Regulating Of The Reaction Interface Structure And Surface Stability Of Carbon-based Air Cathode

As the development of society,the employment of fossil energy and electricity has greatly improved the standard of our living.Up to 70%energy demand is provided by fossil energy（oil,natural gas and coal）which is is extremely important to modern society.However,the overuse of fossil energy will bring out some severe problems（greenhouse effect,smog and so on）which will worsen the natural environment.To alleviate these matters,it is urgent to design an alternative energy system.As a promising new generation battery system,lithium oxygen battery（Li-O₂battery）possesses extremely high energy density.However,there are many drawbacks including capacity and efficiency,stability and practicality,prevent the practical application of Li-O₂battery.These problems are closely related to the interface characteristics of carbon cathodes,including the number of three-phase reaction sites,interfacial catalytic activity and surface stability.Considering the above troubles,the detail research contents are carried out from the following aspects:（1）To improve the number of three-phase reaction sites on the cathode surface and enhance the discharge capacity of Li-O₂ battery,we propose a catalytic directional drilling strategy to controllably and efficiently design the pore structure in carbon by introducing metal nano-drills.During the traditional KOH activation process,the by-product K₂CO₃ will genarate on the surface of the carbon material,preventing the further reaction between KOH and carbon.While in our strategy,metal particles anchored on the carbon support can promote the decomposition of K₂CO₃ layer.After that,the carbon is exposed again and react with the molten KOH.In this paper,Ru metal acts as nano drills and CMT is chosen as carbon framework which has internal hollow channels.After drilling process,the carbon wall of CMT is etched to generate through holes,forming RCMT with three-dimensional structure.Compared with CMT,the discharge capacity of RCMT in Li-O₂ battery is increased by 2.5 times and its specific capacity in the supercapacitor is increased by 2.8 times.The catalytic directional drilling strategy is highly controllable.The depth and size of pores can be adjusted by changing reaction time and precursor concentration.Furthermore,the method is suitable for different carbon materials（SP²-and SP²-SP³ hybridized）and various metal（noble metal Ru,Au,Pt and cheap metal Ni）.（2）Considering the unstability of catalyst during cycles,ultra-dispersed ruthenium（Ru）nanoparticles are partially confined by microtube walls of free-standing carbon textiles to prepare a neoteric chimeric air-cathode（Ru-chimera-CMT）.A series of tests such as XPS,UPS and XANES have proved that the charge will transfer from the CMT carrier to the Ru metal,and the larger the contact area between the Ru metal and the carbon material,the more charge transfer.Critically,according to DFT calculations,the chimeric Ru particles can function as active catalytic centers,drastically modulating the intrinsic Li O₂-absorption abilities and thus fundamentally determining the growth processes and morphologies of the involved Li₂O₂.Nanosheets Li₂O₂ species appear on the Ru-chimera-CMT during ORR process which can be decomposed at lower voltage compared with large particles Li₂O₂.Thus,the Ru-chimera-CMT electrode can afford remarkably high energy efficiency of 84.1%and excellent cyclability(260 cycles at300 m A g^-1).（3）A method of inward vapor growth is used to farbricated TiC skin on multi-walled carbon nanotubes（MWNTs）as a stable cathode（TiC/MWNTs）which can suppress the unstablity of carbon cathode and electrolyte during cycles.During the reaction,the carbon wall will be turn into TiC in the way of“layer by layer”from outside to inside.Thus,the TiC layer can be precisely confined at the nanometer level（～3 nm）by regulating reaction conditions.Thus,the TiC/MWNTs with superthin TiC layer possesses the advantages of TiC layer（stability against O₂^-）and MWNTs（low density,high conductivity and so on）.Furthermore,the catalytic performance of TiC/MWNTs is improved by loading the Ru particles on the surface.Finally,TiC/MWNTs-Ru show almost no capacity recession after 90 cycles,nearly three times than that of MWNTs-Ru.This inward vapor growth method is suitable for a variety of non-carbon materials（TiC,Nb C and Si C）and carbon materials（carbon nanotubes,graphene and Ketjen black）,which proves the universality of the preparation method.（4）To deal with the shuttle effect of LiI and unstability of carbon cathode,we design a composite cathode（KB/TiC-Ru O₂）combining TiC surface layer with Ru O₂particles.TiC surface layer is farbricated by inward vapor growth strategy in research（3）.Ru O₂ can adsorb I₂ molecules to the side of the cathode,preventing the reaction of I₃^-（from the reaction of I₂ and I^-）with the Li metal and thus inhibiting the shuttle effect of LiI.However,the I₂ adsorbed on the cathode will react with the carbon cathode,resulting in a decrease in the soluble catalyst LiI and deterioration of cathode.TiC surface layer shows excellent stability to I₂,the content of C-I bonds on KB/TiC cathode is much less than that of KB cathode after many cycles.The stable TiC layer cooperates with Ru O₂ greatly improve the cycle performance of the KB/TiC-Ru O₂ cathode,which can stablely cycle 180 cycles under 3.8 V,twice than that of the KB-Ru O₂ cathode.