Bifunctional Cathode Catalysts For Lithium Oxygen Batteries

The development of electrochemical energy storage devices with high energy density,safety and environmental friendliness is of great significance for promoting the industrial upgrading of new energy vehicles and realizing the goals of carbon peaking and carbon neutrality.Due to the ultra-high theoretical energy density(3500 Wh kg^-1),low cost,and environmental friendliness,lithium oxygen battery is deemed as the most anticipated high-energy-density energy storage system with great technological breakthrough.Nevertheless,the practical application of lithium oxygen battery is still confronted with thorny challenges and difficulties:The multi-step and multi-electron redox chemistry is subject to high energy barrier.Besides,the discharge product Li₂O₂ is intrinsically insulating and insoluble,leading to sluggish reaction kinetics.These issues result in high polarization and serious parasitic reactions,which deteriorate the electrochemical performance of lithium oxygen battery.Therefore,it is essential to exploit bifunctional catalysts to promote the reversible formation and decomposition of Li₂O₂,so as to enhance the ORR and OER kinetics and thus improve the round-trip efficiency and cycling stability.Transition metal-based materials have great potentials as bifunctional catalysts,because of the controllable composition and structure,variable valence of metal elements,low cost,and high stability.This paper concentrates on the rational design of cathode catalysts and structural modulation of Li₂O₂.Transition metal-based catalysts are modified through the strategies of surface interface optimization,defect modulation,heterostructure engineering,and high-entropy effect,which optimizes the catalytic active site from micro-scale.As a consequence,the charge/mass transportation and catalytic process can be dramatically accelerated,facilitating reducing the energy barrier of Li₂O₂ growth and decomposition,which promises lithium oxygen battery with low polarization,high rate capability,and long-term cyclability.This paper comprehensively investigates the structure-activity relationship between cathode catalysts,Li₂O₂ growth mechanism and electrochemical performance.The main research contents are as follows:1.P-Co₃O₄ catalyst enriched with oxygen vacancies are prepared through heterogeneous atom doping.Aiming at the low conductivity and poor OER activity of Co₃O₄,heteroatoms with high electron-donating ability are introduced to trigger the generation of oxygen vacancies,which improve the conductivity,activity,and stability of Co₃O₄ simultanously.Combined with the first-principle calculation and experimental results,the electron structure and band structure of P-Co₃O₄ are optimized with the synergistic effect of doping and vacancy.The band gap of Co₃O₄ is significantly reduced after heteroatom doping,effectively increasing the charge transfer rate of PCo₃O₄.The adsorption energy of LiO₂ is strengthened due to the higher oxygen vacancy concentration of P-Co₃O₄,which changes the growth path of Li₂O₂ and thus induces the formation of large disc-shaped Li₂O₂ composed of nanosheets.The low impedance reaction interface between P-Co₃O₄ and Li₂O₂ significantly enhances the redox kinetics.The strategy of heteroatom doping to trigger oxygen vacancy dramatically improves the comprehensive electrochemical performance of Co₃O₄ catalysts.The P-Co₃O₄ cathode shows a large initial discharge capacity(7690 mAh g^-1),a smaller polarization potential（1.2 V）and a good cyclability（90 cycles）at the current density of 100 mA g1.2.Self-supported ZnCo₂S₄ nanosheet arrays with hollow structures are fabricated by in situ growth and low temperature sulfuration method.On the basis of sulfides catalysts with higher conductivity and activity,structural design is utilized to improve the charge/mass transportataion kinetics.The 3D array structure of ZnCo₂S₄ is conducive to the infiltration of electrolyte and rapid charge/ion transport,and provides sufficient space for Li₂O₂ deposition.The hollow structure of ZnCo₂S₄ nanosheet further increases the density of catalytic active site and charge/mass transportation channel.The self-supported structure