New exploration based on crystal chemistry of ion migration and energy storage mechanism in anion-close-packed cathode materials for rechargeable battery
Among the electrode materials of secondary batteries,there exists a class of high-performance electrode materials with anionic close or near-close packed structures.The host cations reside and migrate within the interstitial spaces left by the close stack,thus realizing the conversion between electrical energy and chemical energy.Experimental study on the microscopic mechanism of related processes is difficult because it involves the analysis of atomic scale microstructure.Therefore,it is mainly carried out through the combination of crystallographic theoretical analysis,crystal field theory and first principles of quantum mechanics.Existing theories have been developed from the crystal field analysis of transition metal ligands,but relatively little attention has been paid to the electron interaction in the host ion migration.In this study,according to the previous research findings,Pauling's first rule in classical crystallography was extended on the basis of the accurate description of the interstitial space of oxygen ion(the most representative anion)face-centered cubic close packed(FCC)structure,and the accurate analysis of the interstitial volume by differential geometry analytical calculation.Based on the new understanding,we proposed a new topological deformation model that host cations reside and migrate within interstitial sites,as well as developing a novel electron morphology,and a new relationship between the volume of host cations and the radius of packed anions.To further elucidate the electron morphology characteristics of the host ion,we obtained the electron density equipotential map in LiMn2O4 spinel with a typical FCC structure through first principles calculations.The three-dimensional shape of the electron density equipotential distribution in the tetrahedral interstitial space for the host Li ion was also constructed,which was consistent with the new electron morphological characteristics obtained in the crystallographic analysis.Through the analysis of the electron cloud density distribution of the{110}plane group between two closely packed planes,the"S"migration path of Li ion in LiMn2O4 and its influence on the electron cloud density distribution of the nearby Mn ion coordination polyhedron were clearly revealed for the first time.Based on the characteristics of de-intercalation/intercalation of host ion,we proposed a new idea of energy conversion and storage achieving by that the topological deformation of a host cationic electron cloud is compressed by anionic topological polyhedral ligands.The new theoretical energy density for typical electrode materials with close-packed structures has been calculated,all results were consistent with the traditional theoretical values,which provides a new perspective for comprehending energy storage in secondary batteries from the standpoint of crystallography and quantum mechanics.
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