| Lithium-ion batteries are currently the most widely used energy storage systems,but the practical energy density of lithium-ion batteries is usually limited to 250 Wh·kg-1,and they can’t meet the needs of various electronic devices with increasing power consumption.Furthmore,they are far from meeting the demands of electric vehicles（EVs）of high specific energy.Therefore,developing a new recyclable clean energy storage system with high energy density is imperative.The rechargeable lithium-air battery has received a great deal of attention due to its super-high theoretical energy density(2-3 kWh·kg-1)which is 5-10 times of lithium-ion battery,even comparable to that of gasoline,and it is the most promising power system for use in electric vehicles（EVs）.However,the lithium-air battery is still far from commercialization,and several fundamental issues must be solved before its realistic application,such as:large charge-discharge overpotential,low round-trip efficiencies,and poor cycling life,etc.All these problems are mainly caused by the sluggish catalytic activity and poor stability of the cathode catalyst,therefore developing bi-functional catalysts toward both oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）is the key to solve those problems.As one of the transition metal carbides,TiC possesses high electronic conductivity,good chemical stabilities and comparable OER activity to noble metals,which enable it to be widely applied in electrochemistry.Moreover,it is inexpensive and easier to fabricate as a cathode catalyst.Carbon materials have been widely researched as cathode catalysts for lithium-air batteries due to their low cost,low density,and high electrical conductivity.With these factors in mind,we synthesized carbon-coated core-shell TiC@C nanoparticles in situ by DC arc-discharge plasma method under the methane atmosphere.The characterization including X-ray Diffraction（XRD）and Transmission Electron Microscope（TEM）show that TiC@C nanoparticles which with TiC as the core and graphite layer as the shell are core-shell structures,and the graphite layer coated on the surface has lots of defects.X-ray Photoelectron Spectroscopy（XPS）indicates that the surface of TiC@C consists of 3 elements:Ti,C and O.The presence of O is attributed to the passivation of a small amount of air during the preparation,and the purpose of passivation is to stabilize the highly active nanoparticles so that they can stably exist in the air.The results of linear sweep voltammetry（LSV）imply a 2-electron oxygen reduction process on the TiC@C catalyst in 0.1 M KOH solution,and the OER catalytic activity of TiC@C is significantly higher than that of cubic TiC nanoparticles and commercial Pt/C catalyst.The results of electrochemical test in lithium-air batteries show that TiC@C nanoparticles are bi-functional catalysts toward both ORR and OER.The discharge plateau of the TiC@C electrode was ca.110 mV higher than that of Super P（SP）electrode or TiC electrode at a current density of 100 mA·gSP-1.Moreover,the discharge capacity of the TiC@C electrode was up to 1860 mAh·g-1,which was higher than that of the bare SP electrode(1011 mAh·g-1)and TiC electrode(964 mAh·g-1).For the charge process,the voltage plateau of the TiC@C electrode was ca.340 mV lower than that of the SP electrode and ca.90 mV lower than that of the TiC electrode.The TiC@C nano-catalyst significantly improved the discharge capacity and cycling stability of the lithium-air battery:at the current density of 50,100,and 150 m A·gSP-1,the specific discharge capacity of the TiC@C electrode was much higher than that of the TiC electrode;the cycling life of the Ti C@C electrode was twice that of the TiC electrode at 100 mA·gSP-1 with a fixed capacity of 500 mAh·g-1.The characterization of XRD,Fourier transform infrared（FT-IR）and scanning electron microscopy（SEM）show that the formation and decomposition of Li2O2 have great reversibility under the bi-functional catalysis of TiC@C nanoparticles,which can significantly alleviate the accumulation of undesired by-products,and eventually improve the electrochemical performance of Li-air batteries.Furthermore,it was also found that the electrolyte decomposition and cathode passivation were responsible for the performance fading of Li-O2 batteries.This work demonstrates that sufficiently utilizing the double advantages of carbon materials and catalysts is an effective strategy to optimize the performance of Li-O2 batteries,which may be extended to the design of advanced catalysts for metal-air batteries and potentially other energy applications. |
