Optimizing Acidic Reductive Leaching for Lithium Recovery: Enhancing Sustainable Lithium Supply for Energy Markets

Abstract

The growing demand for lithium, driven by its crucial role in energy storage technologies such as lithium-ion batteries for electric vehicles, renewable energy storage, and portable electronics, is intensifying the need for sustainable extraction methods. While lithium is sourced from both primary and secondary resources, particularly from recycled materials, the recovery from spent lithium-ion batteries remains challenging. This article presents acidic reductive leaching as a promising alternative for lithium extraction from secondary sources and unconventional ores, emphasizing its potential benefits, such as higher recovery rates, faster processing, and adaptability to various waste materials. Notably, this method facilitates the selective recovery of lithium before cobalt and nickel, providing a strategic advantage. This study highlights the lack of optimization studies on leaching conditions (e.g., acid concentration, reducing agents, temperature, and time) that could maximize lithium recovery while minimizing environmental and economic costs. The article aims to investigate and optimize the parameters of acidic reductive leaching for more efficient lithium recovery. Additionally, the results contribute to the principles of the circular economy and sustainable supply chains in the energy sector, providing a method to reduce dependency on geopolitically constrained lithium resources and supporting the global energy transition toward cleaner energy solutions.

1. Introduction

The global energy transition, driven by the urgent need to mitigate climate change, has catalyzed a profound transformation in energy markets, particularly in the production, storage, and utilization of energy. At the core of this transformation lies lithium-ion battery (LIB) technology, a cornerstone of renewable energy storage systems and electric mobility. These batteries play a critical role in storing energy generated by renewable grids, stabilizing energy systems when production is variable, and enabling the electrification of transportation. However, this rapid expansion has significantly increased the demand for key materials such as lithium and cobalt, which are essential for their production [1]. With traditional extraction methods struggling to meet this demand, research efforts have increasingly focused on optimizing lithium recovery processes to secure sustainable supply chains for the rapidly evolving energy sector [2,3,4,5].

In this context, secondary sources—particularly spent lithium-ion batteries—represent a promising yet underutilized reservoir of lithium. Recovery from these secondary sources not only alleviates pressure on primary resources but also aligns with the principles of a circular economy, minimizing waste generation and environmental impacts. The strategic recovery of lithium from end-of-life batteries contributes significantly to sustainability goals, as it mitigates the need for environmentally intensive mining operations. The introduction of innovative lithium recovery methods could, thus, lead to a reduction in industrial energy costs, improving the overall efficiency of energy transfer [6,7,8]. However, the complex material composition of spent LIBs and inefficiencies in existing recovery processes present significant technical challenges that demand innovative and scalable solutions.

As mentioned above, the increasing demand for lithium is a direct consequence of its critical role in energy storage technologies, particularly lithium-ion batteries used in electric vehicles (EVs), renewable energy storage systems, and portable electronics. This growing reliance on lithium-ion batteries is expected to drive a significant rise in lithium demand and production in the coming years.

To explore this trend, historical data on lithium production from primary raw materials during the period 2018–2023 were analyzed using a Microsoft Excel spreadsheet. Projections for lithium production for the years 2024 and 2025 were derived using the REFLINX.ETS forecasting function [9]. The predictive model’s accuracy was assessed using mean absolute error (MAE) and root mean square error (RMSE), yielding values of 1.60 and 2.06, respectively. Moreover, the spreadsheet methodology facilitated the determination of confidence intervals, which provided upper and lower bounds for the 2024–2025 production forecasts. The analysis results forecast a steady increase in lithium production, with estimated outputs reaching 20.1 million tons in 2024 and 22.2 million tons in 2025. The presented forecasts are consistent with statistical data presented in the literature, which indicate a large increase in demand for lithium in the coming years [6,8,10,11,12,13,14,15]. In 2020 alone, 56% of the global demand for lithium was for Li-ion battery production, while in 2021, it could be as much as 75% [7,14]. It is estimated that by 2030, global demand for lithium in Li-ion batteries will amount to 300–600 thousand tons per year, which will constitute more than ¾ of the total demand for lithium [15]. This is particularly related to the growing production of lithium-ion batteries, which are used to power portable electronic equipment and electric vehicles, as well as energy storage devices [8]. In 2024, almost 8.3 million pieces of electronic equipment were introduced to the global market, of which 42% were related to computing and 24% were related to mobile phones. In turn, in 2029, this type of equipment will reach 9 million pieces on the global market [16]. In the European Union member states alone, 7.6 million tons of electronic equipment were introduced to the market in 2012, and in 2022—14.4 million tons [17]. This also determines the growing amount of waste electronic equipment; one inhabitant of the European Union produces an average of 11.2 kg of waste electronic equipment per year (2022) [17]. The situation is similar in the field of electric vehicle production. In 2019, almost 5 million battery-powered electric vehicles were introduced to the market, and predictions for 2030 indicate that production will reach 245 million pieces [18]. Lithium-ion batteries are also one of the solutions used for energy storage. By 2050, renewable energy sources could cover up to 50% of global energy demand, which also influences the use of modern technologies, including Li-ion batteries for energy storage [19,20].

