| With the development of modern industries,electric energy has gradually become an indispensable energy source for peoples daily life and industrial production,and is closely related to our lives.In order to adapt to the development of society,the research of electric energy storage and conversion equipment has never stopped.Various new batteries are gradually replacing traditional lead-acid batteries,lithium-ion batteries,sodium-ion batteries,lithium-oxygen batteries,zinc-air batteries,etc.The research on new secondary batteries is attracting the attention of countless researchers.At present,commercial batteries on the market are basically covered by lithium-ion batteries.Of course,the stability and higher specific energy density of lithium-ion batteries make it gradually replaced traditional lead-acid batteries,but its specific capacity is not big enough.The lithium-oxygen battery has a very high theoretical energy density and specific capacity,which is far from the lithium ion battery.Therefore,the research and improvement of lithium-oxygen batteries have become a subject of great practical significance and research value.The basic principle of a lithium-oxygen battery is that Li+in the discharge process react with O2 to form Li2O2,and Li2O2 decomposes into Li+and O2 in the charging process.Its basic composition is also the cathode,the electrolyte and the anode.Therefore,the performance improvement of the lithium-oxygen battery is also based on these three aspects.Among them,finding a suitable positive electrode catalyst is a very effective and direct method to improve the electrochemical performance of lithium oxygen.The cathode material used in the early days was carbon material.Due to its low density,large voids and large specific surface area,it can provide many active sites for electrochemical reactions and a channel structure for lithium ions and oxygen,therefore good electrochemical performance was obtained.However,there are a small amount of side reactions when carbon materials were used as cathode.During the discharge progress,they chemically react with the components in the electrolyte to generate non-decomposable discharge products such as Li2CO3,resulting in short cycle life of the bettery.Then,in order to improve the electrochemical performance of lithium-oxygen batteries,researchers have explored new and higher performance cathode material catalysts.For example,precious metals,transition metals,transition metal oxides,selenides,sulfides,alloy materials,etc.In this paper,in order to improve the performance of lithium-oxygen batteries,two materials are explored to catalyze electrochemical reactions.One of them is MoSi2 which is a narrow bandgap material.As a narrow bandgap material,the internal electrons are easily excited from the valence band to the conduction band.It own great conductivity and a short lithium ion diffusion route,which can improve the electrochemical activity of the battery.Moreover,the density of the molybdenum silicide material is very low,only 6.31 g cm-3,which is lower than 25%of the density of the superalloy.This can greatly increase the specific capacity of the battery.In addition,the molybdenum silicide material also has excellent physical properties such as oxidation resistance,corrosion resistance,high hardness,and high melting point,which can improve the tolerance of the battery working environment and has better practical significance.The other material is a Co atomic doped NC matrix material.According to the former literature,transition metals and transition metal oxides have been widely used in the process of catalyzing electrochemical reactions.Not only lithium-oxygen batteries,but also lithium-ion batteries,zinc-air batteries,water decomposition to produce hydrogen,etc.However,due to the large particle size of the transition metal nanoparticles,the utilization rate of the active sites is relatively low,which is not conducive to the efficient progress of the electrochemical reaction to a certain extent.The metal atomic doping can greatly improve the utilization of active sites and catalyze the efficient progress of electrochemical reactions.Both materials showed excellent catalytic effect and electrochemical performance in subsequent tests.Single atom Co-N-C materials conducted excellent catalytic performance,among them,the NC material with 1.5 wt%own the best catalytic performance which run for 340 cycles under the condition that the current of 100 mA g-1 with a fixed specific capacity of 500 mAh g-1,and got a good rate performance,which reached a specific capacity of 14075 mAh g-1 under 100 mA g-1 current.A layer of Li2O2 film appeared on the electrode after discharge,which disappeared after recharging,which confirmed the good recyclability of the single atom Co-N-C material.The molybdenum silicide material also has good electrochemical performance as a cathode for lithium-oxygen batteries.At a current of 200 mA g-1 with a fixed specific capacity of 1000 mAh g-1,the molybdenum silicide cathode can cycle for 215 cycles,even at a high current of 500 mA g-1 with a fixed capacity of 1000 mAh g-1,and the cycle number of 90 cycles can still be maintained.Besides,at a current of 200 mA g-1 with a fixed specific capacity of 1000 mAh g-1,the overpotential in first cycle of etched MoSi2 is only 0.45 V,which is far lower than that of other samples.Through subsequent exploration,we have revealed the catalytic nature of the material,and provided other researchers with research ideas and references to the essential reaction of lithium-oxygen battery. |
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