Thermodynamics of the double sulfates NaM(SO)·HO (M = Mg, Mn, Co, Ni, Cu, Zn, = 2 or 4) of the blödite-kröhnkite family.

The double sulfates with the general formula NaM(SO)·HO (M = Mg, Mn, Co, Ni, Cu, Zn, = 2 or 4) are being considered as materials for electrodes in sodium-based batteries or as precursors for such materials. These sulfates belong structurally to the blödite ( = 4) and kröhnkite ( = 2) family and the M cations considered in this work were Mg, Mn, Co, Ni, Cu, Zn. Using a combination of calorimetric methods, we have measured enthalpies of formation and entropies of these phases, calculated their Gibbs free energies (Δ °) of formation and evaluated their stability with respect to NaSO, simple sulfates MSO·HO, and liquid water, if appropriate. The Δ ° values (all data in kJ mol) are: NaNi(SO)·4HO: -3032.4 ± 1.9, NaMg(SO)·4HO: -3432.3 ± 1.7, NaCo(SO)·4HO: -3034.4 ± 1.9, NaZn(SO)·4HO: -3132.6 ± 1.9, NaMn(SO)·2HO: -2727.3 ± 1.8. The data allow the stability of these phases to be assessed with respect to NaSO, MSO·HO and HO(l). NaNi(SO)·4HO is stable with respect to NaSO, NiSO and HO(l) by a significant amount of ≈50 kJ mol whereas NaMn(SO)·2HO is stable with respect to NaSO, MnSO and HO(l) only by ≈25 kJ mol. The values for the other blödite-kröhnkite phases lie in between. When considering the stability with respect to higher hydrates, the stability margin decreases; for example, NaNi(SO)·4HO is still stable with respect to NaSO, NiSO·4HO and HO(l), but only by ≈20 kJ mol. Among the phases studied and chemical reactions considered, the Na-Ni phase is the most stable one, and the Na-Mn, Na-Co, and Na-Cu phases show lower stability.