The demand for lithium is also related to numerous changes in the approach to environmental protection in recent years [21]. One of the key missions of the European Union is to carry out climate and energy transformation in pursuit of meeting the sustainable development goals and implementing the circular economy assumptions. To carry out the green transformation, the “Fit for 55” package was developed, which defined the main climate goals for the EU, i.e., reducing emissions by at least 55% in 2030 and achieving climate neutrality in 2050 [21]. To achieve the designated strategies, it is necessary to update and establish appropriate formal and legal acts. The package contains several amendment proposals, including those directly related to the use of Li-ion batteries—in the electrification of the automotive market and the renewable energy sources [21]. The activities of the European Union are also focused on the rational management of raw materials from natural resources, as well as the intensification of the use of secondary raw materials from waste, which are of key importance for the EU economy. Since 2011, every three years, the European Commission has been developing a list of critical raw materials used in the most important EU industrial sectors, but of European resources are limited, and there is a risk of their supply from external countries [22]. The latest list from 2023 contains 34 critical raw materials, including materials related to lithium-ion batteries—graphite, cobalt, and lithium [23]. Closely linked to critical raw materials is the new European Critical Raw Materials Act, which aims to improve the functioning of the internal market by establishing a framework to ensure access to secure, resilient, and sustainable supplies of critical raw materials, including by promoting sustainability, efficiency, and circularity throughout the value chain [24]. In order to achieve diversification of critical raw materials sources and reduce dependence on third countries, the EU has set specific targets for 2030, including in the area of mining and recycling. As established, at least 10% of the annual consumption of critical raw materials in the EU has to come from mining in the EU and at least 25% from domestic recycling [24]. This, in turn, has forced the need to amend new regulations on the management of batteries and their waste. The established rules include the entire life cycle of batteries, from design to end-of-life, in line with the principles of the circular economy model [24]. The regulation concerns, among others, a lithium recovery target of 50% by the end of 2027 and 80% by the end of 2031, and sets mandatory minimum recycled content levels in industrial, vehicle, and machinery batteries for cobalt (16%), lead (85%), lithium (6%), and nickel (6%) [24]. Therefore, one of the main goals of the European Union economy in the coming years will be to efficiently process waste containing critical raw materials for their recovery and reuse in new products, which will reduce the use of raw materials originating from natural resources outside Europe and, thus, ensure the continuity of the EU industrial systems.

