Copper-Mediated Leaching of LiCoO₂ in H₃PO₄: Kinetics and Residue Transformation

Abstract

The recycling of spent lithium-ion batteries (LIBs) requires efficient and sustainable methods for recovering critical metals. In this study, the leaching behavior of LiCoO₂ cathode material obtained from spent LIBs was investigated in phosphoric acid, using copper powder recovered from waste LIBs as a reducing agent. Leaching experiments were conducted under various conditions (temperature, solid-to-liquid ratio, agitation rate) and compared with systems without copper. In the absence of copper, lithium and cobalt, recoveries after 30 min were approximately 77% and 23%, respectively. The addition of copper significantly enhanced leaching, achieving >96% recovery for both metals at 80 °C, with most extraction occurring within the first 30 min. Kinetic analysis using the shrinking core model indicated a mixed-control mechanism involving both surface chemical reaction and product layer diffusion. The calculated activation energies were 20.2 kJ·mol⁻¹ for lithium and 16.1 kJ·mol⁻¹ for cobalt. Solid residues were characterized by X-ray diffraction (XRD) and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS). XRD results revealed that the composition of the residues varied with leaching temperature: Co₃O₄ was consistently detected, whereas Cu₈(PO₃OH)₂(PO₄)₄·7H₂O appeared only when leaching was performed above 50 °C. Thermodynamic calculations supported the reductive role of copper and provided insight into possible reaction pathways. These findings confirm the effectiveness of copper-mediated leaching in phosphoric acid and demonstrate that temperature strongly influences residue phase evolution, thereby offering valuable guidance for the design of sustainable LIB recycling processes.

1. Introduction

The rapid rise of electric vehicles (EVs), portable electronics, and renewable energy systems has led to a sharp increase in LIB production [1,2]. Battery demand is projected to grow more than fourteenfold by 2030 relative to 2018, driven largely by transport electrification [3]. In parallel, the consumption of critical raw materials such as cobalt and lithium is rising significantly; for instance, their use in EV batteries alone is expected to nearly triple between 2020 and 2030. This growing reliance is particularly concerning given that these metals are predominantly sourced from a limited number of countries, increasing the vulnerability of global supply chains and potentially constraining the growth of the battery industry due to geopolitical and economic factors [4]. Moreover, natural deposits of cobalt and lithium often contain lower concentrations than those found in spent lithium-ion batteries, particularly in LiCoO₂ cathodes, which typically contain 5–15 wt% Co and 2–7 wt% Li, making battery waste a particularly rich and strategic secondary resource [5].

In this context, the development of effective recycling strategies is essential not only for economic reasons but also due to the environmental and health risks posed by LIB components. Spent batteries contain toxic and potentially hazardous substances, including heavy metals and organic electrolytes, which can leach into the environment if not properly managed [6,7]. Among various cathode chemistries, lithium cobalt oxide (LiCoO₂) remains one of the most widely used materials, particularly in early-generation commercial batteries [8]. Its layered structure allows reversible Li⁺ intercalation [9], but it also poses challenges for efficient metal recovery when the battery reaches the end of its life [10]. Traditional recycling methods include pyrometallurgy, biometallurgy, and hydrometallurgy [11,12]. Among these, hydrometallurgical techniques are especially appealing because they can recover high-purity metals under relatively mild conditions, while reducing energy use and gas emissions [4]. Recently, direct recycling has also emerged as a promising laboratory-scale method, offering notable environmental and economic advantages over traditional recycling technologies. However, its large-scale commercialization remains limited due to economic and technical challenges [13].

Leaching of LiCoO₂ in acidic media generally requires a reductive environment to solubilize Co³⁺ by reducing it to Co²⁺. In the absence of a strong reducing agent, water acts as the electron donor, resulting in a slow leaching process where Li+ and Co²⁺ are gradually released from LiCoO₂. This process often leads to the formation of a Co₃O₄ crust around the unreacted core, limiting further leaching by hindering diffusion. This highlights the importance of introducing an effective reducing agent to enhance reaction efficiency [4]. Most commonly, hydrogen peroxide (H₂O₂) has been employed as a reductant in combination with mineral acids such as H₂SO₄ or HCl [3,14,15,16]. However, the use of H₂O₂ presents several drawbacks, including instability, non-selective reactivity, and potential safety hazards [17]. Consequently, there has been growing interest in alternative, environmentally benign reductants such as glucose (C₆H₁₂O₆) and ascorbic acid (C₆H₈O₆), which have demonstrated effectiveness in various leaching processes [18,19].

