Recycling and Separation of Valuable Metals from Spent Cathode Sheets by Single-Step Electrochemical Strategy

Abstract

The conventional spent lithium-ion batteries (LIBs) recycling method suffers from complex processes and excessive chemical consumption. Hence, this study proposes an electrochemical strategy for achieving reductant-free leaching of high-valence transition metals and efficient separation of valuable components from spent cathode sheets (CSs). An innovatively designed sandwich-structured electrochemical reactor achieved efficient reductive dissolution of cathode materials (CMs) while maintaining the structural integrity of aluminum (Al) foils in a dilute sulfuric acid system. Optimized current enabled leaching efficiencies exceeding 93% for lithium (Li), cobalt (Co), manganese (Mn), and nickel (Ni), with 88% metallic Al foil recovery via cathodic protection. Multi-scale characterization systematically elucidated metal valence evolution and interfacial reaction mechanisms, validating the technology’s tripartite innovation: simultaneous high metal extraction efficiency, high value-added Al foil recovery, and organic removal through single-step electrochemical treatment. The process synergized the dissolution of CM particles and hydrogen bubble-induced physical liberation to achieve clean separation of polyvinylidene difluoride (PVDF) and carbon black (CB) layers from Al foil substrates. This method eliminates crushing pretreatment, high-temperature reduction, and any other reductant consumption, establishing an environmentally friendly and efficient method of comprehensive recycling of battery materials.

1. Introduction

The global transition toward electrified transportation and energy storage has created an unprecedented reliance on lithium-ion battery (LIB) technologies [1,2]. With the continuous expansion of the new energy market, a large number of spent LIBs are undoubtedly generated. Spent LIBs have both resource properties and environmental hazards: they contain a large number of valuable metals such as lithium (Li), nickel (Ni), cobalt (Co), manganese (Mn), aluminum (Al), and copper (Cu), as well as a plenty of fluorine-containing electrolytes, organic binders, plastics, and other harmful substances [3,4]. Therefore, resource treatment of spent LIBs is of great significance to promote the resource cycle and environmental protection.

The cathode material (CM) particles are firmly attached to the Al foil by organic binders, making them difficult to separate effectively [5,6]. At present, the common methods to separate CMs from Al foils are mainly solvent and heat treatment methods. The solvent method often uses organic solvents such as N-Methyl-2-Pyrrolidone (NMP) to dissolve organic binders so as to achieve the separation of CMs and Al foils [7,8]. However, this method consumes a large amount of toxic organic agents and poses potential secondary pollution. As for the heat treatment process, organic binders are decomposed into gases by controlling the temperature, which can realize the effective separation of CMs and Al foils [9,10,11]. Although heat treatment can achieve the separation of CMs from the Al foil, the treatment of toxic release gases is still a major challenge. Moreover, due to the high-valence transition metals that exist in the CM, CM shows a poor leaching efficiency in acidic solutions without reduction [12]. In order to improve the leaching efficiency, a certain amount of reducing agents such as explosive H₂O₂ [13,14] are generally added in the hydrometallurgical process to reduce the high-valence metals, or reducing substances such as graphite [15,16], coke [17] and biomass [18,19] are used for mixed heat treatment to reduce the high-valence transition metals before acid leaching. However, these conventional processes usually cause high reagent consumption, high energy consumption, and potential environmental pollution. Therefore, there is an urgent need for a green and effective way to recover valuable metals from spent LIBs.

Electrochemical recycling presents a transformative approach for spent LIBs, especially driven by green electricity [20,21,22]. The electrochemical method can promote the separation of CMs and Al foils by generating hydrogen on the surface of Al foil through an external electric field, which is non-destructive to the structure of organic binders [23]. Moreover, high-valence transition metals in the CM can be reduced by obtaining electrons at the cathode. Therefore, electrolysis in acid solution can achieve efficient leaching of the CM [24,25]. However, the current research on electrochemical methods only focuses on the separation of CMs from Al foils or the dissolution of valuable metals from CMs. In fact, when spent CSs are used as the cathode in an electrochemical leaching process, hydrogen will inevitably be generated, and the cathode material will be separated from the Al foil prematurely, leading to the failure of the full reduction of the CMs, resulting in a reduction in the leaching efficiency of valuable metals. Therefore, how to effectively apply electrochemical methods to recycle spent LIBs is of great research significance.

