Systematic Studies on the Substitution of Ge4+ by its Homologue Sn4+ in Li1.5Al0.5Ge1.5(PO4)3 Solid Electrolyte

Abstract

Along with many technologies developed to store energy for electric vehicles and portable electronic devices, solid state batteries are getting serious attention due to advantages such as high energy density and solution to the safety issues. A solid-state battery is a battery that has both solid electrodes and solid electrolytes. Replacing the organic liquid electrolyte with a solid state ionic conductor would improve safety and the direct-series-stacking of cells can result in a high energy density for the battery. Until now many oxides-based super-ionic materials such as LISICON, NASICON, Pervovskite and Garnet type have been investigated as electrolyte. However, interestingly most of them include expensive elements and this can be a barrier for further scaling up.

In this thesis, the focus has been positioned in the Li1.5Al0.5Ge1.5(PO4)3 NASICON electrolyte. Main reasons were high ionic conductivity, wide electrochemical window, easy synthesis and good stability with the environment. However, although this electrolyte has many attractive advantages, it contains the expensive cation Ge4+. For this reason, its inexpensive homologue Sn4+ was considered for substitution, considering its larger ionic radii which can open the pathways for the lithium ion mobility in the structure. Moreover, cost analysis indicated that Sn4+ was 59 times cheaper than Ge4+ which can result in an additional important advantage for the tin-bearing ionic conductors. First, a systematic substitution was carried out in the system Li1.5Al0.5Ge1.5-ySny(PO4)3 with y = 0.00, y = 0.25, y = 0.50, y = 0.75 and y = 1.00 by conventional solid state reaction. X-ray diffraction showed successful substitution until level y = 1.00. It was not possible to fully substitute Ge4+ due to impurity problems. Additionally, from the structural information extracted from the X-ray diffraction pattern, it was seen an increase in the lattice parameters for all tin containing NASICON materials. Furthermore, dense pellets were sintered by classical Spark Plasma Sintering (SPS) and annealing steps were carried out in order to remove graphite contamination and recover the white coloration of all the pellets. The NASICON pellets were characterized by Electrochemical Impedance Spectroscopy (EIS) and the analysis of the electrical data showed that tin addition produced slightly lower ionic conductivity at various concentration levels. In order to elucidate the decrease in the ionic conductivity, Raman spectra were recorded. The results showed that the decrease in ionic conductivity was due to the local disorder in the MO6 units group in the NASICON crystal structure which resulted in a distortion of the bottleneck regions reducing the charge carrier mobility. Moreover, DC-polarization was carried out and the results showed that synthesized materials had lower electronic conductivity compared to the pristine material. This is a desirable property for LIB in order to avoid the self-discharge. Finally, as a contribution from this work it was found three new solid electrolytes in the system Li1.5Al0.5Ge1.5-ySny(PO4)3, among them Li1.5Al0.5Ge0.50Sn1.00(PO4)3 has the higher substitution level and ionic conductivity comparable to the well-known Li1.5Al0.5Ge1.5(PO4)3, this expands more the research on inexpensive oxide solid electrolytes feasible for applications.