Parameter Study on the Recycling of LFP Cathode Material Using Hydrometallurgical Methods

Abstract

The need to recover critical elements from lithium-ion batteries is undisputed. The further development of cathode materials and the move towards cheaper alternatives make it necessary to adapt the corresponding hydrometallurgical recycling processes. In the best case, however, a process is so flexibly structured and designed that it can be used for a variety of cathode materials with different compositions. The leaching of nickel manganese cobalt oxide with sulfuric acid and a reducing agent has already been demonstrated and optimized in previous research work. Based on these data, an evaluation of the process as well as a parameter study for lithium iron phosphate cathode material, which has recently been used with increasing frequency but has a significantly lower valuable metal content, was carried out within the scope of this publication. By using the synergy effects that occur, an optimized parameter combination for the leaching of the critical element lithium could be found and further critical factors identified.

1. Introduction

In order to achieve the climate goals, the European Union has specified far-reaching measures within the framework of the European Green Deal, which include sustainable energy production and a rapid switch to e-mobility. In addition, the idea of establishing high-performance energy storage systems, and the use of lithium-ion batteries (LIBs) is explicitly promoted [1].

To reach the European Union’s climate targets and those of the Paris Climate Agreement, CO₂ emissions in the transport sector must be drastically reduced. Sales of electric vehicles (EVs) have increased strongly in recent years due to corresponding policy measures and this trend will continue. In the time between 2020 and 2021, sales of plug-in hybrid and fully electric vehicles have been doubled. Around 6.6 million vehicles were sold worldwide (see Figure 1), whereby the People’s Republic of China can be seen as the largest market with 3.3 million vehicles sold. In comparison, Europe shows a continuous growth, but still remains a smaller market. In the first quarter of 2022, European sales increased by 25%. Sales of EVs will continue to grow, reaching over 18 million vehicles in 2025 and between 30–45 million in 2030, depending on the various forecast models [2,3].

In any case, developments in the field of lithium-ion battery technology also contribute to the success of electromobility. Different materials are used today in lithium-ion batteries. A subdivision of anode and cathode materials can be made on the basis of their components. Another way to differentiate is given by their crystalline structure. Known representatives of the various cathode materials are lithium cobalt oxide (LCO), lithium nickel manganese cobalt oxide (NMC), lithium nickel cobalt aluminum oxide (NCA), and lithium iron phosphate (LFP) [4].

Considering the strong growth of EVs, the demand for high-grade raw materials such as cobalt, lithium, and graphite, which are classified as critical by the European Commission, as well as nickel and manganese for the production of traction batteries is increasing. Therefore, an efficient and environmentally friendly circular economy is needed to recover the materials. So far, the difficulty lies in establishing a stable recycling process for different types of cathode materials (LCO, NCA; NMC; LFP, etc.) [1,5].

Well-known vehicle manufacturers, such as Tesla, VW, Daimler, and Ford, are increasingly using lithium iron phosphate batteries instead of conventional nickel manganese cobalt cells due to cost and safety aspects. Compared to LFP cells, NMC batteries impress with their specific energy densities. However, lithium iron phosphate cathode material has a higher reversible capacity, improved cycle stability, and is also fast-chargeable. The main arguments for switching to LFP are cost and safety advantages. In addition, such energy storage systems are non-toxic and ecologically safe. The high demand, concerns about supply bottlenecks, and scarce supply capacities for industrial metals, especially high-quality nickel, are currently fueling the price trend. The price of lithium, for example, has now increased more than sevenfold compared to the previous year. Nickel and cobalt prices are very volatile, as the last year in particular showed. After a brief peak, prices are currently about 30% higher than a year earlier. The market price development for battery cells, Li, Ni, and Co is shown in Figure 2 [3,4,6,7,8,9].

Due to the nickel- and cobalt-free cathode material, LFP cathodes are arousing interest. The cost advantage of those materials makes electric vehicles affordable for a larger consumer market.

Table 1. Average composition of LFP and NMC cathode material and prices of the individual components [9,11,12].

Component	Price [US-$/t]	LFP	NMC
Li	76.886	2.78%	4.76%
Ni	2.200	-	7.94%
Co	51.520	-	3.97%
Mn	2.060	-	29.37%
Fe	835	21.53%	-
Al	2.280	42.36%	24.60%
Other	-	-	7.54%
P	320	11.81%	-
O2	-	24.31%	21.83%

shows the prices for the individual materials in NMC and LFP cathode materials and the composition of the black mass. The composition is not based on the element distribution of one defined battery type but rather represents the average value of ingredients from batteries with widely varying sizes and shapes from hybrid, plug-in hybrid, and fully electric vehicles as they currently appear on the market [3,8,10].

