Elucidating Salt Conversion Mechanisms of Lithiated Transition Metal Oxides for Lithium-Ion Battery Recycling

Exploring new methods for recycling spent Li-ion batteries is mandatory for facing the ongoing growing demand for strategic elements. Here, we propose a method to chemically convert lithiated transition metal oxides into readily water-soluble sulfate products, preventing the use of highly acidic media typically used in hydrometallurgy. The process comprises a temperature-driven solid-state reaction between the electrode material and potassium hydrogenosulfate molten salt yielding langbeinite K2M2(SO4)(3) and potassium/lithium sulfates. The salt conversion of the prototype material LiCoO2 was elucidated, highlighting a complex mechanism. Prior to the melting of the salt, we pointed out an ionic exchange occurring between Li ions and protons from the LiCoO2 and KHSO4 reactants. The accumulation of protons within the layered structure is followed by thermal dehydration, leaving undercoordinated Co ions. Concomitantly, the melting of the salt provides reactive species such as HSO4-, leading to the progressive sulfation of Co ions as revealed by the formation of an intermediate reduced hydroxyl-sulfate phase followed by the final stabilization of K2Co2(SO4)(3). Most remarkably, we highlight the compositional versatility of the langbeinite K2M2(SO4)(3) by extending this approach to more complex cathode chemistry with nickel and manganese (NMC, NCA), which opens novel insights into versatile recycling methods featuring lower atom consumption.