Publication date: 10th April 2024
All-solid-state sodium batteries are the one of the most potent energy sources due to the high energy density and safety. To achieve the practical development goal, novel active materials with high energy density are necessary. The activation of anion redox are focused on to improve the capacity. However structural change by anion redox heavily affects ionic transfer intra/inter active materials. In intra ionic diffusion, formation of anion dimer deforms host structure itself and ionic diffusion pathway. In addition, the change of the anion packing induces decrease of the particle size, resulting to the contact loss of particles in the positive electrode.
In this study, we analyze relationship between anion redox and ionic diffusion based on Na2FeS2 [1] and Na2CoS2 with reversible anion redox reaction.
Na2FeS2 and Na2CoS2 were synthesized by the heat treatment from the mixture Na2S (98.0 >, Nagao Co.), Fe (99.9%, Kojundo Chemical)/Co(99.9%, Wako) and S (99.99%, Kojundo Chemical). The two active materials showed similar structure with edge shared FeS4/CoS4 tetrahedra. The linked tetrahedra like a chain formed host structure to insert/extract sodium. The electrochemical properties were evaluated by half cell with liquid electrolytes and solid electrolyte. In each density of charge and depth of discharge, the cells were decomposed and evaluated by XRD and XAFS measurements. Na2FeS2 showed 2 e– reaction with Fe2+/3+ redox and S redox. Na2CoS2 also showed 1.3 e– reaction with Co2+/3+ redox and S redox. During the cation redox in Na2FeS2 and Na2CoS2, the XRD patterns showed crystalline phases in maintaining the host structure. On the other hands, their crystallinities were declined by activation of S redox. Especially, Na2FeS2 showed S-S bonding on the XAFS measurements. The disproportionation suggested that the phase separation was locally induced. Na2CoS2 showed hybridization of sulfur dimer and iron. The reaction suppressed the irreversible change in the host structure. In the poster, the relationships between the ionic diffusion and the structural change in the two types of redox reaction was explained.
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