Easily recyclable lithium‐ion batteries: Recycling‐oriented cathode design using highly soluble LiFeMnPO4 with a water‐soluble binder

Recycling lithium‐ion batteries (LIBs) is fundamental for resource recovery, reducing energy consumption, decreasing emissions, and minimizing environmental risks. The inherited properties of materials and design are not commonly attributed to the complexity of recycling LIBs and their effects on the recycling process. The state‐of‐the‐art battery recycling methodology consequently suffers from poor recycling efficiency and high consumption from issues with the cathode and the binder material. As a feasibility study, high‐energy‐density cathode material LiFeMnPO4 with a water‐soluble polyacrylic acid (PAA) binder is extracted with dilute hydrochloric acid at room temperature under oxidant‐free conditions. The cathode is wholly leached with high purity and is suitable for reuse. The cathode is easily separated from its constituent materials and reduces material and energy consumption during recycling by 20% and 7%, respectively. This strategy is utilized to fabricate recyclable‐oriented LiFeMnPO4/graphite LIBs with a PAA binder and carbon paper current collector. Finally, the limitation of the solubility of the binder is discussed in terms of recycling. This research hopefully provides guidance for recycling‐oriented design for the circular economy of the LIB industry.


| INTRODUCTION
Battery design oriented toward recycling is required to comply with the principles of a circular economy for the lithium-ion battery (LIB) industry. 1 The typical LIB cell is complex and comprises various valuable metals, carbon-based materials, and fluorinated derivatives. 2 LIB cell complexity presents significant challenges to material separation and cathode leaching, making it difficult to recycle while reducing recycling efficiency. 3 Individual components of spent LIBs can be easily separated mechanically. Breaking each component further down to recover its constituent materials in pure form is less straightforward. 4 For example, the disassembly of cathodes is hindered by the properties of the binder and cathode materials. [5][6][7] Recycling research on LIB has emphasized recovering Al foil and cathode materials over other valuable materials (e.g., conductive agents). The LIB recycling processes are costly in terms of material and energy consumption, reducing profitability, and having deleterious effects on the environment. 8 Recycling 1 tonne of LiFePO 4 cells using modern recycling methods would require 10 kL of 1 M HCl, 10 kL of 1 M H 2 O 2 , and 54.73 kJ of energy, with a projected cost of ∼$2400 (16,400 Chinese yuan [CNY]). The primary recovered material would be approximately 55 kg of Li 2 CO 3 , valued at ∼$3400 (23,600 CNY) at the current market price. [9][10][11][12] Unfortunately, recycling LiFePO 4 may become commercially unviable with labor and processing costs, as the profitability is strongly linked to the lithium salts' market price. If the lithium salts' prices stabilize, the revenue recoverd lithium salts could depreciate as low as ∼$1400 (9600 CNY). Approaches to combat the dependency on lithium salt would require reduced labor and processing costs. This can be achieved by designing the next generation of LIBs that are easily separated and oriented for recycling to further support the LIB industry's development.
Next-generation LIBs will require higher energydensity cathode materials to meet the increasing energy demands while being easily recyclable. Cutting-edge research on promising cathode materials based on olivine LiFePO 4 , layered LiCoO 2 , and LiNi x Co y Mn z O 2 is being developed and explored. LiFePO 4 has been one of the most widely used modern cathode materials because of its low cost, favorable operating parameters, and safety. Mn has emerged as a supplementary material for Fe in LiFePO 4 LIBs, increasing energy density by up to 20% and raising the output voltage from 3.5 to 4.1 V ( Figure 1A). [13][14][15] The theoretical energy density of a LiFeMnPO 4 battery is comparable to layered cathode materials and dramatically exceeds the performance of LiFePO 4 batteries. The precursor of LiFeMnPO 4 materials is cheaper than Ni and Co, which could enable a more sustainable LIB production ( Figure 1B).
Herein, LiFe 0.3 Mn 0.7 PO 4 exemplifies a high-energydensity cathode material that can be easily recycled by leaching with hydrochloric acid (HCl) at room temperature under oxidant-free conditions ( Figure 1C). The effects of the HCl concentration, leaching duration, and solid-liquid (S/L) ratio on the effects of leaching efficiencies for Li, Fe, Mn, and P were investigated. Results indicate that the cathode material is easily recycled with a water-soluble binder, 16,17 enabling quick recovery and reuse of the current collector, the cathode material, and the residual carbon materials.