In its directive 2006/66/EC of 6 September 2006, the European Union stipulates a recycling rate of at least 50% of the average weight of a used battery. A revision of the directive is already taking place, therefor a recycling process with high recovery rates of the ingredients will become mandatory for end of life batteries (EoL). Due to the increasing demand for traction batteries, a treatment process should be specifically designed for lithium-ion batteries. So far, only a few recycling activities are known in Europe due to difficulties in processing spent NMC lithium-ion batteries based on the highly differentiated starting materials. Both pyrometallurgical and hydrometallurgical recycling processes are known. In the case of pyrometallurgical treatment, a metal-containing phase is produced, which includes Cu, Ni, and Co, a slag containing Li, Al, and Mn, as well as flue dust. To recover the metals, hydrometallurgical processes are downstream for both the alloy and the slag. However, such a process concentrates purely on the recovery of the valuable metals like cobalt, nickel, and copper from the melt for economic reasons. Research efforts exist with regard to the recovery of lithium by evaporation of the metal, whereby a recovery rate of only 68.4% of the lithium could be achieved with LFP cathode material in a paper by [13]. In contrast, purely hydrometallurgical recycling routes can recover all valuable metals as well as lithium, manganese, and aluminum in product purity. For the application of wet chemical processes, mechanical and thermal pre-treatment steps are necessary to remove organic and halogen components. Afterwards, leaching is carried out by the use of inorganic or organic acids, whereby inorganic acids stand out due to their lower costs and higher leaching rates. Problems exist with regard to waste water, potentially toxic components, and the need for waste water treatment and landfilling. Hydrochloric, sulfuric, phosphoric, and nitric acids are often used together with a suitable reducing agent such as hydrogen peroxide, whereby iron is oxidised by H₂O₂ in an acidic environment. Both because of environmental issues and because of the selective leaching of lithium with respect to product purity, minimization of acid use is obligatory [13,14,15,16,17,18].

1.1. Research Activities in the Field of Leaching of LFP Cathode Material

Metal recovery by wet chemical methods consists of a leaching step with the aim of dissolving all desired elements, separating them with downstream processes, and recovering them in sufficient quality for reuse in batteries. In this context, the leaching of LFP cathode material may be carried out with sulfuric acid. The addition of a suitable oxidant, such as hydrogen peroxide, assists the leaching reaction as illustrated below in Equation (1). In addition, such patents already exist from Umicore and Rockwood for the recovery of lithium and phosphate from spent LFP cathode material [19,20,21].

$${2 \mathrm{LiFePO}}_4+ {\mathrm{H}}_2{\mathrm{SO}}_4+ {\mathrm{H}}_2{\mathrm{O}}_2 \leftrightarrow {2 \mathrm{FePO}}_4+ {\mathrm{Li}}_2{\mathrm{SO}}_4+ {2\mathrm{H}}_2\mathrm{O}$$

(1)

The possibilities for selective recovery of lithium from the cathode material have been published in a wide variety of papers. For example, ref. [17] showed that 95.56% Li can be selectively recovered from lithium iron phosphate cathode material under optimal conditions. Increasing the acid concentration from 0.1 M to 0.3 M raises the efficiency of the leaching process. Although the pH decreases, most of the iron remains as solid. Only at an acid concentration of 1 M H₂SO₄ does iron dissolve in significant amounts. The addition of hydrogen peroxide causes an improvement in the solubility of Li and a decrease in that of Fe [21]. Qin et al. (2019) investigated the recovery and reuse of spent LiFePO₄ through a hydrometallurgical recycling process using sulfuric acid and hydrogen peroxide to dissolve lithium, iron, and phosphorus, and subsequently recover FePO₄ and LiFePO₄ [22]. Lou et al. (2021) demonstrated a leaching rate of more than 91.53% of the cathode material by applying 98 mass% sulfuric acid in their studies [23]. The study by Tang et al. (2020) investigated the leaching behavior of Fe, P, Li, as well as Al with NaClO₃ as oxidizing agent. In ideal conditions, the contained lithium could be transferred into solution with an efficiency of 97.23% [24]. A study investigating the leaching kinetics by [22] demonstrated a high selective leaching rate of lithium from the lithium iron phosphate cathode material when 0.1 M H₂SO₄ with 2 vol% hydrogen peroxide was used as oxidant. 74.74% of lithium and 0.99% of iron dissolved from the black mass within 60 min [25].

Table 2. Selected leaching parameters from relevant literature for the selective leaching of lithium and iron from LFP cathode material [17,20,21,22,27,28,29].

Reference	Acid	Conc.	Oxidising Agent	Conc.	Temperature	Leaching Time	Stirring Speed	Efficiency
Li et al. (2017) [17]	H2SO4	0.3 M	H2O2	2.07 mol% H2O2/Li	60 °C	120 min	-	95.56% Li
Jung et al. (2021) [27]	H2SO4	0.28 M	-	-	85 °C	240 min	-	98.46% Li
Jung et al. (2021) [27]	H2SO4	2.5 M	-	-	60 °C	240 min	-	97% Li
Jung et al. (2021) [27]	H2SO4	-	O2/H2O2-	-	80–120 °C	120 min	-	92% Li
Lou et al. (2021) [20]	H2SO4	0.35 mol% (H2SO4/LFP)	-	-	20 °C	90 min	800 rpm	91.53% LFP
Tang et al. (2020) [21]	H2SO4	1 M	NaClO3	25 mass% NaClO3/LFP	90 °C	60 min	150 rpm	97.23% Li
Wu et al. (2022) [28]	H2SO4	0.6 M	O2	1.3 MPa	120 °C	90 min	-	95.74%Li
Tao et al. (2019) [29]	H2SO4	0.28 M	H2O2	-	85 °C	240 min	-	98.48% Li
Dyana et al. (2020) [22]	H2SO4	0.1 M	H2O2	2 vol%	60 °C	60 min	-	74.74% Li

gives an overview of selected previous leaching experiments for the selective leaching of lithium from lithium iron phosphate [22,23,24,25,26].

