Structural, Thermal, and Electrochemical Studies of Novel Li₂Co_xMn_1-x(SO₄)₂ Bimetallic Sulfates

A novel high-voltage solid solution of Li₂Co_xMn_1-x(SO₄)₂ (x = 0-1) was synthesized using a simple solid-state reaction. Single-phase monoclinic crystal structure was identified, and there was no evidence for the presence of an orthorhombic polymorph at a sample preparation temperature of 450 °C. Interestingly, the bimetallic sulfates were found to be thermally stable up to 550 °C, and it was validated through various characterization techniques. Beyond 550 °C, the materials start to decompose and form Li₂SO₄ and α-CoSO₄ in the case of Li₂Co(SO₄)₂, while Li₂Mn(SO₄)₂ decomposes to form Li₂Mn₂(SO₄)₃. In addition, an important property called sensitivity toward moisture was also characterized, which plays a critical role in the handling of the sulfate-based cathode during the cell fabrication. Thus, the reaction mechanism involved during the absorption of the atmospheric moisture was also investigated carefully. Finally, for the first time electrochemical activity of both Co and Mn systems was discovered where the average redox potential was found to be 5.02 and 4.85 V versus Li, respectively. For a purely cobalt- or manganese-based system, this is the highest voltage reported and is promising for sulfate-based polyanionic systems.