| LiFeMnPO 4 leaching
The results of leaching experiments using LiFeMnPO 4 are shown in Figure 2. Various HCl concentrations (0.5, 1.0, 2.0, and 3.0 mol/L) were tested at room temperature as a solvent for LiFeMnPO 4 leachants, loading 20 g/L with a 1-h leaching time. Figure 2A-D demonstrates that the leaching efficiencies for Li and Mn were around 94% when using 0.5 mol/L HCl, while for Fe and P, they were around 90%. Increasing HCl concentrations improved all elements' leaching efficiencies to 100%, demonstrating that LiFe-MnPO 4 is readily recycled. The chemical equation of the leaching process is in Equation (1). According to Equation (1), ∆fG x x = 1152.27 -952.8(0 < < 1). Given x = 0.3, the Gibbs free energy of formation for this reaction is −607.12 kJ/mol, which means this reaction can occur at room temperature (Table S1).
The effect of varying the S/L ratio on Li, Fe, Mn, and P leaching efficiency was investigated to evaluate the feasibility of industrial-scale LiFeMnPO 4 recovery. The time to effectively leach out the metals was evaluated with various solvent exposure times (10, 30, 60, 120, and 180 min) at 1.0 mol/L HCl. Leaching readily occurs with 68.0%-75.5% leaching efficiencies within 10 min of HCl solvent exposure at 1.0 mol/L HCl ( Figure 2E-H). Extending the reaction time to 30 min increases the Fe and Mn leaching efficiencies to ∼99.0%, while those of Li and P were 87.2% and 94.6%, respectively. Complete dissolution of all elements was achieved within one hour. The leaching efficiency remained nearly 100% for all elements when the S/L ratio was between 10 and 30 g/L. However, when the S/L ratio was 40 g/L, it decreased the leaching efficiency of Li, Fe, Mn, and P below 90% ( Figure 2I,J). The decreased leaching efficiency may be potentially attributed to Le Chȃtelier's principle, where the product of leachants shifts the reaction's direction back toward reactants preventing equilibrium from moving toward leaching. 18 LiFeMnPO 4 leaching was optimally performed using HCl concentrations higher than 1.0 mol/L, with an S/L ratio of less than 30 g/L and a leaching time more than 1-h. Under these conditions, the residual carbon material obtained after leaching was very pure, with Li, Fe, Mn, and P content below 0.02 ppm ( Figure 2K). Separate experiments using 2.0 mol/L H 2 SO 4 as the leachant, instead of HCl, resulted in Li's complete dissolution but were poor at recovering Fe at ∼20% leaching efficiency ( Figure 2L).

