Regulating Pseudo-Jahn-Teller Effect and Superstructure in Layered Cathode Materials for Reversible Alkali-Ion Intercalation br

Abstract

The Jahn-Teller effect (JTE) is one of the most important determinators of how much stress layered cathodematerials undergo during charge and discharge; however, manyreports have shown that traces of superstructure exist in pristinelayered materials and irreversible phase transitions occur even aftereliminating the JTE. A careful consideration of the energy ofcationic distortion using a Taylor expansion indicated that second-order JTE (pseudo-JTE) is more widespread than the afore-mentioned JTE because of the various bonding states that occurbetween bonding and antibonding molecular orbitals in transition-metal octahedra. As a model case, a P2-type Mn-rich cathode(Na3/4MnO2) was investigated in detail. MnO6octahedra are wellknown to undergo either elongation or contraction in a specificdirection due to JTE. Here, the substitution of Li for Mn (Na3/4(Li1/4Mn3/4)O2) helped to oxidize Mn3+to Mn4+suppressing JTE;however, the MnO6octahedra remained asymmetric with a clear trace of the superstructure. With various advanced analyses, wedisclose the pseudo-JTE as a general reason for the asymmetric distortions of the MnO6octahedra. These distortions lead to thesignificant electrochemical degradation of Na3/4Li1/4Mn3/4O2. The suppression of the pseudo-JTE modulates phase transitionbehaviors during Na intercalation/deintercalation and thereby improves all of the electrochemical properties. The insight obtainedby coupling a theoretical background for the pseudo-JTE with verified layered cathode material lattice changes implies that manyprevious approaches can be rationalized by regulating pseudo-JTE. This suggests that the pseudo-JTE should be thought moreimportant than the well-known JTE for layered cathode materials