Sun Yongming's latest Nature sub-issue： high efficiency, environmental protection and low cost! New strategy for recycling waste lithium-ion battery active materials

　　1. Introduction to the background

　　Lithium-ion batteries are the main power source of electric vehicles. After 5-10 years' service, it is estimated that the total amount of discarded LIBs in the world will reach 464,000 tons by 2025, which makes their recycling an urgent task. The cathode active materials of waste lithium batteries contain precious metals (such as lithium, nickel and cobalt), and various recovery methods have been developed, such as the recovery of precious metals through classical thermal metallurgy, hydrometallurgy and biometallurgy, and the emerging direct regeneration technology. Among them, the separation of the active material layer from the anode aluminum foil is one of the most important processes. Effective separation of active substances from aluminum foil is beneficial to subsequent recycling.

　　The separation of aluminum foil from active substances can be realized by chemical corrosion of aluminum foil. However, the continuous reaction between aluminum foil and inorganic acid will produce a large amount of hydrogen, and Al will be dissolved in the reaction solution, and the active substances will also be dissolved. All the above strategies inevitably introduce residual aluminum into active materials, which brings trouble to the subsequent recovery process. Therefore, it is urgent and a challenge for us to develop a new mechanism for separating active materials from aluminum foil with high efficiency, environmental protection and low energy consumption.

　　2. Brief introduction of achievements

　　Recently, the team of Professor Sun Yongming from Huazhong University of Science and Technology proposed a mechanism driven by reactive passivation, which made it easy to separate the aluminum foil from the active material layer. The experimental results show that the separation efficiency between Al foil and LiNi0.55Co0.15Mn0.3O2 layer is more than 99.9% within 5 minutes in a 102 Ah waste battery, and an ultra-thin and compact Al-phytic acid complex layer is formed on Al foil immediately after the Al foil contacts phytic acid, which inhibits the continuous corrosion of Al. In addition, the dissolution of transition metals in LiNi0.55Co0.15Mn0.3O2 can be neglected, and the good structural integrity of LiNi0.55Co0.15Mn0.3O2 is maintained during processing. This study provides a feasible method for the separation of active material layer of positive aluminum foil, which can promote the practical application of green energy-saving recycling of batteries.

　　The achievement was published in the top international journal Nature Communications with the title of "Reaction-Passivity Mechanism Driven Materials Separation for Recycling of Spent Lithium-ion Batteries".

　　3, graphic guide

　　Phytic acid aqueous solution (PA) containing 6 phosphate carboxyl groups and 12 hydroxyl groups was used as the anode separation reagent for lithium ion batteries. As shown in fig. 1a, the strong acidity of PA can induce it to react rapidly with Al _ 2O _ 3 and metal Al2O3 the surface of Al foil, generating Al _ 3+ions and bubbles (Formula 1), resulting in the loss of contact between the positive active material layer and Al foil. Through the weak van der Waals interaction between PVDF and Al foil, the active material layer was adhered to the positive Al foil. PA reacts with Al2O3 on the surface of Al foil and metal Al to form a dense Al-PA layer, which has a strong covalent bond interaction with Al foil, accompanied by bubbling, which destroys the interaction between Al foil and PVDF, the active material layer. Because the size of the cathode material layer is large (several centimeters), it can be easily separated and collected by physical sedimentation, which is beneficial to further operation. PA molecules will immediately chelate with Al3+ to form aluminum-phytic acid complex (Al-PA) (Formula 2), and the further corrosion reaction between PA and surface Al will be terminated. In principle, an Al3+ ion can coordinate with 1 ~ 3 PA molecules, and PA molecules can coordinate with a certain amount of Al3+ ions through their rich phosphate/carboxyl groups (Figure 1b). Due to the various connection modes between PA and Al3+ ions, once a small amount of surface Al is dissolved, a complex [Al-PA] network will be generated in situ. Therefore, PA solution can minimize corrosion through in-situ passivation mechanism.And it has great potential to effectively separate the active material layer and Al foil with low cost and low energy consumption, so as to realize the efficient recovery of the battery cathode material (Figure 1c).

