Thermodynamics of the double sulfates Na 2 M 2+ (SO 4 ) 2 · n H 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 · n H 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 · x H 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 · m H 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.