1.2. Physico-Chemical Fundamentals of Hydrometallurgical Decomposition of LFP

In their paper, Dokko et al. (2007) describe the recovery of a fine-grained LFP powder. In this process, the starting materials are mixed in aqueous solution, heated to 170 °C and kept in a pH range between 4 to 6. In addition, the properties and morphology were studied after separating the solid from the solution. The hydrothermal decomposition of the cathode material is carried out in the same way and can be described by using E-pH diagrams, which are applied for synthesizing the active material [30,31].

In a study by Jing et al. (2019) a new possibility for selective leaching of Li is described. In the calculated Pourbaix diagrams, different stability ranges exist depending on the pH, the electrochemical potential, and the molar quantity of each material. As shown in Figure 3, Li⁺ and Fe²⁺ (I), Li⁺ as well as Fe³⁺ (II), and exclusively Li⁺ (III) can be dissolved. In the first route, lithium and iron ions dissolve using strong acids and reducing agents. Iron removal is then accomplished by increasing the pH as well as the potential through the addition of suitable alkalis and oxidizing agents. The second possibility describes the way of decomposition of LFP into Li⁺, Fe³⁺, and H₃PO₄ under strong acid and oxidative conditions. One possibility is the application of H₂SO₄ with H₂O₂. The work of Jing et al. (2019) investigated the third leaching pathway using different oxidants. By applying hydrogen peroxide, 95.4% lithium and only 0.05% iron can be dissolved [31].

However, based on this literature review, it was found that processes are usually designed and tested specifically for one type of cathode material. The aim of this study is to apply classical hydrometallurgical methods and feedstocks under the consideration of synergy effects. The possibility of leaching NMC material with sulfuric acid at low concentrations and hydrogen peroxide as oxidant has already been demonstrated [5,14]. Above all, the high yields here of the individual valuable metals were convincing. In the optimal case, other cathode materials can also be used in this process and thus enable synergetic use.

2. Materials and Methods

This study used sulfuric acid (96 mass%, Carl Roth, Karlsruhe, Germany) to investigate the leaching behavior of LFP cathode material. The influence of hydrogen peroxide (30 vol% as well as 35 vol%, Carl Roth, Karlsruhe, Germany) as oxidizing agent is also investigated. The lithium iron phosphate cathode material (Lithium Iron Phosphate S19) was purchased from Gelon (DongGuan, China). The aim was to identify an optimum range with high leaching efficiency by varying the leaching parameters such as acid and oxidant concentration, temperature, solid/liquid ratio, and leaching time (see

Table 3. Selected parameter limits for the study.

Parameter	Minimum Value	Maximum Value
H2SO4 [mol/L]	0.3	2.3
H2O2 [vol%]	0	16.5
Temperature [°C]	30	80
S/L ratio [g/L]	50	110
Leaching time [min]	30	240

).

2.1. Experimental Procedure

The leaching experiments were carried out in a double-walled flat-bottomed glass reactor. The target temperature was set using a thermostat (Lauda, P 8C X). The cathode material was first added to the defined amount of distilled water via funnels into the nitrogen-flooded vessel. N₂ was used to set a defined atmosphere so that no additional oxygen potential was provided in the experiment. By using a magnetic stirring unit, the mixing of liquid and solid took place to form a homogeneous suspension. The stirring speed was fixed at 400 rpm for all experiments. Sulfuric acid and hydrogen peroxide could be added to the mixture by using dropping funnels. Figure 4 shows a schematic diagram of the experimental setup.

The leaching start time was defined for all experiments as the time of completed addition of the oxidant. For those experiments without the addition of hydrogen peroxide, the completed addition of the acid marked the starting time. After the leaching time, the solids were separated from the leachate by vacuum filtration. The filter cake was dried in a drying oven at 105 °C for a minimum of 24 h.

2.2. Analytical Methods

A microwave plasma atomic emission spectrometer (4210 MP-AES, Agilent, Santa Clara, CA, USA) was used to evaluate the amounts of lithium and iron contained in the filtrate. The analysis was performed with the help of the statistical program MODDE 12.1 (Sartorius AG, Göttingen, Germany). Within this software, it was possible to create a D- optimal experimental design with five center trials. To determine the influences of the individual target parameters (acid concentration, oxidant concentration, temperature, leaching duration, and S/L ratio), 35 individual tests were performed. Consideration of the interaction of the individual parameters was possible. For the qualitative determination of the residues in the filter cake, a scanning electron microscope was used for the energy-dispersive measurement of secondary electrons.