Currently, the largest producers of lithium in the world are Australia, Chile, China, Argentina, and Brazil [25], while the largest lithium reserves are located in Chile, Australia, Argentina, China, and the USA [26]. Therefore, this highlights that the European raw material economy in the field of lithium relies mainly on imports from external countries. Lithium naturally occurs in different concentrations in various sources on Earth—in rocks, brines, salt marshes, geothermal brines, and water [7]. However, to produce Li-ion batteries, the extracted lithium must be processed into lithium carbonate, which involves the use of many different methods, from raw material extraction to obtaining the final product. Therefore, in addition to the impact of extraction on the environment, such as depletion of the natural resources, generation of mining waste, and water and soil pollution, the use of many different processing and preparation processes also determines significant energy consumption and emissivity [7]. Lithium is extracted not only from primary sources but also from secondary ones, making the development of sustainable lithium recovery methods particularly critical. Such approaches aim to mitigate the limitations of conventional extraction methods from primary sources, but this necessitates innovative solutions. Additionally, the recovery of lithium from spent lithium-ion batteries poses significant challenges, which stem from the complex chemical and physical nature of battery waste, the heterogeneity of material composition, and the stringent requirements for high-purity lithium recovery to meet industrial standards. These obstacles underscore the importance of advancing recovery techniques to create a more circular and sustainable lithium economy. Despite these efforts, although the amount of lithium recovered through recycling has been steadily increasing, it remains insufficient to meet growing global demand. This underscores the urgent need to scale up recycling processes and improve their efficiency to complement primary lithium sources effectively (

Table 1. Advantages and disadvantages of selected conventional and unconventional lithium extraction methods.

Source	Advantages (+) and Disadvantages (−)	Ref.
Hard-rock and clay mining, extraction and concentration	well-known and highly developed mining technologies possibility of lithium carbonate or lithium hydroxide production by-products recovery, e.g., tin and tantalum compounds faster processing than brine mining profitability depending on deposit depth and size, expensive exploitation of vein- and lens-type deposits mechanical pretreatment of mined material	[27] [6] [7]
Li-bearing brine mining, extraction, and concentration	simple mining technology by-products recovery, e.g., phosphate and boron compounds cheaper than hard-rock mining technologies initial product only in form of lithium carbonate strong interference of Mg2+ in the Li recovery climatic restrictions regarding sunlight and precipitation intensity large area demand for evaporation ponds spent brine tailings management	[6] [28] [29] [30]
Recovery from spent batteries	protection of primary Li resources environmental benefits, e.g., proper waste management, emissions, and landfilling reduction legislative support sustainable development and circular economy implementation resource availability is dependent on spent battery waste collection levels high fire hazard during batteries storing and processing, discharging and mechanical pretreatment before Li recovery recovery methods in development phase	[31] [32]

).

In industrial practice, pyrometallurgical or hydrometallurgical methods are used to recover metals from lithium-ion batteries, in combination with preceding mechanical processing of waste cells to separate the electrode material—a polymetallic cathode that may contain, in addition to lithium, cobalt, nickel, or manganese [33]. Pyrometallurgical methods use high temperatures to transform the components of the material into a solid (metal alloy) or gaseous form with subsequent condensation. They are relatively fast and cost-effective for large installations, but lithium recovery is difficult due to its losses in slag. In addition, pyrometallurgy is associated with pollutant emissions and high energy consumption [11,34]. Hydrometallurgical methods most often include the leaching of electrode powder using inorganic or organic acids and reducing agents to separate metals into aqueous solutions. Then, they are selectively separated using, among others, precipitation, solvent extraction, or electrodeposition [35]. The use of hydrometallurgy allows for high efficiency of metal recovery from spent LiBs, including lithium, and their selective separation. The processes are easily scalable and have low temperatures (reduced energy consumption and emission), although the potential generated waste may require proper management, for example, neutralization [6,33,36].

New research directions include, in particular, direct recycling and biometallurgy with microorganisms [11,33]. Direct recycling consists of the regeneration and upcycling of battery components, mainly electrode materials, in order to restore their electrochemical properties [4,37]. In the case of biometallurgy, naturally occurring in the environment microorganisms are used—bacteria, fungi, and algae, which have the ability to live in conditions of high metal concentrations, extreme pH values, or variable temperatures and, thus, have the ability to extract metals from polymetallic waste [38,39]. These new recycling possibilities are very promising, especially in the field of environmental protection, but they still require further laboratory work on the optimization of process parameters and scaling to industrial conditions [40]. Before alternative technologies are implemented, solutions are constantly being sought that can modernize existing LiB waste recycling technologies. In this respect, the greatest emphasis is placed on innovative, green chemistry in hydrometallurgy, i.e., the use of organic components instead of inorganic acids, which have so far been used in LiBs recycling.

