Constructing Multiple Phase Reaction Interfaces In Li-O₂/Li-CO₂ Batteries And Their Action Mechanisms

Li-O₂battery offers an attractive and potential strategy for developing energy conversion and storage devices with high energy density.However,the high overpotential of oxygen evolution reaction（OER）caused by the slugglish decomposition of discharge product Li₂O₂seriously impedes its electrochemical performances.Li-CO₂battery is a new type of energy conversion and storage device by the utilization of greenhouse gas CO₂.Similar to Li-O₂battery,the products Li₂CO₃and C are difficult to be electrochemically decomposed,resulting in poor reversibility which seriously affects the development of Li-CO₂battery.The formation and decomposition of products in the two batteries are complex multiple phase reactions,during which the characteristics of the multiple phase interfaces formed between products and catalysts/electrolyte as well as those of Li₂CO₃/C have very important effects on their decomposition behavior.Therefore,it is of great significance to investigate the products growth mechanism of Li-O₂/Li-CO₂batteries and the interfacial action between products and catalysts/electrolyte,as well as their influence on products decomposition.This paper aims at constructing more reasonable multiple phase reaction interfaces to enhance the formation and decomposition of products in Li-O₂/Li-CO₂batteries by designing cathode materials and using suitable solution phase catalyst（RM）,respectively.The products distribution,morphology changes and the related interfacial actions were studied with the help of scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,density functional theory（DFT）calculation,ab initio molecular dynamics（AIMD）simulation and other research methods.The specific research works are as follows:（1）N,O co-doped carbon sphere shells with folded surface for Li-O₂battery.O doped and N,O co-doped carbon nanoparticles with similar morphology were prepared by hydrothermal combined with thermal treatments.It was found that N and O co-doped carbon can improve the performances of Li-O₂battery.Therefore,a spherical shell-like porous carbon material with connected pores was prepared through direct pyrolysis by using 5’-Guanylic acid disodium salt（5’-GMP,2Na）containing N and O.After being further activated in air,the carbon shell with a finer folded porous surface was produced along with enhanced specific surface area and greatly increased surface N and O contents.By using this carbon material as a cathode catalyst,the morphology of discharge products changed from toroidal to film due to the finer folded porous surface and higher surface N and O contents.The film-like products can form an increased interfacial contact and interaction with the active sites on the carbon surface,which is conducive to the transfer of electrons during the decomposition of Li₂O₂,thus improving the electrochemical performances of battery.（2）Tuning the nucleation and decomposition of Li₂O₂by F-doped carbon vesicles to improve the performances of Li-O₂battery.N,O,S,F co-doped porous vesicle carbon was prepared by a self-template pyrolysis method and applied to Li-O₂battery.It was found that the introduction of appropriate content of F in the carbon matrix containing N,O,S can regulate the morphology of Li₂O₂from rod to film and enhance its contact with the carbon cathode,which is favorable for its decomposition during charge.The optimized F-doped carbon vesicle cathode exhibited low overpotential,large capacity,and good cycle stability（187 cycles）.DFT results showed that the F and C atoms in the C-F bond have strong interactions with the Li and O in Li₂O₂,respectively,which can enhance the transfer of electrons from Li₂O₂to the carbon matrix through the Li₂O₂/C-F interface directly,and generate hole polarons on the surface of Li₂O₂,thereby accelerating the delithiation and decomposition of Li₂O₂.（3）Using CTAB as a bifunctional solution phase catalyst in Li-CO₂battery.Cetyl trimethyl ammonium bromide（CTAB）was introduced into Li-CO₂battery as a bifunctional solution phase catalyst（RM）to optimize both the discharge and charge processes.The results showed that the addition of CTAB significantly improved the electrochemical performances,reduced the discharge and charge overpotentials,especially,the discharge capacity was greatly improved for 10 times.During discharge process,CTA⁺can induce the vertical accumulation of large granular products on the cathode,during charge,Br^-anion can act as a redox mediator（Br^-/Br₂）in solution phase.The outstretched products morphology makes Br₂has more contact sites with the products,which improves the reaction ability of the products interface,promoting the efficient decomposition of discharge products.AIMD simulation results showed that CTA⁺with smaller desolvation energy can form a more stable contact ion pair（CIP）with CO₂^-radical,thus reducing the energy barrier of CO₂reduction reaction and also favorable for the solution phase growth of discharge products,which played a key role to change products morphology.（4）Regulating the nucleation of Li₂CO₃and C by anchoring Li-containing carbonaceous species to improve the performances of Li-CO₂battery.A free-standing cathode constructed by growing MnOOH nanosheet arrays on stainless steel mesh was prepared and used in Li-CO₂battery.Significantly improved electrochemical performances（a stable operation of 138 cycles）were achieved by adjusting the morphology of discharge products and improving the interface contact area between Li₂CO₃and C.Experiments combined with AIMD simulation showed that during discharge process,the spontaneous formation of Li-containing carbonaceous species（Li CO₂,Li CO,Li₂CO₃）in the electrolyte and the bonding between Li atom with the O atom of MnOOH regulated the nucleation behavior of Li₂CO₃and C,resulting in the uniform growth of products on MnOOH.More importantly,fine Li₂CO₃product particles（with the size of about 5 nm）were embedded evenly in C product matrix,which greatly increased their interfacial contact,enhanced the transfer ability of electrons between these two products,thus promoting the synchronous decomposition of them during charge process.