Selective cobalt and nickel electrodeposition for lithium-ion battery recycling through integrated electrolyte and interface control

Global growth in electronic device use has driven a surge in waste lithium-ion batteries containing critical metals such as Li, Co, Ni, and Mn. Future demand, particularly for cobalt and nickel, is projected to outpace identified reserves, creating geographic, environmental, and political pressures on mining and underscoring the importance of sustainable recovery from secondary resources. Hydrometallurgical recycling of multi-metal cathodes like NMC requires selective separation of Co and Ni from leach liquors, but their similar physicochemical properties and close standard reduction potentials (E°Co = −0.277 V, E°Ni = −0.250 V vs SHE) hinder selective electrodeposition in aqueous media. State-of-the-art separations (solvent extraction, precipitation) achieve high selectivity but can incur significant chemical use and waste. Electrochemical recovery offers a potentially sustainable, energy-efficient route, yet aqueous electrodeposition suffers from co-deposition of Co and Ni due to their similar redox potentials. This work investigates a strategy that integrates electrolyte speciation control (using concentrated chloride to impart opposite charges to Co and Ni complexes) with interfacial design (positively charged PDADMA coatings) to split effective deposition potentials and tune molecular selectivity. The goal is to enable potential-dependent, surface-tunable selective electrodeposition of Co and Ni directly from aqueous solutions relevant to battery recycling.

The paper reviews established cobalt/nickel separation approaches used in battery recycling, including solvent extraction, precipitation, adsorption, intercalation, and dialysis. While solvent extraction and precipitation can deliver high selectivity, they often entail high chemical costs, complex speciation control, and waste generation. High-temperature molten salt electrorefining can separate Co and Ni but is incompatible with hydrometallurgical process integration due to operational temperatures of 400–550 °C. Aqueous electrodeposition is desirable but limited by similar Co and Ni reduction potentials, leading to co-deposition and necessitating pre-separation. Concentrated chloride media, ionic liquids, and deep eutectic solvents have been explored for integrated leaching and recovery and can modulate metal speciation. Prior studies on anomalous codeposition of Ni-Co alloys suggest pH- and adsorption-mediated mechanisms can bias deposition toward Co, hinting that electrolyte and interface engineering could provide selectivity at low temperature.

Electrolyte/speciation control: Concentrated chloride (10 M LiCl) was used as background electrolyte to form anionic CoCl4²⁻ while maintaining Ni as [Ni(H2O)5Cl]+, creating opposite charges for interfacial discrimination. Electrochemical testing: Linear sweep voltammetry (LSV) and chronoamperometry were conducted in a three-electrode cell (BASi) at room temperature. Working electrodes were copper foils (1 × 2 cm; effective immersed area 0.5 cm²). Reference electrode: Ag/AgCl (3 M NaCl). Counter electrode: Pt wire isolated by glass and a porous tip. Simulated electrolytes contained CoCl2 and/or NiCl2 (10 or 100 mM) in 0.1 M Li2SO4, 0.1 M LiCl, or 10 M LiCl. Nitrogen purging preceded tests. Onset potentials were determined from LSV by tangent intersection of non-faradaic and faradaic regions. Interfacial design: Copper foils were coated with poly(diallyldimethylammonium chloride) (PDADMA, MW 200,000–350,000 Da) via drop-casting 0.75 µL of PDADMA solutions (0.1–20 mg mL−1 in 1:1 ethanol:water), air-dried 10–12 h to target loadings from 0.0375 to 4.995 mg cm−2. Selectivity tuning was evaluated by varying potential and PDADMA loading. Quantification: After electrodeposition, deposits were rinsed, digested in 10% w/w HNO3, and analyzed by ICP-OES (Agilent 5110). Calibration with 100–5000 ppb standards ensured R² > 0.999; at least 15 replicates per sample. Faradaic efficiencies were calculated using n = 2 electrons for both direct metal and hydroxide formation pathways. Stripping: Deposits were stripped anodically at −0.08 V vs Ag/AgCl into 5 mM NaNO3 (pH 2.9–3.0) until current <10 µA; stripped and residual metals (post-digestion) quantified by ICP-OES to compute stripping efficiency. EQCM: In situ gravimetry used 5 MHz Cu-coated quartz crystals (area 0.2 cm²). Mass changes were derived from frequency shifts via the Sauerbrey equation. Measurements tracked deposition and stripping on Cu, PDADMA/Cu, and PDADMA/Au crystals. Characterization: SEM/EDS (Hitachi S-4700, iXRF), XPS (Kratos Axis ULTRA, Al Kα), and XRF (Shimadzu EDX-7000) were used for morphology, composition, and chemical state analysis. Battery recycling application: Commercial 18650 graphite||NMC cells were pretreated by discharging in 10% w/v NaCl, manual dismantling, NMP treatment at 100 °C for 24 h to dissolve PVDF and separate active material, filtration and drying at 140 °C. Leaching used 30 mL of 10 M HCl with 4 g cathode powder for 2 h at 300 rpm; pH adjusted to 3.0 using LiOH. The leachate composition (Co, Ni, Mn) was measured by ICP-OES. Electrochemical recovery sequence: (1) selective Co deposition at −0.725 V vs Ag/AgCl on thin PDADMA/Cu, followed by anodic stripping to re-solution for up-concentration, (2) second selective Co deposition from the Co-enriched electrolyte, and (3) selective Ni deposition at −0.6 V vs Ag/AgCl from the Ni-enriched raffinate. Technoeconomic analysis: Costs of reagents (including PDADMA and LiOH), energy consumption, and material revenues were estimated; electrolyte recycling strategies were considered to mitigate LiOH costs.

