Electrochemical Lithium Recycling System toward Renewable and Sustainable Energy Technologies

Preparation of WTL cell components

Fabrication of the WTL cell

Testing of WTL cell

Characterizations

Fig. 1 shows the schematic configuration of the WTL cell used in this study. The key strategy in this cell is the use of a multi-layer electrolyte system,^15,16 which is achieved through the use of three compartments in the cell: the waste cathode compartment, the harvesting anode compartment, and the recycling cathode compartment. The multi-layer electrolyte consists of an aqueous liquid electrolyte in the waste cathode compartment, a non-aqueous liquid electrolyte in the harvesting anode compartment, a water catholyte in the recycling cathode compartment, and a dense ceramic electrolyte between each compartment, as shown in Fig. 1. The use of a ceramic electrolyte prevents direct contact between the liquids. In this work, commercially available Li_1+x+yTi_2−xAl_xP_3−ySi_yO₁₂ (LTAP) was used as the ceramic electrolyte, with 1 inch × 1 inch-area and 800 μm-thickness. Waste materials containing Li ions, such as the cathodes made of Li_xFePO₄ or Li_xCoO₂, graphite anodes (Li_xC₆), or organic liquid electrolytes (1 M LiPF₆ in EC-DEC) can be stored in the waste cathode compartment of the WTL cell. Li-containing inorganic solids will sink due to their higher density (>3.5 g/cm³) than water (1.0 g/cm³) and make physical contact with the current collector located at the bottom of the waste cathode compartment. By charging the WTL cell, the Li metal is harvested by extraction from one of the waste Li-containing materials. After Li-harvesting process, the Li metal can be transformed to Li precursor chemicals such as LiOH and Li₂CO₃ by electrochemical reaction with H₂O in the recycling cathode compartment by discharging the WTL cell.

To prove the concept of the WTL system, commercialized solid cathodes such as LiFePO₄, LiMn₂O₄, and LiNi_1/3Co_1/3Mn_1/3O₂ were chosen as waste Li-containing materials. 0.05 g of each sample powder was loaded in the waste cathode compartment, and the cell was charged a current of 0.05 mA/cm². Fig. 2a shows the charge voltage curves for LiFePO₄, LiMn₂O₄, and LiNi_1/3Co_1/3Mn_1/3O₂ powders. The charge voltage curve corresponding to Li extraction is observed to be at 3.5 V for LiFePO₄, 4.0 V for LiMn₂O₄, and 4.0 V for LiNi_1/3Co_1/3Mn_1/3O₂. The specific charge capacity is measured to be 157 mAh/g for LiFePO₄, 111 mAh/g for LiMn₂O₄, and 210 mAh/g for LiNi_1/3Co_1/3Mn_1/3O₂. It is noted that theoretical specific capacity is 170, 148, and 278 mAh/g for LiFePO₄, LiMn₂O₄, and LiNi_1/3Co_1/3Mn_1/3O₂, respectively. This means that 92% (see Fig. 2b), 75% (see Fig. 2c), and 76% (see Fig. 2d) of Li ions, relative to the theoretical capacity, are extracted from LiFePO₄, LiMn₂O₄, and LiNi_1/3Co_1/3Mn_1/3O₂, respectively, during the charge of the WTL cell.

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To confirm the electrochemical delithiation from Li solids during the charging process, each sample power was collected after charging the cell and its powder X-ray diffraction (XRD) pattern was measured and compared with its initial XRD pattern as shown in Fig. 3. The change in the crystal structure was investigated. For the LiFePO₄ sample, our XRD analysis showed orthorhombic olivine structure with lattice parameters of a = 10.30 Å, b = 5.98 Å, and c = 4.68 Å, which are in agreement with literature.^17,18 After charging the cell, the XRD pattern for the Li_1−xFePO₄ sample is changed, as shown in Fig. 3a; the (211) and (020) peaks of LiFePO₄ sample at ∼30° are slightly shifted to greater than 30°, and the (311) peak is also slightly shifted from 35.6° to 36.5° after delithiation of LiFePO₄. The XRD pattern of the delithiated Li_0.08FePO₄ sample with the refined lattice parameters (a = 9.87 Å, b = 5.83 Å, and c = 4.82 Å) is consistent with that of FePO₄ phase reported in literature.^17,18 After charging of the WTL cell, the Li extraction from the Li_1−xMn₂O₄ was also proved by XRD measurement as shown in Fig. 3b. The initial spinel structure of the LiMn₂O₄ with the space group Fd3m remains, but its lattice parameter of 8.216 Å is slightly changed to 8.02 Å for the delithiated Li_0.25Mn₂O₄. This is consistent with the XRD pattern of the delithiated Li_1−xMn₂O₄ reported in literature.^19,20 The XRD patterns of the LiNi_1/3Co_1/3Mn_1/3O₂ and delithiated Li_0.25Ni_1/3Co_1/3Mn_1/3O₂ are compared in Fig. 3c. Its initial XRD pattern is indexed to the Rm space group with lattice parameters of a = 2.84 Å and c = 14.14 Å, which are in agreement with those in literature.²¹ After delithiation of Li_1−xNi_1/3Co_1/3Mn_1/3O₂, the c lattice parameter increases to 14.38 Å, which indicates the electrostatic repulsion among the MO₂ (M = Ni, Co, Mn) slabs along the z-axis-direction of the unit cell after Li extraction.²¹ The XRD patterns of the delithiated Li_0.25Ni_1/3Co_1/3Mn_1/3O₂ are also in accord with that reported in literature.²¹ The XRD results corroborate the electrochemical Li-extraction behaviors of the Li solids during charging of the WTL cell (Fig. 2).

