Operando Electrochemical Dilatometry (ECD)—Monitoring the Dynamic Behavior of Carbon Materials in Sodium-Ion Batteries During Cycling

Due to the continuously increasing demand for energy storage systems research on beyond-lithium technologies has gained much attention. In this field, sodium-ion batteries (SIBs) play an important role due to the greater availability of sodium compared to lithium, no need of critical cobalt as positive electrode material and the possible use of aluminum as current collector, which is cheaper and lighter compared to the commonly used copper.^[1] Carbon electrodes have been effectively applied as negative electrode material in this case. The focus of this abstract is on the investigation of graphite and hard carbon as negative electrode material using operando electrochemical dilatometry (ECD). ECD is a technique that recently gained attention due to the improvement in measurement setups. In contrast to other analytical methods, like X-ray diffraction, that provide information on the structural changes of the active material, ECD gives information on the expansion of the complete electrode, also including inactive components and morphological characteristics. It has been recently used to investigate several electrode materials for sodium- and lithium-ion batteries (LIBs).^[2]<br/>Graphite, which is the standard material of choice in LIBs, can not be used with the same, carbonate-based electrolytes, in SIBs, as the formation of binary graphite intercalation compounds is thermodynamically not favorable.^[3] To overcome this issue it was demonstrated that the use of ether-based electrolytes results in the formation of stable ternary graphite intercalation compounds, where the solvent molecules co-intercalate together with the sodium ions into the graphitic lattice.^[4] This reaction results in a large electrode expansion which can be easily followed using ECD. Even though the reaction is highly reversible a decrease of the electrode expansion is favorable, especially in full cell applications. Herein, the influence of different binder materials as well as the addition of a co-solvent on the electrode expansion has been investigated. The use of CMC (sodium salt of carboxy methylcellulose) as well as the addition of 10 vol% ethylenediamine has shown promising outcomes.^[5]<br/>Hard carbon is so far the standard material in commercial SIBs and benefits from its redox potential close to Na⁺/Na, a storage capacity which is typically between 150-350 mAh g^-1 and especially low cost.^[6] Unlike graphite carbonate-based as well as ether-based electrolytes can be used in this case. However, a lot of research has been done to reveal the storage mechanism no clear answer has been found yet. ECD has been applied to get a further understanding of the storage mechanism in hard carbon electrodes. In this case ECD showed that the intercalation process of sodium ions can be divided in three steps, proposing an intercalation – pore filling – plating mechanism. In contrast, a two-step mechanism has been found in the case of lithium ions, assuming an intercalation – pore filling process.^[7]<br/>In conclusion, ECD was found to be a powerful operando tool to investigate the influence of different components (binder, co-solvent) on the co-intercalation of sodium ions in graphite as well as the charge storage mechanism in hard carbon electrodes.<br/>[1] C. Vaalma, D. Buchholz, M. Weil, S. Passerini, <i>Nat Rev Mater </i><b>2018</b>, <i>3</i>, 1-11.<br/>[2] I. Escher, M. Hahn, G. A. Ferrero, P. Adelhelm, <i>Energy Technology </i><b>2022</b>, <i>10</i>, 2101120.<br/>[3] O. Lenchuk, P. Adelhelm, D. Mollenhauer, <i>Phys. Chem. Chem. Phys. </i><b>2019</b>, <i>21</i>, 19378-19390.<br/>[4] B. Jache, P. Adelhelm, <i>Angew. Chem., Int. Edit. </i><b>2014</b>, <i>53</i>, 10169-10173.<br/>[5] I. Escher, Y. Kravets, G. A. Ferrero, M. Goktas, P. Adelhelm, <i>Energy Technology </i><b>2020</b>, <i>9</i>, 2000880.<br/>[6] X. Dou, I. Hasa, D. Saurel, C. Vaalma, L. Wu, D. Buchholz, D. Bresser, S. Komaba, S. Passerini, <i>Mater. Today </i><b>2019</b>, <i>23</i>, 87-104.<br/>[7] I. Escher, G. A. Ferrero, M. Goktas, P. Adelhelm, <i>Advanced Materials Interfaces </i><b>2021</b>, <i>9</i>, 2100596.