- Speciation control in 10 M LiCl created anionic CoCl4²⁻ and cationic [Ni(H2O)5Cl]+, splitting effective deposition behavior. LSV showed distinguishable onset potentials in 10 mM single-metal baths: Co at −0.68 V and Ni at −0.59 V vs Ag/AgCl, enabling a window for Ni-selective deposition; Co showed a more negative shift due to complex stabilization. - In binary 10 mM Co(II)+Ni(II), low-to-moderate chloride (0.1 M Li2SO4 or 0.1 M LiCl) yielded Co/Ni ≈ 1–2 across −0.80 to −0.55 V, indicating co-deposition. In 10 M LiCl, moderate potentials (−0.60 to −0.55 V) favored Ni-rich deposits, whereas more negative potentials (−0.65 to −0.80 V) induced cobalt-selective anomalous deposition; at −0.75 V Co/Ni reached 3.18. At 100 mM total metals, Co/Ni rose up to 14 at −0.725 V, confirming strong anomalous deposition in concentrated chloride. - EQCM indicated high faradaic efficiencies (>90%) for Co near its onset in 10 M LiCl. Initial m/z ≈ 51.2 g mol−1 for Co suggested Co(OH)2 formation via local pH increase, followed by a decrease consistent with direct Co deposition and/or catalytic H2 evolution on Co. Ni exhibited low m/z (~10 g mol−1) and lower faradaic efficiency under comparable conditions, consistent with sluggish dehydration of [Ni(H2O)5Cl]+. - Interfacial PDADMA tuning: Small PDADMA loadings (≤0.075 mg cm−2) enhanced Co selectivity at cobalt-favored potentials; higher loadings suppressed Co deposition and favored Ni. At −0.725 V, the surface Co/Ni decreased from 2.3 on pristine Cu to 0.40 with 4.995 mg cm−2 PDADMA, evidencing tunable selectivity by polymer loading. - LSV and chronoamperometry with single-metal solutions showed PDADMA had minimal effect on Ni onset/current but markedly suppressed Co deposition and shifted Co onset by −0.02 V. At −0.725 V for 0.5 h, Co deposition on PDADMA/Cu was about 7% of that on pristine Cu, while Ni amounts were similar. - Diffusion analysis: Adding 0.01 wt% PDADMA to electrolyte reduced CoCl4²⁻ diffusion coefficient from 2.50 × 10−8 to 4.19 × 10−10 cm² s−1, indicating strong electrostatic stabilization and mobility reduction; Ni diffusion changed slightly (1.56 × 10−8 to 1.43 × 10−8 cm² s−1). Tafel slopes increased for Co (47→67 mV dec−1) but were nearly unchanged for Ni (149→147 mV dec−1) with PDADMA, corroborating selective interfacial effects. - Synergy of electrolyte and interface: In 10 M LiCl at −0.6 V, Ni/Co on pristine Cu was 1.81 and increased to 7.05 with PDADMA (0.75 mg cm−2). At −0.725 V and 100 mM Co+Ni, Co/Ni reached 14.08 on pristine Cu and 16.73 with thin PDADMA (0.07 mg cm−2). XRF and EDS corroborated high Co/Ni (16.0 and 18.4, respectively); XPS confirmed selectivity enhancements (Ni/Co 1.45→3.01 at −0.6 V; Co/Ni 10.19→12.41 at −0.725 V). - Morphology control: Without PDADMA, needle-like dendrites formed; PDADMA yielded rough/grainy or wrinkled morphologies without sharp dendrites, consistent with surface conduction and modified ion transport. - Reversibility: Stripping efficiencies >90% for both metals; EQCM showed ~96% mass recovery upon stripping, with minor PDADMA loss (<0.3% of loading after prolonged stripping). - Application to spent NMC: Leachate (10 M HCl, pH adjusted to 3.0) had Co:Ni:Mn = 1.00:6.52:0.50. After selective Co deposition and stripping, the stripping electrolyte shifted to 1.00:0.60:0.02, evidencing Co up-concentration. Second Co-selective deposition achieved 96.4 ± 3.1% Co purity. From the Ni-enriched raffinate, Ni-selective deposition at −0.6 V achieved 94.1 ± 2.3% Ni purity. Mn co-deposition was negligible. - Technoeconomic analysis: Assuming 95% metal recovery, material revenue ~$2.230 per kg NMC powder; energy consumption 29.4 kWh kg−1 with electricity cost ~$2.027 kg−1; PDADMA cost contributed 3.61% of the benefit and 0.41% of total material costs; LiOH for pH adjustment dominated material costs but could be mitigated via LiCl recovery. Overall profit estimated at ~$0.2 per kg of waste NMC powder, with potential to improve via electrolyte recycling and process optimization.

