Thermodynamics of the double sulfates Na(2)M(2+)(SO(4))(2)·nH(2)O (M = Mg, Mn, Co, Ni, Cu, Zn, n = 2 or 4) of the blödite–kröhnkite family

The double sulfates with the general formula Na(2)M(2+)(SO(4))(2)·nH(2)O (M = Mg, Mn, Co, Ni, Cu, Zn, n = 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 (n = 4) and kröhnkite (n = 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(f)G°) of formation and evaluated their stability with respect to Na(2)SO(4), simple sulfates MSO(4)·xH(2)O, and liquid water, if appropriate. The Δ(f)G° values (all data in kJ mol(−1)) are: Na(2)Ni(SO(4))(2)·4H(2)O: −3032.4 ± 1.9, Na(2)Mg(SO(4))(2)·4H(2)O: −3432.3 ± 1.7, Na(2)Co(SO(4))(2)·4H(2)O: −3034.4 ± 1.9, Na(2)Zn(SO(4))(2)·4H(2)O: −3132.6 ± 1.9, Na(2)Mn(SO(4))(2)·2H(2)O: −2727.3 ± 1.8. The data allow the stability of these phases to be assessed with respect to Na(2)SO(4), MSO(4)·mH(2)O and H(2)O(l). Na(2)Ni(SO(4))(2)·4H(2)O is stable with respect to Na(2)SO(4), NiSO(4) and H(2)O(l) by a significant amount of ≈50 kJ mol(−1) whereas Na(2)Mn(SO(4))(2)·2H(2)O is stable with respect to Na(2)SO(4), MnSO(4) and H(2)O(l) only by ≈25 kJ mol(−1). 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, Na(2)Ni(SO(4))(2)·4H(2)O is still stable with respect to Na(2)SO(4), NiSO(4)·4H(2)O and H(2)O(l), but only by ≈20 kJ mol(−1). 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.