Potential regulation strategy enables ferrocene as p-type redox mediator for direct regeneration of spent LiFePO4 cathode

Conventional metallurgical technologies for recycling cathode materials from retired Li-ion batteries go against carbon neutrality owing to massive material input and energy consumption. Although featuring with simplified process, direct regeneration technology still fails to bypass high-temperature driving forces for Li⁺ compensation of degraded cathodes. Herein, chemical re-lithiation strategy mediated by ferrocene is proposed to directly regenerate the Li-deficient spent cathodes. Ferrocene and its derivatives, the so-called p-type redox mediators, can be oxidized spontaneously from neutral molecules to stable cations under ambient conditions, allowing them to function as electron donors. Meanwhile, lithium salts serve as Li⁺ donors to ensure charge neutrality of the cathode lattice. The effects of solvation and substituent are thoroughly investigated to precisely regulate the potential of a series of ferrocene-based reductants. Chemical re-lithiation is driven thermodynamically by the intrinsic potential gap between ferrocene and degraded cathodes, thus fundamentally realizing a rapid lithiation reaction (taking less than 20 min at 25 °C), while avoiding the involvement of high-temperature operation. Diverse characterizations have been performed to explore the Li⁺-electron concerted re-lithiation mechanism. The regenerated LiFePO₄ cathode demonstrated comparable Li⁺ storage capability to commercial cathode. Life-cycle analysis verifies the economical and environmental superiority of our chemical re-lithiation strategy to metallurgy in practical industry. The thermodynamically spontaneous chemical re-lithiation provides competitive options for greener recycling of retired batteries in the future.