3. Results

The evaluation of concentration analyses of iron and lithium by means of MP-AES was carried out by the statistical test planning and evaluation programme MODDE 12.1. For a better overview, the results of the data analysis of the evaluated concentrations of Li as well as Fe in the pregnant leach solution are presented in contour diagrams in the first section. The results are standardized in the diagrams with the test temperature in dependence of the leaching time at a uniform solid/liquid ratio of 100 g·L⁻¹ with correspondingly varying acid and oxidizing agent concentrations. In the second part of this section, selected residual solids are evaluated using SEM/EDS analysis. In the third part, factor analyses are carried out with the statistical software at a confidence interval of 95% to graphically display the influences of acid concentration, oxidizing agent, leaching temperature, S/L ratio, as well as holding time on the leaching efficiency of lithium.

3.1. Evaluation of Leaching Efficiency

In this section, the results of the leaching behavior of lithium and iron are discussed. In particular, the main focus is on a realisable yield of lithium from the LFP cathode material when varying the concentrations of sulfuric acid and hydrogen peroxide. The dissolved iron content in the pregnant leach solution is of great importance in order to draw possible conclusions about the selectivity of the process. The concentrations measured in the individual tests using MP-AES are listed in

Table 4. Statistical experimental design for the investigations of the leaching behavior of LFP cathode material.

Nb.	Acid Concentration	Oxidant Concentration	Leaching Time	Temperature	Solid/Liquid Ratio
-	[mol/L]	[vol%]	[min]	[°C]	[g/L]
N1	1.6	10.7	100	46.7	71.3
N2	1.0	5.2	170	46.7	69.6
N3	1.0	5.2	100	46.7	87.0
N4	1.6	5.3	170	46.7	89.1
N5	1.6	5.3	100	63.3	71.3
N6	1.0	10.4	170	63.3	87.0
N7	1.9	2.7	65	38.3	63.3
N8	0.6	12.9	65	38.3	60.0
N9	0.6	2.6	205	38.3	94.3
N10	1.9	13.6	65	71.7	63.4
N11	0.6	2.6	205	71.7	600
N12	0.6	2.6	65	71.7	94.3
N13	1.9	13.6	205	71.7	99.6
N14	2.3	0.0	30	30.0	55.2
N15	0.3	15.2	30	30.0	50.7
N16	0.3	0.0	240	30.0	50.7
N17	2.3	16.5	240	30.0	55.2
N18	0.3	0.0	30	30.0	101.5
N19	2.3	16.5	30	30.0	110.3
N20	2.3	0.0	240	30.0	110.3
N21	0.3	15.2	240	30.0	101.4
N22	0.3	0.0	30	80.0	50.7
N23	2.3	16.2	30	80.0	55.1
N24	2.3	16.2	240	80.0	55.2
N25	0.3	15.2	240	80.0	50.7
N26	2.3	0.0	30	80.0	110.3
N27	0.3	15.2	30	80.0	101.4
N28	0.3	0.0	240	80.0	101.5
N29	2.3	16.5	240	80.0	110.3
N30	1.3	7.9	135	55.0	79.3
N31	1.3	7.9	135	55.0	79.2
N32	1.3	7.9	135	55.0	79.3
N33	1.3	7.9	135	55.0	79.5
N34	1.3	7.9	135	55.0	79.3
N35	1.3	7.9	135	55.0	79.5

. The lithium contents vary between 466.34 mg·L⁻¹ and 8661.65 mg·L⁻¹. The maximum iron concentration amounts to 28,469.08 mg L⁻¹. These data form the basis for the evaluation in the statistical program. Based on the metal contents in solution, statements can be made about the leaching behavior.

3.2. Leaching Behavior of Lithium

At a sulfuric acid concentration of 0.3 M without the addition of hydrogen peroxide at an S/L ratio of 100 g·L⁻¹, it is shown that the amount of dissolved lithium increases with increasing leaching time. Furthermore, increasing the leaching temperature does not improve the leaching behavior. After 75 min, the content of lithium exceeds 2600 mg·L⁻¹ in solution, and there is a further increase in content with a correspondingly longer leaching time. By raising the temperature, the leaching behavior can only be improved at the beginning of the reaction, but the highest concentrations of lithium occur at low temperatures and long leaching times. Figure 5a shows the corresponding evaluation in the context of a contour diagram. Therefore, lithium contents up to 3200 mg·L⁻¹ can be dissolved from the cathode material at low acid concentrations without the addition of oxidizing agents with correspondingly long leaching times at temperatures as low as 30 °C.

Figure 5c shows the result with an acid concentration of 2 M of sulfuric acid without the addition of hydrogen peroxide as oxidizing agent. By increasing the acid concentration, more lithium is dissolved than at lower concentrations. Changing the sulfuric acid concentration from 0.3 M to 2 M leads to a leaching efficiency increase of about 35%. The highest leaching rate of lithium is found at low temperatures of about 30 °C and long leaching times. Increasing the temperature can enhance the lithium yield to a maximum of 3800 mg·L⁻¹ for short leaching durations. The lithium content decreases for holding times longer than 100 min and bath temperatures above 65 °C. The highest yield of Li leached from the LFP cathode material is therefore approx. 4200 mg·L⁻¹ at a test duration of 240 min and a temperature of 30 °C.