Based on the above, the acidic reductive leaching method emerges as a promising alternative for lithium extraction from secondary sources and unconventional ores. The rationale for implementing this method lies in the potential benefits, such as higher recovery rates, faster processing, and adaptability to different waste materials. Additionally, it enables the recovery of lithium before cobalt and nickel, which is a crucial advantage.

Lithium recovery, even if not with high efficiency in the first individual stages of the recycling processes of battery masses from mechanical processing, is of key importance for cell manufacturers. This is related to the fact that currently, in hybrid and electric cars, mainly LFP cells are used, containing only lithium without cobalt and manganese. This encourages recyclers to create new technological solutions that focus on the recovery of commercial lithium products, i.e., LiOH, Li₂CO₃, and Li₂SO₄, and not the previously pure battery NiSO₄, CoSO₄, and MnSO₄ [41,42,43,44,45,46,47,48].

As a result of the analysis, the research gap/technological implementation gaps were established: many existing solutions rely on simpler, yet more expensive methods, such as thermal pre-treatment, and roasting at temperatures of 350–650 °C with reducing agents. These methods often lead to additional environmental issues, such as hazardous emissions, higher energy consumption, dust, noise, troublesome solid waste, and complications in the entire metal recovery system. Furthermore, there is a lack of optimization studies concerning leaching conditions (e.g., acid concentrations, reducing agents, temperature, and time) that would maximize lithium recovery while minimizing environmental and economic costs. Therefore, the objective of this article is to investigate and optimize the parameters of acidic reductive leaching of electrode powder from spent lithium-ion batteries for effective lithium recovery. For this purpose, green chemistry was used; organic reagents—formic acid (leaching agent) and adipic/succinic/glutaric acid— were used as reducers. It has been shown that these agents can be an attractive alternative to inorganic agents commonly used in hydrometallurgy. Furthermore, it has been established that it is possible to recover other metals in an optimized process, such as cobalt, aluminum, and copper—strategic raw materials in current technologies, which are essential in the construction of Li-ion batteries [49]. The research findings align with the principles of the circular economy and sustainable supply chains in the energy sector. The implementation of the method proposed in this work would reduce dependence on geopolitically restricted lithium resources and support the global energy transition toward clean energy sources.

Current processes for obtaining lithium from primary sources require substantial amounts of energy. However, technologies being developed by scientists and engineers for lithium recovery from secondary raw materials, including waste LIBs, are becoming more efficient. This advancement will reduce the impact on the energy balance while simultaneously conserving natural resources of this metal.

Transformations in the lithium recovery sector, focusing on the selective hydrometallurgical recovery of lithium using green reagents such as weak organic acids, hydrogen peroxide, and even water, are reducing carbon dioxide emissions. This shift is altering the way energy transfer occurs on a global scale. Additionally, utilizing renewable energy sourced from on-site photovoltaic farms for lithium recovery can mitigate the negative environmental impact of traditional recovery methods.

2. Materials and Methods

2.1. Research Material

2.1.1. Mechanical Processing of Spent Li-Ion Batteries

The research material consisted of spent Li-ion batteries of the 18650 type from laptops, obtained from a recycling company specializing in electrical and electronic waste, based in the Lower Silesian Voivodeship, Poland. Li-ion cells from a single manufacturer, with an average weight of 417 g, underwent initial separation and fragmentation into individual components:

• External plastic casing (37.2 g);
• Individual cylindrical cells (46.1 g; each battery contained 8 cells);
• Plastics, foils, and paper (Σ 0.9 g);
• Wires and cables (1.2 g);
• Metal components (4.8 g);
• Printed circuit boards (4.1 g).

Further mechanical processing was applied only to the cylindrical Li-ion cells to isolate the paramagnetic fraction (battery mass) for subsequent research stages. The average battery mass content per cell was 21.0 g, while the remaining components accounted for 45.5%. These findings align with industrial reports from companies involved in processing and recycling various types of spent Li-ion cells, such as Elemental Strategic Metals Sp. z o.o. [50,51], Eneris B&R (Batteries and Recycling) [52], and Botree Tech. [53,54].

