Safety concerns on conventional lithium ion batteries are attracting a lot of attentions from both academia and industry as the electric automobile market is fast growing. Replacing the organic, liquid, flammable electrolytes in conventional lithium ion batteries with inorganic, solid, nonflammable electrolytes can potentially eliminate the fire hazard of the batteries. However, currently the performance of all solid state lithium ion batteries are limited by a number of factors, including the low ionic conductivity and high interfacial resistance. On the other hand, low cost electrochemical energy storage technologies beyond lithium chemistry are also much desired for large scale applications.

                In this work, we report the synthesis and electrochemical testing of several sodium tin sulfides as solid electrolyte for sodium ion batteries, aiming to combine the advantages in safety for solid state batteries and the low cost of the sodium chemistry. Na₄SnS₄ was synthesized by solid state reactions in pure phase. In situ XRD study on the synthesis process were used to reveal the formation mechanism of this compound. The Na ion conductivity of this compound at various temperatures was measured by using electrochemical impedance spectroscopy. The relationship between the conductivity and synthesis condition and starting materials are discussed. Full cells with using this compounds as the electrolyte were also fabricated with various methods and the electrochemical cycling performances of the cells were tested. The mechanical properties of the electrolyte and the full cells were also measured and discussed. Several other sulfides are also successfully synthesized in pure phase. Their conductivities were also measured and compared with Na₄SnS₄. These novel Na ion conductors provides new promise for developing high performance all solid state sodium ion batteries.

Figure caption: Figure 1. X-ray diffraction pattern of Na₄SnS₄ sample