Mechanochemical Activation as a Key Step for Enhanced Ammonia Leaching of Spent LiCoO₂ Cathodes

Abstract

The growing demand for lithium-ion batteries (LIBs) has led to an urgent need for sustainable recycling strategies for spent cathode materials. In this study, a mechanochemical approach was developed for the recovery of lithium and cobalt from end-of-life LiCoO₂ cathodes using high-energy ball milling. For the first time, aluminum and carbon were employed as internal reducing agents, facilitating the in situ decomposition of LiCoO₂ into CoO, Li₂O, and metallic Co. X-ray diffraction analysis confirmed significant structural disorder, phase transitions, and the formation of CoO, AlCo, and spinel-like CoAl₂O₄. The Taguchi method was applied to optimize milling parameters, identifying 800 rpm, 60 min, and a ball-to-powder ratio of 50:1 as the most effective conditions for structural activation. Subsequent ammonia leaching under fixed conditions (3.0 M NH₃·H₂O, 1.0 M (NH₄)₂CO₃, 60 °C, 25 mL/g, 6 h) demonstrated high recovery efficiencies: up to 94.6% for lithium and 83.7% for cobalt in the best-performing samples. These results highlight the synergistic benefits of mechanical activation and reductant-assisted phase engineering for enhancing metal recovery. The proposed method offers a simple, scalable, and eco-friendly route for the hydrometallurgical recycling of LIB cathodes without requiring extensive chemical pretreatment.

1. Introduction

Rechargeable lithium-ion batteries, which commonly utilize graphite as the anode and lithium cobalt oxide (LiCoO₂) as the cathode, are extensively used in portable electronic devices and electric vehicles [1,2]. Nevertheless, their relatively short operational lifespan—typically limited to 500–1000 charge–discharge cycles—leads to a growing volume of spent battery waste [3,4,5]. This waste not only poses environmental concerns due to toxic metal content, but also serves as a promising secondary resource for recovering critical elements like lithium and cobalt [6,7,8,9].

Several recycling technologies have been developed for lithium-cobalt batteries, including pyrometallurgical [10,11,12], hydrometallurgical [13,14,15,16], and mechanochemical methods [17,18,19,20]. Nevertheless, the question of which approach offers the most efficient and sustainable solution remains unresolved.

In this context, mechanochemical routes provide a promising alternative. They are solvent-free, energy-efficient, and enable solid-state reactions at room temperature, including phase decomposition, redox transformations, and amorphization [21,22,23,24,25]. The recent study [26] demonstrated efficient recovery of lithium and cobalt from NMC cathodes using a choline chloride/pyrogallol-based DES. The authors [27] have also demonstrated the effectiveness of ultrasound-assisted leaching with deep eutectic solvents for enhanced lithium and cobalt recovery from spent LIBs, further emphasizing the importance of novel hydrometallurgical strategies in battery recycling. Notably, the mechanochemical approach has been successfully applied to oxide systems such as ZnO, MnO₂, and TiO₂, yet remains underexplored for complex cathode materials containing multiple interacting components [28].

Recent studies have highlighted the potential of mechanochemistry to induce structural breakdown and enhance leaching efficiency without high-temperature processing. Moreover, the use of dual-function components, such as aluminum and carbon—commonly present in spent cathodes—as internal reducing agents offers a route to simplify recycling steps. Previous research demonstrated the effectiveness of aluminum and carbon in facilitating the breakdown of LiCoO₂ via carbothermic and aluminothermic reactions [29,30,31]. However, most of these studies employed high-temperature roasting, necessitating significant energy input and reagent separation steps.

Apart from the primary active material LiCoO₂, lithium-cobalt battery cathodes generally contain aluminum current collectors, conductive additives like carbon, and polymeric binders. One major obstacle in processing these cathodes is the difficulty of effectively removing the aluminum foil and other auxiliary components. In 2019, researchers from China introduced an innovative approach for extracting cobalt and lithium from end-of-life lithium-cobalt batteries without the preliminary separation of aluminum and carbon [32]. Rather than eliminating these materials, they utilized them directly as reducing agents through aluminothermic and carbothermic mechanisms to break down LiCoO₂. Although not all reaction routes—such as those involving CO formation—were thoroughly examined, the study highlighted the feasibility of employing aluminum and carbon to convert LiCoO₂ into more reactive cobalt and lithium oxides, along with the formation of metallic cobalt.