XRD Analysis of Battery Mass

The battery mass was subjected to qualitative analysis by X-ray diffraction (XRD) in accordance with the PN-EN13925-1:2007 standard (excluding point 8 [A]) [55], using an Empyrean X-ray diffractometer equipped with a PIXel3D detector from PANalytical (XRD-DADD-000046 A) (Malvern, Worcestershire, UK). The XRD analysis results presented in Figure 1 confirmed the literature data on the phase composition of the tested material of the used Li-ion cells: the dominant components were lithium and cobalt in the form of LiCoO₂ (24.9%), and the remaining part was carbon. By comparing the obtained results of quantitative chemical XRD analyses with the data presented by other authors, it can be clearly confirmed that the tested material is a battery cathode mass [44,56,57]. For example, in the work of Yang L. et al. [58], the scientists showed that the waste cathode material they studied consists of LiCoO₂ and a small amount of CoO₂, which was formed as a result of the decomposition of the main component of the battery mass during its preparation for analytical tests. Similar results were presented by researchers in the work green recycling of spent LiCoO₂ cathodes using a water-based deep eutectic solvent [59], which stated that the research material consists of LiCoO₂ well-leached in acidic reagents. Wang J. et al. doped waste material containing LiCoO₂ with Al₂O₃ and MgO to obtain a substrate for a more efficient cell [60].

SEM/EDS Analysis of the Investigated Battery Mass

The investigated material was also subjected to quantitative and qualitative chemical analysis using Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) at the Nanores Sp. z o.o. Sp.k laboratory [61], prior to individual acidic reductive leaching processes. The obtained results indicate over 50% cobalt content in the battery mass, and are presented in Figure 2 and in

Table 2. Mass fractions (in %) and associated errors of elements in the paramagnetic fraction of the tested battery mass obtained through SEM/EDS analysis..

Element	Mass [%]	Mass Norm. [%]	Atom [%]	Abs.error [%]	Rel.error [%]
Oxygen	17.68	18.59	28.13	1.94	10.95
Cobalt	53.03	55.73	22.90	1.34	2.54
Carbon	22.69	23.85	48.09	2.69	11.84
Calcium	0.09	0.10	0.06	0.03	30.68
Phosphorus	0.26	0.27	0.21	0.04	13.96
Iron	0.92	0.97	0.42	0.05	5.27
Copper	0.47	0.49	0.19	0.04	7.93
Sum	95.14	100.00	100.00

. These findings align with data reported in studies by Meng Q. et al. [47], Santana I.L. et al. [62], and Pietrzyk-Thel P. et al. [63].

Microwave Mineralization of Battery Mass

Quantitative and qualitative analysis of the solution obtained after microwave mineralization, using the DigiPREP Jr mineralization system (heating block), was carried out for the solid sample using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) with an optical spectrometer featuring a horizontally positioned plasma burner from Agilent, model 720. This analysis revealed that the cathode mass consists of 259 g/kg Co, 25.9 g/kg Li, 2.727 g/kg Al, 1.333 g/kg Cu, 0.1829 g/kg Ni, 0.2561 g/kg Ca, 0.01070 g/kg Cr, 0.06665 g/kg Fe, 0.8984 g/kg Mg, 0.0963 g/kg Mn, 0.04306 g/kg Zn, 4.037 g/kg Si, and 0.2077 g/kg Na. It can be assumed that, according to the XDR results presented above (Figure 1), the dominant component of the tested battery mass is carbon/graphite, as well as organic compounds constituting the electrolyte and binder components [51,64].

2.2. Research Methodology

2.2.1. Acid Leaching of Paramagnetic Fraction

The experiments were designed using the DOE (Design of Experiments) module of the Minitab statistical software ver. 17. DOE allows for the simultaneous study of the impact of multiple variables on the outcome (response). The experimental plan includes information on a series of experimental runs (tests), in which intentional changes are made in the input variables or factors, and data are collected. For the experimental part of the project, an experiment was designed with 4 variables, each at three levels. Each experiment was repeated twice or three times according to the model, as shown in

Table 3. Leaching efficiencies of metals Co, Li, Al, and Cu after acidic non-reductive leaching of the battery mass, in %.