　　Fig. 1 is a schematic diagram of separation of aluminum foil and active material layer driven by reactive passivation. A) PAssivation mechanism a) Al foil with pa. B) connectivity between pa and Al ions. B1 and b2 represent the chelation between an Al ion and three phosphate groups in three and two PA molecules, respectively. B3 represents the chelation between an Al ion and two phosphate groups in two PA molecules. B4 shows the chelation between an Al ion and a phosphate group in a PA molecule. C) Schematic diagram of the separation process of active substances from aluminum foil in PA solution.

　　3.1 Separation operation of aluminum foil active substances

　　The separation process of Al foil-active material layer driven by reactive passivation was monitored by in-situ optical microscope (Figure 2a). LiNi0.55Co0.15Mn0.30O2 (Ni55) layer is completely separated from Al foil within 5 minutes. In order to prove the feasibility of this method, the separation experiment of Al foil active material layer was carried out on Ni55 anode with a total mass of ~705 g in a 102 Ah retired waste battery (Figure 2). Due to the rapid penetration of PA solution into the active material layer, the Ni55 layer was completely peeled off from the Al foil within 5 minutes (Figure 2b). Interestingly, the surface of the obtained aluminum foil is clean without any residual active substances and damage (Figure 2c). The measurement results of inductively coupled plasma mass spectrometry (ICP-MS) show that the Al content in the PA solution used is extremely low. The above results verify that after the initial reaction in PA solution, the aluminum foil is separated from the active material layer, which can passivate the aluminum foil instead of continuous corrosion. After separation, the Ni55 layer presents an irregular flake with a size of several centimeters in PA solution, which is easy to be collected by physical sedimentation. Finally, Ni55 tablets with a total mass of ~642 g are obtained (Figure 2d). Based on the total mass of Ni55 cathode, the losses of Li, Ni, Co and Mn are 0.21, 0.27, 0.08 and 0.17 wt% respectively. The residual amount of Al in the separated Ni55 is as low as 0.026 wt%, which once again supports the passivation of Al foil during processing (Figure 2e). On the contrary,Without the protection of in-situ Al-PA layer, Al foil was completely leached in HCl solution, and Ni55 was obviously damaged, and the total loss rate of Li, Ni, Co and Mn was as high as 21.8% wt% (Figure 2e). Therefore, the separation method of Al foil-active material layer driven by reactive passivation can minimize the corrosion of Al foil and the damage of Ni55 in the separation process.

　　Furthermore, X-ray diffraction (XRD) test was carried out to characterize the separated Ni55 material. The results of fig. 2f show that the separated Ni55 has a clear hexagonal -NaFeO2 structure, which is close to that before PA treatment. The pure phase of S-Al foil was verified by XRD. Therefore, the good integrity of aluminum foil and active material layer is realized by reactive passivation mechanism, which has great advantages for further treatment of material regeneration.

　　[Figure 2] Separation of 2】Ni55 anode aluminum foil from active material layer. A) A digital image of separation process of Al foil-Ni55 layer in PA solution. B) 11.5 m long anode of 102 ah waste battery. C) 11.5 m long Al foil and d) separated Ni55 layer. E) the Contents of Al, Li, Ni, co and Mn in PA solution after separation of al foil and ni55 layer. F) XRD spectrum of Al foil and Ni55 layer after separation.

　　3.2 Separation Mechanism of Active Substances in Aluminum Foil

　　In the separation process of Al foil-Ni55 layer, PA reacts with surface Al2O3 and metal Al, forming an ultra-thin and compact Al-PA layer on Al foil, which can stop their further reaction (Figure 3a). The composition and chemical state of Al-PA layer on S-Al foil were studied by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Fig. 3b verifies the bonding among O, P and Al, indicating that Al-PA layer is formed on the surface of s -Al foil. The thickness of Al-PA layer of S-Al foil was analyzed by studying the change of P content during argon plasma etching with XPS depth detection (Figure 3c). The thickness of Al-PA layer is estimated to be ~20 nm. In order to verify the effect of Al-PA layer on inhibiting Al foil corrosion, the potentiodynamic polarization tests were carried out on F-Al and S-Al foils. Fig. 3d shows that the S-Al foil is passivated by the Al-PA layer.

