Preparation And Investigation Of NiMn-LDHs As Cathode Catalyst For Lithium Oxygen Battery

Lithium-oxygen(Li-O2)batteries have the high theoretical energy density(4-5 times that of lithium-ion batteries),which have been considered as one of most promising energy storage devices.However,the Li-O2 battery still suffer from some disadvantages,such as low round-trip efficiency,poor rate performance,and large overcharge potential,which severely limit the practical process of Li-O2 battery.It has been shown that the catalyst can reduce the overpotential of the oxygen electrode,decrease the polarization,and increase the rate of oxygen cathode reaction,thereby improving the energy conversion efficiency.Therefore,developing an efficient dual-function catalyst have attracted great attention during last years.In this paper,we mainly focused on:(1)The optimal conditions for synthesis of NiMn-LDHs through hydrothermal method were studied,and the electrochemical performances of the as-prepared NiMn-LDHs catalysts for Li-O2 battery cathode were studied.It has been found that the molar ratio of nickel chloride and manganese chloride,reaction temperature and time affect severely the crystallinity of morphology and electrochemical properties of the NiMn-LDHs.When the molar ratio of nickel chloride and manganese chloride is 3:1,the reaction temperature is 100?,reaction time is 12 h,the as-obtained NiMn-LDHs shows thin sheet morphology and good crystallinity.The first discharge specific capacity reaches 1845 mAh/g at a current density of 100 mA/g and a voltage range of 2.0-4.5 V.(2)NiMn-LDH nanosheets(NiMn-LDH-NS)were prepared in formamide,then combined with MWCNTs to form NiMn-LDH-NS/MWCNTs composites.The charge and discharge test results show that the initial discharge specific capacity of the composites prepared at a current density of 100 mA/g and a voltage range of 2.0-4.5 V can reach 3477 mAh/g.When the specific capacity is limited to 500 mAh/g,the battery can be stably cycled 31 times.The fine electrochemical performance of NiMn-LDH-NS/MWCNTs electrode is attributed to the fact that the composites assume more active sites and its good lamellar structure,making it has higher catalytic activity during the oxygen reduction reaction(ORR)and oxygen evolution reaction(OER).(3)To improve further the electrocatalytic properties of NiMn-LDHs in Li-O2 batteries,NiMn-LDHs/KB composites were prepared by adding different amount of Ketjen black(KB)in the precursor suspension of nickel chloride,manganese chloride and hexamethylenetetramine for hydrothermal.It was found that the initial discharge specific capacity of the NiMn-LDHs/KB composite electrodes are higher than KB electrode(2683 mAh/g)at 100 mA/g current density.The NiMn-LDHs/50KB composite electrode has the highest discharge specific capacity of 5685 mAh/g.When the specific capacity was fixed to 500 mAh/g,the battery can be stably charged/discharged over 35 cycles.The reason might be that the NiMn-LDHs increased the conductivity NiMn-LDHs/50KB composite,and also provide more reactive sites for ORR and OER.The aforementioned results illustrated the broad prospects of NiMn-LDHs materials as catalysts in Li-O2 batteries.Meanwhile,the work opened a novel research approach ways to develop high-efficiency non-precious metal cathode catalysts for Li-O2 batteries.