No. Expt.	Leaching Parameters				Leaching Efficiency of Metals, %
	T, °C	t, min	s/L, g/mL	C, M	Co	Li	Al	Cu
1	25	120	1/10	5.00	1.72	21.38	0.00	73.98
2	25	120	1/5	1.50	2.23	18.92	0.00	46.71
3	25	30	1/5	5.00	0.71	18.07	0.00	73.76
4	55	120	1/10	5.00	4.11	24.98	0.00	76.62
5	25	120	1/5	5.00	4.20	20.46	0.00	49.84
6	55	120	1/5	1.50	1.40	18.33	0.00	74.71
7	55	120	1/10	1.50	2.80	21.20	0.00	78.96
8	25	120	1/10	1.50	1.42	18.47	0.00	72.77
9	25	30	1/10	1.50	0.57	16.65	0.00	72.92
10	55	30	1/10	5.00	3.58	20.70	0.00	74.08
11	25	30	1/10	5.00	0.90	16.57	0.00	75.21
12	55	30	1/5	5.00	1.87	17.95	0.00	80.79
13	25	30	1/5	1.50	0.61	14.62	0.00	19.45
14	55	30	1/10	1.50	1.35	17.40	0.00	72.51
15	40	75	1/7.5	3.25	2.63	17.74	0.00	54.03
16	40	75	1/7.5	3.25	1.76	17.73	0.00	76.66
17	40	75	1/7.5	3.25	1.36	15.74	0.00	71.05
18	55	120	1/5	5.00	3.79	17.91	0.00	75.10
19	55	30	1/5	1.50	0.82	12.86	0.00	69.60

.

2.2.2. Parameters for Acidic Non-Reductive Leaching

The acidic leaching of waste Li-ion batteries was conducted under the following conditions:

• leaching agent: 1.50 M, 3.25 M, and 5.00 M formic acid (CH₂O₂);
• initial leaching temperature: 25.0 °C, 40.0 °C, and 55.0 °C;
• leaching time: 30 min, 75 min, and 120 min;
• solid-to-liquid phase ratio: 1:5.0, 1:7.5, and 1:10 (s/L = s—mass of the paramagnetic fraction/l—volume of leaching agent);
• mixing speed: 500 rpm.

2.2.3. Parameters of Acidic Reductive Leaching

Step 1. In the first step, after analyzing both the metal leaching degrees into solutions and the Pareto charts from the DOE analysis for the various experimental series, it was determined that further leaching would be conducted under the following parameters:

• leaching agent: 5.00 M formic acid (CH₂O₂);
• initial temperature (T_init.) = 55 °C;
• leaching time (t) = 120 min;
• solid-to-liquid ratio (s/L) = 1/10;
• mixing speed = 500 rpm.

The selection of organic reducers was made after calculations in the HSC Chemistry 9 program, which indicated that the selected organic compounds, i.e., adipic acid (C₆H₁₀O₄), glutaric acid (C₅H₈O₄), and succinic acid (C₄H₆O₄), have lower standard oxidation-reduction potentials compared to the lithium-cobalt material being studied. This confirmed their thermodynamic suitability for the processes of acidic leaching of Li and Co from the paramagnetic fraction. Pure analytical grade organic acids in solid form were added to the leaching process in stoichiometric amounts with a 25% excess at the third minute of the experiment.

Step 2. In the second step of optimizing the parameters for acidic reductive leaching, after analyzing the metal leaching degrees into solutions, it was concluded that the best results were obtained for the leaching agent + reducer pair: CH₂O₂ + C₅H₈O₄, and these were further investigated. Additional series of experiments were planned and carried out for formic acid and glutaric acid, in which the effect of a higher initial temperature of the process (T_init. = 85 °C) and the addition of 3% vol. of perhydrol (H₂O₂) on the acidic reductive leaching process was examined, while maintaining the other process parameters.

Reactions of acidic non-reductive and reductive leaching processes.

