First-Principles Study Of High Efficient Catalysts In Lithium-Oxygen Batteries

Lithium-oxygen（Li-O₂）batteries have high theoretical energy density,high working voltage and high safety,and present great potential in the field of efficient energy storage and conversion devices.However,there are still some obstacles and challenges in the current Li-O₂ batteries,which severely hindering the development of large-scale commercial applications of Li-O₂ batteries.These problems are mainly caused by the sluggish formation and decomposition kinetics of the discharge product Li₂O₂,which is electrically insulated on the cathode electrode and insoluble in the organic electrolyte.Because the electrode surface is easily deactivated by the film-like Li₂O₂,the actual discharge capacity of Li-O₂ battery is much lower than its theoretical value.In addition,the poor electronic conductivity of Li₂O₂ is not conducive to its complete decomposition during the charge process,and the high charge voltage will lead to deterioration of battery performance and low energy efficiency.In order to improve the charge/discharge kinetics of oxygen evolution reaction（OER）and oxygen reduction reaction（ORR）in Li-O₂ batteries,various catalysts have been developed.Although the electrochemical performance of Li-O₂ batteries has been improved to a certain extent,the reaction efficiency of Li-O₂ batteries is still low,and the internal reaction mechanisms are still unclear.Therefore,in order to improve the charge/discharge reaction kinetics of Li-O₂ batteries,different kinds of efficient solid catalysts and soluble redox mediators（RMs）for Li-O₂ batteries have been investigated by employing first-principles calculations and simulations.The main research contents are as follows:（1）Graphitic carbon nitride supported single transition metal atoms（TM₁/g-C₃N₄）as potential single-atom catalysts（SACs）for Li-O₂ batteries have been studied by using first-principles calculations.The strong metal–support interactions（SMSI）of TM single atom on the unsaturated pyridine nitrogen site of g-C₃N₄ substrate ensures the good thermodynamic and kinetic stabilities of TM₁/g-C₃N₄ based SACs,which helps to improve the cycling durability of cathode materials in Li-O₂ batteries.Among the15 candidate materials,Ru₁/g-C₃N₄ based SACs have good metal-like electronic conductivity and the lowest ORR/OER overpotentials,which can improve the rate performance,increase the discharge capacity and reduce the charge voltage of Li-O₂batteries.Electronic structures analyses demonstrate that the“charge-spin”synergistic catalysis between TM atom and Li₂O₂ promotes the electrochemical decomposition of Li₂O₂.This study provides a theoretical basis for the use of Ru₁/g-C₃N₄ based SACs as efficient cathode catalysts for Li-O₂ batteries.（2）Electromagnetic structures and catalytic properties of nitrogen-doped graphene supported three homonuclear/heteronuclear TM dual atoms as efficient double-atom catalysts（DACs）for Li-O₂ batteries have been investigated by using first-principles calculations.It is found that heteronuclear TM dual atoms based DACs（Co₁Ru₁@N₈）can break the electron distribution symmetry of the catalyst surface,and form a“metal-metal”pairing with a“high spin-low spin”state.The enhanced electronic conductivity and electromagnetic polarization effect can improve the rate performance and catalytic activity of Li-O₂ batteries.Detailed calculations show that the synergistic interaction between Co₁Ru₁ double-atoms plays an important role in regulating the adsorption strength of Li_2-xO₂（x=0,1 and 2）intermediates,which can reduce the ORR/OER overpotentials,and facilite the charge/discharge kinetics of Li-O₂ batteries.The investigations of Li₂O₂ decomposition at the interface of electrolyte|Co₁Ru₁@N₈provide useful informations for studying the OER process of Li-O₂ batteries under the near-real experimental conditions.（3）Based on the first-principles calculations,the redox potential and OER catalytic activity of iron phthalocyanine（Fe Pc）based metal-organic RM in Li-O₂batteries have been regulated by using the functional group modification strategies.It is found that the electron donating groups modified Fe Pc can increase the highest occupied molecular orbitals energy level,reduce its oxidation potential,and reduce the charge overpotential of Li-O₂ batteries.Among the different functional groups,the oxygen anion radical（-O^-）can regulate the redox potential of Fe Pc obviously.Comparing with the pristine Fe Pc,Fe Pc modified with-O^-radical（Fe Pc-O^-）can break the electron distribution symmetry,enhance the chemisorption of O₂ and weaken the adsorption of Li₂O₂,which is conducive to the reversible formation and decomposition of Li₂O₂.Ab initio molecular dynamics simulations present that the decomposition time of Li₂O₂ under the solvent effect of dimethyl sulfoxide（DMSO）is shortened,the Li₂O₂decomposition kinetics is accelerated,and the whole decomposition of Li₂O₂ is realized in the electrolyte containing Fe Pc-O^-.Therefore,Fe Pc-O^-can promote the decomposition kinetics of Li₂O₂ in the electrolyte of Li-O₂ batteries.（4）Keggin type polyoxometalate supported TM single atom（TM₁/POM）as ORR/OER bifunctional high efficient inorganic RM for Li-O₂ batteries has been studied by using first-principles calculations.It is found that TM₁/POM with“electron sponge”property can maintain good thermodynamic stability in the oxidized/reduced states during the charge/discharge processes of Li-O₂ batteries.As the active site of“adsorption-catalysis”,TM single atom can effectively reduce the thermodynamic free energy barriers of ORR/OER in Li-O₂ battery.The electronic structures analyses show that Co as an“electron transfer bridge”can not only effectively improve the electronic conductivity of POM,but also regulate the bonding/antibonding occupation of[Co₁/POM-Li₂O₂]^ox near the Fermi level of electronic density of states,so as to regulate the adsorption strength of Li₂O₂ on[TM₁/POM]^ox.Therefore,Co₁/POM shows good catalytic performance for Li₂O₂ decomposition.Comparing with the vacuum condition,the solvent effect of DMSO can enhance the charge transfer between Li₂O₂ and[Co₁/POM]^ox,and improve the OER catalytic performance of Co₁/POM based inorganic RM in Li-O₂ batteries.