From Additivity to Phase-Boundary Control in Lithium-Ion Battery Recycling: Implications for Selective Metal Recovery

Introduction:
Hydrometallurgical recycling of lithium-ion batteries requires moving from additive descriptions to phase-boundary control of multicomponent aqueous systems in which metal ions, ligands, and competing species interact under tightly coupled conditions. Process design often relies on additive assumptions, although deviations from additivity frequently determine metal selectivity, recovery efficiency, and reagent use. A predictive description of these effects is essential for reliable and scalable recycling.

Methods:
A thermodynamic framework was developed by integrating aqueous speciation with heterogeneous phase equilibria relevant to leaching and separation processes. A reference additive system was constructed by suppressing multicomponent interactions under identical physicochemical conditions, enabling separation of cooperative contributions from pH dependent and binary effects. Calculations were performed across operational pH ranges and ionic strengths representative of industrial recycling streams.

Results:
The analysis shows that the transition from additive behavior to phase-boundary control governs precipitation, dissolution, and complexation processes in multicomponent recycling systems. These effects define narrow operational domains where small variations in composition lead to pronounced changes in metal recovery and selectivity. Additive models fail to capture these regimes, particularly near critical conditions relevant for separation of Co, Ni, and Mn. In contrast, the proposed framework identifies stability domains and enables reduction of co precipitation and improved selectivity under practically relevant conditions.

Conclusions:
The proposed framework provides a predictive and operationally relevant tool for hydrometallurgical recycling of lithium-ion batteries. By identifying robust operating windows, it enhances selectivity, reduces uncertainty, and supports efficient resource recovery. This approach establishes phase-boundary control as a practical design principle for transforming multicomponent complexity into predictable and optimized recycling performance.