The objective of this study is to develop and evaluate an integrated mechanochemical recycling approach for spent LiCoO₂ cathodes by utilizing aluminum and carbon as in situ reducing agents during high-energy ball milling. The central hypothesis is that mechanical activation in the presence of these additives will induce structural breakdown of the LiCoO₂ lattice and promote the formation of reactive secondary phases such as CoO, CoAl₂O₄, and AlCo, thereby enhancing the leachability of lithium and cobalt in subsequent hydrometallurgical treatments. This approach is expected to simplify the recycling process by eliminating the need for prior separation of aluminum and carbon additives, reducing overall energy consumption by enabling solid-state reduction at ambient temperature, and providing an environmentally friendly and scalable method for the sustainable recovery of critical metals from end-of-life lithium-ion batteries.

2. Materials and Methods

2.1. Materials

All batteries used in the study were Japanese-made Apple 3.82 V batteries (Figure 1) used and discarded from iPhone mobile phones (Apple Inc., Cupertino, CA, USA). The net weight of the batteries, measuring 10 samples, is 27.02 ± 3.05 g. The voltages of the used batteries were preliminarily measured to assess their residual charge.

Subsequently, the batteries were dismantled (Figure 2) and discharged in a 10% sodium chloride (NaCl, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) solution for 12 h. During the discharge process, the evolution of hydrogen gas bubbles was observed. The endpoint of the discharge was determined by the cessation of gas evolution. Upon completion of the discharge, the batteries’ contents were air-dried for 24 h, followed by the separation of individual components. The active material (LiCoO₂) (Figure 2a) was isolated from other battery components, including the anode (copper foil), cathode (aluminum foil), and the separator positioned between them. No external additives were introduced. The cathode composite contains conductive carbon, and the aluminum current-collector was co-milled with the active powder, generating fine Al fragments. Thus, the black mass intrinsically includes Al and C, which, under high-energy milling, act as in situ reductants and mixing agents.

2.2. Mechanical Activation

The planetary ball mill Pulverisette 7 premium line (Fritsch, Idar-Oberstein, Germany) was used for the mechanochemical activation of the cathode material of the spent batteries. Taguchi orthogonal array (3⁴) [33] was created using the Minitab14 software version 14.13 (Minitab, Ltd., Coventry, UK) and subsequently 16 experiments with different combinations were planned. The experimental details regarding the mechanochemical activation of spent batteries for Taguchi designed experiments were as follows: loading of the mill—balls with 4 mm, 6 mm, 8 mm, 10 mm in diameter; ball charge—24 g; material of the milling chamber and balls: iron; volume of the milling chamber: 20 mL; rotation speed of the planet carrier: 500 rpm; 600 rpm; 700 rpm; 800 rpm; air atmosphere; laboratory temperature; milling time: 15–60 min; with an overall mass of 10 g initial cathode material; ball-to-powder ratio: 35, 50, 65, 80.

The experimental details regarding the mechanochemical activation of the cathode material with carbon and aluminum were as follows: loading of the mill—balls with 8 mm in diameter; ball to powder ratio—80; material of the milling chamber and balls: iron; volume of the milling chamber: 20 mL; rotation speed: 800 rpm; air atmosphere; laboratory temperature; milling time: 60 min.

2.3. Analytical Techniques

Phase composition and structural characteristics were recorded with an X’Pert Pro diffractometer (Panalytical, Almelo, The Netherlands), equipped with a X’Celerator detector (Panalytical, Almelo, The Netherlands), automatic divergence slits, and CuKα₁ and CuKα₂ radiation sources (λ₁ = 0.15406 nm, λ₂ = 0.15443 nm). A sample changer with interchangeable apertures and monochromators was used. Data were collected at a step size of 0.017° with a counting time of 400 s per point. Samples were prepared on silicon zero-background holders. The measured intensities were converted from automatic to fixed divergence slits (0.25°) for subsequent evaluation. Peak positions and profiles were analyzed using the Pseudo-Voigt function in the HighScore Plus software package verion 4.9 (Panalytical, Almelo, The Netherlands).

