| To address the issues of the fossil-fuel shortage and environmental pollution,great efforts have been made to seek the next generation of sustainable and safe energy.In the field of electrochemical energy storage technologies,rechargeable lithium-ion batteries have been widely used as high performance power sources for portable electronic devices owing to their high energy densities and cycleabilities.However,the state-of-the-art lithium-ion batteries fall far behind the requirements for electric vehicles.Among various energy storage systems,lithium-oxygen battery seems to be one of the most promising next-generation batteries due to very high theoretic specific energy(3505Wh Kg-1).Although nonaqueous lithium-oxygen battery have been widely investigated in the past decades,there are also several barriers to be overcomed to realize its application.For example,the insoluble discharge product Li2O2accumulated on porous cathode hinder the continued diffusion of oxygen,which results in a low utilization of cathode;the decomposition of electrolyte in the presence of intermediates and discharge products leads to the insulated by-products?Li2CO3 etc.?,resulting in poor cycling performance;the organic electrolyte employed in lithium air battery is flammable.To solve the oxygen diffusion problem,we introduce a thick LOB cathode consisting of packed gel particles.The packed gel particles are electronic/ionic biconductive and maintain the electrochemical activity.Meanwhile,since gels do not have capillary effect,the gaps between the gel particles may serve as oxygen diffusion expressways so that the inner part of thick cathode is more efficiently utilized.The model gel in our study consists of carbon nanotubes?CNTs?and lithium bis?trifluoromethane sulfonyl?imide-tetraethylene glycol dimethyl ether-polyurethane?LiTFSI-TEGDME-PU?gel electrolyte.The bulk gel was manually cut into50-250?m particles and piled up.When they are piled up,the gaps between these particles serve as excellent oxygen diffusion channels are steady and evenly distributed in the whole cathode.The packed particles were compressed with different volume ratios to reduce the interfacial resistance.However,the continuity of gas channel decreased with the volume ratio increasing.To balance the ionic conductivity and the gas channel continuity,we chose the 30%compressed sample in the further studies,which gives the composite gel good electronic conductivity of 6.5×10-4 S cm-11 and volume ratio of the gaps?porosity?of 23%.Benefited from our design,the LOB cathode as thick as 1 mm has been fabricated,which shows a specific capacity as high as 46,400 mAh gcarbon-1 at the current density of 100 mA g-1(or 2 mA g gel-1),almost four times higher than that of the bulk cathode with similar size.For the bulk gel cathode,the content of discharge product Li2O2 on cathode surface is higher than that at bottom side.However,for gel particles,Li2O2 mainly accumulated at the inner part of cathode.It demonstrates the efficiency of oxygen diffusion of gel particles cathode.Based on the gel particles,we designed and fabricated a cylindrical lithium-air battery with a discharge current of 0.3 mA and a discharge capacity of 54.5 mAh,corresponding to battery energy density of 303 Wh kg-1.Most organic electrolyte used in lithium air batteries are of high volatility and unstable to O2 and its reduced species.The electrolyte would decompose on oxygen cathode with mixture of lithium carbonates.We had explored inorganic molten salt or ionic liquid based on bis?fluoroalkylsulfonyl?amide anions as a new class of electrolyte applied in lithium air batteries.We focused on the ternary mixtures of alkali metal bis?trifluoromethylsulfonyl?amides,MFSA?M=Li,K,Cs?,for which eutectic point is 323 K at?xLi,xK,xCs?=?0.30,0.35,0.35?.This moderate temperature was suitable for lithium air battery.With no carbon contained,this inorganic ternary molten salt would prevent the insulated and insoluble Li2CO3 from accumulated on cathode during the cycling.Furthermore,this ternary molten salt was nonflammable.The cesium bis?trifluoromethylsulfonyl?amides were obtained by one-step reaction.This molten salt had much higher ionic conductivity?262 mS cm-11 at 55??and showed good electrochemical stability in Li-O2 battery.the lithium air batteries based on molten salt had achieved a discharge capacity of 4,258 mAh g-1,at 50 mA g-1.the columbic efficiency of charge is 95%,without any addition of catalyst or redox medium.X-ray diffraction?XRD?measurements showed only lithium oxides and no carbonates?Li2CO3?were found during cell cycling. |
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