| With increasing market of electric vehicles and growing demands on fossil energy in the past several decades,the coupled resource depletion and critical environmental pollution have raised an urgent need for alternatives to fossil fuels and combustion engines.Among various energy storage systems,lithium air and lithium sulfur batteries have been considered as the most desirable systems and drawn tremendous attention due to their high specific energy densities which are 3,458 and 2,567 Wh kg-1,respectively.Although progressive improvements have been achieved for both lithium air and sulfur systems,they are still hindered from practical application by some intrinsic problems.In lithium air battery,most aprotic electrolytes undergo decomposition in the presence of intermediates and discharge products,resulting in poor cycling performance;and the insulating discharge products(Li2O2/ Li2O)accumulated on the air cathode during cycling leads to low capacity,high overpotential and poor cyclability.In this dissertation,the air cathode structure and aprotic electrolytes were specifically investigated to solve these problems.On the other hand,the drawbacks such as the electrically insulating nature of sulfur and Li2 S together with the dissolution of lithium polysulfides into organic liquid electrolytes and its “shuttle effect” result in low electrochemical utilization of the active material and poor rate capability.The thesis mainly focused on lithium sulfide cathode and its electrochemical performance.The main research results are presented as below:(1)We designed a lithium air battery with novel structure that delivers high capacity and works stably under air atmosphere.A free-standing,highly porous Pd-modified carbon nanotube(Pd-CNT)sponge was used as cathode.The Pd-CNT sponge was synthesized through a chemical vapor deposition growth followed by an electrochemical deposition process.To build a full lithium-air battery,the air cathode was integrated with a ceramic electrolyte-protected lithium metal anode and non-volatile ionic liquid electrolyte.The lithium anode was stable during the operation and long-time storage and the ionic liquid was chemically inert.By controlling the amount of ionic liquid electrolyte,the carbon nanotube sponge was wetted but not fulfilled by the liquid electrolyte.Such configuration offered a tricontinuous passage for lithium ions,oxygen and electrons,which was propitious to the discharge reaction.In addition,the existence of Pd nanoparticles improved the catalytic reactivity of the oxygen reduction reaction.The battery was durable to a wide humidity range(0-95%)and delivered a capacity as high as 9,092 mA h g-1.(2)Many solid cathode catalysts have been developed and proved not to be sufficient to solve some serious problems in lithium air battery such as the accumulation of discharge products on cathode,slow kinetics and poor cycling property.To alleviate the above problems,organic-electrolyte-dissolved iron phthalocyanine(FePc)was used as the solution-phase catalyst.FePc worked as a molecular shuttle of(O2)-species between the surface of the electronic conductor and the insulator Li2O2,facilitating both oxygen reduction reaction(ORR)and oxygen evolution reaction(OER).According to the proposed working mechanism “(FePc-O2)?(FePc-O2)-?(FePc-Li OOLi)”,FePc can stabilize reduced O2 species and hence improved stability of electrolyte.Meanwhile,Li2O2 was observed to grow and decompose without direct contact with carbon,which greatly enhanced the electrochemical performance.The comparison results showed that the solution-phase catalyst FePc was compatible with solid catalyst and achieved better cell performance.The use of catalytically active molecular shuttles may bring a chance to practical application for lithium air rechargeable batteries.(3)Highly conductive and stable Li2 S nano spheres anchored to single-layered graphene(Li2S/G@C)was synthesized to address some problems in lithium sulfur battery,such as the dissolution of polysulfide in most electrolytes and insulating nature of S/Li2 S that further lead to low utilization of sulfur,poor rate capability and cycle life.In Li2S/G@C composite,Li2 S nanospheres were uniformly deposited on the surface of single-layered graphene and further formed a durable protective carbon layer on the surface of the Li2 S particles by using a facile CVD method.Reduced lithium diffusion limitations can be achieved by decreasing particle size of Li2 S.The graphene substrate and conformal carbon coating can improve not only the electronic conductivity but also the mechanical stability of cathode structure.With Li2S/G@C as cathode,remarkable rate capability and cycling stability were obtained.The capacities based on Li2 S were 993,773 and 743 mA h g-1,corresponding to 1,420,1,105 and 1,062 mA h g-1 based on S,at 0.5 C,1 C and 2 C rates,respectively(1 C = 1,166 mA h g-1,Li2S).After 1,000 cycles at a rate of 2 C,the capacity was still retained to 314 mA h g-1 of Li2S(451 mA h g-1 of S).(4)Aiming at the commercialization of lithium sulfur battery,Li2 S cathode,conductive additive and electrolyte/lithium sulfide(E/Li2S)ratio were studied to optimize the cell performance.With the assistance of polyvinyl pyrrolidone(PVP),large-scale produced Li2S/Ketjen balck(Li2S/KB)with excellent electrochemical performance was synthesized via a facile method.Highly conductive KB with high surface area was coated on the Li2 S surface,giving rise to high conductivity and stability of Li2S/KB cathode material.The cell showed highest rate capability and cycling stability when Li2 S ratio in Li2S/KB was 80 wt.%,which capacity was 650 mA h g-1 of Li2S(930 m A h g-1 of S)after 60 cycles at 0.5 C.The addition of a small amount of carbon nanotues(CNT)can remarkably enhance the rate capability and capacity due to the long-range electronic conductivity of CNT.The experiment revealed that the ratio of E/Li2 S can dramatically affect the cell performance.With optimized ratio of E/Li2 S,the Li2 S cathode can achieve ideal energy density. |
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