Synthesis, Structure and Properties of Sodium and Lithium All-Solid-State Electrolytes for All-Solid-State Battery Applications

Abstract

All-solid-state batteries have received a keen interest as emerging attractive alternatives to conventional liquid electrolyte cells, because of their potential in improving battery safety and electrochemical properties. The discovery and development of high-performance solid-state electrolytes -which lie at the heart of the solid-state battery concept- is critical for the realization of all-solid-state battery technology. Among the variety of different solid electrolyte chemistries, sodium and lithium thiophosphates are of great interest for all-solid-state batteries because of their high ion conductivity, ductility and processability.

Herein, the synthesis, structure and properties of new sodium and lithium-ion conductors, namely Na11Sn2PnQ12 with Pn=P and Sb; Q=S and Se, Li4.29P0.71Si0.29S4I and Li3.3P0.85Al0.15S4 are presented. Detailed structure analysis of each solid electrolyte was obtained by a combination of powder neutron diffraction, single crystal and powder X-ray diffraction techniques. The mobile ion dynamics of the synthesized solid electrolytes was studied by electrochemical impedance spectroscopy and direct current polarizability techniques, whereas their thermal stability was investigated by differential scanning calorimetry. Computational chemistry was carried out to obtain the topology of the potential ion-migration pathways in the different crystal structure frameworks.

For solid state synthesis, the heat treatment parameters including, temperature and holding time strongly influence the phase purity, relative density and ionic conductivity of the final product. By optimizing the synthesis conditions of the Na-ion conductor Na11Sn2PS12, its ionic transport properties were successfully enhanced, from 0.5 mS·cm−1 to 1.39 mS·cm−1. The enhanced conductivity was attributed to a reduction of the grain boundary resistance.

The structure analysis of the Sb-analogue, Na11Sn2SbS12, enabled a further understanding the dynamics of ion transport in this structure type. The findings addressed the role of interstitial site occupation in fast-ion conduction for Na11Sn2PnS12 with Pn=Sb and P. Single crystal X-ray diffraction studies of the isostructural Na-ion conductors, Na11Sn2PnS12 with Pn=Sb and P, allowed the identification of the reasons responsible for the difference in their ionic conductivity. The Se version of Na11Sn2SbS12 was studied by isovalent Se-substitution within the Na11Sn2SbS12−xSex with x=1, 6 and 12; where for each composition, the structure-property relations were identified through single crystal and powder X-ray diffraction studies. The homogeneous distribution of Se on all three available lattice sites, 32g Wyckoff positions, indicates no site preference and the absence of a solubility limit for selenium within Na11Sn2SbS12−xSex.

Examination of the series Li4+xP1−xSixS4I with x=0, 0.12, 0.29 and 0.4 led to the discovery of the fast-ion conductor, Li4.29P0.81Si0.29S4I. The aliovalent substitution in Li4+xP1−xSixS4I, (Li+ + Si4+) for P5+, induced a highly defected Li sublattice. This structural change significantly improved the ionic transport properties of the pristine Li4PS4I poor ion conductor by lowering the energy barrier for Li+ diffusion. This work shows a promising strategy towards the design and development of fast-ion conductors by inducing mobile-cation site disorder using elemental substitution; besides, it highlights the role of point defects on ion transport mechanism in crystalline solid electrolytes.

A version of the β-Li3PS4 structure, a high-temperature phase, with a more disordered Li sublattice was stabilized at room temperature by Al substitution in Li3+2xP1−xAlxS4. The solubility of Al in the lattice was determined by the analysis of the series Li3+2xP1−xAlxS4 with x=0.15, 0.2 and 0.33. Single crystal X-ray diffraction revealed that Al substitution led to structural splitting of one of the Li sites, effectively creating a disorder Li+ distribution, which resulted in an increase in the ionic conductivity versus that of the bulk β-Li3PS4 solid electrolyte. The findings obtained in this study shed light on the predicted via first principle calculations, Li11AlP2S12 fast-ion conductor, by elucidating the greater thermodynamic stability of the Li9.9Al0.45P2.55S12 phase and the limited solubility of Al in this thio-LISICON structure. All the data presented in this thesis were obtained during my studies, unless attributed in-text to another researcher.