minimizes the interfacial contact impedance between catalyst and current collector.Therefore,the kinetics of charge/mass transportation and catalytic transformation process can be enhanced significantly.According to the structural characterization,Co is vital to maintaining the hollow structure during sulfuration process,while Zn promotes the exposure of Co3+ active sites on the surface of ZnCO₂S₄,which enhance the bifunctional catalytic activity and induce the formation of disc-like Li₂O₂ composed of stacked nanosheets.The abundant surface/interface of Li₂O₂ promotes rapid ion/charge transport and oxygen release,which markedly reduces the OER reaction energy barrier.ZnCo₂S₄ exhibits excellent charge-discharge capacity and cycle stability when served as the self-supported cathode of lithium oxygen battery.At the current density of 100 mA g^-1,the specific discharge capacity is up to 9505 mAh g^-1.Besides,it also achieves a decreased overpotential of 1.02 V and a longer cycle life of 90 cycles under a cutoff capacity of 1000 mAh g^-1.3.Based on oil phase co-reduction and solvothermal strategy,PtCoNi@Ni（OH）₂ composite catalyst is prepared by in-situ growth of Ni（OH）₂ membrane on PtCoNi nanowires.Due to the limited bifunctional activity of transition metal compounds,the noble metal elements are introduced to alloy with transition metals to further promote the catalytic properties.The one-dimensional alloy nanowires exhibit excellent catalytic activity and stability due to its inherent anisotropy,higher flexibility and electrical conductivity.Meantime,the large specific surface area of nanowires facilitates the in-situ growth of Ni（OH）₂,enabling abundant redox catalytic sites.Besides,the wrinkled Ni（OH）₂ membrane prevents the PtCoNi nanowires from agglomeration and inactivation,promoting the catalytic activity synergistically.Combined with theoretical calculation and experimental analysis,the electron transfer and redistribution at the interface of PtCoNi@Ni（OH）₂ induce the generation of buildin electric field,facilitating rapid interfacial charge transfer.At the same time,the adsorption energy of LiO₂ can be optimized at interface,which prompts the formation of nano flower-like Li₂O₂ with small size via solution/surface mediated mechanism.The PtCoNi@Ni（OH）₂-based Li-O₂ battery exhibits superior electrochemical performance with excellent rate capability and long cycle stability.At the limited capacity of 1000 mAh g^-1 and current density of 100 mA g^-1,PtCoNi@Ni（OH）₂ delivers a minimal polarization potential of 0.31 V.In addition,the charge potential is lower than 3.5 V even the current density increases to 800 mA g^-1.Therefore,PtCoNi@Ni（OH）₂ can suppress the parasitic reactions at high potential,achieving long cycle life of 140.4.The HEA@Pt（HEA:PtRuFeCoNi）core-satellite heterostructure catalyst is synthesized by two-step oil-phase reduction method.High-entropy alloys,consisting of five or more metal elements,exhibit superior catalytic activity than traditional alloys,which can serve as seeds to grow Pt nano-dendrites.Pt dendrites grow along HEA lattice at a certain angle to form low angle boundary,which provides abundant heterointerface catalytic active sites for redox.Due to the difference of work functions,electrons transfer takes place at the heterointerface from HEA to Pt,engineering the electron distribution and band structure.As a result,the interfacial charge transport can be promoted during charge and discharge process.In addition,the electron distribution modulates the d band center of interfacial metal elements,which regulates the adsorption energy of reaction intermediate and thus alters the growth mechanism of Li₂O₂.The energy barrier of Li₂O₂ decomposition is reduced due to the formation of nanoflower-like Li₂O₂ with low interfacial impedance,which plays a significant role in reducing polarization,inhibiting side reactions,and improving rechargeability and stability.As expected,the HEA@Pt based lithium oxygen battery delivers a high discharge capacity of 8400 mAh g^-1 at 100 mA g^-1,lower overpotential（0.37 V）and long-term cyclability（210 cycles）under a cutoff capacity of 1000 mAh g^-1,superior than those of HEAs and noble metal-based catalysts.