A new recycling strategy for spent lithium-ion batteries is based on a hydrometallurgical process in neutral solution. This allows lithium and other valuable metals to be leached in an environmentally friendly, highly efficient and cost-effective way, as reported by a Chinese research team in the journal Angewandte Chemie. The leaching efficiency is effectively increased by a solid-solid reduction mechanism, a so-called battery effect and the addition of the amino acid glycine. The common hydrometallurgical reprocessing of spent lithium-ion batteries is based on acid and/or ammonia leaching processes. However, the excessive and repeated use of acids and bases increases environmental pollution and safety risks. A pH-neutral process would be safer and more environmentally friendly.
In order to achieve a neutral approach, the team led by Lei Ming and Xing Ou from Central South University in Changsha, Zhen Yao from Guizhou Normal University and Jiexi Wang from the National Engineering Research Central of Advanced Energy Storage Materials had to dig deep into their bag of tricks, as the aggressive reagents required for classic leaching processes are not so easy to replace.
Trick one: they create "micro-batteries" in situ, which help to break down the spent cathode material of the batteries - nickel-cobalt-manganese oxide (NCM) loaded with lithium. The NCM particles are mixed in a neutral liquid with an iron(II) salt, sodium oxalate and the amino acid glycine. As a result, a thin, solid layer of iron(II) oxalate is deposited on the particles. This "shell" acts as an anode, the NCM cores as a cathode (battery effect).
The direct, close contact makes it easy to transfer electrons. The coating also prevents unwanted by-products from being deposited on the particles. The battery effect drives an electrochemical reaction in which iron(II) ions oxidize to iron(III) ions and oxygen ions of the oxidic NCM particles are reduced to hydroxide ions with water. This breaks up the NCM layers and the lithium, nickel, cobalt and manganese ions stored in the structure are released into the solution.
Here they are "captured" - trick two - by glycine in the form of complex compounds. The glycine also has another task: it buffers the pH value of the solution in the neutral range. Within 15 minutes, it was possible to leach 99.99 % lithium, 96.86 % nickel, 92.35 % cobalt and 90.59 % manganese from spent cathodes.
Efficient leaching in a neutral solution could open up new ways of realizing large-scale, environmentally friendly recycling of used batteries. Hardly any harmful gases are produced and the glycine waste water is suitable as a fertilizer. Significantly less energy is consumed and the costs are lower than with conventional processes.