This study developed a sandwich-structured electrochemical reactor tailored to the spatial configuration of spent CSs, effectively mitigating premature separation of CM particles and Al foils caused by hydrogen evolution while achieving metal leaching and components separation. A comprehensive analysis of architectural features of the spent CSs was first investigated, through which the influence of applied current on metal leaching efficiency was evaluated. After single-step electrochemical treatment, three products were obtained, namely metal-rich leachates, leaching residues, and metallic Al foils. Then, characteristics of the products were characterized using advanced analytical techniques to reveal the electrochemical leaching mechanisms. Finally, a novel single-step electrochemical strategy was proposed to leach valuable metals with component preservation.

2. Experimental Section

2.1. Materials and Reagents

The investigation utilized spent 18650-type LIBs obtained from a local recycling company. Following a 48 h discharge in 10% NaCl solution, these LIBs were air-dried and manually disassembled to isolate CSs for experiments. Selected CS samples underwent complete metal dissolution through aqua regia digestion (HCl: HNO₃ = 3:1 v/v), with elemental quantification via an AGILENT 7900 inductively coupled plasma mass spectrometry (ICP-MS, United States) confirming the CM composition as LiNi₀.₅Co_0.2Mn_0.3O₂ (NCM523), containing 7.88 wt% Al foil (

Table 1. The main metal components in the spent CSs.

Element	Li	Ni	Co	Mn	Al
Content wt.%	6.69	28.26	11.10	15.62	7.88

). Acidic chemical reagents (H₂SO₄, HNO₃, HCl) were analytical grade from Xilong Reagent Co., Ltd., Shantou, China, with aqueous solutions prepared using laboratory-grade deionized water. In addition, the Al foil was separated by dissolving the organic binder in spent CSs with N-methylpyrrolidone (NMP, analytical grade), which was then compared to the electrochemical leaching method.

2.2. Electrochemical Leaching Process

The electrochemical leaching process (schematically illustrated in Figure 1) utilized spent CSs in a 100 mL beaker equipped with a 304 stainless steel working electrode (4 × 4 × 0.1 cm, perforated with 0.1 cm diameter holes to enhance mass transfer between CMs and H₂SO₄ solution), a dual platinum counter electrodes (1 × 1 × 0.1 cm), and a magnetic stirrer operating at 150 rpm. CSs were precision cut into 4 × 4 cm sections and sandwiched between working electrodes for electrochemical treatment under controlled conditions: 50 °C thermostatically regulated via a water bath (Zhengzhou Ketech, DF-101S, Zhengzhou, China), with potential applied using a power supply (Guwei Electronics, GPP-4323 DC, Suzhou, China). In this study, a dilute H₂SO₄ solution was used as the electrolyte and leaching agent, and the leaching time was maintained at 120 min. Post-leaching solutions were analyzed by ICP-MS to quantify metal concentrations, with leaching efficiencies calculated using Equation (1):

$$E={\displaystyle \frac{CV}{mw}}\times 100\%$$

(1)

where E is the leaching efficiency, where C represents the metal concentration (g/L), V denotes the volume of leaching solution (L), m indicates the initial mass of crude CSs (g), and w corresponds to the mass percentage of target metal in raw CSs.

2.3. Measurement and Characterization

Material characterization employed advanced analytical techniques: phase composition analysis utilized a BRUKER D8 ADVANCE X-ray diffractometer (XRD, Karlsruhe, Germany) for original and leaching residues; surface topography and elemental mapping were conducted using a TESCAN GAIA3 XMH high-resolution scanning electron microscope (SEM, Brno, Czech Republic) coupled with a OXFORD Ultim Max 65 energy dispersive spectrometer (EDS, London, United Kingdom); chemical state evolution was probed through an ESCALAB 250Xi X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, Waltham, MA, USA) before/after leaching. Metal ion quantification in leachates employed an ICP-MS.