　　Another important parameter to verify the reaction passivation mechanism is the change of Al content in PA solution with time, the change of positive electrode weight and the change of PA solution recoverability. We used ICP-MS to monitor the change of Al content in PA solution during the separation of Al foil -Ni55 layer (Figure 3e). The Al content in PA solution rapidly increased from 0 to 1.8 wt% within 1 minute and then remained unchanged. This result supports the rapid formation of dense Al- PA layer on the surface of aluminum foil, which inhibits the continuous dissolution of Al. In order to verify the ability to generate a stable Al-PA layer, the formed Al-PA layer was polished and then treated with fresh PA solution under the same conditions as the initial treatment. The concentration of Al3+ in this PA solution is close to that when the initial Al foil active material layer is separated. Therefore, once the Al-PA layer is formed on the surface of aluminum foil, the corrosion of aluminum foil is significantly suppressed. The author further studied the gas production of Al foil -Ni55 layer separation under different sample masses of 20 ~ 80 g (Figure 3f). It is observed that there is a linear relationship between mass and gas production. The ultra-thin Al-PA passivation layer and the used PA solution have low residual Al content, which supports the ultra-low loss of PA solution and shows its recyclability. The same PA solution was repeatedly used in the separation experiment of Al foil -Ni55 layer (5 times). The separation time was constant (~5 min), and the separation efficiency remained above 99.9% (Figure 3g). After the PA solution is reused for 5 times,It shows the same characteristic vibration band as the original solution (Figure 3h). The above results show that the constant reactive dose of PA for Al foil and the robust reactive passivation mechanism drive the separation of Al foil -Ni55 layer, making it extensible and sustainable. In other positive electrodes such as LiCoO2 and LiFePO4, this active material-aluminum foil separation method is also used to separate the Al foil from the active material layer (Figure 2).

　　[Figure 3] Characteristics of aluminum foil and PA solution under different conditions. A) Schematic diagram of passivation mechanism a) Al foil-Ni55 layer separation reaction. B) FT-IR spectra of f-al and S-Al foils. C) High resolution P2XPS spectrum of S-Al foil during Ar+ sputtering. D) Tafel curves of f-al and S-Al foils. E) Al content in PA solution when Ni55 is dissolved in the separation process of Al foil -Ni55 layer. F) 20 ~ 80 g of gas production separated from Al-Ni55 layer with different mass. G) diagram of the relationship between separation efficiency and cycle times for separating Al foil-Ni55 layer by repeatedly using the same PA solution. H) FT-IR spectrum of the above reused PA.

　　3.3 Regeneration of Active Materials

　　The separated Ni55 has a good structure and composition, which provides an easy regeneration basis for its hybrid direct annealing with Li salt (Figure 2f). The anode of waste battery Ni55 before the aluminum foil-active material layer separation operation is recorded as degraded Ni55, as shown in Figure 2. The results of scanning electron microscope (SEM) showed that the cracks of degraded Ni55 particles were cured after recovery. In addition, HRTEM and XRD results verify that the regenerated Ni55 is a layered α-NaFeO2 pure phase with R-3m space group (Figures 4a and b). As shown in fig. 4c, the discharge capacity of the regenerated Ni55 is obviously higher than that before regeneration (the discharge capacity of Ni55 before and after regeneration is 166 and 152 mAh g?1, respectively). In addition, the discharge capacities of the regenerated Ni55 at 0.3, 0.5, 1.0 and 2.0C are 161, 157, 146 and 132 mAh g?1, respectively, which are far better than those of the Ni55 before regeneration (for example, the discharge capacity at 2.0C is 91 mAh g?1, as shown in Figure 4d). In addition, 94% of the regenerated Ni55 can be retained at high capacity after 100 cycles at 0.3C (Figure 4e).

　　Fig. 4 Characterization of regenerated Ni55. A) SEM image and b) XRD image of regenerated Ni55. C) the voltage-capacity curve of the first cycle at 0.1C, d) the rate performance at different current densities of 0.1, 0.3, 0.5, 1.0 and 2.0C, and e) the cycle performance at 0.3c..