Previous studies have demonstrated the effectiveness of metallic copper as a reducing agent for LiCoO₂ leaching in sulfuric acid media [17,20,21,22,23,24]. For example, Peng et al. [24] reported that copper and aluminum foils recovered from spent LIBs significantly enhanced the leaching efficiencies of cobalt and lithium by acting as reductants in sulfuric acid solutions. Their approach exploited the naturally present Cu and Al in the waste material itself, eliminating the need for expensive additives such as hydrogen peroxide or ascorbic acid, and achieving over 99% extraction of the target metals. Similarly, Porvali et al. [17] investigated the kinetics of LiCoO₂ dissolution in sulfuric acid media enhanced with metallic copper and catalytic amounts of ferrous ions, concluding that copper effectively donates electrons to ferric ions, which then mediate the reductive dissolution of LiCoO₂. Although these studies strongly suggest the potential of copper in sulfuric systems, to the best of our knowledge, no investigations have yet explored the use of copper powder as a reductant in phosphoric acid systems.

Unlike stronger mineral acids, it generates fewer toxic fumes and enables potential recovery of valuable phosphate-based by-products. However, its interactions with metallic reducing agents such as copper have not been extensively studied, particularly in the context of lithium cobalt oxide leaching.

Given the limited application of phosphoric acid in combination with metallic copper, this study focuses on investigating the leaching kinetics of Li and Co from LiCoO₂ in the presence of copper powder. Special emphasis is placed on temperature effects, solid residue characterization, and thermodynamic interpretation. This approach contributes to the development of efficient and environmentally sustainable recycling methods for spent LIBs.

2. Materials and Methods

2.1. Materials and Reagents

The cathode material used in this study was obtained from lithium-ion batteries recovered from discarded laptops of various models and manufacturers. Copper powder, serving as the reducing agent, was prepared by manually grinding copper foil separated from the current collectors of spent LIBs.

Phosphoric acid (H₃PO₄, Zorka Pharma, Šabac, Serbia) was used as the leaching agent. To determine the lithium and cobalt content in the cathode material, samples were digested using an aqua regia mixture of nitric acid and hydrochloric acid in a 1:3 volume ratio (HNO₃:HCl, Merck, Darmstadt, Germany).

All reagents were of analytical grade, and all solutions were prepared with deionized water.

2.2. Analytical Methods

The chemical composition of the initial cathode material and leachates was determined using inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 8300, PerkinElmer, Waltham, MA, USA).

The phase composition of both the pristine material and the leaching residues was examined using X-ray diffraction (Rigaku MiniFlex 600, Tokyo, Japan), equipped with a Cu Kα radiation source (λ = 0.154 nm) and operated at 40 kV and 15 mA.

Surface morphology and microstructural features were analyzed using scanning electron microscopy (JSM-IT300LV, JEOL, Tokyo, Japan) operated at an accelerating voltage of 20 keV. Elemental composition was further assessed by energy-dispersive X-ray spectroscopy (EDS), integrated with the SEM system. Data acquisition and analysis were performed using Aztec software (Aztec 3.1 SP1).

Thermodynamic analyses were performed using HSC Chemistry 9.9.2.3 to complement the experimental findings and to verify that the presence of copper powder thermodynamically favors cobalt reduction and leaching. The Gibbs free energy change (ΔG) for the relevant reactions was calculated to evaluate the spontaneity and feasibility.

2.3. Preparation of the Cathode Material

For this study, 40 LIBs from laptop computers of various manufacturers were collected. On average, each battery contained six cylindrical LIB cells. Battery identification was performed according to the procedure described by Medić et al. [25]. Only batteries with LiCoO₂ cathodes were selected for further processing.

The dismantling of LIBs into individual cells was manually carried out using standard handheld tools. After full discharge, the cell terminals were removed with a hacksaw, followed by a longitudinal cut along the casing. Once the metal casing was removed, each cell was disassembled into its components: plastic casing, metal casing, cathode, anode, and separator.

To remove the organic binder, the cathode materials (still attached to aluminum foil) were thermally treated at 580 °C for 10 min in a muffle furnace under atmospheric air conditions. After cooling to room temperature, the cathode materials were carefully collected. A representative image of the aluminum foil and resulting thermally treated cathode powder is shown in Supplementary Materials Figure S1.

In some cases, portions of the cathode material remained adhered to the separator. To minimize material loss, the separators were annealed at 300 °C. Figure S2 shows a separator with residual cathode material. The recovered material was combined with the cathode powder previously obtained from aluminum foil and subjected to an additional calcination step at 630 °C for 6 h in air to ensure complete removal of residual carbon.

The prepared cathode materials were then ground using an agate mortar and pestle. To avoid contamination, the mortar was cleaned with quartz sand before each use.

In addition, copper foil from the anode material was mechanically separated from the separator, rinsed with distilled water, dried at room temperature, and milled using a laboratory grinder to obtain the copper powder used in this study. A representative image of the manually separated copper foil is shown in Figure S3.

