Universal and efficient extraction of lithium for lithium-ion battery recycling using mechanochemistry

The study addresses the urgent need for efficient, profitable, and environmentally benign recycling technologies for lithium-ion batteries (LIBs) as their production rapidly scales for electric vehicles and stationary storage. Existing industrial approaches—pyrometallurgy (high temperature, high energy, emissions), hydrometallurgy (complex multi-step chemistry, corrosive reagents, co-extraction losses notably of Li), and biohydrometallurgy (promising but nascent)—each have significant drawbacks. Mechanochemistry (MC) offers a solvent-free, low-energy pathway to induce solid-state reactions by mechanical force. Building on prior work showing mechanochemical reduction of LiCoO2 with Al, the research question is whether a universal, acid-free, low-temperature MC process using Al (the cathode current collector) as a reducing agent can efficiently recover lithium as high-purity Li2CO3 across major cathode chemistries (LCO, NMC, LMO, LFP) and their mixtures, while simplifying processing and reducing environmental impact.

- Pyrometallurgy: Operates above 1000 °C to form metal alloys and lithium-rich slags; advantages include minimal pre-treatment but high capital/energy costs and pollution; Li recovery from slag is energy-intensive. - Hydrometallurgy: Leaching with acids/bases/salts followed by purification (precipitation, solvent extraction, ion exchange) and recovery (precipitation/crystallization/electrowinning). Advantages are milder conditions and high recoveries, but processes are complex, chemistry-dependent, and generate significant waste. Co-extraction leads to Li losses (>20%) with Ni/Co/Mn in practice. - Biohydrometallurgy: Uses microbes or bioacids for leaching; lower cost and greener, but currently limited by efficiency, scalability, and separations. - Mechanochemistry in recycling: Recognized for low cost, scalability, unique kinetics/thermodynamics, and solvent-free operation. Prior MC work enhanced subsequent hydrometallurgy or directly converted LCO to Co metal and Li-derivatives via reduction. This study extends MC to a universal Li recovery across multiple cathodes, aiming to eliminate corrosive leachants and reduce steps/energy.

Two acid-free mechanochemical recycling processes were developed to recover lithium as Li2CO3 using Al as a reductant. Materials: Commercial powders of LiCoO2 (LCO), Li(Ni0.33Mn0.33Co0.33)O2 (NMC), LiMn2O4 (LMO), and LiFePO4 (LFP, carbon-coated) and aluminum foil (cut to 1–2 cm pieces). Used as received. Reactive milling: ~2 g of cathode/Al mixtures in molar ratios tailored to redox stoichiometry were ball-milled 0.5–3 h in air in a 65 mL hardened steel vial with 20 g steel balls using a SPEX 8000 shaker mill. Ratios: LCO:Al = 1:1; NMC analogous to LCO; LMO:Al = 1:2.33; LFP:Al = 1:3. Mechanochemical reduction forms a magnetic metallic d-element composite, Li2O/Al2O3, and transient LiAlO2. Representative reactions: - LCO/NMC: 2LiMO2 + 2Al → 2M(metal composite) + {Li2O + Al2O3}; Li2O + Al2O3 → 2LiAlO2. - LMO: 2LiMn2O4 + 4.66Al → 4Mn + {Li2O + 2.33Al2O3}. - LFP (air): 2LiFePO4 + 6Al + 0.25O2 → Li2O + Fe2P + AlP + 2.5Al2O3. Process 1 workflow: Reactive milling → room-temperature aqueous leaching (DI water, stir minutes) → filtration → evaporative crystallization at 70 °C → purification by heating at 350 °C for 3 h in air to decompose Li2Al4(CO3)(OH)12·3H2O (LACHH) to Li2CO3 + Al2O3 → water dissolution and filtration to separate water-soluble Li2CO3 from insoluble Al2O3 → recrystallize Li2CO3 by evaporation. Observations: Li2CO3 and LACHH typically form in soluble fraction; Al(OH)3 may appear (LMO, LFP). Metallic composite is magnetically separable. Process 2 workflow: Reactive milling → carbonatization (mix with DI water, heat to 90 °C for 1 h while exposed to ambient CO2; dry at 70 °C overnight) to convert amorphous LiAlO2 into LACHH and Li2CO3 via 4LiAlO2 + 9H2O + 2CO2 → Li2Al4(CO3)(OH)12·3H2O + Li2CO3 → heat to decompose LACHH (to Al2O3 + Li2CO3) → room-temperature aqueous leaching, filtration, and recrystallization to obtain pure Li2CO3. Process 2 reduces steps and improves Li recovery by rendering previously insoluble Li phases water-soluble as carbonate. Characterization: XRD (STOE Stadi P, Cu Kα, 10–70° 2θ, step 0.015°) for phase ID; SEM for microstructure; ICP-OES for Li2CO3 purity and elemental impurities (Li, K, Al, Co, Ni, Mn, Fe, P). Lithium yield calculated from final Li2CO3 mass vs theoretical Li content of starting mixtures (no gas loss assumed during milling).