3. Results and Discussion

3.1. Characterization of the Spent CSs

The spatial architecture of spent CSs was first investigated through morphological and elemental distribution analyses of their surfaces and cross-sections. As shown in Figure 2a, spherical CM particles are observed on the surface, interconnected by abundant organic binders within particle gaps. These binders ensure robust particle agglomeration, resulting in an exceptionally smooth and compact surface (Figure 2b). Cross-sectional analysis (Figure 2c–e) reveals a distinct sandwich structure: an Al foil core flanked by composite layers consisting of CM particles, binders, and conductive additives. EDS semi-quantitative analysis (Figure 2d) of the cross-section confirms the substantial presence of valuable metals, with Ni, Co, Mn, and Al collectively accounting for 63.62% of the total mass. Non-metallic components include fluorine (4.78%) and carbon (12.39%), primarily attributed to organic binders and conductive additives. EDS mapping (Figure 2e) demonstrates spatial overlap of Ni, Co, Mn, and O elements, confirming their exclusive origin from CM particles. Carbon signals primarily derive from the binder’s carbon backbone and conductive carbon black (CB), while fluorine originates from polyvinylidene difluoride (PVDF) binders and residual LiF electrolyte traces. Minor phosphorus content suggests a residual electrolyte on the surface, and the isolated Al distribution corresponds to the current collector foil. Therefore, the spatial structure of the spent CS was obtained, that is, a sandwich structure composed of the CS, PVDF and CB on both sides of the Al foil. This multi-layered structure hinders the effective recovery of valuable metals by traditional methods, which have complex procedures that often require the removal of binders and the physical separation of CMs from Al foil by crushing and screening.

3.2. Electrochemical Leaching and Separation Valuable Metals from Spent CSs

The electrochemical leaching process eliminates conventional pretreatment requirements for binder removal or Al foil separation. The organic binder inherently stabilizes CM particle agglomeration, preventing material detachment into the leaching solution, while the embedded CB network facilitates electron transfer to CMs through enhanced conductivity. This dual functionality enables simultaneous particle immobilization and electrochemical activation during metal dissolution.

The effects of the H₂SO₄ concentration (0.2–1.0 mol/L) and applied current (320–1600 mA) on electrochemical leaching performance were investigated. As shown in Figure 3a, at a constant current of 960 mA, leaching efficiencies of Li, Mn, Ni, and Co increased progressively with the H₂SO₄ concentration: from 65%, 30%, 54%, and 62% at 0.2 mol/L to 97%, 93%, 94%, and 95% at 0.8 mol/L, respectively. This significant enhancement in metal dissolution is attributed to two factors: (1) H₂SO₄ acts as an electrolyte, where higher concentrations improve mass transfer within the electrolytic cell; and (2) increased acidity promotes the dissolution of valuable metals following electrochemical reduction. Further increases to 1.0 mol/L yielded only marginal efficiency gains. Notably, Al dissolution remained at 10% to 14% across all concentrations, indicating effective cathodic protection. Consequently, 0.8 mol/L was identified as the optimal H₂SO₄ concentration.

As can be seen in Figure 3b, without an electric field, baseline leaching efficiencies reached 52% for Li, 29% for Mn, 45% for Co, and 45% for Ni, accompanied by 60% Al dissolution. This partial Al corrosion acted as a reductant, indirectly promoting metal leaching but compromising Al recovery. When an external electric field was applied, leaching efficiencies increased proportionally with the current increase, demonstrating the critical role of current in electrochemical leaching. At 320 mA, metal leaching efficiencies rose to 84% Li, 73% Mn, 76% Co, and 79% Ni, peaking at 960 mA with 97% Li, 93% Mn, 94% Co, and 95% Ni. Beyond this threshold (1280–1600 mA), the leaching efficiencies of Li, Ni, Co, and Mn changed slightly. Therefore, 960 mA was selected as the optimal current in this study. Moreover, the electrochemical system simultaneously addressed Al preservation. While natural Al dissolution reached 60% in conventional acid leaching processes due to its reductive role [26], cathodic protection under optimal conditions reduced this to 12%, preserving 88% of the Al foil as recoverable metal. However, localized alkalization at higher currents gradually increased Al corrosion, emphasizing the need for precise parameter control. This integrated approach achieved high-yield metal extraction (>93% for Li, Mn, Co, Ni) while maintaining Al integrity and retaining binders for simplified downstream processing, demonstrating significant advantages over traditional methods.

