All solid state battery has gained recently much attention because of their potential to store more energy while being safer to operate compared to liquid electrolyte Li-ion batteries. The development of solid electrolytes with superior electrochemical properties is highly desirable. Representatively, sulfide and oxide-based solid electrolytes have been vigorously researched and demonstrated for high-performance all-solid-state batteries. The oxide-based solid electrolytes have superior electrochemical stability compared to the sulfide one, but relatively low ionic conductivity and low processability owing to their mechanically rigid and brittle features which must be taken into consideration to achieve high-performance all-solid-state batteries. In this presentation, different synthesis and processing approaches will be presented to design efficient Li-ion conducting ceramic. Their ionic conductivity measured at room temperature will be discussed as function of their microstructure (porosity, pparticle size, chemical composition,...). First, we study the synthesis, the fabrication of dense ion-conducting ceramic such as (Li 1.3 Al 0.3 Ti 1.7 (PO4) 3 ) LATP and (Li 7 La 3 Zr 2 O 12 ) LLZO and their electrochemical characterization. Starting from “sol-gel” like synthesis coupled with heating at high temperature, we have succeeded to achieve dense ceramic with ionic conductivity of ~10 -4 S/cm for LLZO and for LATP, attesting good quality of the purity and crystallinity. More interestingly, we have also developed an innovative synthesis and processing approach to sinter LATP with suitable properties in one single step, with the aid of microwave heating. Pure and crystalline LATP ceramics are achieved in only one single step starting from amorphous, compacted LATP’s precursors powders. Despite a relative density of 88%, the ionic conductivity is about 3.14 x 10 -4 S.cm -1 at ambient temperature. With the objective of developing hybrid organic-inorganic membrane, we extend our knowledge on the synthesis of 3-D network of crystalline LATP or LLZO fibers by coupling an electrospinning process with the sol-gel synthesis followed by an optimized thermal treatment (atmosphere, temperature and dwelling time). Depending on the processing parameters of electrospinning, different fiber mats organization and density have been achieved. The Li + ion conductivity of these fibers mats was evaluated by impedance spectroscopy at ambient temperature. As an example, the Li ion conductivity for LATP was estimated to be 3 × 10 −4 mS/cm; note that this value is not affected by the water content used in the electrospinning chamber and therefore by the microstructure of the LATP mat. Of course, this value is low compared to that observed on the dense ceramic; this is linked to the porosity of the LATP fiber mat of about 60%. Taking into account the porosity of the fiber mats, the conductivity values are corrected and finally the corrected conductivity value is 1 order of magnitude higher than the one achieved on ceramic with low relative density. This result shows that good particle−particle contact is achieved along the LATP fibers and that most of the LATP fibers are connected to each other to ensure a continuous Li ion pathway throughout the porous ceramics. References: Lancel et al., Langmuir 2017, 33, 37, 9288–9297 Hallopeau et al., Journal of Power Sources 2018, 378:48-52