The morphology of the selected products was investigated by a scanning electron microscope, Quanta 3D 200i Dual system, FEI, using an accelerating voltage of 20 kV.

The registration and processing of IR spectra were carried out using a VERTEX 70 FTIRspectrometer (Bruker Qptik GmbH, Ettlingen, Germany) in the frequency range of 4000–500 cm⁻¹ with a PlKE MIRacle ATR (PlKE Technologies, Madison, Wl, USA) accessory equipped with a germanium crystal for single-reflection attenuated total reflectance (ATR) measurements.

The elemental composition was determined using a wavelength-dispersive X-ray fluorescence (WDXRF) spectrometer (Axios 1 kW, PANalytical, Almelo, The Netherlands).

2.4. Leaching Experiments

Ammonium carbonate followed by aqueous ammonia leaching was applied to the optimized mechanically activated sample containing 0.25 g of carbon and 0.5 g of aluminum powder. The leaching was conducted using a solution of 3.0 M aqueous ammonia (NH₃·H₂O) and 1.0 M ammonium carbonate ((NH₄)₂CO₃) at 60 °C, with a liquid-to-solid ratio of 25 mL/g and a total duration of 6 h under continuous stirring. These conditions ensured the formation of soluble ammine and carbonate complexes. The improved leaching performance was attributed to the structural breakdown of LiCoO₂, phase transformation, and the generation of more reactive intermediates through high-energy ball milling and additive-assisted mechanochemical reduction.

3. Results and Discussion

3.1. LCO Cathode Material Characterization

According to the results of X-ray phase analysis, the following phases were identified in the initial cathode material: LiCoO₂ and carbon (Figure 3). The elemental composition (

Table 1. Chemical composition of initial LCO cathode material determined by X-ray fluorescence (WDXRF) spectrometer.

Component	(wt.%)	Component	(wt.%)
O	26.278	P	0.38
Na	5.595	Cl	5.222
Mg	0.303	Ti	0.164
Al	10.198	Cu	3.155
Si	0.077	Zn	0.015
Fe	0.038
Co	44.891
Li	3.684
Total	100

), in wt.%, varies as follows: Li—3684; Co—44,891; Na—5595; Cl—5222; Al—10,198; Cu—3155; etc. The presence of Al and Cu is attributed to residual current collectors and possible cross-contamination during cell disassembly, consistent with findings from Nowak et al. [34].

The X-ray diffraction (XRD) analysis confirms that the starting cathode material predominantly consists of lithium cobalt oxide (LiCoO₂). Within the 2θ range of 10° to 80°, distinct diffraction signals appear, corresponding to the layered hexagonal crystal structure typical of LiCoO₂, classified under the R-3m space group (PDF No. 001-083-9084). Pronounced peaks are detected near 18.7°, 36.5°, 38.6°, 45.6°, 59.1°, and 65.3° 2θ, which align with the (003), (101), (006), (104), (018), and (110) planes, respectively. The sharpness and intensity of these peaks suggest that the material exhibits high crystallinity and maintains a well-defined layered arrangement.

Notably, the presence of dominant (003) and (104) peaks supports the identification of a well-ordered α-NaFeO₂-type structure—commonly observed in stoichiometric LiCoO₂. In addition to the main phase, low-intensity and broadened signals near ~26.5° and 43.5° 2θ indicate the existence of graphitic carbon, matching the (002) and (100) planes as per PDF No. 00-026-1080. This implies the inclusion of conductive carbon additives in the cathode formulation. No other crystalline cobalt-containing byproducts or impurities were detected, confirming both the structural purity and integrity of the original LiCoO₂ phase.

Figure 4 presents scanning electron microscopy (SEM) images of the as-received lithium cobalt oxide (LiCoO₂) powder, recorded at magnifications of 10,000× and 500×, respectively. The micrographs reveal irregularly shaped particles with a broad distribution in particle size and morphology, typical of commercial LiCoO₂ cathode materials.