The statistical evaluation (see Figure 5f) shows that with an addition of 7.5 vol% H₂O₂ to the leaching solution with a solid/liquid ratio of 100 g·L⁻¹ and an acid concentration of 2 MH₂SO₄, a maximum of 3600 g·L⁻¹ of lithium is dissolved, assuming that the leaching duration amounts to 240 min and the temperature is 30 °C. Extending the leaching temperature to improve the kinetics does not appear to be effective, since already doubling the leaching duration results in a significant increase in the lithium content. However, the addition of oxidizing agent lowers the achievable yield by 500 mg·L⁻¹, as can be seen from the comparison in Figure 5g,h. Lower acid concentrations with an addition of 7.5 vol% H₂O₂ in turn lead to lower lithium contents in solution.

A further increase of the content of oxidizing agent to 15 vol% hydrogen peroxide in the 2 M sulfuric acid leach solution with an S/L ratio of 100 g·L⁻¹ again reduces the amount of recoverable lithium from the pregnant leachate. Figure 5i shows the statistical metal content of lithium in the solution depending on the process temperature and leaching time. It can be seen that the highest contents of valuable metal occur at long times and low temperatures. Thus, up to 3000 mg·L⁻¹ Li can be recovered from the pregnant leaching solution. Doubling the oxidant addition of H₂O₂ reduces the yield by about 700 mg·L⁻¹.

It can therefore be stated that an increase in the oxidizing agent reduces the leaching efficiency of lithium. The opposite behavior is seen with the addition of sulfuric acid. Higher concentrations of acid favour an increase in leaching efficiency. A rise in temperature did not show an improved leaching rate of lithium in any case, which is an important reason why a possible economic benefit can be realised by a longer leaching time.

3.3. Leaching Behavior of Iron

The statistical evaluation of the results shows that the behavior of iron in different acid and oxidant concentrations differs from that of lithium. Thus, at an acid concentration of 2 mol·L⁻¹, an oxidizing agent content of 15 vol% and an S/L ratio of 100 g·L⁻¹, a maximum concentration of dissolved iron occurs in a temperature range between 35 and 50 °C with a leaching time between 150–220 min. The statistical evaluation of this behavior is shown in Figure 6. This demonstrates that small amounts of iron are dissolved from the powder at short residence times of the LFP cathode material as well as at high heat input. In addition, the iron content in the pregnant leach solution increases at a low leaching temperatures and long times. The achievable iron contents therefore vary between 2500 and 60,000 mg·L⁻¹ depending on the holding time and temperature.

When the volume fraction of hydrogen peroxide in solution is reduced to 7.5 vol% at an acid concentration of 2 M and an S/L ratio of 100 g·L⁻¹, a very similar behavior to that with 15 vol% of H₂O₂ is seen, with the only difference being that higher absolute amounts of iron are dissolved. Figure 6 shows the statistical evaluation for such a leach composition as a function of temperature and leaching time. When the holding time is increased, the content of Fe in the pregnant leach solution increases before it begins to decrease from about 150 min. The situation is similar with regard to the set leaching temperature. Thus, iron contents of up to more than 90 g·L⁻¹ are achievable. Such a maximum is found in a temperature range within 40–60 °C and a leaching time between 125–200 min.

The statistical evaluation at a sulfuric acid concentration of 2 M and a solid/liquid ratio of 100 g·L⁻¹ without the addition of hydrogen peroxide is shown in Figure 6g. The results demonstrate a shift of the maximum achievable iron concentrations in the solution to higher temperatures. Similar to the evaluation with the same molarity of the acid and the same S/L ratio with an H₂O₂ concentration of 7.5 vol%, the iron content initially increases at an experimental temperature of 30 °C before it begins to reduce from about 100 min leaching time and a value below 30,000 mg·L⁻¹ is reached at 240 min. Realisable amounts of iron in solution, however, are 70,000–120,000 mg·L⁻¹ at higher temperatures (see Figure 6h).

At a low acid concentration of 0.3 M without the addition of an oxidizing agent, the statistical evaluation with MODDE 12.1 at a solid/liquid ratio of 100 g·L⁻¹ gives the distribution shown in Figure 6a. It can be seen that the content of iron increases with longer leaching times, as in all other treated cases, in the beginning and decreases after a defined holding time. Within this parameter combination, a reduction of the iron content in solution occurs at 175 min and at a temperature of 30 °C. It can be seen in comparison to higher acid concentrations that Fe contents of a maximum of 25,000 mg·L⁻¹ can be achieved with appropriate heat input and leaching time. To reach this concentration peak, a leaching time of 150–225 min and a temperature of 35–50 °C are required. The smallest amounts of iron dissolve with short leaching times and high heat input.