Figure 3c exhibits a stable 3.2 V potential during 960 mA leaching, surpassing the water decomposition threshold (1.23 V vs. SHE) and confirming persistent cathodic hydrogen evolution. The resulting bubbles mechanically detach CMs from Al foil while enhancing solution turbulence, thereby accelerating solid–liquid proton exchange and metal ion transfer into the leaching medium. Crucially, the sandwich-structured electrochemical reactor prevents premature layer separation, ensuring complete electrochemical metal reduction prior to dissolution.

3.3. Properties of the Leaching Residue

XRD analysis of raw material and residue from electrochemical leaching under an optimal current revealed critical insights into structural evolution. Figure 4 shows that the untreated CSs exhibited diffraction peaks closely aligned with the standard layered structure. Both the (006)/(102) and (018)/(110) peaks are obviously split in the raw spent CSs, indicating that the crystal structure of the CM is complete. After electrochemical leaching, the two sets of (006)/(102) and (018)/(110) doublets were combined together, indicating structural distortion caused by electrochemical leaching. Notably, the detection of PVDF characteristic peaks in the leaching residue indicates an increase in PVDF content, proving that valuable metal components were leached out from spent CSs. No peaks corresponding to monovalent oxides or elemental forms of transition metals were detected in leaching residue, confirming the direct dissolution of CMs into the acidic solution without intermediate compound formation.

XPS technology was applied to investigate the surface properties of the leaching residue; the results are presented in Figure 5. XPS analysis of fluorine speciation in solid residues provides critical insights into binder stability during electrochemical leaching. The pristine cathode surface exhibited two distinct fluorine states: a C-F bond at 687.6 eV, characteristic of PVDF binder; and a Li-F component at 685.2 eV, attributed to residual electrolytes. Post-leaching analysis revealed the complete disappearance of the Li-F signal, confirming the dissolution of electrolyte residues into the solution. Notably, the persistent C-F signature at 687.6 eV demonstrates structural preservation of the PVDF binder throughout the leaching process. This chemical stability enables direct recovery of intact PVDF from solid residues, bypassing conventional binder decomposition requirements. Furthermore, XPS analysis of Ni, Co, and Mn valence states before and after electrochemical leaching reveals synchronized reduction–dissolution dynamics. For Ni 2p spectra, characteristic peaks at 854.8 eV (Ni²⁺) and 856.1 eV (Ni³⁺) remained unchanged throughout the process, indicating the continuous reduction of higher valence states during the leaching process. Similarly, Co 2p spectra maintained dual oxidation states at 779.9 eV (Co²⁺) and 781.2 eV (Co³⁺), while Mn 2p consistently showed a single +4 valence state at 642.0 eV. This persistent coexistence of multiple oxidation states demonstrates real-time electrochemical reduction of higher-valent metal species (Ni³⁺, Co³⁺, Mn⁴⁺), followed by immediate protonation and dissolution into the acidic medium. The absence of stable intermediate phases in XRD patterns corroborates this instantaneous reduction–dissolution mechanism, where reduced metal species transition directly into ionic solutions without forming detectable crystalline intermediates. These findings collectively validate the proposed electrochemical leaching pathway involving concurrent electron transfer and acid dissolution processes.

Further analysis of the leaching residues’ surface morphology and elemental distribution reveals critical insights into the electrochemical leaching performance (Figure 6). Post-leaching residues exhibit a loosely packed, porous structure, indicative of efficient CM dissolution. EDS analysis confirms a dominant carbon–fluorine composition (92.64%), corresponding to the preserved PVDF-CB network. Notably, no residual transition metals (Ni, Co, Mn) were detected on the surface, demonstrating near-complete dissolution of cathode active materials into the solution. These observations collectively validate the effectiveness of the electrochemical leaching process in selectively extracting valuable metals while retaining the polymeric–conductive matrix as a recoverable byproduct. The porous architecture further suggests minimal structural degradation of the binder framework during leaching, aligning with prior XPS findings on PVDF stability.