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After charging of the WTL cell, Li metal with a shiny surface and gray color was observed on the current collector when the cell was disassembled in the glove box (Fig. 4a). The purity of the harvested Li metal was characterized by ICP. Fig. 4b shows that its purity is 98.9–99.2% Li with 0.81–1.09% phosphorus. The phosphorus might be introduced from the liquid electrolyte of LiPF₆ in EC-DMC. In this work, we added a small amount of LiPF₆ in the waste cathode compartment by simulating practical conditions in which ordinary Li-ion cells contain liquid electrolytes, such as LiPF₆ in EC/DMC, in addition to anode and cathode materials. However, LiPF₆ can quickly react with water and form toxic HF (LiPF₆ + H₂O → POF₂(OH) + LiF + 3HF), which may induce safety issues in the system. This issue can be resolved by using neutralizing agents, such as KOH and CaCO₃, for HF.

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The Li metal obtained from the WTL cell has a purity of ∼99%, similar to commercially available Li metal (Aldrich, lithium granular 99%). However, commercial Li metal is generally produced from Li ores after undergoing complicated chemical and heat-treatment processes.²² Fig. 4c shows a typical method for Li metal production. First, Li ores containing 1–3% Li are converted to α-LiAlSi₂O₆ by the flotation method to increase its Li concentration, and α-LiAlSi₂O₆ is heated at ∼1100°C to form β-LiAlSi₂O₆, which has a lower density than the α-phase. Then, β-LiAlSi₂O₆ is washed with H₂SO₄ and water-leached at 250°C to form Li₂SO₄·H₂O, followed by treatment with Na₂CO₃ and HCl to produce Li₂CO₃ and LiCl. Finally, by electrolysis of the mixture at ∼450°C in an argon atmosphere, commercial Li metal is obtained. Although the two approaches for production of pure Li metal utilize different raw materials, the WTL system (Fig. 4d) can produce high-purity (∼99%) Li metal via a straightforward, one-step electrochemical process at room temperature, compared to the typical process (Fig. 4c).²²

Since the Li metal is unstable and very reactive in a moisturized air atmosphere, it must be stored and carefully handled in a dry room or argon filled glove box, which limits its direct use in many applications, such as lithium grease and Li electrode materials for LIBs. For this reason, LiOH or Li₂CO₃ have been mostly used as the Li precursors for such applications.¹⁴ In our WTL system, air-stable Li precursor chemicals LiOH and Li₂CO₃ can be also obtained by simple discharge (oxidation) of the harvested Li metal in water (Fig. 1). Fig. 5a shows the charge/discharge curves of the WTL cell when using the Ni mesh current collector in the harvesting anode compartment and LiFePO₄ powder in the waste cathode compartment. The empty symbol curve corresponds to the formation (harvesting) of Li metal on the Ni mesh collector. By performing the discharge process (the red solid symbol curve), Li ions pass through the LATP membrane and react with water in the recycling cathode compartment. The discharge curve plateau ∼3.0 V is thought to be due to the following chemical reaction.

According to the above reaction, the discharge product in the recycling cathode compartments would be LiOH. The white powder of the discharge product (Fig. 5b) was collected from the recycling cathode compartment after discharging the WTL cell. However, the XRD measurement identified its phase as Li₂CO₃ rather than LiOH, as shown in Fig. 5c. It is thus believed that the LiOH (aq) reacts with CO₂ dissolved in water, forming Li₂CO₃ (aq), according to the following reaction route:

It is noted that CO₂ is soluble in water and present in the form of a dissolved gas, and its solubility (1.45g/L) at room temperature is ∼200 times of that of O₂ (0.0083g/L). The formation of Li₂CO₃ in LiOH (aq) solution is well known.²³ Li₂CO₃ (s) powder was obtained after drying the discharged water solution in a vacuum oven at 80 °C. The surface morphology and XRD powder pattern of the Li₂CO₃ powder are shown in Figs. 5b and 5c, respectively. The photograph of the acquired Li₂CO₃ powder is shown in the inset of Fig. 5b. The particles had a pillar shape. As shown in Fig. 5c, the XRD pattern was in agreement with a typical Li₂CO₃ phase [JCPDS #22-1141]. The final production yield of Li₂CO₃ powder from the harvested Li metal was estimated to be ∼25%, based on the coulombic efficiency of the charge/discharge curves (Fig. 5a). This low yield could be due to the use of Ni mesh as the current collector, where Li metal is deposited during charging process. It is believed that small amounts of initially formed Li metal on the current collector could be easily oxidized via reactions with the liquid electrolyte and form byproducts, such as a solid-electrolyte interface (SEI) layer,²⁴ consequently leading to the loss of the harvested Li metal. To resolve the problem and improve the Li-harvesting efficiency of the WTL system, a representative Li-intercalation electrode, graphite (C₆), was employed as the Li-harvesting agent, which leads to Li ion storage instead of Li metal formation. Fig. 5d shows the charge and discharge curves of the WTL cell using a graphite (C₆) electrode in the recycling anode compartment and LiFePO₄ electrode in the waste cathode compartment. During the charging process, the Li ions extracted from LiFePO₄ are stored in the structure of graphite through the electrochemical chemical reaction below.

By the subsequent discharge process, the stored Li ions in the graphite anode are converted into LiOH (aq) via electrochemical reactions with water and oxygen as below.

The formed LiOH (aq) reacts with carbon dioxide to form Li₂CO₃ (aq) as mentioned in reaction 2. Graphite is a well-known anode material for Li ion batteries because it can reversibly intercalate and de-intercalate Li ions within its structure through the stable SEI layers on its surface.²⁵ As a result, the Li-harvesting efficiency of the WTL cell was significantly improved, up to ∼98%, when the Ni was replaced by the graphite anode as shown in Fig. 5d. Besides, other Li-intercalation and Li-alloy materials which have been investigated as the anodes for Li-ion batteries,^26,27 can be applied as the harvesting anode for the WTL system to improve its performance.

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The feasibility of the WTL system was also tested by using real waste Li-ion batteries. Fig. 6a shows the photograph of the selected waste Li-ion battery pack. From our chemical analysis (not shown), it is assumed that it contains a graphite anode, LiCoO₂ cathode, and carbonate-based liquid electrolyte. After disassembling the waste Li-ion battery, all parts of the battery, including pouch film, current collectors, electrodes, separator, and liquid electrolytes, were subjected to sonication in distilled water to separate the electrode materials from the current collectors (Fig. 6b). Finally, a solution in which to disperse the electrode powders and Li salts from the liquid electrolyte was prepared after removing the remaining compartments of the battery (Fig. 6c). This solution was then poured into the waste cathode compartment of the WTL cell with a graphite anode and the charge process was performed. Fig. 6d shows the charge/discharge curves of the WTL cell with the water containing waste Li materials for 100 h at 0.1 mA/cm². The charge voltage plateau (the empty symbol curve) was observed at ∼4.0 V. This indicates that the most of Li was harvested from the LiCoO₂ cathode particles and inserted in the graphite anode. The discharge curve (the solid symbol) had a voltage plateau of ∼3.0 V, which could be attributed to the formation of LiOH by the electrochemical reaction with water (Reaction 4). After discharging the WTL cell, the discharge products were collected from the recycling cathode compartment and identified as Li₂CO_3. These results demonstrate that the WTL cell can work as an efficient and novel device for Li harvesting and recycling with commercially available Li₂CO₃ from various waste Li-containing materials, including waste Li-ion batteries.

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The WTL system was developed to recycle waste Li or Li-containing materials in a simple and environmentally friendly electrochemical process. This system operates at room temperature without using any acid solution. It was demonstrated that high-purity (∼99%) Li metal, comparable to commercial Li metal products, can be harvested from the Li-containing waste materials or stored in the Li-intercalation electrode in the harvesting anode compartment during the charging process. More than 75% of Li in the Li-containing waste materials was found to be extracted, on the basis of the Coulombic efficiency and XRD analysis. In discharging, the harvested Li metal can be transformed to commercially applicable Li₂CO₃ via electrochemical reactions with O₂ and CO₂ dissolved in water. The WTL system features straightforward, one-step electrochemical processes operable at room temperature, compared to existing complicated, energy-intensive processes for Li metal production. In addition, the system can be easily scaled up due to the simplicity of the device. Therefore, the WTL system shows great promise as a low-cost, efficient Li-recycling system that not only enables the production of highly pure Li metal and Li precursors, such as LiOH and Li₂CO₃, but also minimizes environmental impacts.

The work performed at Ulsan National Institute of Science and Technology was financially supported by National Research Foundation of Korea (NRF-2014R1A2A1A11052110).