Theoretical Study On Crystal Structure Prediction And Properties Of Lithium Oxide Li₃O₂

Lithium-air batteries have attracted tremendous research attentions because of their superhigh theoretical energy density.However,the current technology level of lithium-air batteries is still far from practical,and facing enormous scientific and engineering challenges.First,the charging voltage on the carbon electrode is too high lead to energy efficiency is very low,the discharge voltage of 2.5 3.0 V,while the charging voltage is as high as 4.0 4.5 V,so the charging voltage was significantly higher than the discharge voltage,and the energy efficiency is only 65%.Second,the current lithium-air battery has a poor reversibility,and the discharge product（which is generally considered to be Li₂O₂）can not be dissolved in organic electrolyte after several cycles,resulting in deposition in the porous carbon channels,blocking the channel to prevent the entry of O₂ makes the capacity completely lost.Furthermore,the power density is very low,only 0.1 1 mA/cm2,at least one order of magnitude lower than the power required for practical applications.In addition,there are side reactions of the effects of H₂O and CO₂ in atmosphere,the protection of lithium metal anode and the dendrite of lithium,and too high over-voltage may lead to electrolyte decomposition.These unresolved engineering and basic scientific problems have seriously hampered the commercial use of lithium air batteries.A key scientific problem in the basic research of lithium-air batteries is the determination of the types of cathodic discharge products of lithium-air batteries.In recent years,a series of studies have shown that the cathode discharge products of lithium-air batteries may have other Li-O compounds other than the commonly accepted Li₂O₂,Li₂O,and LiO₂.Recently,some researchers have predicted a new possible cathode discharge product for lithium-air batteries by theoretical means based on the first principle: Li₃O₄,the combination of Li and O atoms is very different from the atomic bonding scheme and the bonding modes of O atoms in Li₂O₂,Li₂O,LiO₂ compounds which have been reported.In addition,from the valence of the O atom,the O atom in Li₃O₄ is-3/4 valence,which between Li₂O（O atom is-2 valence）and LiO₂（O atom is-1/2 valence）.Although the crystal structure of Li₃O₄ has not been obtained at present,the proposal of Li₃O₄ compound provides a new idea for the study of the composition and species of cathode discharge products in lithium-air batteries.The purpose of our study is to investigate the crystal structure of a lithium oxygen compound Li₃O₂,which has not been obtained from experimental crystal structure analysis and theoretical research,by means of quantum chemical calculation based on the first principle.Based on the theoretical study of the crystal structure of this lithium-oxygen compound,on the one hand,we have successfully determined the combination styples of O atom and Li atom in this type of lithium-oxygen compound;on the other hand,based on the theoretical study of the crystal structure of the Li₃O₂ compound obtained from the theoretical prediction,compared with the reported electron emission spectra of cathode discharge products in lithium air batteries,we propose that Li₃O₂ is likely to be a new unreported discharge product for lithium air batteries.The main contents and conclusions of our study include the following two parts:1)Theoretical prediction of crystal structure of Li₃O₂ compound.Although the researchers had found that Li₃O₂ compound were presence in the residue of anhydrous lithium hydroxide decomposition more than 40 years ago,which may belong to orthorhombic system,and the lattice parameters of the compound are about a = 10.84 ± 0.04 ?,b = 12.84 ± 0.05 ?,c = 10.36 ± 0.04 ?.But so far,the exact crystal structure of Li₃O₂ compound has not yet been fully determined.The combination of Li and O atoms in Li₃O₂ needs to be further explored.Based on this,the first part of our study used the CALYPSO software package based on particle swarm algorithm developed by Professor Ma Yanming and density functional theory is applied to the theoretical exploration of Li₃O₂ crystal structure.The possible crystal structures of Li₃O₂ compounds in more than 100 different space groups were simulated by conditional model size and simulation parameters.A candidate crystal structure with highly matched lattice parameters was preliminarily screened by comparing with the XRD spectra determined by early experiments.The lattice parameters of the calculated structure is very close to the experimental value（a = 10.97 ?,b = 12.47 ?,c = 10.30 ?）,and the crystal does belong to the orthorhombic system（space point group 35）.The XRD spectra of theoretical prediction of Li₃O₂ crystal structure was highly consistent with the XRD patterns of the compounds determined by the early experiments,which indicating that the theoretical prediction of the crystal structure can better explain the early experimental results.2)Study on the structure and properties of Li₃O₂ compounds.Based on the theoretical calculation of the possible crystal structure of Li₃O₂,we further explored the unique physicochemical properties of Li₃O₂ compounds.It was found that the internal oxygen atoms in the compound had two forms: independent oxygen atoms and oxygen molecules.The valence of the O atom is 3/2,between Li₂O₂（O atom is-1 valence）and Li₂O（O atom is-2 valence）.So Li₃O₂ can be seen as a mixture of lithium oxide and lithium peroxide,and is clearly distinguished from the existence of experimental and theoretical reports of Li-O compounds.Based on the theoretical prediction of the crystal structure of Li₃O₂,we further simulates the K-edge spectrum of the O atom of the compound.The strong peaks at 535 eV in the theoretical simulation spectra are highly consistent with the measured spectra of the discharged products,and also provide evidence for confirming that Li₃O₂ is a possible discharge product for lithium-air batteries.