- Universality: Both processes work across LCO, NMC, LMO, LFP, and their mixtures. Al acts as an in situ reductant (also ubiquitous as current collector), forming magnetic metallic composites (Co/Ni/Mn/Fe-based), Al2O3, and lithium compounds convertible to Li2CO3. - Lithium recovery yields: - Process 1: Low yields due to insoluble Li phases (LiAlO2), with overall range 29.8–39.6% depending on cathode chemistry. - Process 2: Introducing carbonatization roughly doubles recovery, yielding approximately 55.6–75.9% across LCO/NMC/LMO; mixture achieved ~75.2%. (LFP Process 2 yield not reported due to complex phosphate phases.) - Product purity (ICP-OES): Li2CO3 purity >99 wt% for LCO, NMC, LMO, and mixed-cathode systems (e.g., Mix-Pr2 ~99.72%). LFP systems showed lower purities due to P and Al impurities (e.g., LFP-Pr1 ~93.10%; LFP-Pr2 ~71.81%), attributed to Li3PO4 carryover and X-ray amorphous Al2O3 passing filtration; additional dissolution/filtration can improve purity. - Process simplification: Process 2 reduces the number of unit operations versus Process 1 while increasing yield, improving economic viability. - Phase evolution: Transient γ-LiAlO2 forms early during milling and becomes XRD-amorphous with time; carbonatization converts residual LiAlO2 to LACHH and ultimately Li2CO3. Metallic composites are readily magnetically separable and remain as solids through leaching. - Environmental advantages: Acid-free, ambient-pressure processing at relatively low temperatures (≤350 °C in P1; 90 °C step in P2), avoiding corrosive leachants and high-temperature smelting.

The work demonstrates that mechanochemical reduction using Al can universally convert major LIB cathode chemistries (and their mixtures) into a product suite from which lithium can be efficiently recovered as Li2CO3 without corrosive leachates or high-temperature smelting. The primary bottleneck in Process 1 was lithium retained in poorly soluble LiAlO2 (often XRD-amorphous). Incorporating a carbonatization step (Process 2) transforms this lithium into LACHH and Li2CO3, enabling its full recovery upon decomposition and leaching, thereby increasing yields into the 56–76% range and simplifying the flowsheet. The resulting Li2CO3 meets high purity standards (>99 wt%) for most systems, supporting re-entry into battery supply chains. The universality across chemistries and mixtures could eliminate sorting requirements at recycling plants, addressing practical constraints of heterogeneous feedstocks. Industrial considerations include tuning milling parameters for real black mass containing binders, graphite, Cu, and F-containing species (e.g., LiPF6), which may necessitate extended milling or additional purification to remove species like LiF. The approach also produces a magnetic metallic composite, enabling subsequent recovery of transition metals (to be addressed separately). Overall, the findings substantiate mechanochemistry as a scalable, cleaner alternative or complement to conventional LIB recycling routes.

A universal, acid-free mechanochemical method using aluminum as a reductant was developed to recover lithium from LCO, NMC, LMO, LFP, and mixed cathode feeds. Two flowsheets were established: Process 1 (milling → leaching → purification) and Process 2 (milling → carbonatization → leaching). Process 2 both reduces processing steps and significantly improves lithium recovery (to ~56–76%) relative to Process 1 (~30–40%) by converting residual LiAlO2 into water-soluble carbonate species. High-purity Li2CO3 (>99 wt%) was obtained for LCO/NMC/LMO and mixed feeds, while LFP required further purification due to phosphate and alumina carryover. The technique is simple, energy-efficient, and broadly applicable, potentially eliminating feedstock sorting. Future work will optimize handling of complex black masses, enhance purification for LFP-containing streams, and address separation and valorization of the metallic composite (d-elements).

- Process 1 exhibits low Li yield due to retention of lithium in insoluble/amorphous LiAlO2; Process 2 mitigates but may still require optimization for certain feeds. - LFP-containing systems form phosphate/phosphite phases (e.g., Li3PO4, Fe2(HPO3)3, AlFe(PO4)O) upon carbonatization/leaching, complicating separation; Process 2 Li yield for LFP was not reported and requires additional purification steps. - Al and P impurities (likely amorphous Al2O3 and phosphate residues) reduce Li2CO3 purity in LFP cases; extra dissolution/filtration may be needed. - Real black mass contains binders/electrolytes (e.g., LiPF6) and inert components that may insulate reactants, necessitating longer milling, different ball-to-powder ratios, or process modifiers; possible LiF formation may require tailored purification. - Separation and recovery of the metallic composite (d-elements) are not addressed in this paper and remain future work.