リチウム回収を最大化する直接リサイクル：残留リチウムを用いた層状酸化物カソードの再リチウム化

Self-saturating direct recycling techniques, such as hydrothermal relithiation, offer promising pathways for scalable lithium-ion battery recycling due to their lower resource demands and reduced environmental impact compared with conventional recycling methods. However, they often require excess lithium salt additives to drive relithiation. In this study, we present a hydrothermal relithiation strategy driven by residual lithium compounds retained in the cathode black mass after cell disassembly, eliminating the need for external lithium salts during the hydrothermal step. Using spent LiNi_0.33Co_0.33Mn_0.33O₂ (NCM 111) cathodes as a model system, we first identify residual lithium components and demonstrate that an alkaline KOH solution facilitates their reintegration into the bulk structure. This treatment not only restores lithium stoichiometry and crystallinity in degraded cathodes but also simultaneously enables the removal of impurities such as conductive carbon and binder, outperforming other solution compositions for hydrothermal relithiation. Mechanistic studies further validate that Li₂CO₃, LiF, and LiPF₆ each contribute distinct dissolution pathways that drive relithiation within the layered oxide structure. Further structural and electrochemical characterizations confirm the restoration of other cathode materials by using a similar process, including LiCoO₂ (LCO), LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622), and LiNi_0.87Co_0.05Mn_0.08Al_0.03O₂ (NCMA). Removing external lithium salts during hydrothermal relithiation results in an estimated ∼20% reduction in direct recycling costs according to EverBatt analysis, underscoring the sustainability and economic promise of this approach.