A graphite electrode is the conductor for huge electrical currents that are required to melt steel or scrap metal during the electric arc furnace (EAF) process. EAF is an industrial method to make steel, which uses electricity to melt the material and to react with oxygen producing carbon dioxide gas (CO) emission from slag. Carbon in the slag can also be recovered as a by-product of the carbon reaction. This improves the efficiency of steelmaking process and enhance the quality of the products.
Graphite electrodes are consumed slowly during the EAF process because of sublimation at high temperatures as well as chemical reactions that occur with the steel liquid and slag. About 2/3 of total graphite electrode use is due to the loss of oxidation. Replacing them with renewable sources could reduce GHG emissions and make the EAF more sustainable, but it requires engineering the electrode morphology and properties to ensure adequate performance.
The graphite electrode could be degraded mechanically by the combination of cyclic volumetric contraction and expansion that is caused by Li-ion injection into the active material and extraction as well through the severe bending of the electrode's cantilever. The hysteresis loops depicted in Figure 5a reflect these events. The first two cycles show more hysteresis loops that the subsequent ones, suggesting that the composite electrode suffers more damage in the beginning.
To study the relation between the cyclic degradation process and extreme bending commercially-available composite electrodes that include a Celgard 2400 separator (Kejing Star Technology Company Ltd., Shenzhen China) were re-cycled under various electrochemical conditions. To observe the deformation of bending in situ an elongated bilayer cantilever containing a the Celgard 2400 separator woven into it was designed.
The cyclic degradation as well as bending deformation were analyzed by measuring the variation of the elastic modulus of the graphite electrode during the charging and discharging process with the BKB6808 battery test system. The modulus evolution was correlated with the state of charge (SOC) of the graphite electrode over different cycle times.
The results indicated that the SOC of the graphite electrode at the beginning of the experiment correlated with the elastic modulus. During charging, the elastic modulus increases in proportion to the SOC and decreases when the SOC is reduced. These results suggest that the elastic modulus is affected through the chemomechanical coupling within the electrochemical cycle which can be used as a measure for the design of lithium-ion batteries. The results also show that SOC is closely related to the highest curvature of the bilayer-cantilever. This measurement's evolution rate is identical for the charging and discharging processes and indicates that graphite has high stability in its structure. These findings offer valuable insight into the degradation of graphite electrodes, as well as their deformation in bending due to mechanical coupling between chemistry and chemistry. This will aid in optimizing the performance of these materials in future applications.
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