2.4. Leaching Procedure

Leaching experiments were carried out in a 250 mL double-walled glass reactor equipped with a condenser to prevent evaporation of the leaching solution and a thermometer to ensure precise temperature control (Figure S4). A volume of 50 mL of leaching solution at the desired concentration was thermostated to the target temperature. Once the set temperature was achieved, a pre-weighed amount of cathode material was added to the reactor, depending on the experimental conditions. In all experiments, 0.2 g of copper powder was used as a reducing agent, except in the tests investigating the effect of the reducing agent, where no copper powder was introduced. Magnetic stirring was maintained at 600 revolutions per minute (rpm) in all experiments, except in those designed to study the influence of stirring speed on the leaching efficiency of Li and Co.

Aliquots (1.0 mL) of the leaching solution were collected at predetermined time intervals (5, 15, 30, 45, and 60 min), filtered, diluted to 50 mL in volumetric flasks, and subsequently analyzed for metal content using ICP-OES.

All solid residues were collected by filtration, thoroughly rinsed with distilled water, and dried. Selected leach residues were further characterized by SEM and XRD analysis.

3. Results and Discussion

3.1. Characterization of the Cathode Material

ICP-OES analysis of the post-calcined cathode material revealed cobalt and lithium contents of 59.89 wt.% and 6.97 wt.%, respectively. The XRD presented on Figure 1 identified LiCoO₂ as the predominant crystalline phase, along with a minor amount of Co₃O₄. XRD characterization was performed on the calcined material because the as-received sample contains non-active electrode constituents such as binder and carbonaceous matter, which significantly elevate the background signal and obscure the characteristic diffraction peaks of LiCoO₂. Calcination at 630 °C significantly reduces these residues, lowering the background signal and enabling clearer phase identification. The presence of Co₃O₄ is attributed to the partial degradation of LiCoO₂ during battery operation, as well as to phase transformations induced by thermal treatment of the spent material. According to Li et al. [26], Co₃O₄ is frequently observed as a degradation product formed under extended electrochemical cycling. This is further supported by Medić et al. [27], who reported that thermal decomposition of LiCoO₂ contributes to the formation of Co₃O₄ in heat-treated cathode residues.

Figure 2 shows the SEM image of the prepared cathode powder for leaching, with particle contours overlaid using ImageJ 1.54p, and the resulting particle-size distribution histogram with cumulative curve and characteristic diameters (D10, D50, D90).

Image analysis of the SEM micrograph in Figure 2b using ImageJ 1.54p showed that the cathode powder consists predominantly of fine particles with a relatively narrow size distribution. The mean Feret diameter was 3.37 µm with a standard deviation of 1.76 µm, while the minimum and maximum measured values were 1.4 and 18.2 µm, respectively. The powder is strongly dominated by small particles—the D10, D50 and D90 values were 1.8, 2.9, and 5.6 µm, meaning that half of the particles are smaller than about 3 µm, and nearly all (90%) remain under 5.6 µm. The histogram in Figure 2c shows that the largest fraction of particles falls within the 2.5–5 µm interval, whereas only a very small fraction (<1%) exceeds 10 µm, confirming the presence of a fine powder with a minimal share of particles above 10 µm. Such a particle-size distribution is expected to provide a high specific surface area and thus favorable conditions for subsequent leaching of the cathode material.

3.2. Optimal Leaching of Waste Cathode Material

3.2.1. Effect of Copper Powder on the Leaching Efficiency of Li and Co

To investigate the role of the reducing agent, the leaching efficiencies of lithium and cobalt were compared in systems with and without copper powder. All experiments were conducted using 50 mL 0.4 mol·L⁻¹ H₃PO₄ as the leaching agent at 80 °C, with a stirring speed of 600 rpm. The mass of cathode material was 0.4 g. In the test with the reducing agent, 0.2 g of copper powder was added to the reaction system. As shown in Figure 3, the presence of copper significantly enhanced the leaching efficiency of both metals, in agreement with previous findings that metallic Cu increases the dissolution efficiency of both Li and Co in acidic leaching systems [17,24]. In the absence of copper (H₃PO₄ only), lithium extraction remained stable at around 75%, while cobalt extraction stayed below 25% throughout the 60 min leaching period. The addition of copper powder significantly enhanced the extraction of both metals. Lithium recovery rapidly exceeded 94% within the first 15 min and remained high thereafter. Although the final extraction percentage was lower for cobalt, the effect of copper addition was more pronounced in its case-cobalt extraction increased to over 84% within just 15 min.

