Sodium (Na) batteries are promising devices for smart grids and electric vehicles due to cost effectiveness arising from the overall abundance of and its even geographical distribution. The energy density of Na-ion batteries is limited by the positive electrode chemistry [1,2]. The Natrium Superionic CONductor (NaSICON) is known for its wide range of electrochemical potentials, its structural and thermal stability and its high conductivity [1-3]. The electrolyte analogue Na1+xZr2SixP3–xO12 (0 ≤ x ≤ 3) is stable toward other NaSiCON electrodes. Despite the share of research on NaSICONs, the structural and thermodynamic properties of these materials require better understanding.I will analyse the thermodynamic properties of the Na1+xZr2SixP3-xO12 electrolyte by constructing its phase diagram, based on DFT and Monte Carlo simulations. I will discuss the phase diagram as a function of temperature and composition for the rhombohedral phase, which is common in Na3Ti2(PO4)3, Na3V2(PO4)3 and Na3Cr2(PO4)3 [4].I will chart the chemical space of NaSICON electrodes NaxMM’(PO4)3 (M, M’= Ti, V, Cr, Mn, Fe, etc.), to analyse their thermodynamic stabilities and the intercalation voltages for Na+ ions in 28 distinct NaSICON compositions. I will discuss anomalies in previously reported experimental data in this diverse and important chemical space [5].[1] Hong, H. Y.-P. Crystal structures and crystal chemistry in the system Na1+xZr2SixP3–xO12. Mater. Res. Bull.1976, 11, 173– 182.[2] Goodenough, J. B.; Hong, H. Y.-P.; Kafalas, J. A. Fast Na+-ion transport in skeleton structures. Mater. Res. Bull.1976, 11, 203– 220.[3] Boilot, J. P.; Collin, G.; Colomban, P. Relation structure-fast ion conduction in the NASICON solid solution. J. Solid State Chem. 1988, 73, 160– 171.[4] Deng, Z.; Gautam, G. S.; Kolli, S. K.; Chotard, J. N., Cheetham, A. K.; Masquelier, C.; Canepa, P. Phase Behavior in Rhombohedral NaSiCON Electrolytes and Electrodes. Chem. Mater. 2020, 32, 18, 7908–7920.[5] Singh, B.; Wang, Z.; Park, S.; Gautam, G. S.; Chotard, J. N.; Croguennec, L.; Carlier, D.; Cheetham, A. K.; Masquelier, C.; Canepa, P. A Chemical Map of NaSiCON Electrode Materials for Sodium-ion Batteries. J. Mater. Chem. A. 2020, 10.1039/D0TA10688G