Recycling Batteries with Citric Acid

A simple, highly efficient, inexpensive, and environmentally friendly process could provide a viable pathway for the sustainable recycling of depleted lithium-ion batteries (LIBs): No chemicals beyond citric acid need to be added to leach out and separate over 99 % of the lithium, nickel, cobalt, and manganese metals contained in NCM batteries (NCM = mixed nickel, cobalt, and manganese oxides). The resulting recycled material can be directly converted into NCM electrodes. This process was developed by Ruiping Liu, China University of Mining and Technology, Beijing, Guangmin Zhou, Tsinghua University, Shenzhen, China, and colleagues.

Processing Spent Batteries

From smartphones to electric vehicles, lithium-ion batteries are everywhere in our daily lives. They are also an important component of our transition to renewable energy, as they are used to store excess solar and wind energy and send it back into the power grid on demand. The downside is that their limited lifespan results in vast numbers of spent LIBs that contain dangerous heavy metals and other hazardous materials. In addition, metal resources are being depleted.

Most recycling processes suffer from drawbacks such as high energy usage, high emissions, and limited or low-quality recovered material. They can also require very large amounts of chemicals, be complicated and expensive, and/or produce toxic gases and run-off. Leaching with biocompatible acids like citric acid is one alternative to these processes. However, conventional processes (such as the so-called chelation-gel process) require a significant excess of the acid, and the pH value must constantly be adjusted with ammonia—which is complicated and not very environmentally friendly.

Two-Step Dissociation

The team developed a novel citric-acid-based method for the leeching, separation, and reclamation of metals from NCM cathodes. NCM is a mixed oxide containing nickel, cobalt, and manganese in a lamellar structure. Lithium ions are enclosed between the layers.

The trick to their method: Instead of leeching with an excess of citric acid like conventional methods, they use a relatively small amount. Because of this, only two of the three acid groups in the citric acid dissociate. The released protons break up the lithium-oxygen bonds, releasing lithium ions from the NCM into the solution. Bonds between the other metal ions and the oxygen ions are also broken. Nickel, cobalt, and manganese enter the solution, where they are bound into stable complexes by the citric acid anions.

Fischer-Lactonization-Driven Mechanism

The third acid group of the citric acid then reacts with the hydroxyl group on the same molecule. A ring closure occurs in an intramolecular esterification (Fischer lactonization) reaction. This facilitates the reaction of the intermediates with each other to make a polyester, which gels into solid particles that can easily be separated out. Energy consumption and CO₂ emissions are significantly lower than in conventional hydrometallurgical recycling processes.

The gel can subsequently be heated to burn off the organic fragment. This results in a new NCM lamellar framework with included lithium ions, which can be used as a high-quality electrode material.