The evaluation of the iron contents at different acid concentrations and oxidant additions at an S/L ratio of 100 g·L⁻¹ shows that increasing the molarity from 0.3 to 2 M by adding sulfuric acid increases the amount of dissolved iron by a factor of 4.5. Similar to the behavior of lithium, it can be observed that the addition of oxidizing agent decreases the achievable iron content. After an initial increase, the evaluation shows a decrease in iron content with longer leaching times, so that at 240 min lower amounts of iron are dissolved than in a leaching interval of 100–225 min at a temperature regime of 30 °C. This seems to make it possible to achieve a higher selectivity of the process.

3.4. Analysis of Solid Residues

As already mentioned, the leaching efficiency depends on the acid concentration, oxidant addition, S/L ratio, leaching time, and process temperature. In addition, the Pourbaix diagrams allows conclusions to be drawn about the solid residual masses that can be expected. The solids obtained correlate with the pH value and the oxidant concentration. In addition, the leaching time plays a determining role. It can be seen that at pH values below 1.59, the leaching efficiency is the highest (66.67%) of the tests carried out and thus the residue masses are the smallest. These are in the range of 1.4–2.3% related to the weighed masses of LiFePO₄. At higher pH values, a stronger influence of holding time and oxidant application is recognisable. In two experiments, pH values of 4.76 (N27) and 5.42 (N21) were obtained. It was found that in the weakly acidic range, the effect of H₂O₂ as an oxidizing agent for lithium is strongest and iron dissolves only in small quantities. The residue masses obtained were in the range of 96.16% (pH 4.76) and 94.36% (pH 5.42) related to the weighed mass of cathode material.

The qualitative evaluation of the residual solids was carried out by energy dispersive measurement of secondary electrons (EDX) at a scanning electron microscope (SEM) as well as with a statistical analysis of the mass distribution of the element. The detection of lithium is not possible in the used SEM. An assessment can be made via morphological examinations. As already shown the dissolved amount of iron in the pregnant leach solution varies over a broad range. With increasing acid concentration and moderate application of hydrogen peroxide as well as temperatures between 35–60 °C, an iron content of 100,000 mg·L⁻¹ in the pregnant leach solution can be achieved. Figure 7 shows the element distribution in the solid of experiment N15, which was carried out at an acid concentration of 0.32 MH₂SO₄, 15.2 vol% of H₂O₂ at 30 °C with a leaching time of 30 min and an S/L ratio of 50.75 g·L⁻¹. The energy dispersive measurement shows that iron is the main residue next to carbon (42 mass%). In addition, Al and Si also appear as impurities in the LFP cathode material with 0.1 mass% each.

3.5. Parameter Study

As already illustrated, the amount of dissolved lithium increases with increasing acid concentration. Higher acid concentrations favour a more efficient leaching of the valuable metals from the LFP cathode material. The following statistical analyses carried out with MODDE 12.1 show the dependencies of the individual factors on each other, considering a 95% confidence interval. The dotted lines indicate the upper and the dashed lines the lower limits. The solid curve is the expected amount of lithium in the solution for a given parameter. These parameters form the foundation for factor optimization of the process. The width of the confidence interval permits also other progressions, which is why further experiments in the particular parameter field are necessary for a more precise prediction.

3.5.1. Influence of the Acid Concentration

Figure 8a shows the statistical evaluation of the tests with regard to the increase of the lithium content in solution with rising acid concentration, whereby the amount of dissolved lithium changes linearly with an elevation of the acid concentration. The acid content thus has a decisive influence on the recoverable amount of valuable metal from the LFP cathode material. Accordingly, a pregnant leach solution with 2600 mg·L⁻¹ can be obtained from an acid concentration of 2.2 mol·L⁻¹.

3.5.2. Influence of Oxidizing Agent

From the literature an increasing content of valuable metals would be expected from the addition of hydrogen peroxide. However, from the factor analysis of the addition of H₂O₂, see Figure 8b, it appears that for the leaching of lithium iron phosphate cathode material the content of lithium decreases linearly with rising oxidant concentration. Thus, the Li content in solution decreases from over 2600 mg·L⁻¹ to approx. 2000 mg·L⁻¹ with the addition of 16.5 vol% hydrogen peroxide. Instead of reacting as an oxidant for lithium, H₂O₂ causes a reduction of iron. This means that the addition of oxidant does not add any economic value due to the costs of the chemical itself and the reduced yields for lithium. It even reduces the selectivity of the leaching step. As can be seen from Section 3.3, the iron reduction and thus the iron content is dependent on the acid concentration and the application of oxidant.

3.5.3. Influence of the Leaching Temperature

Figure 8c shows the influence of the temperature on the amounts of lithium to be recovered graphically with a confidence interval of 95%. It can be seen that an increase in the leaching temperature leads to a linear elevation in the lithium content in solution, but by a maximum of 200 mg·L⁻¹ at a temperature of 50 °C. It is already clear from the evaluation in Section 3.2 that a high lithium yield can be expected at low temperatures and long residence times. With regard to the economic mapping of a possible industrial process due to rising energy prices, this aspect is of decisive importance.