LiCoO₂ + 12HCOOH = 4LiOOCH + 4Co(OOCH)₂ + O₂ + 6H₂O

(1)

14LiCoO₂ + 36HCOOH + C₆H₁₀O₄ = 14LiOOCH + 14Co(OOCH)₂ + 20H₂O

(2)

10LiCoO₂ + 25HCOOH + C₅H₈O₄ = 10LiOOH + 10Co(OOCH)₂ + 14H₂O

(3)

6LiCoO₂ + 14HCOOH + C₄H₆O₄ = 6LiOOCH + 6Co(OOCH)₂ + 8H₂O

(4)

2.2.4. Process of Acidic Non-Reductive and Reductive Leaching

The acidic non-reductive or reductive leaching processes were carried out in glass beakers with a capacity of 250 or 500 mL, using a magnetic stirrer with heating (WIGO ES 24 H). To the heated solution of inorganic acid, the battery mass was added in such a way that the solid-to-liquid phase ratio was 1:5, 1:7.5, or 1:10. The process was conducted for a specified time (30, 75, or 120 min), with continuous stirring at 500 rpm. After the leaching process, the samples were filtered, the final pH was measured, and the remaining mass was rinsed with distilled water. The resulting filtrates were quantitatively transferred to volumetric flasks and diluted with distilled water to a final volume of 250 cm³ or 500 cm³. The concentrations of metal ions (Co, Li, Cu, and Al) were then determined using the ICP-OES method.

3. Results

The results of the various stages of optimization for the acidic non-reductive and reductive leaching of the paramagnetic fraction derived from spent lithium-ion batteries are presented in Table 3 and

Table 4. Leaching Efficiency of Metals Co, Li, Al, and Cu After Acidic Reductive Leaching of Battery Mass, % (Step 1 and Step 2).

No. Expt	Leaching Parameters				Leaching Efficiency of Metals, %
	T, °C	t, min	s/L, g/mL	C, M	Co	Li	Al	Cu
Step 1
CH2O2 + C6H10O4
A *	55	120	1/10	5.00	4.37	23.13	X **	X
CH2O2+ C5H8O4
B	55	120	1/10	5.00	0.70	16.38	X	X
CH2O2 + C4H6O4
C *	55	120	1/10	5.00	5.28	16.55	X	X
				Step 2
				CH2O2
D	85	120	1/10	5.00	12.03	36.14	100.0	28.20
CH2O2 + C5H8O4
E	85	120	1/10	5.00	23.38	59.63	61.06	47.00
CH2O2 + H2O2
F	85	120	1/10	5.00	9.40	27.29	52.64	86.49
CH2O2 + C5H8O4 + H2O2
G	85	120	1/10	5.00	31.00	68.40	100.0	12.89

Notes: * In the solutions from experiments A and B during the acidic reductive leaching processes, a significant amount of precipitate was observed, likely consisting of organic cobalt salts. ** In the solutions from experiments A, B, and C, the concentrations of Al and Cu metal cations were not determined.

.

Research on the optimization of the leaching process has shown that the optimal parameters for acidic reductive leaching of the paramagnetic fraction obtained after the mechanical processing of Li-ion batteries include a single-step leaching process conducted at an initial temperature of 85.0 °C, using 5.00 M CH₂O₂ with the addition of a mixture of reductants (C₅H₈O₄ and H₂O₂). The process is characterized by a solid-to-liquid ratio (s/L) of 1:10, a leaching duration of 120 min, a stirring speed of 500 rpm, and a strongly acidic environment. Under these conditions, the leaching efficiencies of metals were as follows: Co—31.00%, Li—68.40%, Al—100%, and Cu—12.89%.

Additionally, results obtained from experiments using a leaching mixture of formic acid, glutaric acid, and hydrogen peroxide indicated selective leaching of lithium. An example of the SEM-EDS analysis results for the leaching of battery mass in glutaric acid is presented in Figure 3 and in the

Table 5. Mass fractions (in %) and associated errors of elements in the paramagnetic fraction for the leaching of battery mass in glutaric acid obtained through SEM/EDS analysis.

