Thimothée Fabre PH'D Defense

All-solid-state Li-ion battery technology is attractive because of its promise of higher energy density and greater safety than conventional Li-ion batteries. Among the solutions studied, systems based on ceramic oxides are faced with manufacturing problems due to the long sintering at high temperature required to form the interfaces and densify the different layers making up the battery, with the formation of interphases that are detrimental to charge transport. To limit interfacial reactivity, one solution could be to use ultra-fast sintering such as flash sintering (FS), a process that densifies ceramics in a few seconds. However, using FS with purely cationic conductors remains achallenge. To address this, the concept ofelectrochemical FS was recently demonstrated with a green pellet of Li1.4Al0.4Ti1.6(PO4)3 (LATP) inserted between two pellets of a mixed electronic/Li+ conductor, LiCoO2, as a reversible electrode4. With such a stack, a flash occurs thanks to reversible intercalation reactions, and this system is equivalent to the architecture of an all-solid-state battery. The aim of this thesis is therefore to obtain a battery system in just a few seconds thanks to electrochemical FS using materials with good chemical and electrochemical compatibility at temperature: phosphate-based phases with LATP as the electrolyte and Li3V2(PO4)3 (LVP) as the active material, the latter having the advantage of being able to be used as both a positive and negative electrode. The first step was to master the conventional sintering of the two materials separately, before combining them in composite electrodes (LVP+LATP) and, finally, in a complete system. One of the critical parameters turned out to be the atmosphere: LATP tends to be sensitive to reducing conditions, and conversely, LVP is sensitive to oxidising conditions. For the FS, we were confronted in all cases with a problem inherent to the process: the location of the current flow leading to ‘hot spots’ and heterogeneous densification7. To better understand its origin, a systematic analysis of the microstructure was carried out using SEM imaging and X-ray microtomography, combined with a numerical simulation of the phenomenon. In addition, a new current/voltage generator capable of operating at up to 1 MHz was developed, along with a heating cell to limit the inductive effects obtained at high frequencies. Preliminary results show the formation of a high-frequency flash on cationic conductive materials with blocking platinum electrodes. Above 100 kHz, excitation activates
ion transport processes in the material at high frequency, while limiting electrode blocking. These results open up a wide range of possibilities for the development of this technique. Finally, multilayer composite/LATP/composite systems were densified by electrochemical FS, demonstrating the concept with another combination of materials. SEM cross-sections confirmed good densification of the system and EDX mapping showed structural stability with little interdiffusion at the interfaces.