The high-magnification image (Figure 4a) shows a compact, agglomerated secondary particle composed of smaller primary grains. The particle surface appears rough, with visible porosity and irregular, undulating features. Such morphology is indicative of a layered oxide structure and suggests that the material retains its crystallinity in the as-received state.

In the lower-magnification image (Figure 4b), the powder exhibits a broad particle size distribution, with sizes ranging from approximately 8.19 µm to 102.32 µm. Individual particles display irregular shapes with angular and blocky features, while larger agglomerates are also observed. The presence of both coarse and fine fractions implies the material has undergone limited post-synthesis classification. This heterogeneity in size and morphology may significantly influence the uniformity of mechanochemical activation and the kinetics of subsequent leaching processes.

The combination of well-faceted grains and agglomerated clusters in the as-received powder indicates a mixture of crystalline integrity and structural disorder, which could affect the accessibility of internal surfaces during chemical treatment steps.

The infrared (IR) spectrum of the initial cathode material (Figure 5) reveals distinct absorption features corresponding to its main constituents—LiCoO₂ and carbon.

Vibrational modes detected in the 610–675 cm⁻¹ region are characteristic of Co–O bond stretching within CoO₆ octahedral units, confirming the structural presence of lithium cobalt oxide. Meanwhile, the broad absorption bands spanning 1350–1600 cm⁻¹ are linked to disordered and graphitic forms of carbon, associated with the D and G bands, respectively. These signals point to the inclusion of imperfect or partially graphitized carbon, commonly added to enhance conductivity in electrode materials.

Additional spectral peaks near 1028 cm⁻¹ and 1720 cm⁻¹ suggest C–O and C=O stretching vibrations, implying the existence of oxygen-containing surface groups likely formed during cycling or synthesis. Furthermore, the presence of absorptions around the 2800–3000 cm⁻¹ range is indicative of C–H stretching modes, respectively, which may originate from organic binder residues or adsorbed moisture. Collectively, these IR features affirm the coexistence of LiCoO₂ and carbon in the cathode, along with trace amounts of surface-bound organic or environmental species [35,36].

3.2. Characterization of the LCO Cathode Material After Mechanical Activation

3.2.1. Optimization of Mechanical Activation Conditions

The Taguchi design DOE was applied to identify optimum mechanochemical milling parameters for the breakdown of the LiCoO₂ structure. It was chosen (

Table 2. Taguchi design levels for each factor—3⁴ orthogonal array.

Factor	Level 1	Level 2	Level 3	Level 4
Rotation speed (rpm)	500	600	700	800
Milling time (min)	15	30	45	60
Ball-to-powder ratio	35	50	65	80
Ball diameter (mm)	4	6	8	10

and

Table 3. Experimental setup according to the Taguchi design.

№	rpm (A)	Time (B)	BPR (C)	Ball Diameter (mm) (D)	Relative Intensity of LiCoO2 (003) Diffraction
T1	500	15	35	4	1.00
T2	500	30	50	6	0.88
T3	500	45	65	8	0.73
T4	500	60	80	10	0.66
T5	600	15	50	8	0.55
T6	600	30	35	10	0.77
T7	600	45	80	4	0.61
T8	600	60	65	6	0.50
T9	700	15	65	10	0.42
T10	700	30	80	8	0.33
T11	700	45	35	6	0.29
T12	700	60	50	4	0.25
T13	800	15	80	6	0.19
T14	800	30	65	4	0.16
T15	800	45	50	10	0.11
T16	800	60	35	8	0.08

) due to its efficiency in determining the optimum combination of multiple parameters with a reduced number of experimental runs. It enables identification of the most influential factors affecting the intensity of the (003) peak, serving as an indicator of phase transformation. Specifically, the response value was calculated as the ratio of the normalized (003) peak intensity of the given sample (Iₛₐₘₚₗₑ) to the corresponding peak intensity in the reference material (I_reference), which represents the maximum observed value. This approach enables a comparative assessment of structural changes across samples subjected to different processing conditions, using the initial cathode as a baseline.