The study demonstrates that combining speciation control (concentrated chloride) with interfacial charge engineering (PDADMA) effectively splits the deposition behavior of Co and Ni in aqueous media despite their similar standard potentials. Concentrated chloride transforms Co into an anionic complex (CoCl4²⁻) while Ni remains cationic, creating an electrostatic handle at the electrode interface. This enables potential-dependent selectivity windows: moderate potentials favor Ni deposition, whereas more negative potentials in chloride promote anomalous deposition that prioritizes Co via hydroxide/adsorption-mediated pathways. PDADMA coatings further bias selectivity by stabilizing and immobilizing CoCl4²⁻ within the positive polymer layer, suppressing Co deposition at higher loadings and modestly enhancing Co selectivity at low loadings. The approach reduces hydrogen evolution, improves faradaic efficiency for Co near onset, and enables reversible deposition/stripping with high efficiency. Applied to leachates from spent NMC cathodes, sequential selective electrodepositions produced high-purity Co and Ni deposits without reliance on solvent extraction, pointing to an electrochemically driven, potentially recyclable process that can complement hydrometallurgical trains. The findings also reveal interfacial control over deposit morphology, offering broader implications for materials processing and dendrite suppression in metal electrodeposition.

Speciation engineering with concentrated chloride, coupled with positively charged PDADMA interfacial layers, enables tunable, potential-dependent molecular selectivity between cobalt and nickel during electrodeposition in aqueous media. This synergy splits effective deposition behavior of metals with near-identical redox potentials and allows selective recovery directly from complex leachates. The method achieved Co and Ni purities of 96.4 ± 3.1% and 94.1 ± 2.3%, respectively, from spent NMC cathodes, with high stripping efficiencies and reversible operation. Beyond separations, the interfacial strategy controls deposit morphology, suggesting utility in broader electrochemical materials processing. Future work should optimize polymer coating uniformity, interfacial and cell designs, and electrolyte recycling (e.g., LiCl recovery) to improve selectivity, throughput, and cost, and to scale the process for industrial implementation.

- Selectivity and recovery rates require improvement to compete with state-of-the-art solvent extraction/precipitation processes; process optimization and uniform PDADMA coating are needed. - Material costs are dominated by LiOH for pH adjustment after HCl leaching; effective LiCl/Li recovery and recycling are necessary to enhance economics. - Energy consumption and costs, particularly associated with LiCl harvesting and drying, need reduction via scale-up and heat integration. - Some PDADMA dissolution occurs initially; although small (<0.3% loss after prolonged stripping), long-term polymer stability and reuse cycles require further validation. - Experiments were performed at laboratory scale with unoptimized cells; mass transfer limitations and concentration polarization may affect throughput and scalability. - Process performance may depend on specific leachate composition (e.g., Ni-rich NMC feeds); broader feed variability and impurity impacts need assessment.