Solid-state glass-ceramic electrolytes for next-generation rechargeable batteries

Renewable energy sources (RESs) represent a striking energy supply that can help reducing the environmental impact of electricity production. However, some of them (e.g. solar or wind) are intermittent by nature. To overcome this relevant issue, energy storage is an efficient way to maintain a constant electric supply. Batteries used as electrochemical storage represent one of the best solutions. Solid-state-batteries (SSBs) are devices relying on solid electrolytes to store energy. Their integration to systems based on RESs represent an effective way to overcome their intermittence. This solution is progressively replacing the commonly used liquid electrolytes since they suffer from low thermal stability being volatile and may cause serious safety issues. Furthermore, SSBs offer better cycling stability and lifetime at lower processing costs. However, SSBs have some intrinsic drawbacks, which include low ionic conductivity and instability at the interface with the electrode materials. Recently, solid electrolytes with a NASICON) structure have been widely investigated by the scientific community, due to their electrochemical stability and appreciable ionic conductivity. In this context, the aim of the present Master Degree Thesis is to develop and characterize a new NASICON composition, based on the LAGP (Lithium Aluminum Germanium Phosphate) glass ceramic electrolyte material. The addition of silica and magnesium into the LAGP structure was proposed, thus producing the glass composition Li1.5(Al0.3Mg0.1)Ge1.6(P2.9Si0.1)O12 (LAMGPS). The effects of dopants were investigated, which could lead to a better chemical stability and higher ion mobility. More in details, starting from properly selected reactants, samples having two different glass compositions (LAMGPS and LAMGPS10) were produced through a melt-quenching synthesis method. The produced glasses were physico-chemically and thermally characterized by Differential Thermal Analysis (DTA) and Heating Stage Microscopy (HSM) to assess their sinter-crystallization behavior. Bulk samples and pellet (uniaxial pressed powders) samples were successively divitrified and sinter-crystallized, respectively, with proper heat treatments, to produce glass ceramics form the parent glass. X-ray Diffraction (XRD) analysis allowed to study the chemical composition and crystal structure of the samples, thus understanding which phases were present. Scanning Electron Microscope (SEM) was used to understand the morphology of the glass-ceramic samples, their densification and their conductivity. To determine the ionic conductivity of gold-sputtered samples, Electrochemical Impedance Spectroscopy (EIS) was carried out. Through the Ohm’s Law, ionic conductivity reached values in the order of 10-5 S cm-1 at 20°C. Overall, although the proposed solid state electrolyte did not reach outstandingly high ionic conductivity values, its performances are consistent and similar to analogous results in the literature for LAGP-type electrolytes. Summarizing, even if further trials have to be addressed to promote the penetration of SSBs in the electrochemical energy storage market. To our knowledge, the proposed synthetic route is simple and efficient, and the comprehensive characterization could shed light in the understanding of the relationship between the processing, the structure and the properties of glass ceramic solid-state electrolytes, an utmost important and up-to-date area of solid-state battery research.