The maximum intensity of the (003) peak of LiCoO₂ (response), as can be seen from Figure 3, was located in the 2θ range of 18.5–19.0°. The (003) peak intensity declines monotonically with rpm when data are grouped by rotational-speed level (not by sample order). The normalized intensity of the (003) reflection decreased from 1.00 (sample 1) to 0.08 (sample 16), corresponding to a 92% reduction, indicative of substantial X-ray amorphization and structural degradation.

Figure 6 presents the main effects plot derived from the Taguchi analysis, revealing that higher rpm levels promote more effective lattice breakdown. The optimum parameters for subsequent mechanical activation: 800 rpm, 60 min, B:P ratio—50, ball diameter—8 mm.

The optimum was determined by the standard Taguchi criterion: for each factor, the level corresponding to the lowest mean and lowest S/N (smaller-the-better) in the main-effects plots of Figure 7 was selected. The values are derived directly from the data in Table 3 and the Taguchi analysis.

The ANOVA analysis (

Table 4. One-way ANOVA for the effect of factors.

Source	DF	SS	MS	F	p
rpm	3	1.0946	0.3649	32.64	0.000
Error	12	0.1341	0.0112
Total	15	1.2287
S = 0.1057; R − Sq = 89.08%
time, min	3	0.0794	0.0265	0.28	0.841
Error	12	1.1493	0.0958
Total	15	1.2287
S = 0.3095; R − Sq = 6.46%
BPR	3	0.022	0.007	0.07	0.973
Error	12	1.207	0.101
Total	15	1.229
S = 0.3171; R − Sq = 1.80%
ball size, mm	3	0.016	0.005	0.05	0.984
Error	12	1.213	0.101
Total	15	1.229
S = 0.3179; R − Sq = 1.27%

) revealed that milling speed has a statistically significant effect on the structural degradation of LiCoO₂, as reflected by the decreasing intensity of the (003) XRD peak (p < 0.001, R² = 89.08%). This indicates that higher rotational speeds lead to greater amorphization and lattice disruption. In contrast, milling time (p = 0.841), ball-to-powder ratio (p = 0.973), and ball size (p = 0.984) showed no significant influence within the tested ranges, suggesting their limited impact on phase transformation under the selected conditions. Therefore, the optimized parameters (800 rpm, 60 min, BPR 50, ball size 8 mm) were selected for subsequent experiments involving reductive mechanochemical activation with aluminum and carbon additives, as discussed in Section 3.2.2.

3.2.2. Structural Changes After Mechanical Activation

Figure 3 shows the XRD pattern of the initial cathode material, which is dominated by sharp and intense diffraction peaks characteristic of a highly crystalline layered LiCoO₂ phase (space group R-3m). The main reflections are observed at 2θ ≈ 18.7°, 36.5°, 44.7°, 59.0°, and 65.5°, corresponding to the (003), (101), (104), (018), and (110) planes. These peaks confirm the presence of well-ordered layered LiCoO₂ with minimal lattice distortion.

In comparison, the XRD pattern of sample No. 6 (T6) after mechanical activation (shown in Figure 7) exhibits significant peak broadening and a decrease in peak intensity, particularly for the (003) and (104) reflections. This behavior indicates a reduction in crystallite size and an increase in lattice strain, both of which result from the mechanical stress induced by milling. Additionally, the appearance of new peaks near 36.8°, 42.2°, and 61.5° corresponds to the formation of CoO, suggesting that partial decomposition of LiCoO₂ has occurred.

In more intensively activated samples, such as sample No. 13 (Figure 7), the XRD pattern reveals a marked loss of crystallinity, with the characteristic LiCoO₂ peaks becoming almost undetectable. Instead, the pattern shows a broad amorphous halo, indicating a transition to a predominantly amorphous or nanocrystalline structure. Minor reflections in the 2θ range of 31–37° and 55–65° are consistent with CoAl₂O₄ spinel and possible AlCo intermetallic phases, which are likely to form in systems containing aluminum.

These structural transformations clearly demonstrate that mechanical activation disrupts the long-range order of the LiCoO₂ lattice, promotes the formation of secondary phases, and enhances structural reactivity. Such modifications are highly beneficial for subsequent hydrometallurgical recovery processes, particularly lithium and cobalt leaching.