Element	Mass [%]	Mass Norm. [%]	Atom [%]	Abs.error [%]	Rel.error [%]
Oxygen	35.35	33.93	60.14	3.73	10.55
Cobalt	17.17	16.48	21.93	1.84	10.71
Carbon	50.88	48.84	17.64	1.29	2.54.
Iron	0.78	0.75	0.29	0.04	5.73
Sum	104.19	100.00	100.00

. This outcome led to subsequent studies focused on the selective recovery of lithium from the solution, aiming to develop a comprehensive and universal hydrometallurgical method for recovering this element from waste cells.

4. Discussion

The experiments presented in this study on acidic non-reductive and reductive leaching of battery mass from lithium-ion cells demonstrate the feasibility of using an alternative approach to the processing of this type of polymetallic waste stream. Priority is given to the recovery of lithium over other metals present in the battery mass, such as cobalt, nickel, and manganese. This focus is particularly critical in the context of lithium-ion battery recycling. The reason is that new lithium-based energy storage technologies introduced to the market contain progressively lower mass percentages of cobalt, nickel, and manganese—such as LCO, LMO, and NMC—or exclude them completely, as in the case of LFP [65]. Due to the importance of lithium-ion battery technologies and its high market value, it is considered a critical metal for sustainable energy supply in a rapidly developing world. The introduction of a two-stage acid leaching process into industrial technologies—enabling selective lithium recovery using weak organic acids in the first stage and subsequent recovery of Co, Ni, and Mn using inorganic acids in the second stage—increases efficiency, versatility, and modularity [66,67,68]. The trend of designing such recycling processes for battery masses from used LCO, NMC, and LFP can be seen in both scientific works and implemented technologies. For example, in the work of Zhao T. et al. [69] on direct selective leaching of lithium from industrial-grade black mass of waste lithium-ion batteries containing LiFePO₄ cathodes, the authors showed that a mixture of formic acid and hydrogen peroxide leads to selective lithium leaching at the level of 97% with insignificant leaching of metals Fe, Cu, Al, Ni, Co, Mn, and Ni. Zhao T.’s team confirmed the results of laboratory work obtained on a pilot scale by conducting a five-stage continuous process of selective lithium leaching using a mixture of HCOOH and H₂O₂ solutions, demonstrating that it is possible to obtain a lithium solution with the content of impurities of other metals at the level of <1% [70]. The discussed technological solution is confirmed by the results of research work by Zhang G. et al. [71], which indicate that multi-stage water leaching is necessary for selective recycling of lithium in the form of Li₂CO₃ at the level of 87.15% from thermally reduced electrode materials. Similar conclusions were reached by the authors of the comparative study for selective lithium recovery via chemical transformations during incineration and dynamic pyrolysis of EV Li-ion batteries [72], indicating that selective lithium recovery by multi-stage hydrometallurgical processes can be preceded by thermal treatment, which increases the efficiency of lithium transfer to the solution already in the first leaching stage to 60%.

At the same time, this method minimizes the environmental impact, reduces the costs of subsequent processing, and facilitates the separation of metals by chemical metallurgy methods such as electrolysis [73], solvent extraction [74,75,76], co-precipitation [54] and ion flotation [77].

5. Conclusions

The obtained research results lead to the following conclusions:

• The leaching processes for the paramagnetic fraction (battery mass) derived from mechanically processed spent lithium-ion cells can be effectively conducted using formic acid with the addition of a novel (previously unexamined) reducing agent composed of glutaric acid and hydrogen peroxide. The recovery rates for the analyzed metal cations are as follows: cobalt (Co)—31.00%, lithium (Li)—68.40%, aluminum (Al)—100%, and copper (Cu)—12.89%.
• The development of modular hydrometallurgical recycling technologies enables the rapid adjustment of process parameters to accommodate variations in the chemical composition and quantity of battery waste streams. Unlike thermal methods, these technologies offer greater flexibility and scalability, making them better suited for the dynamic nature of evolving waste profiles in lithium-ion battery recycling.
• Designers and researchers of new lithium and non-lithium batteries must prioritize eco-design principles to enhance sustainability. Key focus areas include ensuring reusability to extend product lifecycles, minimizing the presence of hazardous substances to address environmental and recycling challenges, promoting energy efficiency and resource conservation during production and use, and reducing the carbon and environmental footprint of batteries throughout their lifecycle.