Alkali Ions Transport into Lignin-Based Hard Carbon Fibers

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Hard carbons are considered as preferred negative electrodes in the development of sodium- and potassium-ion batteries because of low working potential, high capacity, as well as good cycling stability. A further advantage is that they can be made from renewable precursors, in this case lignin. The structure changes of the precursors during synthesis of the hard carbons have been extensively studied1–3. The alkali ions insertion mechanism broadly accepted by the community is the insertion into the carbonaceous structure in the slope region, and a pore filling mechanism in the plateau region of the galvanostatic profile. The diffusion of sodium ions and potassium ions into hard carbon electrodes has been investigated using Galvanostatic Intermittent Titration Technique (GITT) combined with analytical models4,5. However, a physics-based model can be used instead in order to analyze the experimental data and investigate potassium- and sodium ion transport into hard carbon electrodes in greater details.In this work a physics-based single particle model combined with GITT experimental data has been used to investigate the effects of carbonization temperature and state of charge on potassium ions diffusion into an electrode made from pure lignin based hard carbon fibers. Electrospun fiber mat electrodes carbonized in the temperature range from 800 °C to 1700 °C have been investigated. The parameters extracted from the physics-based model have been used to shed further light on the insertion mechanism. Finally, the study was extended to sodium ions to investigate the impact of various alkali ions.The results show that diffusion coefficients as well as electrochemical rate constants depend of carbonization temperature and state of charge. The physics-based model cannot reproduce the experimental data well by only considering diffusion, and a second time constant needs to be considered in the model. The coupling of parameters and open circuit voltage (OCV) curves extracted from the GITT experimental data supports that the slope region refers to the insertion into the carbonaceous structure and the plateau region refers to the pore filling mechanism. References Morikawa, Y., Nishimura, S., Hashimoto, R., Ohnuma, M. & Yamada, A. Mechanism of Sodium Storage in Hard Carbon: An X‐Ray Scattering Analysis. Adv. Energy Mater. 10, 1903176 (2020).Kubota, K. et al. Structural Analysis of Sucrose-Derived Hard Carbon and Correlation with the Electrochemical Properties for Lithium , Sodium , and Potassium Insertion Structural Analysis of Sucrose-Derived Hard Carbon and Correlation with the Electrochemical Properties fo. (2020) doi:10.1021/acs.chemmater.9b05235.Buiel, E. R., George, A. E. & Dahn, J. R. Model of micropore closure in hard carbon prepared from sucrose. Carbon N. Y. 37, 1399–1407 (1999).Jian, Z., Xing, Z., Bommier, C., Li, Z. & Ji, X. Hard Carbon Microspheres: Potassium-Ion Anode Versus Sodium-Ion Anode. Adv. Energy Mater. 6, 1501874 (2016).Li, Y., Hu, Y. S., Titirici, M. M., Chen, L. & Huang, X. Hard Carbon Microtubes Made from Renewable Cotton as High-Performance Anode Material for Sodium-Ion Batteries. Adv. Energy Mater. 6, 1–9 (2016). Figure 1