3.2.3. Changes in Morphology After Mechanical Activation

SEM images of the mechanically activated samples (Figure 8) reveal significant morphological transformations. Mechanical treatment led to fragmentation of the initial particles into finer, more irregularly shaped aggregates. The particle sizes were drastically reduced, with most samples exhibiting a broad distribution in the submicron to low-micron range (e.g., 0.6–6 µm). The surfaces of the particles became rough, porous, and in some cases amorphized, indicating partial structural collapse and enhanced reactivity. Specifically: sample (a), activated at 500 rpm for 60 min with a ball-to-powder ratio of 80:1 (T4 sample), showed a heterogeneous distribution of particle sizes (0.67–4.99 µm, P80 ~ 5.0 µm) with evident surface roughness; sample (b), activated at 600 rpm (T8 sample), exhibited more uniform, submicron particles (~0.88–2.02 µm, P80 ~ 2.5 µm) and reduced aggregation; sample (c), activated at 700 rpm (T12 sample), showed increased aggregation and size dispersion (0.86–5.96 µm, P80 ~ 1.8 µm), with signs of partial amorphization. Sample (e) subjected to shorter activation time for 15 min (T 13 sample) retained larger fragments (~6–8 µm, P80 ~ 1.0 µm), although signs of surface disruption were still present.

SEM analysis confirmed the fragmentation of coarse cathode particles into fine, irregular aggregates, with particle sizes ranging from submicron to low-micron levels (0.6–6 µm). Higher rotational speeds and longer activation times led to increased amorphization and surface roughening, indicating enhanced structural disruption and reactivity. These morphological modifications are expected to improve subsequent leaching efficiency due to the increased surface area and defect density.

3.2.4. Influence of Aluminum and Carbon Additives on the Phase Composition of the Initial Cathode Material

The mechanochemical activation in the presence of aluminum and carbon additives significantly affects its phase composition. The rationale for employing these additives lies in their established role as in situ reductants: aluminum is a strong reducing agent that promotes the transformation of LiCoO₂ into CoO and spinel-type CoAl₂O₄ through aluminothermic pathways, while carbon facilitates partial carbothermic reduction and induces structural disorder [29,30,31]. These previous studies demonstrated that both Al and C can simplify recycling by eliminating the need for prior separation steps and by enhancing phase decomposition, providing the basis for their application in the present work.

Upon the introduction of aluminum during mechanical activation, notable changes are observed in the IR (Figure 9) and XRD patterns (Figure 10 and Figure 11). XRD (Figure 11 and Figure 12) and IR patterns (Figure 10). The appearance of secondary phases such as CoO, spinel-type CoAl₂O₄, and possibly intermetallic AlCo suggests active redox interactions between aluminum and cobalt oxide. These transformations are indicative of a partial reduction of Co³⁺ to Co²⁺ facilitated by the strong reducing nature of metallic aluminum, especially under the high-energy milling conditions. Moreover, the mechanical impact promotes atomic-scale mixing, enhancing the formation of new phases through solid-state reactions.

Carbon incorporation during mechanical activation also induces structural changes, though via a mechanism distinct from that of aluminum. Post-treatment XRD analysis reveals the presence of CoO and a noticeable decrease in the intensity of LiCoO₂ reflections, along with signs of increased structural disorder and partial amorphization. These effects can be linked to carbon’s moderate reducing nature and its facilitation of enhanced mechanical disruption.

Unlike aluminum, carbon does not participate in the formation of new crystalline compounds with cathode constituents. Instead, it contributes to destabilizing the layered lattice of LiCoO₂, leading to its partial breakdown. While aluminum promotes the development of distinct phases such as CoO, CoAl₂O₄, and potentially AlCo—indicative of strong redox reactivity and alloy formation—carbon mainly fosters lattice collapse and CoO formation without generating crystalline byproducts. This contrast highlights aluminum’s role as a more potent reducing and alloying agent in mechanochemical processes compared to the structurally disruptive, but chemically milder, nature of carbon.

