Lithium Aluminum Germanium Phosphate Solid Electrolyte and Interfaces in All Solid-State Cell

Abstract

Li ion batteries are prevalently applied to mobile electrics from mobile phones to electric vehicles. As the performance of these devices has been developed, they need better performing batteries. However, the improvement of battery performance is relatively slow and requires some breakthroughs in the aspect of materials. Solid electrolytes are focused on substituting liquid electrolytes with many fundamental advantages including safety, stabilities, and efficiency. However, present solid electrolytes don`t have enough properties to be commercialized yet and even need more scientific understandings. The objective of this dissertation is to understand the several properties and mechanisms of the solid electrolyte to be developed and applied to the all solid-state cell or solid-state air cell. The ionic conducting properties of the solid electrolyte in bulk and grain boundary were studied and the interfacial phenomena with cathode and anode were studied. Overall introductions to help understand my thesis are carried in chapter I. It contains the background, components and fundamental principles about Li ion batteries. In addition, fundamental limitations of liquid electrolyte in the Li ion batteries are mentioned. Furthermore, the information about solid electrolytes including advantages, related theories, ideal properties and about representative types of solid electrolytes is introduced. Then, the fundamental information about the Li1.5Al0.5Ge1.5(PO4)3 (LAGP) solid electrolyte is introduced. Chapter II deals with the Li ionic conduction mechanism in the bulk LAGP structure by DFT calculation and experimental supports. The mechanism of the conductivity enhancing with Al3+ doping to LiGe2(PO4)3 (LGP) is understood. A new Li site in the LAGP was found and preferred Li pathways are calculated. The reduced activation energy was originated not just from increased carrier concentration but also by newly created diffusion paths. Cooperative motion with at least two Li was proposed to understand mechanism for the activation energy reduction. Chapter III deals with the increased Li ionic specific grain boundary conductivity in the LAGP with excess Li. Both bulk and grain boundary conductivity is significant in the oxide based solid electrolytes. However, the conductivity in the grain boundary is lower than in the bulk and that reduces total Li ionic conductivity. Therefore, increase in grain boundary conductivity can improve total conductivity effectively. Addition of excess Li without keeping charge balance segregates Li ions into the grain boundary and the Li-rich grain boundary reduced Li ionic resistance even with lowering pellet density, which has advantages in ionic conductor in electrode composites. Chapter IV deals with the failure mechanism of LAGP from the direct contact of Li metal as an anode. I demonstrated the relationship between the formation of the interphase between LAGP and Li and the increased interfacial resistance. LAGP forms a black mixed conducting interphase from the Li metal and its components are analyzed as including stoichiometrically changed LAGP, Li2O, Li2O2 and Li2CO3. The chemically formed interphase can be threatening during electrochemical test. The local reduction of interphase with increased volume cracks LAGP continuously. Furthermore, I checked the thermal stability of the interphase. It was ignited and burned at about 200 °C even without external oxygen gas, although LAGP has very high thermal stability over 1000 °C in the air.