This remarkable improvement in leaching efficiency is consistent with the presence of metallic copper in the system, which promotes substantially faster cobalt dissolution compared to leaching without Cu. While the exact electron-transfer pathway cannot be confirmed within the scope of this study, the early increase and subsequent decrease in dissolved Cu²⁺ indicate that copper actively participates in processes that enhance cobalt release. As shown in Figure 4, the concentration of dissolved Cu²⁺ in the solution increased sharply within the first 15 min, indicating active oxidation of copper powder and its participation in redox reactions. This coincided with the steep rise in lithium and cobalt extraction, confirming the critical role of copper in promoting metal dissolution. After reaching peak values at around 30 min, the Cu²⁺ concentration gradually decreased, suggesting that copper was either consumed within the solution via precipitation or complexation. Despite this decline, the leaching efficiencies of both lithium and cobalt remained high (as presented in Figure 3), indicating that the majority of the leaching process occurred during the initial phase when copper activity was at its peak. These findings confirm that the presence of copper not only accelerates the dissolution kinetics but is also essential for achieving high extraction yields under the applied experimental conditions.

Previous studies have demonstrated that in acidic leaching systems, especially sulfate-based, metallic copper facilitates cobalt reduction often through indirect mechanisms involving ionic mediators such as Fe²⁺/Fe³⁺ or Cu⁺ species stabilized by chloride. For instance, Joulie et al. [21] reported that galvanic interactions between transition metal oxides and conductive components such as copper and aluminum can significantly enhance the leaching efficiency of lithium nickel manganese cobalt oxide (NMC) cathodes, even in the absence of externally added reducing agents or thermal activation. Similarly, Chernyaev et al. [22] and Porvali et al. [17] showed that copper can indirectly enable the reduction of Co³⁺ through redox mediation by Fe³⁺/Fe²⁺ cycling, where Cu regenerates Fe²⁺ from Fe³⁺, and Fe²⁺ subsequently reduces the active material. Partinen et al. [23] further highlighted the importance of chloride ions in stabilizing Cu+ species, which then catalyze electron transfer to LiCoO₂.

In contrast, our results demonstrate that in a phosphoric acid medium, and in the absence of both iron species and chloride ligands, copper powder alone is sufficient to induce rapid and complete leaching of both cobalt and lithium. The mechanism underlying this efficiency remains unclear, as the exact electron transfer pathway between Cu and LiCoO₂ is not yet identified. Nevertheless, the sharp increase in extraction rates, combined with the concurrent decrease in soluble copper concentration, strongly indicates that copper is directly involved in redox reactions and is progressively consumed during the process.

3.2.2. Effect of Cathode Material Mass on the Leaching Efficiency of Li and Co

The influence of cathode material mass on the leaching efficiency of lithium and cobalt was investigated in a 50 mL solution of 0.4 mol·L⁻¹ H₃PO₄ with the addition of 0.2 g of copper powder as a reducing agent. The experiments were conducted at a temperature of 80 °C and a stirring speed of 600 rpm. The obtained results are presented in Figure 5.

A clear decreasing trend in leaching efficiency was observed with increasing mass of the cathode material. When 0.4 g of LiCoO₂ was used, the leaching efficiencies of lithium and cobalt were 99.98% and 99.86%, respectively. As the mass increased to 0.6 g, lithium leaching dropped to 89.81% and cobalt to 76.42%. Further increasing the mass to 0.8 g resulted in lithium and cobalt extraction efficiencies of 85.28% and 45.23%, respectively. At the highest tested mass of 1.0 g, the leaching efficiency of lithium fell to 78.07%, while cobalt dropped drastically to only 26.34%.

The influence of the solids increase refers only to the cathode material mass, while the copper powder addition (0.2 g) was kept constant in all experiments. As the cathode mass increases, copper rapidly oxidizes and precipitates as insoluble copper-phosphate, reducing the available electrons for Co³⁺ reduction and intensifying diffusion resistance at the particle surface. Thus, the decline in cobalt leaching efficiency is primarily attributable to early copper precipitation and restricted electron availability, rather than to the modest increase in the total solids content of the suspension.

Considering these findings, the cathode material mass of 0.4 g was identified as the optimal value and was therefore used in all subsequent experiments.

3.2.3. Effect of Stirring Speed on the Leaching Efficiency of Li and Co

The effect of stirring speed (0, 200, 400, and 600 rpm) on the dissolution of cathode material was examined in a 50 mL solution of 0.4 mol·L⁻¹ H₃PO₄ with the addition of 0.2 g of copper powder. The experiments were conducted at a temperature of 80 °C using 0.4 g of LiCoO₂. The results are shown in Figure 6.

The obtained data indicate a positive correlation between stirring speed and leaching efficiency. In the absence of stirring (0 rpm), the leaching efficiencies of lithium and cobalt were significantly lower, with lithium extraction at 55.50% and cobalt at 41.64%. As the stirring speed increased to 200 rpm, Li and Co leaching efficiencies improved to 86.29% and 87.41%, respectively. With further increases to 400 rpm, the efficiencies continued to rise, reaching 87.79% for Li and 98.61% for Co. The highest leaching efficiencies were achieved at a stirring speed of 600 rpm, with Li extraction reaching 99.98% and Co reaching 99.86%.

