Densification of Na-ion compounds by conventional and cold sintering

NZSP (NASICON) solid electrolyte has good conductivity for Na-ion solid-state batteries (SSSBs), enabling efficient ion transport at room temperature. NZSP crystal symmetry and stability contribute to its high ionic conductivity (10-3 – 10-4 Scm-1) at room temperature, promoting durability and enhanced battery performance. Na3Zr2Si2PO12 (NZSP) have been widely studied; to date, Na1+xZr2SixP3-xO12 compositions invariably contain m-ZrO2 as a secondary phase. Here, the solid-state method is used to prepare single-phase NZSP by modifying the mole fraction of the ZrO2 reactant, thereby creating Zr and O vacancies. In addition, the effect of m-ZrO2 reactant in suppressing the impurity phase and promoting the ionic conductivity of NZSP was investigated and compared with the literature. X-ray diffraction, scanning electron microscopy, Raman and FTIR spectroscopy, dilatometry studies, and Electrochemical impedance spectroscopy are used to characterise the structural, morphology and electrical properties of NZSP and results compared with ‘stoichiometric’ Na3Zr2Si2PO12 and literature.
In addition, NZSP was densified by the cold sintering method. The density, phase assemblage, morphology and conductivity of cold sintered samples were investigated using a combination of Archimedes/geometric density measurement, XRD, SEM and impedance spectroscopy. Furthermore, the impact of post-annealing on the morphology and conductivity of the cold-sintered NZSP ceramics was studied and compared with conventional sintered NZSP.
Overall, cold sintering had only a limited effect on NZSP densification, with post-annealing required at temperatures > 400 °C to obtain a reasonable relative density (89%) with conductivity lower by 10-fold compared to the conventionally sintered NZSP.
The NASICON-type Na1.3Ti2P3O12 (NTP) has been widely investigated for various applications. In this context, Na1.3Ti1.7Al0.3P3O12 (NATP) was studied as a potential solid electrolyte for Na-ion solid-state batteries. Sintering studies by the conventional method resulted in a ceramic with a relative density of ~ 92.0%. Phase identification study by XRD revealed a rhombohedral structure with an R-3c space group. Morphology and shrinkage studies were analysed by scanning electron microscopy and dilatometer, respectively. Impedance spectroscopy plots resulted in the RT conductivity value of 10 -7 S/cm.
The densification of NATP by cold sintering method was studied, and the effect of different transient solvents (aqueous and organic solvents) on NATP ceramic was investigated. The density, structure, morphology, and impedance response were studied as a function of the transient solvents, post-annealing and pressing pressure. The densification results were compared with the NATP ceramic sintered conventionally and the literature.
Sodium cobaltate (Na0.7CoO2, NCO) is a well-known cathode material in Na-ion batteries. This study explored its properties as a possible solid electrolyte by densifying the NCO ceramic using the conventional sintering method. The relative density studies achieved only ~ 92.0%, and the resulting phase, morphology, magnetic and electrical properties were investigated using XRD, SEM, SQUID magnetometer and Impedance spectroscopy. The conventionally sintered NCO ceramics exhibited a hexagonal structure with a P63/mmc space group. The magnetic studies resulted in a classic paramagnetic behaviour down to 6K with a change in magnetic behaviour to spin-glass. The impedance spectroscopy studies exhibited an ionic conductivity of ~ 10-2 Scm-1 at room temperature with a low Ea of 0.027 ± 0.015 eV.
Densification of NCO by cold sintering technique was investigated, and the effect of different transient solvents on the phase, morphology, magnetic and electrical properties were studied, and the results were compared with conventionally sintered NCO ceramic. The density of the aqueous cold sintered ceramic achieved a ρr of ~ 98.0%, which exceeded that of the conventional sintered (ρr ~ 92.0%) NCO ceramics whilst maintaining high values of conductivity (between ~10-1 to 10-2 Scm-1), with subtle differences observed with conventionally sintered NCO samples. The magnetic studies resulted in a classic paramagnetic behaviour with no significant changes in the susceptibility as a function of temperature. The impedance spectroscopy studies of the aqueous cold sintered NCO exhibited an ionic conductivity of ~ 10-2 Scm-1 at room temperature with an Ea of 0.037 ± 0.017 eV, similar to the conventional sintered NCO. Overall, the transient solvents for the densification of NCO ceramic produce different but complimentary responses.