In the case of aluminum addition (1 g Al), XRD and IR data suggest partial reduction of LiCoO_2, accompanied by the formation of secondary phases such as CoO, CoAl₂O₄, and potentially intermetallic AlCo. These changes can be attributed to the strong reducing nature of aluminum, which reacts with Co³⁺ ions in LiCoO₂, reducing them to Co²⁺ and facilitating the formation of spinel-type and mixed metal oxide phases upon subsequent calcination. The presence of CoAl₂O₄ indicates successful incorporation of Al³⁺ into the cobalt oxide matrix, suggesting a solid-state reaction pathway during mechanical activation.

In contrast, carbon addition (0.5 g C) results in less extensive phase transformation. The XRD and IR spectra reveal the presence of LiCoO₂ as the major phase, with minor formation of CoO and CoAl₂O₄, indicating that carbon acts as a milder reductant compared to aluminum. The lower intensity of secondary phase peaks suggests a slower or less complete reduction process, potentially due to the limited reactivity of carbon at the mechanochemical conditions applied.

Such suppression of LCO reflections together with the emergence of CoO/spinel features after high-energy milling is consistent with prior mechanochemical studies that report defect generation, amorphization, and reductive phase evolution in LCO and other oxide systems, which in turn accelerates subsequent leaching [21,22,29]. Similar activation-assisted enhancements of alkali extraction have also been shown for silicate matrices (lepidolite, microcline) [37,38,39].

The sample processed with Al and C (Figure 12) shows a significant suppression of the initial crystalline reflections, accompanied by the appearance of a broad, diffuse background signal—indicative of extensive amorphization. This transformation suggests that high-energy milling in the presence of reducing agents (Al and C) disrupts the long-range order of the LiCoO₂ structure, likely through partial reduction of cobalt ions and the formation of intermediate phases (e.g., CoO, CoAl₂O₄) and spinel-like or amorphous structures.

The near-complete disappearance of characteristic LiCoO₂ peaks confirms that the combined action of aluminum and carbon facilitates not only mechanical fragmentation but also redox-induced structural degradation, thereby enhancing the material’s reactivity for subsequent hydrometallurgical extraction

3.3. Ammonia Leaching of Spent LCO Cathode Material

Ammonia leaching of mechanochemically activated LiCoO₂ cathode material proceeds through the formation of soluble ammine and carbonate complexes of lithium and cobalt. The mechanochemical treatment disrupts the layered LiCoO₂ structure, partially reducing Co³⁺ to Co²⁺ and producing CoO, CoAl₂O₄, and amorphous phases, which are more reactive in ammonia media. The primary reactions can be expressed as:

LiCoO_2(s) + 2NH_3(aq) + H₂O → Li⁺_(aq) + 2NH₄⁺_(aq) + CoO(OH)_(s).

(1)

Li₂O_(s) + (NH₄)₂CO_3(aq) → 2Li⁺_(aq) + CO₃²⁻_(aq) + 2NH_3(aq)

(2)

CoO_(s) + 6NH_3(aq) → [Co(NH₃)₆]²⁺_(aq)

(3)

[Co(NH₃)₆]₂⁺_(aq) + CO₃²⁻_(aq) → [Co(NH₃)₆]CO_3(aq)

(4)

Thus, lithium predominantly dissolves as free Li⁺ stabilized by carbonate ions, while cobalt forms hexaammine complexes, enhancing solubility under mild leaching conditions [40,41,42,43].

To evaluate the effect of mechanochemical activation on the leachability of lithium and cobalt, a series of ammonia leaching experiments was conducted. Five representative samples were selected: initial cathode material (B); mechanically activated sample under optimum Taguchi conditions (C); sample activated with the addition of 0.25 g of carbon powder, serving as a reducing agent and potential conductive additive (D); sample activated with 1 g of aluminum powder, acting as a chemical reducer to facilitate structural breakdown (E); sample jointly activated with carbon (0.25 g) and aluminum (1 g), enabling synergistic reduction and mechanical fragmentation (F).

Leaching tests were conducted under controlled parameters derived from previous studies [40,43] to achieve efficient dissolution of cobalt and lithium. Constant agitation was applied to maintain a stable pH environment and to inhibit the formation of cobalt hydroxide precipitates. These operational conditions favored the generation of soluble metal–ammine and carbonate complexes, thereby enhancing the recovery of lithium and cobalt from the pre-activated cathode material.

