As the acceptance of electric vehicles increases, so does the need for battery recycling. Recycling is usually the process of breaking the battery down into purely chemical components that can be reconstituted for brand new battery materials. But what if that is – at least for some battery chemistries – excessive?
A new study by Panpan Xu at the University of California at San Diego shows an entirely different technology for lithium iron phosphate (LFP) batteries. This is not the most energy dense type of lithium ion battery, but it is economical and long lasting. (This is the chemistry Tesla wants to rely on in, for example, shorter range vehicles and grid storage batteries.) The low cost cuts both ways – less expensive ingredients mean less profit from recycling operations. However, it seems possible to rejuvenate the lithium iron phosphate cathode material without decomposing it and starting over.
The idea behind the study is based on the knowledge of how the capacity of the LFP battery deteriorates. On the cathode side, the crystal structure of the material does not change over time. Instead, lithium ions increasingly cannot find a way back into their slots in the crystal during battery discharge. Iron atoms can move and take their place and clog the lithium pathway. If you could convince iron atoms to return to their assigned seats and repopulate with lithium atoms, you could have cathode material that is literally "as good as new."
To test a method of performing this reset, the researchers took off-the-shelf batteries and charged and discharged them until the batteries had lost half their capacity. (A reduction to 80 percent capacity is often the defined marker for the end of the service life.) Then the researchers dismantled the batteries and harvested the LFP cathode powder.
The first step is "relithiation", in which the powder is bathed for several hours in a heated lithium solution that also contains a little citric acid. The warm temperature (approx. 80 ° C) and the citric acid help the iron atoms to return to their houses in the crystal lattice and to bring the lithium ions back into position.
After washing and drying the powder, the team tested new cathodes made from the recycled material. While this showed "like new" capacity, it deteriorated fairly quickly. So the researchers added a second step: tempering the dry powder at much higher temperatures.
The rejuvenated powder was heated to 600 ° C (1,112 ° F) over several hours, held there for a while, and then cooled again. This improved the order and stability of the crystal structure in the powder particles, and the cathodes made using this process kept their capacity over 300 charge cycles identical to the brand new ones.
Enlarge /. The direct recycling process can skip some steps, but it leaves behind a more valuable product.
This "direct recycling" process has a strong economic advantage over typical methods. The researchers say it uses 80-90 percent less energy and is therefore associated with around 75 percent fewer greenhouse gas emissions. For LFP batteries, the researchers estimate that hydrometallurgical processes (based on the dissolution of materials and their chemical separation) occur with a net loss of about $ 1.40 per kilogram. Pyrometallurgical processes (which start by melting everything down) are even worse and cost around $ 2.60 per kilogram. However, the researchers' direct recycling process can be profitable, generating a little more than $ 1 per kilogram.
This technique may not be limited to LFP batteries either. In particular, the researchers mention lithium-manganese oxide chemistry as a likely candidate. This is another common type of lithium-ion battery that is used in a number of applications.
As with any laboratory technology, there will certainly be challenges in scaling this up into a commercial recycling facility. However, if possible, this approach could help expand the range of batteries that can be economically recycled and reduce reliance on mining for the supply of virgin material.
Joule, 2020. DOI: 10.1016 / j.joule.2020.10.008 (Via DOIs).