　　Environmental and economic analysis of 3.4PA regeneration

　　The flow charts of different recovery methods show that they include PA direct recovery (PA-direct, Figure 5a), General direct recovery (general -direct, Figure S40), Pyro recovery (Figure S41) and hydrometallurgical recovery (Hydro, Figure S42). Based on the treatment of 10000 tons of waste Ni55 batteries, the life cycle assessment (LCA) and technical and economic analysis (TEA) of the above recycling process were carried out by using the everbat model developed by Argonne National Laboratory. The high energy consumption of anode pretreatment in industrial recovery methods (such as crushing, grinding/heat treatment and screening) is not required, and the total energy consumption of PA-direct is 5.84 MJ kg-1, which is far lower than that of General-direct, Hydro and Pyro methods (Figure 5b). At the same time, compared with other recovery methods, the author's PA-direct completely separates the anode from other battery components, thus avoiding the additional greenhouse gas emissions from burning mixed graphite and smelting Al and Cu. Therefore, PA-direct has the lowest greenhouse gas emission (Figure 5c). PA solution is recyclable, which makes the water consumption low. Only 2.65 L kg-1 of water is needed to treat one kilogram of battery, which is equivalent to the water consumption of Pyro process and far lower than that of Hydro process and General-direct process (Figure 5d).

　　Fig. 5e shows the costs of the different recovery methods mentioned above. The cost of using lithium salt for PA-direct repair of cathode materials is slightly higher, which is 6.78 $ kg?1. PA-direct has the advantages of high separation efficiency, production of cathode materials and direct output of high-performance regenerated cathode materials. Therefore, compared with other recovery methods, its revenue and net profit are 15.79 and 9.01 $kg?1, respectively (Figures 5f and g). Taking the production of 1kg-Ni55 anode as an example, the TEA experiment was carried out, which provided reference for industrial production. Fig. 5h-i shows the cost and profit of manufacturing 1 kg-Ni55 cathode from raw materials (Ni salt, Co salt, Mn salt and Li salt) and recycled materials (degradation of Ni55). It is pointed out that more lithium salt is needed to synthesize active cathode materials from the products of Pro and Hydro. The cost of preparing Ni55 cathode by PA-direct is only 16.07$ kg?1, which is far lower than that of Virgin and other processes (26.41$ kg?1). Therefore, a high profit of 16.80 $ kg?1 can be obtained from the production of Ni 55 by PA-direct method, which is 2.68 times that of the production of Ni55 by raw material method. Therefore, the author's PA-direct provides a promising way for the rapid separation of aluminum foil from active material layer and the regeneration of active material, which can promote the recycling of energy-saving, environmental protection and high-value batteries to practical application.

　　[Figure 5] Economic and environmental analysis of 5】PA-direct and other recycling methods. A) PA-direct diagram. B) energy consumption, c) greenhouse gas emissions, d) water consumption, e) cost, f) revenue and g) profit. H) The total cost of manufacturing 1kg of Ni55 cathode from raw materials and recycled materials. I) Comprehensive comparison of different recycling methods.

　　4. Summary and prospect

　　A reactive passivation driving mechanism is proposed to separate aluminum foil and positive active material layer from waste LIB efficiently. In the experiment, 60g Al foil and 636 g Ni55 were separated from 102 Ah waste battery with PA solution, and the separation efficiency was over 99.9%. After separation, the Al foil keeps a complete structure, and the dissolution loss of metal ions of Ni55 is small during processing. The reaction between PA and Al leads to the formation of a dense Al-PA layer with a thickness of 20 nm on Al foil during the separation process, which inhibits the continuous corrosion of Al foil. After direct annealing of degraded Ni55 and Li salt, the regenerated Ni55 was cycled 100 times at 0.3 C, with a reversible capacity of 161 mAh g?1 and a capacity retention rate of 94%. This work provides a promising way for the rapid separation of metal current collector and active material layer, which is different from the traditional method, and can promote the green and energy-saving recovery of batteries to practical application.

　　references

　　Chen, Z., Feng, R., Wang, W. et al. Reaction-passivation mechanism driven materials separation for recycling of spent lithium-ion batteries. Nat Commun 14, 4648 (2023).

　　DOI：10.1038/s41467-023-40369-9

　　https://doi.org/10.1038/s41467-023-40369-9

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