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 LiFeSO4OH 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 SO42− 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 C3v to C2v.