Phase Behavior in NaSiCON Electrolytes and Electrodes

The replacement of the presently used liquid electrolytes by a non-flammable solid electrolyte is an important avenue to create safer batteries. The Natrium Superionic CONductor (NaSiCON) Na_1+xZr₂Si_xP_3-xO₁₂ (0 < x < 3) that displays high bulk ionic conductivity and good stability towards other NaSiCON-based electrodes is a good solid electrolyte in NaSiCON-based batteries. Despite the sizeable share of research on Na_1+xZr₂Si_xP_3-xO₁₂, the structural and thermodynamic properties of NaSiCON require better understanding for more efficient synthesis and optimization as a solid electrolyte, which often follows chemical intuition. Here, we analyze the thermodynamic properties of the NaSiCON electrolyte by constructing the Na_1+xZr₂Si_xP_3-xO₁₂ phase diagram, based on density functional theory calculations, a cluster expansion framework, and Monte Carlo simulations. Specifically, we build the phase diagram as a function of temperature and composition (0 < x < 3) for the high-temperature rhombohedral structure, which has been also observed in several positive electrode materials, such as Na₃Ti₂(PO₄)₃, Na₃V₂(PO₄)₃ and Na₃Cr₂(PO₄)₃. Through the phase diagram, we identify the concentration domains providing the highest Na⁺-ion conductivity and previously unreported phase-separation behavior across three different single-phase regions. Further, we note the similarities in the phase behavior between Na_1+xZr₂Si_xP_3-xO₁₂ and other NaSiCON-based mono-transition metal electrodes and discuss the potential competition between thermodynamics and kinetics in experimentally observed phase separation. Our work is an important addition in understanding the thermodynamics of NaSiCON-based materials and in the development of inexpensive Na-ion batteries. From our results we propose that the addition of SiO₄^4– moieties to single-transition metal NaSiCON-phosphate-based electrodes will slow significantly the kinetics toward phase separation.