3.5.4. Influence of the Solid/Liquid Ratio

The analysis of the results shows a significant increase of the lithium content in solution due to an elevation of the S/L ratio (see Figure 8d). The factor analysis leads to a linear increase of the expected recoverable amount of lithium in the pregnant leach solution with rising solid/liquid ratio. Doubling the cathode material causes an increase in the yield of 85.6%. Thus, with an S/L ratio of 50 g·L⁻¹, about 1700 mg·L⁻¹ Li can be dissolved in the liquor. On the other hand, with an addition of 100 g·L⁻¹ solid material to the leaching process, about 3.2 g·L⁻¹ of lithium can be found in the pregnant leach solution.

3.5.5. Influence of the Leaching Time

The importance of the leaching time for the leaching behavior of lithium is shown in Figure 8e. The analysis indicates that with increasing residence time of the cathode material in the leaching process, the amount of dissolved lithium rises. This behavior is reflected in the evaluation in Section 3.2. Accordingly, the leaching time is a viable method to increase the content of lithium in the pregnant leach solution without a massive increase in the energy demand required for the process. The statistical parameter analysis shows an increase in lithium yield of 180 mg·L⁻¹ by changing the leaching time from 30 to 240 min. In addition, the content of iron in solution decreases at longer leaching times, as demonstrated in Section 3.3.

4. Discussion

Within the scope of the investigation of the leaching of LFP cathode material, it became apparent that an analysis of the target parameters as well as their interaction is necessary for the determination of an optimal leaching point. From the factor analysis it can be concluded that the five parameters investigated influence the leaching behavior of LFP cathode material to different degrees:

• Acid concentration
• Amount of oxidant
• Temperature
• Solid/liquid ratio
• Leaching time

The acid concentration has the greatest influence on the lithium content in solution. This makes it easy to increase the amount of lithium in the pregnant leach solution. The statistical evaluation shows that low additions of oxidizing agents do not lead to a serious deterioration of the leaching behavior of lithium, whereas higher contents of up to 15 vol% cause a loss of the recovery potential of Li. Nevertheless, in weakly acidic media, from a thermodynamic point of view lithium should dissolve from the lithium iron phosphate according to Equation (2) within the formation of the hardly soluble FePO₄.

$${2 \mathrm{LiFePO}}_4{+\mathrm{H}}_2{\mathrm{O}}_2+2 \mathrm{H}+\leftrightarrow { 2 \mathrm{FePO}}_4{+2 \mathrm{H}}_2\mathrm{O}+2 \mathrm{Li}+$$

(2)

In contrast to the reaction with iron, lithium has no catalytic influence on the decomposition of hydrogen peroxide as long as it is in a neutral set form. It is clear from the evaluation that high additions of hydrogen peroxide lead to lower yields. For economic process control, it is advantageous to know that only the smallest amounts of oxidant cause a considerable increase in efficiency. Further addition of hydrogen peroxide also dissolves iron to a greater extent. It can be concluded that H₂O₂ acts both partly as an oxidant for lithium and as a reducing agent for the Fe³⁺ ion in the undissolved FePO₄. The Pourbaix diagrams show that at room temperature Fe³⁺ is present as FePO₄, Fe²⁺ as well as Li⁺ in solution at corresponding pH values. Higher temperatures lead to a shift in the stability ranges. In addition, the decomposition of hydrogen peroxide by Fe³⁺ is a catalytic process started by Equation (3) and corresponds to the Haber–Weiss mechanism in a sulfuric acid milieu.

$${\mathrm{Fe}}^{3+}{+\mathrm{H}}_2{\mathrm{O}}_2\to { \mathrm{Fe}}^{2+}+\mathrm{HOO}·{+\mathrm{H}}^{+}$$

(3)

Fe²⁺ is oxidized by the further presence of hydrogen peroxide according to Equation (4). The resulting OH· again forms HOO· and both radicals are chain carriers for the catalytic decomposition. The high conversion rate could be explained by this mechanism.

$${ \mathrm{Fe}}^{2+}{+\mathrm{H}}_2{\mathrm{O}}_2{+\mathrm{H}}^{+} \to { \mathrm{Fe}}^{3+}+\mathrm{OH}·{+\mathrm{H}}_2\mathrm{O}$$

(4)

$$\mathrm{HO}·+ {\mathrm{H}}_2{\mathrm{O}}_2 \to \mathrm{HOO}·+ {\mathrm{H}}_2\mathrm{O}$$

(5)

This in turn leads to a continuation of reaction, which results in the formation of Fe²⁺, H⁺, as well as O₂ according to the equation through the surplus of ferric iron in the cathode material.

$${ \mathrm{Fe}}^{3+}+\mathrm{HOO}· \to { \mathrm{Fe}}^{2+}{+\mathrm{H}}^{+}{+\mathrm{O}}_2$$

(6)

This reaction can be seen as a competing reaction to the oxidation of lithium and the associated leaching. Lithium and bivalent iron have a wide stability range and are present dissolved at the adjusted pH values. Due to this circumstance, the concentration of iron increases due to the elevated addition of hydrogen peroxide in addition to the moderate increase of the lithium content in the pregnant leach solution. The observed violent gas evolution by adding H₂O₂ underlines this possible reaction sequence.