The results (Figure 13a) demonstrate that mechanochemical activation significantly enhances lithium leaching efficiency. After 6 h, lithium recovery increased from 34.7% (initial) to 79.7% (Taguchi-optimized sample). The addition of reducing agents further improved extraction, with the aluminum-activated sample reaching 91.8%, and the combined Al + C sample achieving the highest lithium recovery of 94.6%.

Similarly, cobalt leaching (Figure 13b) improved substantially following activation. The recovery of cobalt from initial LiCoO₂ was limited to 18.8% after 6 h. However, activated samples exhibited markedly enhanced performance: 67.3% for the Taguchi optimized sample, 80.1% with Al addition, and 83.7% for the combined Al + C case.

To evaluate the effect of temperature, additional leaching experiments were performed on the Al + C sample at 40 °C, 60 °C, and 80 °C. As shown in Figure 14, the extent of lithium and cobalt extraction increased with temperature. At 40 °C, lithium recovery reached 84.2% and cobalt 72.4% after 6 h. At 80 °C, nearly complete lithium (97.0%) and cobalt (87.9%) extraction was achieved within the same timeframe, indicating enhanced leaching kinetics at elevated temperatures.

The enhanced recoveries obtained in this study (94.6% Li and 83.7% Co under optimized Al + C mechanochemical conditions) compare favorably with values reported in the literature. For instance, Liu et al. [40] achieved ~85% Li and ~70% Co recovery through ammoniacal leaching without prior mechanochemical activation. Similarly, Wang et al. [41] reported Li recovery of 80–85% and Co recovery below 70% when using mechanochemical pre-treatment without reductants. Guan et al. [42] demonstrated that mechanochemical activation improved cobalt dissolution to ~75%, but lithium recovery remained below 90%. In contrast, the synergistic combination of Al and C additives in the present work resulted in nearly complete lithium extraction and significantly higher cobalt recovery, highlighting the critical role of reductant-assisted mechanochemical activation in enhancing subsequent ammonia leaching.

These findings confirm that mechanochemical pretreatment significantly promotes the breakdown of the LiCoO₂ structure and facilitates metal recovery. The synergistic use of aluminum and carbon as reducing and disrupting agents further boosts extraction, particularly under thermally optimized leaching conditions.

4. Conclusions

In this study, an integrated mechanochemical approach was developed for the selective recovery of lithium and cobalt from spent lithium cobalt oxide (LiCoO₂) cathode materials. For the first time, aluminum and carbon, commonly present as additives in spent batteries, were directly utilized as reducing agents during high-energy ball milling. This in situ reduction enabled the decomposition of LiCoO₂ into CoO, Li₂O, and metallic cobalt, which significantly enhanced the reactivity of the material toward ammonia leaching.

X-ray diffraction analysis confirmed the formation of intermediate phases, including AlCo and CoO, while the possible presence of CoAl₂O₄ was inferred in samples with high aluminum content. The application of the Taguchi design enabled the identification of optimal milling parameters (800 rpm, 60 min, ball-to-powder ratio of 50:1), leading to a sample with improved phase transformation and reactivity.

Ammonia leaching experiments were performed on selected samples using 3.0 M NH₃·H₂O and 1.0 M (NH₄)₂CO₃ at 60 °C for 6 h with a liquid-to-solid ratio of 25 mL/g. The results demonstrated significantly enhanced metal recovery from mechanochemically treated materials compared to the initial LiCoO₂. In particular, the sample processed under optimal milling conditions with combined aluminum and carbon additives achieved lithium and cobalt recovery rates of 94.6% and 83.7%, respectively. This improvement was attributed to the synergistic effects of mechanical amorphization, reductive decomposition, and increased surface reactivity.

The proposed method provides a promising and environmentally benign route for recycling LiCoO₂ cathodes without the need for extensive pre-separation steps. The use of aluminum and carbon as internal reductants not only simplifies the process but also reduces chemical consumption. These findings offer a scalable and cost-effective strategy for the sustainable management of lithium-ion battery waste.