Possible correlation between enthalpies of formation and redox potentials in LiMSO₄OH (M = Co, Fe, Mn), Li-ion polyanionic battery cathode materials

The thermodynamic stabilities of lithium hydroxysulfates of general formula LiMSO₄OH (M = Co, Fe, Mn) with layered and tavorite structures have been investigated using isothermal acid solution calorimetry. These compounds have been explored as sustainable F-free alternatives to F-based flurosulfate cathode materials. The energetic trends for layered LiMSO₄OH (M = Co, Fe and Mn) samples generally showed a decrease in stability with an increase in ionic radius (Co²⁺ to Mn²⁺), reflecting weaker M-O bonds and increasing structural distortions. The low symmetry tavorite LiFeSO ₄OH with a structure containing corner-shared octahedral chains is less stable than layered LiFeSO₄OH with a more symmetric edge-shared octahedral structure. Structural distortions within the metal octahedra as well as changes in sulfate bonding and symmetry of the SO₄^2- groups appear to control the thermodynamic and electrochemical behavior of LiMSO₄OH (M = Co, Fe and Mn) materials. Both redox potential and thermodynamic stability of layered LiMSO₄OH (M = Co, Fe and Mn) can be correlated to the lowering of the sulfate bonding symmetry in the structure from C_3v to C_2v.