| Cathode separation
The excellent leaching properties of LiFeMnPO 4 suggest that it could be used to create easily recycled cathodes for enhanced recyclability. Separating traditional cathode materials is generally inherent to using polyvinylidene fluoride (PVDF) as a binder due to poor solubility in water and organic solvents. Water-soluble polyacrylic acid (PAA) was explored as a potential cathode binder ( Figure 3A,B) to avoid the solubility issues of PVDF; this allows the cathode materials to be separated from the Al foil simply by washing with water, as shown in Figure 3B.
The cathode materials that remain after removing the Al foil require further processing to separate the active cathode material from the residual carbon materials, both of which are valuable. Conventional LIB cathodes comprise the active cathode material, PVDF, and a conductive nanomaterial. While the majority of the active cathode material can generally be recovered by acid leaching, residual metal elements can significantly affect the performance and properties of the recovered material if the leaching efficiency is below 100%. Furthermore, the poor solubility properties of PVDF binders increase the complexity of recovering the conductive nanomaterial. Utilizing more soluble cathode active materials and binders thus facilitates the recycling of the residual carbon materials. Figure 3 illustrates that the residual material obtained after removing the Al foil and water-soluble binder from LiFeMnPO 4 cathodes consists of carbon-coated LiFe-MnPO 4 particles mixed with the conductive agent. Due to strong interactions between these materials, this mixture cannot be separated based on density differences. However, the residual LiFeMnPO 4 can be removed by completely leaching the active cathode material, leaving only carbon materials. While these residual carbon materials include the conductive nanomaterial and nano-coating layers from LiFeMnPO 4 particles, the mixture could theoretically be repurposed as a conductive agent.
Images with the electron microscope were used to evaluate the practicality of reusing the recovered carbon materials. As shown in Figure 3B, with the assistance of a water-soluble PAA binder, cathode materials are readily separated from the Al foil physically. Figure 3C shows that the LiFeMnPO 4 particles were coated with graphene layers and that the graphene remaining after leaching contained no detectable Fe, Mn, or P. Accordingly, the residual carbon material obtained from recycled cathodes after leaching was highly pure: Figure 3D shows that it consisted of the conductive agent (super P) and graphene layers from LiFeMnPO 4 particles, with no detectable metallic impurities at the micron scale. This material's energy-dispersive X-ray spectroscopy analyses confirmed the absence of characteristic peaks for Fe and P at 0.8 and 2.0 keV, respectively ( Figure 3E).
The only noncarbon peak present is a minor peak corresponding to trace quantities of O (0.53 keV) at the edges of the carbon material and indicating the potential suitability of reusing the recovered carbon residue as a conductive agent. The viability of recycled carbon residue as a conductive agent for LIBs was assessed by assembled NMC333 half cells. Figure 3F shows that these NMC333 cells had an initial specific capacity of 188 mAh/g with a Coulombic efficiency of 83.5%. For comparative purposes, commercial NMC333 half cells had a slightly lower specific capacity of 179 mAh/g and a negligible higher initial CE of 84.8%. The minor differences in NMC333 cells may be due to the increased oxygen content of the carbon residues. Cycling performance reveals that the recycled carbon showed similar stability to super P:NMC333 half cells made using recycled carbon, and super P retained 86.7% and 88.3% of their stable capacity after 80 cycles, respectively ( Figure 3G).

| Environmental and economic analysis
Their recycling was analyzed quantitatively to illustrate the advantages of LiFeMnPO 4 -based cathodes over traditional methodologies. Recycling efficiencies nearing full recovery were achieved for the active cathode material, conductive agent, and Al foil. In contrast, conventional pyrometallurgical, hydrometallurgical, and direct physical recycling methods have recovered active cathode materials and Al foil with many impurities and below 90% recycling efficiency. 19 The conductive agents account for 1%-2% of the mass of LIBs and 3% of the overall cost (Tables S2 and S3). Pyrolysis of the conductive agent in spent LIBs is predicted to increase greenhouse gas emissions by 10% during recycling. Thus, pyrolysis of the conductive agent has no economic benefit and is a potential source of environmental pollutants. The LiFeMnPO 4 -based cathode with watersoluble binder design outlined here significantly reduces material and energy consumption during the cathode separation process. The leaching of the active cathode material is feasible without oxidants or heating compared to other cathodic materials utilizing PVDF binders. This reduces the material and energy consumption required for recycling by 20% and 7%, respectively. This method reduces CO2 emission by ∼11.1%, or 162 g per kg of material, compared to traditional hydrometallurgical methods (Table S4).
In comparison, the economic value of recycling LiFePO 4 and LiMn x Fe 1−x PO 4 cathodes are comparable due to the low valuation of metal elements. The precise separation of the easily dissembled cathode can significantly improve recycling profitability. For instance, with precise separation, the profitability of $3231.9 + 10.59x (0 < x < 1) (22160.9 + 72.62x (0 < x < 1) CNY) can be achieved for 1 tonne of degraded LiMn x Fe 1−x PO 4 cathode, with ∼$133 (918 CNY) per kg of cathode revenue from conductive agent recycling (Table S5). Therefore, bringing in recycle-oriented designs for cathodes could dramatically improve recycling efficiency, reduce environmental impact, and increase the profitability of LIB recycling.

