RESUMEN
Lithium-ion batteries and related battery concepts show an expansion and shrinkage ("breathing") of the electrodes during cell cycling. The dimensional changes of an individual electrode or a complete cell can be continuously measured by electrochemical dilatometry (ECD). The obtained data provides information on the electrode/cell reaction itself but can be also used to study side reactions or other relevant aspects, e.g., how the breathing is influenced by the electrode binder and porosity. The method spans over a wide measurement range and allows the determination of macroscopic as well as nanoscopic changes. It has also been applied to supercapacitors. The method has been developed already in the 1970s but recent advancements and the availability of commercial setups have led to an increasing interest in ECD. At the same time, there is no "best practice" on how to evaluate the data and several pitfalls exist that can complicate the comparison of literature data. This review highlights the recent development and future trends of ECD and its use in battery and supercapacitor research. A practical guide on how to evaluate the data is provided along with a discussion on various factors that influence the measurement results.
RESUMEN
The electrochemical intercalation/deintercalation of solvated sodium ions into graphite is a highly reversible process, but leads to large, undesired electrode expansion/shrinkage ("breathing"). Herein, two strategies to mitigate the electrode expansion are studied. Starting with the standard configuration (-) sodium | diglyme (2G) electrolyte | graphite (poly(vinylidene difluoride) (PVDF) binder) (+), the PVDF binder is first replaced with a binder made of the sodium salt of carboxymethyl cellulose (CMC). Second, ethylenediamine (EN) is added to the electrolyte solution as a co-solvent. The electrode breathing is followed in situ (operando) through electrochemical dilatometry (ECD). It is found that replacing PVDF with CMC is only effective in reducing the electrode expansion during initial sodiation. During cycling, the electrode breathing for both binders is comparable. Much more effective is the addition of EN. The addition of 10 v/v EN to the diglyme electrolyte strongly reduces the electrode expansion during the initial sodiation (+100% with EN versus +175% without EN) as well as the breathing during cycling. A more detailed analysis of the ECD signals reveals that solvent co-intercalation temporarily leads to pillaring of the graphite lattice and that the addition of EN to 2G leads to a change in the sodium storage mechanism.
RESUMEN
There is currently intense research on sulfur/carbon composite materials as positive electrodes for rechargeable batteries. Such composites are commonly prepared by ball milling or (melt/solution) impregnation to achieve intimate contact between both elements with the hope to improve battery performance. Herein, we report that sulfur shows an unexpected "spillover" effect when in contact with porous carbon materials under ambient conditions. When sulfur and porous carbon are gently mixed in a 1:1 mass ratio, complete surface coverage takes place within just a few days along with the loss of the sulfur bulk properties (crystallinity, melting point, Raman signals). Sulfur spillover also occurs in the presence of a liquid phase. Consequences of this phenomenon are discussed by considering a sodium-sulfur cell with a solid electrolyte membrane.