3.4. Properties of the Recovered Al Foils

A comparative evaluation of Al foil surfaces obtained through different processing methods reveals critical advantages of the electrochemical approach (Figure 7). Organic solvent (NMP) treatment effectively removes bulk CM particles from the Al foil surface (Figure 7a,b), yet fails to eliminate particles deeply embedded within the Al matrix, compromising Al foil purity. In contrast, electrochemical leaching produces Al foil surfaces with minimal CM particle residue and distinct micro-corrosion pores (Figure 7c,d). This phenomenon is caused by the separation of CM from Al foil enhanced by hydrogen evolution and Al foil dissolution during electrochemical leaching, which removes CM particles adhered to the surface of Al foil, and finally obtains high-purity Al foils.

XRD phase analysis of the Al foil recovered through electrochemical separation (Figure 8) revealed its phase composition, with characteristic peaks detected at 38.5°, 44.8°, 65.2°, and 78.3° within the 20–90° scanning range. These diffraction patterns align precisely with the standard PDF card for elemental Al, while the absence of detectable impurity phases demonstrates the effectiveness of the electrochemical process in preserving Al integrity. In addition, the purity of the recovered Al foil reaches 99.2% through ICP measurement. The clean spectral profile not only validates the foil’s structural fidelity but also highlights the method’s capability to achieve material recovery without introducing secondary contaminants.

3.5. Electrochemical Leaching Mechanisms and Valuable Metals Recovery Process

The electrochemical leaching mechanism underlying valuable metal dissolution and component separation in spent CSs has been systematically elucidated through comprehensive analysis, as schematically depicted in Figure 9. Capitalizing on the intrinsic sandwich configuration of spent CSs, the application of an electric field on both surfaces of the spent CS enables the efficient reduction and leaching of high-valence transition metals (Ni³⁺, Co³⁺, Mn⁴⁺) within the acidic medium. Concurrent hydrogen evolution at the cathode interface generates microbubble-induced mechanical agitation that synergistically enhances CMs, PVDF, and CB liberation from the Al foils, while compressive forces exerted by 304 stainless steel plates prevent premature structural disintegration of this structure. Crucially, cathodic protection preserves the Al foil in its metallic state throughout the acidic environment.

This coordinated process achieves triphasic separation through three self-reinforcing mechanisms: (1) electrochemical reduction converts high-valent metals into acid-soluble species for subsequent ionic dissolution, (2) hydrogen-mediated physical disengagement weakens the adhesive interface between PVDF and CB and Al foils, and (3) potential controlled surface passivation maintains Al integrity. The self-propagating separation between the PVDF and CB composite layer and Al foil during leaching facilitates effortless post-treatment segregation. Consequently, a single-step electrochemical treatment simultaneously yields three distinct product streams: a metal-enriched leachate containing Li⁺, Ni²⁺, Co²⁺, and Mn²⁺; intact metallic Al foil retaining over 88% of the initial mass; and separable PVDF and CB residues. This potential regulated approach demonstrates unprecedented selectivity in recovering strategic metals while preserving both Al and polymeric components, establishing a paradigm-shifting alternative to conventional sequential recovery processes.

4. Conclusions

This study demonstrates an innovative electrochemical strategy that achieves simultaneous high-efficiency recovery of strategic metals (Li, Ni, Co, Mn) and high-purity Al foil from spent CSs. By exploiting the intrinsic sandwich architecture of spent CSs, the proposed method eliminates conventional pretreatment requirements while enabling three self-sustaining separation mechanisms: (1) electrochemical reduction of high-valence transition metals (Ni³⁺→Ni²⁺, Co³⁺→Co²⁺, Mn⁴⁺→Mn²⁺) coupled with acid dissolution, (2) hydrogen bubble-assisted physical liberation of PVDF and CB from Al foils, and (3) cathodic protection-mediated preservation of the Al foil integrity. At 960 mA, the process attained exceptional leaching efficiencies exceeding 93% for target metals alongside 88% metallic Al recovery. The proven synergy between selective electrochemical and self-driving material separation offers an environmentally friendly pathway to sustainable battery recycling, which does not require complex pretreatment processes and expensive reducing reagents.