Preparation Of Co/Mn-Based Oxide Catalysts For Li-O₂ Cells

Li-O₂ cells are receiving intense interest because of their extremely high energy density.However,great challenges still remain to develop Li-O₂ cells with high energy density and long cycle life,such as poor cycle performance,low rate capability,serious polarization,electrolyte evaporation and decomposition,Li anode corrosion and so on.A highly efficient catalyst for oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）is a key factor influencing the performance of lithium-oxygen cells.The main purpose of this work is to develop high-performance catalysts to guide Li2O2 controllable growth and decomposition,thereby to reduce the overpotential,improve the cycling stability and rate capability.Detailed work are as follows:（1）Liquid/solid cocatalysis was used to promote the formation/decomposition of Li2O2.In this part,conformal growth of thin-layered Li2O2 on Co3O4 nanowire arrays（C03O4 NAs）during discharge is realized through the cocatalytic effect of solid/immobile Co3O4 NAs and mobile Pd nanocrystals（Pd NCs）,rendering easy decomposition of Li2O2 during recharge.Meanwhile,high discharge capacity is also ensured with the unique array-type design of the catalytic cathode despite the surface growth mode of Li2O2.The Li-O₂ cells can deliver a high discharge capacity of 5337 mAh g-1 and keep a stable cycling of 258 times at a limited capacity of 500 mAh g-1.The achievement of excellent electrochemical performance is attributed to the highly efficient cocatalytic ability of Co3O4 NAs and Pd NCs as well as the desirable array-type architecture of the catalytic electrode free of carbon and binder.The cocatalytic mechanism of Co3O4 NAs and Pd NCs is clarified by systematic electrochemical tests,microstructural analyses and Zeta potential measurements.（2）A highly efficient CeO₂-decorated two-dimensional（2D）5-MnO₂（CeO₂/δ-Mno2）catalyst was prepared which is composed of graphene-like 5-Mno2 with ultrafine Ceo2 nanocrystals decorated.Li-O₂ cells with the CeO₂/O-MnO₂ catalyst exhibit superior electrochemical performance,including high discharge specific capacity（8260 mA h g-1 at 100 mA g-1）,good rate capability（735 mA h g-1 at 1600 mA g-1）,and excellent cycling stability（296 cycles at a limited capacity of 500 mAh g-1）,which is much better than that with the bare 8-MnO₂ catalyst.The achievement of excellent electrochemical performance is attributed to the highly efficient cocatalytic ability of δ-MnO₂ and CeO₂,the desirable graphene-like architecture of 2D CeO₂/δ-MnO₂ catalyst,as well as the formation of the thin-layered discharge product Li2O2 which is loosely stacked on the surface of CeO₂/5-MnO₂.（3）Pd-modified 5-MnO₂ was prepared and their synergetic catalytic mechanism was investigated.In this part,a hybrid nanostructure composed of 2D δ-MnO₂ nanosheets and Pd NCs was prepared and investigated as a promising catalyst for Li-O₂ cells.Li-O₂ cells with the Pd/8-MnO₂ hybrid catalyst exhibit low polarization（terminal charge/discharge voltages 4.2 V/2.58 V at 1600 mA g-1）,good rate capability（2400 mAh g-1 at a high current density of 1600 mA g-1）,and long cycle life（133 cycles/247 cycles at limited capacities of 1000 mAh g-1/500 mAh g-1）due to the synergetic catalytic effect between δ-MnO₂ and Pd.Density functional theory（DFT）calculations clarify that the charge transfer between δ-MnO₂ and Pd underlies their synergetic catalytic mechanism,where the presence of Pd on the one side of δ-MnO₂ sheets facilitates the formation of Li2O2 on the opposite side of the δ-MnO₂ sheets.The DFT calculations also suggest that it is energetically favorable for the formation of stable,electronically conductive LiO2 on the 5-MnO₂ sheets with Pd on the opposite side.This work gives a mechanistic insight into the synergetic catalytic effect between noble metal and metal oxide,and provides an effective strategy for designing high-performance catalysts especially those with a 2D configuration.In addition,the influence of crystal orientation and shape of Pd NCs on the catalytic performance of Pd/5-MnO₂ was also investigated.For the Pd（200）/5-MnO₂ and Pd-NS/5-MnO₂ catalysts,Li2O2 adopts a conformal growth mode on the Pd-decorated MnO₂ sheets and exhibits a thin-layered morphology.However,Li-O₂ cells with Pd（200）/5-MnO₂（or Pd-NS/5-MnO₂）show rather inferior cycling performance,which indicates that the crystal orientation and shape of Pd NCs are closely correlated with their catalytic activity towards ORR and OER.（4）Quasi-solid electrolyte was prepared and used as the electrolyte for solid Li-O₂ cells.Pure phase Li1.3Al0.3Ti1.7（PO4）3（LATP）solid electrolyte was synthesized at 600℃ through a sol-gel method.The particle size is about 100 nm,but agglomeration of particles occurs.After modification with surfactant（CTAB,P123 or F127）,nanostructured LATP electrolyte was obtained with clear outline.Then,LATP powder was densified by spark plasma sintering（SPS）,hot pressing sintering and pressureless sintering,respectively,and the effects of sintering methods on ionic conductivity were compared.The result shows that pellet density and ion conductivity（2.9×10-4 S cm-1）prepared by SPS are the highest.However,SPS and hot pressing sintering cause carburization.Therefore,it need remove carbon at high temperature,which reduces the pellet density.Pressureless sintering can avoid carburization,but the pellet density is limited.LATP with nano-treatment can enhance its ionic conductivity,because the particles are easier to grow up with each other,then the density is improved.In addition,the effects of sintering temperature and time on ionic conductivity were also investigated.Finally,a quasi-solid-state electrolyte with good flexibility and excellent ionic conductivity was prepared using LATP.The result shows Li-O₂ cell with Pd/δ-MnO₂ catalyst and the solid electrolyte shows good cycling stability.