This can be attributed to improved mass transfer, better particle suspension, and enhanced contact between the solid phase and the leaching solution at higher stirring rates [28,29]. Efficient mixing ensures uniform distribution of both the leaching agent (H₃PO₄) and the reducing agent (Cu), which collectively promote more effective dissolution of the active material.

Based on these results, a stirring speed of 600 rpm was selected as the optimal condition and was therefore used in all subsequent experiments.

3.2.4. Effect of Temperature and Time on the Leaching Efficiency of Li and Co

The effects of temperature and leaching time on the extraction efficiencies of lithium and cobalt were systematically investigated in a 50 mL 0.4 mol·L⁻¹ H₃PO₄ solution with the addition of Cu powder, under constant stirring conditions. The results are presented in Figure 7.

The data reveal a positive correlation between temperature and leaching efficiency. At 35 °C, after 60 min, lithium and cobalt achieved extraction efficiencies of 92.29% and 89.09%, respectively. As the temperature increased to 50 °C, the leaching efficiencies slightly improved to 97.29% for lithium and 95.53% for cobalt. Further enhancement was observed at 60 °C, with lithium extraction reaching 99.07% and cobalt 98.40%. At 70 °C, the efficiencies rose to 99.92% for lithium and 99.59% for cobalt, while the highest values were recorded at 80 °C—99.98% and 99.86%, respectively.

These findings are consistent with the well-established influence of temperature on leaching kinetics. According to the Arrhenius equation, the reaction rate increases exponentially with temperature as a result of the higher kinetic energy of the reacting species, leading to a greater frequency of effective collisions. Elevated temperatures also enhance ion mobility, improve diffusion processes, and accelerate surface reactions between the solid phase and the leaching agent. Furthermore, the increased reductive activity of copper at higher temperatures may facilitate the more efficient reduction of Co(III) to Co(II), thereby promoting cobalt dissolution. Similar effects were reported by Sahu et al. [30], who demonstrated that elevated temperatures significantly enhance the leaching efficiencies of lithium and cobalt from spent lithium-ion battery cathode materials.

In addition to temperature, leaching time had a significant impact on the extraction efficiencies of lithium and cobalt. At 80 °C, over 82% of Li and 59% of Co were extracted within 5 min, increasing to ~95% (Li) and ~84% (Co) after 15 min, and exceeding 97% (Li) and 96% (Co) after 30 min. At lower temperatures, extraction was slower but also improved with time, reaching ~92% (Li) and ~89% (Co) after 60 min at 35 °C. Although most extraction occurred within the first 30 min, a leaching time of 60 min was selected as optimal to ensure maximum and consistent metal recovery across varying conditions. This observation is further supported by the kinetic analysis presented in Section 3.3. The mass balance of Co, Li and Cu under the optimized leaching conditions is provided in Table S1.

3.3. Kinetics of Li and Co Leaching

The leaching kinetics of lithium and cobalt from LiCoO₂ in phosphoric acid in the presence of copper powder was investigated to determine the rate-controlling mechanism and assess the influence of temperature. A series of experiments was carried out at temperatures ranging from 35 to 80 °C under constant operating conditions: 50 mL 0.4 mol·L⁻¹ H₃PO₄ as the leaching agent, 0.4 g of cathode material, 0.2 g of copper powder, and a stirring speed of 600 rpm. The leaching process was monitored over a period of up to 60 min.

Given the fluid–solid, non-catalytic nature of the system, where solid LiCoO₂ reacts with a liquid leaching medium across a phase boundary, the process falls into the category of heterogeneous chemical reactions. Such systems are typically described using the shrinking core model (SCM), which accounts for possible control by surface chemical reactions, diffusion through a liquid film, or diffusion through a product (ash) layer [31,32,33]. Kinetic data were analyzed using SCM expressions for all three classical mechanisms, as well as a hybrid (mixed-control) model, which considers simultaneous chemical reaction and product layer diffusion:

$${\displaystyle \frac{1}{3}}ln\left(1-x\right)+\left[{\left(1-x\right)}^{{\displaystyle \frac{1}{3}}}-1\right]=kt$$

(1)

where x is the fraction of metal extracted at time t, and k is the apparent rate constant.

