Thermodynamics of the double sulfates Na2M2+(SO4)2·nH2O (M = Mg, Mn, Co, Ni, Cu, Zn, n = 2 or 4) of the blödite–kröhnkite family†
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Abstract
The double sulfates with the general formula Na2M2+(SO4)2·nH2O (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 (ΔfG°) of formation and evaluated their stability with respect to Na2SO4, simple sulfates MSO4·xH2O, and liquid water, if appropriate. The ΔfG° values (all data in kJ mol−1) are: Na2Ni(SO4)2·4H2O: −3032.4 ± 1.9, Na2Mg(SO4)2·4H2O: −3432.3 ± 1.7, Na2Co(SO4)2·4H2O: −3034.4 ± 1.9, Na2Zn(SO4)2·4H2O: −3132.6 ± 1.9, Na2Mn(SO4)2·2H2O: −2727.3 ± 1.8. The data allow the stability of these phases to be assessed with respect to Na2SO4, MSO4·mH2O and H2O(l). Na2Ni(SO4)2·4H2O is stable with respect to Na2SO4, NiSO4 and H2O(l) by a significant amount of ≈50 kJ mol−1 whereas Na2Mn(SO4)2·2H2O is stable with respect to Na2SO4, MnSO4 and H2O(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, Na2Ni(SO4)2·4H2O is still stable with respect to Na2SO4, NiSO4·4H2O and H2O(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.
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