Thermodynamic stability and correlation with synthesis conditions, structure and phase transformations in orthorhombic and monoclinic Li₂M(SO₄)₂ (M = Mn, Fe, Co, Ni) polymorphs

Rational design and development of new Li-polyanion battery materials by exercising synthetic control have led to a new class of Li₂M(SO₄)₂ compounds in monoclinic (M = Mn, Fe, Co) and orthorhombic (M = Fe, Co, Ni) polymorphic forms, using solid state (ceramic) and ball milling methods respectively. The enthalpies of formation from binary sulfates determined using isothermal acid solution calorimetry are positive, and show decrease in energetic metastability with increase in ionic radius for both monoclinic and orthorhombic (except for Ni) polymorphs. The higher symmetry orthorhombic polymorphs with Fe and Co are energetically less stable than the corresponding monoclinic polymorphs. The vibrational/rotational disorder of the SO₄ tetrahedra is identified as the most likely cause of the entropy term (TΔS) of the free energy to overcome the positive enthalpy of formation in monoclinic and orthorhombic phases. Driven by thermodynamic metastability, the orthorhombic Li₂M(SO₄)₂ (M = Fe, Co) polymorphs transform irreversibly into the monoclinic phase on heat treatment. Orthorhombic Li₂Ni(SO₄)₂, formed by a ceramic route is thermodynamically stable and does not transform to the monoclinic phase on heating. The formation of metastable orthorhombic samples by ball milling is consistent with earlier thermodynamic studies on other Li-hydroxy/fluorosulfate systems, for which metastable tavorite polymorphs could be formed only by mild chemical synthetic approaches. This work demonstrates that the entropy term can play a key role for the synthesis, stability and phase transformation among polymorphs of Li-polyanionic compounds.