| LiFeMnPO 4 /graphite LIBs with PAA and carbon paper
Separation of the battery from the materials is ultimately a matter of separating the current collector, active materials, conductive agent, and binder. A readily soluble binder facilitates the separation of the current collector and subsequent materials. These electrode separation processes are designed to ensure that the current collector does not mix with other materials. Otherwise, the recycled materials require further purification. Metals, including copper and aluminum foils, are a potential source of contaminants that have caused significant problems during recycling. Unfortunately, the copper and aluminum foils are fragile and are readily incorporated into recyclable materials. Aluminum foil is also prone to oxidation and corrosion, which increases the potential for contamination of recycled materials. As a result, it remains challenging to ensure that the metal content of the recycled materials is below two parts per million.
One approach to avoid potential metal contamination is to substitute metal for carbon paper in the current collector so the recovered materials from LIBs lack metal impurities, making them more suitable for subsequent reuse. As shown in Figure 4, the discharged LIBs can be disassembled into shell, separator, cathode, and anode ( Figure 4A-E). From the disassembled cathode and anode, the current collector and active materials/ conductive agent can be separated very easily by simply immersing them in water ( Figure 4F-J). Even if the separated cathode materials contain carbon, impurities can be removed directly by heat treatment or physical methods.

| Limitation of the soluble binder
The water-soluble binder PAA allows for the separation of the cathode material and the current collector. However, the PAA binder in the material cannot be completely dissolved in water. Figure 5A,B show the PAA detection in the recovered cathode materials after multiple water washes and ultrasonic treatment. Figure 5C shows that the recycled cathode material decomposes by 4.17% before 800°C in an Ar atmosphere, which agrees with the decomposition of PAA ( Figure 5D). The TGA decomposition of PAA leads to the mass percentage of 12.17 carbon materials, indicating that only a fraction of PAA (∼5%) in the recycled cathode materials is dissolved in water. This could be due to the strong interaction between the binder, active materials, and conductive agent, possibly forming irreversible chemical bonds. Fortunately, the decomposition of PAA increases the carbon content in the conductive agent by 11.56%, which does not significantly change the conductive agent's properties. Therefore, extensive research is still required to select suitable binders and processing methods for efficiently developing recyclable LIBs.

| CONCLUSIONS
This research represents a proof-of-concept for the design of an easily recyclable LIB cathode using LiFeMnPO 4 as the active cathode material and a water-soluble PAA binder. The cathode is easily separated into its constituent materials, recovered in high-purity form, and suitable for reuse, which could significantly improve the circularity of the LIB industry. The water-soluble binder allows for the separation of the Al foil current collector from the cathode materials by a simple water wash. The separated cathode can be leached with acid extraction that separates the active materials from the residual carbon, resulting in a high-purity carbon material that can be reused as a conductive agent without significantly impacting performance. The methodology could benefit recycling manufacturing scraps, which account for 5%-10% of the wasted materials in spent LIBs.
Due to environmental concerns associated with the use of PVDF, environmentally friendly and watersoluble binders are being developed for use in LIBs. Water-soluble binders have already been successfully used in the anode materials in LIBs, resulting in nearly 100% recycling of copper foil, graphite, and conductive agents. If the same approach is applied to cathode materials, the recycling of LIBs could be significantly simplified. Future research focusing on developing soluble cathode materials and binders with collaboration between researchers and battery manufacturers is fundamental to the next generation of LIBs to achieve LIBs with high performance oriented toward recyclability. Ensuring a circular economy for the LIB industry is critical for securing energy security, managing resource consumption, and delaying the natural depletion of lithium reserves.