What are some new ways to recycle recycled lithium-ion battery positives?

Lithium-ion batteries are widely used in mobile electronic devices and electric vehicles due to their high energy density. Due to capacity decay, lithium-ion batteries will reach their service life in a few years. Economically, recycling lithium-ion batteries can significantly reduce their costs(electric vehicles cost more than 20 cents from positive materials). From an environmental perspective, flammable and toxic waste(organic solvents, heavy metals) from used batteries can cause serious environmental pollution. Therefore, recycling and remanufacturing of lithium-ion batteries to achieve sustainable energy storage is imperative.

The traditional method of lithium battery recovery is mainly based on hydrometallurgical process, which involves acid dissolution and chemical precipitation. However, the heavy use of acid will generate additional waste and complicate the recycling process. More importantly, in this destructive recycling process, the energy of the positive material is lost. Due to the higher capacity and lower cost, the nickel-cobalt manganese acid lithium ternary material(NCM) is the dominant positive material in lithium batteries. So far, the recycling of NCM is mainly based on hydrometallurgical processes.

Therefore, there is an urgent need to develop an energy-saving, lossless method to directly recover NCM cathode materials. Recently, Professor Chenzheng of the University of California, San Diego, successfully recovered NCM particles that had been severely attenuated in capacity by hydrothermal treatment and short-term high-temperature sintering of positive materials. The article was published in the top international energy Journal ACSENERY Letters(Impact factor 12.277) and the first author was postdoctoral researcher Shiyang. The recycled NCM particles maintained their original morphology and had a high capacity, stable cyclic properties, and high magnification properties. The various electrochemical indexes completely returned to the original materials.

The authors first stripped two types of NCM positive particles(NCM111: LiNi1/3Co1/3 Mn1/3O2 and NCM523: LiNi 0.5 Co 0.2 Mn0 .3 O2) with capacity decay greater than 20 % from the set fluid, It is compared with raw materials that have not been recycled. It can be seen from Figure 1 that there is no significant difference in morphology and particle size distribution between positive polar materials with severe capacity decay and the original material, but the surface of the recycled material has undergone changes in the crystal structure. For the original material, both the body phase and the surface phase are layered structures, while for the recycled material, although the body phase is still a layered structure, the surface becomes a spinel and rock salt structure. These two structures have low lithium ion conductivity, and this structural transformation of the surface layer is an important reason for capacity attenuation.

Figure 1. (a) SEM image of original NCM523 particles(b) Size distribution of original NCM523 secondary particles(c) Size distribution of original NCM523 secondary particles(d) Size distribution of NCM523 secondary particles after circulation(E) HR-TEM of original NCM523 particles Image(f) HR-TEM image of NCM523 particles after circulation.

Another important reason for capacity attenuation is that lithium is gradually lost during the positive material cycle as the SEI layer gradually thickens. As shown in Figure 2, 22 % of the recycled NCM particles are lost. The author added the recycled material to the lithium hydroxide solution and added lithium by hydrothermal method. At 220 degrees Celsius, the content of lithium can be supplemented to the original value by 4 hours of water heat. However, materials that are directly treated with water and heat are less crystalline and require a short high-temperature sintering process (850 °C 4 hours) to improve the crystallinity of the material.

Figure 2. (a) A schematic diagram of lithium supplementation to a positive electrode material, and(b) the content of lithium in a positive electrode material over water heat time

After the process of water heat and sintering regeneration, not only can the lithium content in the material return to the original level, but the spinel and rock salt structures on the surface can also be converted into layered structures. As shown in Figure 3, the recycled particles still retain their morphology and size distribution, and the crystal structure of the surface returns to the layered structure. The author uses direct sintering method to compare with the former in addition to using hot water sintering method for the regeneration of positive polar materials. The direct sintering method is to directly sinter a certain amount of lithium carbonate with the recycled material for a long period of time at a high temperature(850 degrees Celsius 12 hours) and perform it in both air and oxygen atmosphere. The authors found that the direct sintering method in oxygen can also change the surface structure back to the layered structure. However, after the NCM523 particles with high nickel content are directly sintered in the air, there is still a rock salt phase on the surface, and it can not completely change the loop structure. For low nickel content NCM111, the effect of direct sintering in oxygen and air is the same. The authors found that the nickel content in the positive electrode material has a great influence on the regeneration conditions. The higher the nickel content, the greater the effect of oxygen partial pressure on the regeneration process.

Figure 3. (a) SEM images of regenerated NCM523 particles, and(b) dimensions distribution of regenerated NCM523 secondary particles, (c) HR-TEM image of NCM523 particles regenerated by water heating and Sintering(d) and HR-TEM image of NCM523 particles regenerated by direct sintering in air, (E) HR-TEM images of NCM523 particles regenerated by direct sintering in oxygen, and(f) XPS spectra of the original, recycled and regenerated NCM523.

Subsequently, the authors performed electrochemical tests on the cycling properties and rate performance of the original material, the cyclically decayed material, and the regenerated material. As shown in Fig. 4, the attenuated NCM material has poor cycle performance. The hydrothermal sintering method and the material after direct sintering regeneration in oxygen can completely restore the cycle properties of the original material, while the hydrothermal sintering method regenerates the material. Has better rate performance. For NCM523 positive electrode with high nickel content, direct sintering regeneration in the air cannot restore its cycle performance, which is related to the rock salt phase existing on the surface.

Figure 4. (a) Cyclic performance of NCM111,(b) cyclic performance of NCM523,(c) doubling the performance of NCM111,(d) doubling performance of NCM523, and(E) voltage-capacity curve of NCM111 at 5C, (f) Voltage capacity curve of NCM523 at 5C.

To sum up, the work shows a new type of lithium battery recovery technology. After the heat and water sintering process, the material has maintained its original morphology and particle size, and the lithium lost during the cycle has been supplemented. The spinel and rock salt structures formed during the cycle can be converted back to layered structures. Due to its cyclic decay, the component defects and structural defects were repaired during the regeneration process. The recycled material completely restored the electrochemical properties of the original material. This method not only is simple and environmental protection but also has low energy consumption. It has obvious advantages compared with the traditional wet metallurgical battery recovery method and lays an important foundation for the sustainable manufacturing of energy materials.

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