The evaluation indicates that, in addition to the highest possible acid concentration, a long leaching time and a higher S/L ratio have a positive effect on the leaching efficiency of Li. The temperature essentially influences the content of iron in the solution. The aim of an economic process should be to obtain the highest possible yield of the valuable metal with low energy input and simultaneously high selectivity of the leaching step. From the analysis of the results, it can be determined that this can be realised through appropriate temperature control in combination with a long leaching time. Accordingly, the iron content in solution at 30 °C drops after an appropriate holding time of about 240 min. A low energy input could possibly lead to an economic mapping with long leaching times, so that an economic process control for the recycling of LFP cathode material is enabled. In addition, higher yields of lithium can be realised by increasing the solid/liquid ratio.

For the representation of an efficient leaching step, an optimization calculation was carried out with the statistical software so that the highest amounts of lithium occur with the lowest contents of iron in the solution. The optimized values of the parameters are listed in

Table 5. Parameter of the optimization calculation of the leaching step.

Parameter	Value	Unit
Acid concentration	2.00	mol/L
Oxidant concentration	0.02	vol%
Leaching time	240	min
Solid/liquid ratio	96.88	g/L
Temperature	30.02	°C

.

5. Conclusions

Battery technology, both for primary and secondary cells, is essential in modern life. In addition, the demand for resources in this market segment is increasing due to the economic growth in many countries and the permanently rising prosperity worldwide. In this regard, the market is being driven by the ecological rethinking due to climate change which is influencing international and national legislation. For example, the transport sector is responsible for a quarter of greenhouse gas emissions within the European Union. This is why extensive changes and interventions are needed here in particular. Energy storage systems are necessary for all these measures, and batteries are explicitly mentioned alongside storage systems for hydropower and gases, which in the best case are 100% recyclable.

Lithium-ion batteries are interesting as storage media precisely because lithium is, on the one hand, very light and, on the other hand, it has a high specific charge with a low standard potential. LIBs are reactive, easily scalable, can be installed decentrally, and, due to different cell compositions, can be used for various applications depending on the requirements for safety, cycle stability, and recyclability.

Compared to conventional cathode materials, lithium iron phosphate cathode active material has high cycle stability, but poorer electrical and lithium ion conductivity. This can be improved by appropriate measures, whereby the cathode material is additionally protected against unwanted oxidation. In general, LFPs show exceptional thermal stability and are also fast charging and discharging.

For the recycling of various lithium-ion batteries, several processes already exist that aim to recover valuable metals such as copper, nickel, and cobalt. The latest research also deals with the regeneration of special metals, which include strategic and sometimes critical raw materials such as lithium, manganese, or graphite. A recycling process to be developed, places corresponding demands on the quality of the newly generated products as well as on economic efficiency. Due to high adjustable selectivity and producible qualities, a hydrometallurgical process route is desirable.

Although some studies in this research area are already available as described, some important open points can be clarified within the framework of this research. The optimization of the leaching parameters as well as the statistical evaluation of the experiments enable an effective improvement of the data situation and show the possibility of a selective leaching of lithium. Furthermore, synergy effects with similar processes are examined in order to generate a sustainable approach for the recycling of different types of lithium-ion batteries.

For the experimental investigation of a hydrometallurgical process for the leaching of lithium iron phosphate cathode material, it was determined by means of literature research that higher concentrations of acid and minimal use of oxidants are essential for selective leaching of lithium.

The leaching tests carried out at different solid/liquid ratios, bath temperatures, acid and oxidant concentrations, and residence times show that a high acid concentration of 2 M is essential for efficient leaching of the cathode material. It can be seen that an increase in temperature has a negative effect on the leaching efficiency of lithium from the cathode powder. An optimization analysis for the highest possible lithium yield with minimal iron solution indicates minimal necessary concentrations of oxidant, which increases the alkali metal yield. In contrast, more iron is brought into solution with an increase in temperature. A high yield of lithium from the cathode material also requires a long residence time of the suspension in the reactor. Maximum contents of lithium in the filtrate are found at a leaching time of 240 min. An increase in the lithium content is possible by increasing the solid/liquid ratio, but the amount of dissolved lithium in the pregnant leaching solution only increases by 81.3% with a doubling of the S/L ratio. Due to these characteristics of dissolving lithium from the lithium iron phosphate material, economic process control seems possible at higher concentrations of sulfuric acid and long residence times due to the low process heat.

The recovery of lithium from the pregnant leach solution will be a future research question, since on the one hand there are high quality requirements regarding the purity of lithium as lithium carbonate for renewed use in energy storage systems, and on the other hand the first attempts of iron precipitation as hydroxides by adjusting the pH value did not lead to desirable results. Iron could be qualitatively detected in the solution even at a pH value of 7 after appropriate filtration.