Research On Catalysis Performance Of Cobalt Spinel Oxide With Special Morphology For Nonaqueous Lithium-air Battery

Energy shortage and worsening environment are two issues exreting tremendous pressure on the traditional electric vehicles（EVs）.The widesperead commercialization of EVs is limited by the low specific energies.An effective strategy to solve this problem is replacing Li-ion battery by Li-air battery（LAB）,which can deliver much higher energy density.Catalysts for the oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）play an important role in improving LAB performance,such as lower overpotential,larger capacity and stable cycling performance.Herein,spinel oxides NiCo₂O₄ is choosen as cathode catalyst in Li-O₂ battery（LOB）,aiming to induce the formation of film like Li₂O₂ and promote its decomposition,realizing the low overpotential and cycle stability of LAB.Finally,the influence of CO₂,H₂O and O₂ on the LOB is studied.The conclusions are summarized as follows:（1）Study on ORR performance and mechanism of spinel oxides.Herein,we report hierarchical macroporous/mesoporous NiCo₂O₄ nanosheets synthetized by hydrothermal method for exploring the influence of the surface and structure on ORR mechanism,including specific surface area,oxygen vacancy and pores distribution.The observations clearly indicate that the mesopores in NiCo₂O₄ nanosheets can supply numerous active sites that facilitate the oxygen diffusion and electrolyte impregnation.The macropores formed among loosely interconnected NiCo₂O₄ nanosheets can provide abundant space to accommodate Li₂O₂ and then increase the specific capacity.Meanwhile,the high specific surface area(146.19 m² g^-1)and abundant oxygen vancancy of NiCo₂O₄ nanosheets can enhance the O₂ chemisorption and O₂^?-desorption,improving ORR reaction rate.When investigating as the cathode catalyst in LOBs,the batteries exhibit higher specific capacity(11980 mAh g^-1)and charge capacity(10662 mAh g^-1)at a current density of 200 mA g^-1.Also deliver good cycling stability which can run 76 times at a current density of 400 mA g^-1.（2）Optimization on OER performance of spinel oxides.Studies on NiCo₂O₄ nanosheets in LOBs have revealed its insufficient OER activity.The key factor to improve the OER catalytic activity is to find bifunctional catalysts to promote discharge and charge process.Here,a bifunctional catalyst with RuO₂nanoparticles well dispersed on NiCo₂O₄ nanosheets has been prepared.The excellent electrochemical performance of LOBs is strongly correlated with the nanosheets-like NiCo₂O₄ for easy O₂ diffusion,electrolyte permeation,sufficient space provision for Li₂O₂and RuO₂ nanoparticles that benefit the formation of dispersed film-like Li₂O₂.More importantly,the dispersed Li₂O₂ film can be decpmposed at a lower charging potential through the compact contact with RuO₂@NiCo₂O₄ catalyst.Besides,RuO₂@NiCo₂O₄catalyst that exhibits excellent ORR and OER catalytic activity can further lower overpotentials during charge process.While assembled in LOBs,a large specific discharge capacity as high as 17633 mAh g^-1 can be obtained at a current density of 200 mA g^-1 with a coulombic efficiency of 91%,and the charge overpotential can decrease to 0.6 V.Also,the structure and morphology of Li₂O₂ can affect the charge process of LOBs to a large extent.The LOBs with NiCo₂O₄ nanosheets is found to form large Li₂O₂ clusters,resulting in higher charge overpotential.The active sites distribution on dandelion like NiCo₂O₄ microspheres tends to form amorphous Li₂O₂ films after discharge process.This amorphous Li₂O₂ is easily decomposed at lower overpotential and its intimely contact with NiCo₂O₄ microspheres further accelerates Li₂O₂ decomposition,which enhance the OER activity of NiCo₂O₄ microspheres.Correspondingly,the assembled LOBs deliver a excellent capacity of 12500 mAh g^-1 at the current density of 200 mA g^-1 and a lower charge overpotential of 0.67 V.（3）Analysis of CO₂,H₂O and O₂ on LOBsLOBs are considered as the next-generation power sources for many applications,but the commercialization of LOBs is hindered by a variety of technical hurdles,expecially the influence of air.Herein,with an excellent catalyst NiCo₂O₄ designed as 3D dandelion structure,we investigate the influence of CO₂,H₂O and O₂ on the electrochemical performance of LOBs.It is found that the capacity performance and cycle stability of LOBs can be enhanced through the reaction between CO₂ and O₂^?-,avoiding its attack on electrolyte and carbon.The calculation and experiment data indicate that the battery performance is highly enhanced by NiCo₂O₄ catalyst under 30%CO₂ atmosphere.In this system,a specific discharge capacity as high as 22000 mAh g^-1 and a long-term cycling performance of 140times can be obtained at current density of 400 mA g^-1 without obvious deterioration.With the high specific surface area(72.69 m² g^-1)and abundant oxygen vacancy of dandelion like NiCo₂O₄ microspheres,O₂ will be adsorbed selectively on the surface,forming an atmosphere with higher oxygen concentration and then undergo the ORR process without degration.The results also indicate that even at high humidity of 60%,the batteries can still cycle steadily for 40 times with a little degradation.