches for the controlling of electrolyte in lithium air battery.(2)Lil is a widely used redox mediator in lithium air battery.However,its reactivity with the discharge products such as Li2O2 and LiOH is in doubt.We studied the charge reaction and the reactivity between LiI3 and discharge products in lithium air batteries containing Lil as redox mediator by thermodynamics analysis,capacity calculation,O2 release speed monitoring,SEM,XRD and UV-Vis techniques,and proved that it is infeasible to cycle a lithium air battery based on the formation and decomposition of LiOH via Lil/Lil3 mediation.By conbining the charge/discharge method and UV-Vis analysis,we proved that the concertration of I3--in electrolyte is increasing along with cycle numbers during charge process,while small amount of I3-consumes during discharge process.The discharge product LiOH still exists on the surface of electrode after cycling.Meanwhile,we also verified that the reaction between LiOH and LiI3 could happen if H2O2 exists.This work overthrowed the results proposed by Grey et al.which has been evaluated as the "Top 10 Progresses of Science and Technology in 2015”by Thomson Reuters.(3)To solve the self-discharge issue and improve the stability of carbon electrode during charge process,we designed a novel dual oxygen electrodes lithium air battery to separate the ORR and OER processes.Compared with conventional single cathode structure,this design could suppress the self-discharge of redox mediator and improve the cycling stability.The battery can stable operate for 15 cycles under a flat charge potential of 3.7 V.Beside,it can reversible discharge and charge under a 71%DOD with low overpotential.Meanwhile,there is little side products such as Li2CO3 generated after cycling compared with single electrode structrure.Our dual oxygen electrodes design is suitable for most of the redox mediators,and could improve the cycle performance of lithium air battery.This design provides a new way to develop high energy density rechargeable lithium air battery.(4)To solve the oxygen diffusion and the high electrolyte/cathode weight ratio(E/C ratio)problems which restrict the development of partical high energy density lithium air battery,we prepared fur-like carbon nanotube(CNT)coated 3D hierarchical porous graphene aerogel(GA)as the cathode material for lithium air battery.Graphene sheets provide an elastic skeleton with high electronic conductivity.CNTs uniformly grown on the surface of graphene surface forming a fur-like coating.In the CNT/GA structure,the CNT coating can be easily wetted by small amount of liquid electrolyte that forming large amount of O2/electrolyte/CNT reaction sites,while the macropores between graphene sheets will not be flooded by the electrolyte which is benefit for O2 diffusion.Meanwhile,the hierarchical porous structure enabled us to control the E/C ratio in lithium air battery,thus could improve the specific capacity of the battery based on the total weight of electrolyte and cathode.This kind of calculation method based on the total weight of electrolyte and cathode is more objective to reflect the performance of lithium air battery and can promote the development of high energy density lithium air battery.