Investigation of Iron-Additives for Low-Temperature Graphitization of Coffee Ground and its Application in LIBs

Summary:

Graphite is the state-of-the-art negative electrode material (anode) for lithium-ion batteries (LIBs) and will likely dominate the anode market in the form of either natural graphite (NG) or synthetic graphite (SG) for at least the next decade. However, while NG is a limited source and is declared as “critical” raw material by the European Union, SG is typically obtained by high-temperature graphitization (above 2500ºC) of non-sustainable precursors such as petroleum coke, a by-product of the oil refinery. The high energy consumption and long processing times required for production of SGs, as well as the use of environmentally unfriendly precursors has sparked the interest to find alternative approaches to synthetize graphite from more sustainable and highly abundant precursor materials.
In this work, graphitization of coffee ground as sustainable precursor was promoted at low temperature (2000 °C) using six different iron sources (chloride, nitrate, acetylacetonate, oxalate, oxide and pure iron powder) acting as “graphitization additives” at different Fe:C weight ratio. The differences on graphitization efficiency between a wet-mixing or physical-mixing approach of the carbon precursor with the iron additive was deeply investigated via XRD as well as Raman spectroscopy measurements. The degree of graphitization (DOG) was obtained by evaluation of the Raman spectroscopy data. Physical mixing of Fe-powder to the coffee ground led to the highest DOG of 72%. Initial electrochemical characterization has shown, that the DOG is also a first indicator of the 1st cycle Coulombic efficiency (CE) which is an important observation for future material optimization. Furthermore, the additive distribution inside the heat treated materials was examined regarding the size of the precursor particle via focused ion beam scanning electron microscopy (FIB-SEM). The movement of the additive is observed as proposed by the “solution-precipitation” mechanism, but no differences in the additive distribution can be stated by simply comparing two 2D images. A FIB-SEM tomography would clarify the Fe-distribution depending on the precursor size. Electrochemical investigation of the graphitized samples from coffee grounds as anode materials in LIB cells show the best results are obtained by 100CG (Fe(NO3)3) (wet mixing) and 100CG (Fe-powder) (physical mixing) resulting in initial specific delithiation capacities of 286.7 mAh g 1 and 312.6 mAh g 1, respectively. Both samples are further compared with petroleum coke graphitized at 2800 °C as a soft carbon reference and indicated even higher delithiation capacity values at high current rates. These results can pave the way for further optimization of the graphitization of alternative biomass precursors for the production of synthetic graphite anode active materials for LIBs.