Among the tested models, the hybrid model provided the best fit for both lithium and cobalt, with correlation coefficients (R²) above 0.95 across the tested temperature range (Figure 8). Correlation coefficients for the other kinetic models are provided in Table S2, confirming that mixed control best describes the leaching mechanism under the given conditions. Although the surface chemical reaction model also provided relatively high R² values, the hybrid model more accurately reflects the evolving nature of the process. The fitting suggests that, initially, chemical reaction at the particle surface dominates, but as the reaction progresses, the progressive formation of a solid product layer increases the role of diffusion resistance. Consequently, the leaching process transitions from reaction-controlled to mixed-controlled, making the hybrid shrinking core model the most representative description of the mechanism under the tested conditions. A similar kinetic behavior was reported by Fan et al. [34], who investigated the leaching of LiNi_xMn_yCo_1-x-yO₂ cathode material in malonic acid medium. They demonstrated that leaching initially proceeds under surface-reaction control, but as the product layer grows, diffusion resistance becomes dominant; consequently, the system is best represented by a hybrid shrinking-core model. These findings suggest that the interplay between interfacial reaction and diffusion is a general characteristic of leaching processes involving complex multicomponent oxides such as LiCoO₂ and LiNi_xMn_yCo_1-x-yO₂, regardless of the acid medium or reducing agent used.

To further quantify the temperature dependence, the activation energy (Ea) was calculated using the Arrhenius equation:

$$k=A e^{-Ea/RT}$$

(2)

where k is the apparent rate constant (1/min), A is the frequency factor, Ea is the activation energy (J·mol⁻¹), R is the gas constant (8.314 J·mol⁻¹·K⁻¹), and T is the temperature (K). The activation energy represents the minimum energy barrier required for the leaching reaction to proceed.

The Arrhenius plots of ln k versus 1/T were constructed for both Li and Co based on rate constants obtained from the mixed-control model. As shown in Figure 9, linear regression of these plots yielded straight lines with high correlation, confirming the validity of the Arrhenius approach for this system. The calculated activation energies were 20.2 kJ·mol⁻¹ for lithium and 16.1 kJ·mol⁻¹ for Co, which, according to the classification by Jafari et al. [31], indicates a mixed-control reaction mechanism. In their study, activation energies between 13 and 42 kJ·mol⁻¹ were characteristic of systems governed simultaneously by chemical reaction and diffusion through a product layer (shrinking core model), without a single dominant mechanism. Therefore, the obtained values confirm that a mixed mechanism is operative.

The lower Ea for cobalt compared to lithium is attributed to the role of copper as a reducing agent, which accelerates the Co³⁺ → Co²⁺ reduction and facilitates its dissolution. In contrast, lithium, although not requiring reduction, may become partially trapped within solid residues (e.g., Co₃O₄, Cu₈(PO₃OH)₂(PO₄)₄·7H₂O), thereby facing diffusion barriers that increase its apparent activation energy.

3.4. Phase Evolution in Solid Residues During Cathode Material Leaching in the H₃PO₄–Cu System at Different Temperatures

XRD analysis of solid residues obtained after leaching the cathode material at different temperatures (35–80 °C) revealed changes in the crystalline phase composition, which were further examined to understand the influence of temperature (Figure 10).

XRD analysis of the residue obtained at 35 °C revealed the presence of Co₃O₄ and metallic copper. Since Co₃O₄ was already present in the original cathode material, its detection at this temperature does not necessarily indicate new formation but rather incomplete transformation or dissolution under mildly reducing conditions. The coexistence of Cu⁰ and Co₃O₄ suggests that redox reactions between metallic copper and cobalt species were initiated but remained incomplete. Similar observations were reported by Golmohammadzadeh et al. [35], who demonstrated that Co₃O₄ can remain stable during the degradation of LiCoO₂ under limited reducing conditions. Cerrillo-Gonzalez et al. [36] further showed that Co₃O₄ may persist as a decomposition product or precipitate from Co²⁺ under mild reducing conditions.

At 50 °C, both Co₃O₄ and metallic copper were still clearly detectable, with no substantial change in peak intensity compared to 35 °C. This indicates that the overall solid-phase composition remained largely unaffected by the moderate temperature increase, and that a portion of cobalt remained stabilized as Co₃O₄, either as an unreacted residual phase or a product of LiCoO₂ transformation.

At 60 °C, the XRD pattern revealed the emergence of peaks corresponding to Cu₈(PO₃OH)₂(PO₄)₄·7H₂O, a hydrated copper phosphate. The formation of this phase is attributed to the oxidation of Cu⁰ and its subsequent reaction with phosphate ions from the leaching medium. Chen et al. [37,38] reported that this compound can form on copper surfaces immersed in phosphoric acid even at room temperature, due to gradual oxidation and precipitation of Cu²⁺ with phosphate species. Although Co₃O₄ remained present in the residue, the appearance of the copper phosphate phase indicates additional transformation of copper at this temperature, independent of the extent of redox interaction with cobalt.

At 70 °C and 80 °C, the copper phosphate phase became dominant in the solid residues. However, Co₃O₄ remained consistently present, with no significant reduction in its diffraction peak intensity. This persistence suggests that a fraction of cobalt remains immobilized in oxide form even under more favorable conditions for redox transformation. Metallic copper was not observed in the XRD pattern, indicating that any remaining Cu was likely present below the detection limit or had been largely converted into the identified phosphate phase Cu₈(PO₃OH)₂(PO₄)₄·7H₂O.

