This Keeps Energy in Circulation
Optimum Recycling of Graphite From Old Lithium-Ion Batteries
Helmholtz scientists have researched how the mineral graphite from old batteries can be processed so that it can be reused for new batteries with the same performance. In tests, the purified graphite showed excellent capacity retention, which is comparable to new graphite.
Lithium-ion batteries (LIBs) contain many important raw materials. In addition to the lithium that gives them their name, they contain metals such as copper, nickel, cobalt, aluminium and graphite. The latter is the anode material, which accounts for 15 to 25 per cent of the total battery weight. It is applied to a copper foil and forces the lithium ions to find a fixed place in the lattice structure of the material during charging. This has the following advantages: higher cycle stability, better performance during fast charging and higher quality consistency compared to other battery types such as lead batteries.
The purer the graphite, the better this mechanism works. Synthetic graphite, which is produced using an energy-intensive coke-based process, fulfils this task particularly well due to its optimised and adaptable properties. However, both natural (ore-derived) and synthetic anode graphite come largely from China and have a significant environmental footprint.
Recycling is therefore essential in order to recover the raw materials and at the same time minimise dependence on China. Currently, around 100 kilotonnes of used batteries are recycled in Europe every year.
Cleaning and reuse of graphite
To recover graphite from used batteries, the batteries are first shredded, leaving behind what is known as black mass. The graphite is extracted from this fine powder using froth flotation. The process is based on what is known as selective hydrophobisation, i.e. the water-repellent properties of minerals and the adhesion of these particles to gas bubbles, which are then discharged via a foam.
The resulting concentrate is purified. Inorganic acids such as hydrofluoric acid are often used for this purpose, which can cause environmental damage. In this case, the Australian company EcoGraf cleaned the graphite particles using an environmentally friendly process that does not require highly toxic hydrofluoric acid. The researchers at the Helmholtz Institutes in Ulm and Freiberg analysed these graphite particles with regard to purity and reuse as anode material.
»We were able to prove with the tests that the electrochemical performance of the recovered graphite from discarded LIBs matches that of new anode graphite. Structure and morphology are insignificantly changed in comparison. Above all, the recycled graphite has a remarkable storage capacity of more than 350 mAh/g despite minor impurities from the recycling process,« says Dr Anna Vanderbruggen, a scientist at the Helmholtz Institute Freiberg for Resource Technology (HIF) at the time of the study, explaining the test results.
Tests with newly assembled battery cells made from recycled graphite also show excellent cycle stability. The test cell was subjected to 1,000 charging and discharging cycles, with a capacity retention of 80 per cent. This is comparable to the performance of reference full cells made of pure material.
Important step for the circular economy
The results are an important step towards fulfilling the requirements of the European Battery Regulation, which was only amended last year. It provides for an increase in the minimum recycling efficiency from 50 to 70 per cent by 2030.
As graphite accounts for up to 25 per cent of the total mass of LIBs and will remain an essential component of LIBs in the coming years, this recycling process is an important step towards fulfilling the European Green Deal requirements, but above all for the circular economy.
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