In recent years, the interest in sodium battery technology has been increasing, mainly due to the high abundance of sodium, which positions the sodium battery as a complementary technology to the current lithium battery.
In this work, we focus on the development of a sodium solid-state battery, in which we study and evaluate the behavior of the closo-borane Na4B12H12B10H10 as a potential candidate for a solid-state electrolyte. The raw materials Na2B10H10 and Na2B12H12 show a monoclinic phase at room temperature, which becomes face-centered cubic (FCC) and body-centered cubic (BCC) phases at 180 °C and 300 °C respectively. Na4B12H12B10H10 is an equimolar mixture of Na2B12H12 and Na2B10H10 and it has ionic conductivity of 0.9 mS/cm-1 at 20 °C and a thermal stability up to 500 °C. The resulting mixture Na4B12H12B10H10 leads to the stabilization of the high temperature cubic conductive phases of Na2B12H12 and Na2B10H10 at room temperature, which results in high conductivity at 20°C.
The design and evaluation of the processing of Na4B12H12B10H10 for battery development must consider the chemistry of the battery cell and the impact of different atmospheres (for instance glovebox, dry room and standard atmosphere) on its electrochemical performance.
In our work, we have evaluated the solubility of Na4B12H12B10H10 and the binder polyvinyl butyral (PVB) in different solvents as potential candidates for the processing of the electrolyte and the binder at room temperature. We have selected alcohols as a suitable class of solvents and we have investigated their processability by varying the type of solvent and its quantity, the time of stirring and the impact of different atmospheres such as argon glovebox, dry room and standard atmosphere. For the calcination process of Na4B12H12B10H10, we have also conducted a systematic study by varying the temperature and time to see the impact on its electrochemical performance. The systematic studies were complemented by electrochemical impedance spectroscopy (EIS) to determine the ionic conductivity of the samples. Scanning Electron Microscopy (SEM) of Na4B12H12B10H10 shows a homogeneous distribution of the particles after the calcination process. The processing of Na4B12H12B10H10 in ethanol followed by the calcination at 180°C for 18 hours with an applied pressure of 500 MPa leads to a conductivity of 1.5 mS/cm-1, 0.9 mS/cm-1 and 0.4 mS/cm-1 at 25°C respectively in inert, dry room and standard atmosphere.
Future studies will be conducted by the use of thermal analysis and spectroscopies to evaluate the thermal stability of the electrolyte and eventually the formation or the absence of its decomposition products.