It is important to note that LiCoO₂ was not detected in any of the residues, confirming its complete decomposition under all investigated conditions. This highlights the effectiveness of the H₃PO₄-Cu system in breaking down the layered cathode structure and releasing lithium and cobalt into solution.

The evolution of crystalline phases with increasing temperature highlights two key mechanistic aspects of the leaching process:

• The persistence of Co₃O₄ in the leaching residues at all investigated temperatures strongly suggests that this phase behaves as an essentially inert component inherited from the original cathode material.
• The progressive formation of Cu₈(PO₃OH)₂(PO₄)₄·7H₂O, reflecting the oxidation of Cu⁰ and serving as an indicator of redox activity within the system—an effect also observable visually: the residue was black at 35 °C and 50 °C, developed a grayish tone at 60 °C, and turned green at 70 °C and 80 °C due to the dominance of copper phosphate phases.

Notably, both Cu₈(PO₃OH)₂(PO₄)₄·7H₂O and Co₃O₄ are known for their photocatalytic properties, suggesting that the leaching residues may exhibit photocatalytic potential and therefore hold additional value beyond process waste.

3.5. SEM/EDS Characterization of the Leaching Residue at 80 °C

SEM imaging and EDS elemental analysis (Figure 11) provided further insight into the composition and morphology of the leaching residue obtained at 80 °C. The SEM micrograph revealed a heterogeneous mixture of morphologically distinct particles, including aggregates of fine-grained crystalline material and well-developed plate-like structures. The observed morphological heterogeneity reflects the simultaneous formation of multiple secondary phases, generated through the interaction of copper powder with phosphoric acid and dissolved metal ions.

Quantitative EDS spectra acquired from selected points confirmed that copper is the dominant element (~70 wt%), while phosphorus and cobalt were also detected in lower amounts. These results are in good agreement with the phase identification obtained from XRD analysis. Elemental mapping reveals overlapping distributions of Cu, P, and Co within the solid residue, which is consistent with the formation of copper phosphate phases. The presence of cobalt in these regions may indicate its partial incorporation into secondary phases or its retention in trace amounts on surface-associated domains.

These Co-containing areas are likely associated with residual Co₃O₄, as suggested by the presence of weak Co₃O₄ reflections in the XRD patterns. No distinct Co-phosphate phases were identified, implying that if cobalt is incorporated into secondary products, it is either in minor amounts or within poorly crystalline or amorphous structures.

Combined SEM/EDS and XRD results indicate that, under optimal leaching conditions (80 °C, 0.4 mol·L⁻¹ H₃PO₄, 0.2 g Cu), lithium and cobalt are almost completely extracted into solution, whereas copper predominantly precipitates as low-solubility phosphate phases. These precipitates are most likely dominated by Cu₈(PO₃OH)₂(PO₄)₄·7H₂O, as confirmed by XRD, and the spatial overlap of Cu and P in the elemental maps further supports this assignment. Trace amounts of cobalt may be physically trapped or co-precipitated within these secondary phases, but the majority of cobalt is removed via dissolution.

3.6. Interpretation of Solid-Phase Changes and Redox Behavior: Insights from XRD, SEM/EDS, and Thermodynamic Analysis

Based on the experimentally determined optimal conditions, thermodynamic calculations were performed to confirm the observed phase transformations and clarify the role of copper as a reductant. The XRD results (Figure 10) enabled the identification of crystalline phases in the leaching residues, while SEM images and EDS elemental analysis (Figure 11) provided information on surface morphology and local elemental distribution. Combining these methods confirmed the formation of phases such as Cu₈(PO₃OH)₂(PO₄)₄·7H₂O and provided insights into the behavior of cobalt and copper under the applied leaching conditions.

Thermodynamic calculations were carried out for the solution composition corresponding to the optimal conditions at 80 °C, focusing on two representative reactions. Reaction 1 describes the dissolution of LiCoO₂ in phosphoric acid without a reductant and shows limited thermodynamic favorability:

LiCoO₂ + 4H₃PO₄ → Li⁺ + 2H₂O + Co³⁺ + 4H₂PO₄⁻ ΔG (80 °C) = −67.2 kJ·mol⁻¹

(R1)

In the presence of copper powder (Reaction 2), Co³⁺ is reduced to the more stable Co^2+, and copper precipitates as Cu₃(PO₄)₂·3H₂O, which significantly increases the thermodynamic driving force of the process.

