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Nanostructured Lithium Conducting Materials

Final Report Summary - NANOLICOM (Nanostructured Lithium Conducting Materials)

The main objective of the “Nanolicom” IRSES exchange program was to strengthen collaborations, which had been already initiated through individual and very short projects or workshops, between European research organizations from France, Lithuania and Spain, and an Ukrainian institute. All the laboratories involved in this project work in the domain of Materials Science and more precisely in the domain of Solid State Ionics. They form a complementary group of researchers, chemists, physicists and materials engineers.
In this field of research, many applied perspectives are linked to the storage of energy (electrochemical devices) and require a lot of improvements. For example, the lack of high-quality batteries slows down the development of electric cars. These improvements may come from the technology itself but also from the design of new materials and particularly of nanomaterials, called in this field nanoionics.
In this context, the scientific objectives of this joint program were to prepare nanostructured powders, ceramics and films, of ionic conductors and to investigate the grain size effect and the influence of the crystallinity on ionic transport in these conducting materials.
The relevance of the collaboration between the Ukrainian team and the European research organizations came from their great expertises on chemical synthesis of powders ceramic and nanomaterials, and on physical and chemical characterization of ionic conductors.

During this program, we mainly focused our attention on solid state lithium ion conductors. These materials are of relevant interest particularly for electrochemical devices such as lithium secondary batteries and micro power sources due to the possibility to obtain high specific energy batteries, pH and gas sensors or electrochromic devices. Nowadays, the most commercially available Lithium-ion batteries can be dangerous under some conditions and can pose a safety hazard as they contain flammable liquid or gel electrolytes implying risks of corrosion and/or leakage and limiting their thermal stability range. This context explains the great attraction for all solid state devices. However, to be useful in batteries and electrochemical devices, a solid lithium ion conductor has to fulfill some requirements as a high lithium conductivity at high temperature, a negligible electronic conductivity, a grain boundary resistance as low as possible, a high electrochemical stability window and a chemical stability with both electrodes.
Therefore, the aim of this project was to study the effect of nanostructuration of lithium electrolytes on their properties and then to improve their stability against metallic lithium electrodes using protective layers. The main materials which were considered in this study were already well known to present some interestingly high bulk conductivities (~10-3 S.cm-1 at 25°C) for microstructured samples: the perovskites LixLa2/3-xTiO3, the NASICON Li1+x(M’xM2-x)(PO4)3 (with M = Zr, Ti and M’ = Al, Sc, Y…) and LiPON (Li3PO4 in which some oxygens are substituted by nitrogens).
The first step (Work package 1) of this study was to explore, develop and optimize some synthesis routes for the preparation of nanopowders. Several processes of soft chemistry were considered, as they are well known to favor the formation of small particles. Thus, some perovskite and NASICON powders were prepared using polymerizable complex method (Pechini), precipitation or reverse microemulsion. A careful investigation and many experiments allowed to determine the best synthesis parameters (reagents, solvents, heating treatments…) leading to well crystallized and pure nanoparticles. In this work package, the elaboration of thin and thick films was also undertaken using such techniques as spin-coating from gels, tape casting or RF magnetron sputtering.
The obtained nanopowders were then characterized (Work package 2) by X-ray diffraction, thermal analyzes, SEM, TEM, solid state NMR or Raman spectroscopy. These analyzes showed that pure crystallized powders were obtained. The grain sizes were around 15 nm for perovskite compounds while they were evaluated between 100 and 150 nm for NASICON compounds. The chemical environments of the mobile species in the different crystalline structures were also investigated.
The third step of this program (Work package 3) involved the processing of the obtained nanopowders. In order to prepare dense ceramics while preserving the nanostructuration of the powders, several sintering techniques were used: low temperature sintering, flash sintering, spark plasma sintering, use of sintering aids…
The characterization of these dense ceramics was the object of Work package 4. First of all, the SEM images and XRD diagrams indicated that the growth of grains was not totally limited even with the use of flash sintering techniques. Secondly, the electrical properties of the ceramics were investigated by complex impedance spectroscopy. No change in bulk conductivity was evidenced in comparison with microstructured compounds. On the other side, these measurements allowed to bring out the importance of the grain boundaries, their nature and their volume. Especially, some hopeful results were obtained with NASICON ceramics sintered from powders presenting different crystallinities or with the use of boric acid as sintering aid.
The characterization of the thin and thick films was carried out in Work package 5. The SEM and confocal Raman spectroscopy showed that most of the thin films prepared by spin coating presented some composition inhomogeneities together with cracks or pores. This result prevented us to perfom efficient electrical measurements. However, the thick films prepared by tape casting were well sintered, homogeneous and dense. Concerning the NASICON compound Li1.3(Al0.3Ti1.7)(PO4)3 the complex impedance spectroscopy lead to conductivity values close to those obtained for ceramics.
Finally, the Work package 6 was focused on the study of devices implying LiPON layers. First of all, the LiPON thin films were optimized and characterized. Some devices Pt-LiPON-perovskite LLTiO-LiPON-Pt and Li-LiPON-LLTiO-LiPON-Li were then elaborated and their electrical properties and electrochemical stability were determined. The first results are very interesting since they showed that the LiPON film can act as a protective layer which prevents the LLTiO to be reduced by the metallic lithium electrode while the total conductivity of the device is slightly decreased.

To conclude, in addition to the interesting collaborations between European and Ukrainian research teams which were established or consolidated, this project allowed to study the effect of the nanostructuration on lithium conducting materials. Some promising synthesis and sintering processes were developed. The main factors influencing the total conductivities of ceramics or thick films were evidenced and the multilayer Li-LiPON-LLTiO systems lead to first very interesting results.
These results lead our group of partners to prepare a new project on the application of such materials in new architectures of all solid state lithium batteries.