LiCoO₂ + 1.833H₃PO₄ + 0.500Cu → 1Li⁺ + 1.500H₂O + Co²⁺ + 1.500HPO₄²⁻ + 0.167Cu₃(PO₄)₂·3H₂O    ΔG (80 °C) = −117.9 kJ·mol⁻¹

(R2)

Since the HSC Chemistry 9.9.2.3 database [39] does not contain the phase Cu₈(PO₃OH)₂(PO₄)₄·7H₂O, which was identified experimentally by XRD, the thermodynamically similar compound Cu₃(PO₄)₂·3H₂O was used in the modeling. Although these two phases are not fully identical—Cu₈(PO₃OH)₂(PO₄)₄·7H₂O contains both phosphate and phosphite groups—their comparable Cu:P ratio, hydration degree, and low solubility justify this substitution. Although this approximation adds some uncertainty into the absolute ΔG values, it is sufficient to determine the qualitative thermodynamic trend. Neverth`eless, the exact hydrated copper phosphate phase formed, the calculations consistently show that the reaction pathway involving copper yields a substantially more negative ΔG than the copper-free route. Therefore, the presence of copper thermodynamically favors cobalt leaching under the analyzed conditions, which in correspondence with the experimentally observed enhancement in cobalt dissolution.

To investigate the existing species in the leaching solution under the defined conditions, Pourbaix’s diagrams (potential vs. pH) calculated by Hydra/Medusa software (version 2010) [40] are illustrated in Figure 12.

The initial diagram inputs were taken from the optimized leaching experiment in order to indicate the region in which Li⁺ and Co²⁺ remain stable in the leaching solution, while Cu powder remains outside the dissolved-ion fields and is directed toward the low-solubility solid phase. The Eh–pH diagrams for cobalt and lithium show that the marked yellow pH–Eh operating range (pH ≈ 3–4, Eh ≈ 0.4–0.8 V) lies fully within the aqueous stability fields of dissolved ions, where cobalt is thermodynamically stabilized as soluble Co²⁺ and lithium as Li⁺ throughout this region, confirming that these conditions thermodynamically allow their transfer into solution. The copper diagram indicates that the added metallic copper powder (Cu⁰) originates from the lower Eh domain, where the metal is stable but becomes unstable when introduced into the oxidative leaching environment, initiating its surface-localized oxidation, commonly represented by the electron-releasing step Cu⁰ → Cu²⁺ + 2e⁻. The diagram further shows that upon increasing pH within a similar Eh range, the formed oxidized copper species shift toward general low-solubility solid stability domains of copper–phosphate phase. Copper leaves the aqueous phase through fast redox-driven precipitation, rather than remaining dissolved. This process mainly shifts with increasing pH, with no need for large changes in Eh, while Co²⁺ and Li⁺ stay stable in solution during leaching. These diagrams indicate thermodynamic feasibility only and do not constitute direct mechanistic evidence for electron-transfer steps.

4. Conclusions

This study demonstrated that copper powder is an effective reductant for the leaching of Li and Co from LiCoO₂- and Co₃O₄-containing cathode material in a phosphoric acid medium. Under mild conditions, more than 84% of both metals leached within 15 min, with kinetics following a mixed-control mechanism.

The results of this work also provide a new understanding of the role of metallic copper in phosphate-based leaching systems. According to the study, copper not only accelerates the reductive dissolution of LiCoO₂ but also transforms into the phosphate-rich phase Cu₈(PO₃OH)₂(PO₄)₄·7H₂O. This makes the Cu–H₃PO₄ system’s limitations and reaction path more understandable. These findings expand the process-level understanding of the recovery of Li and Co with Cu addition and underline the novelty of using metallic copper as both a reductant and a recyclable resource within a single process.

Compared to common reductants such as H₂O₂, glucose, and ascorbic acid, copper in the H₃PO₄ system proved particularly favorable. While H₂O₂ enables rapid kinetics and nearly complete extraction, it is unstable, costly, and requires careful handling due to safety risks. Glucose and ascorbic acid are inexpensive and safe, but their weaker reducing power and degradation during the process limit kinetics and increase reagent consumption. In contrast, metallic copper allows efficient dissolution under mild conditions and can be directly sourced from waste streams. This dual role of copper—as a reductant and a recyclable raw material—enhances the sustainability of the process and distinguishes the H₃PO₄–Cu system from peroxide- and organic-based reductants.

Thermodynamic analysis also confirmed that the leaching process is spontaneous, with negative Gibbs free energy values supporting the copper-assisted reduction of Co³⁺ to Co²⁺ in the H₃PO₄ medium.

Although the process was investigated at the laboratory scale, the results indicate its potential for industrial application. The identification of Co₃O₄ and Cu₈(PO₃OH)₂(PO₄)₄·7H₂O, both reported as photocatalytic phases, suggests additional opportunities for value-added utilization of leaching residues in environmental or functional applications. However, if there is no practical interest in obtaining Cu₈(PO₃OH)₂(PO₄)₄·7H₂O, further optimization of process conditions by adjusting excess phosphoric acid and copper powder is recommended to ensure more efficient reagent utilization and minimize unnecessary consumption.