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.
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.
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.
The possibility to co-intercalate sodium ions together with various glymes in graphite enables its use as a negative electrode material in sodium-ion batteries (SIBs). However, the storage mechanism and local interactions appearing during this reaction still needs further clarification. 1 H, 13 C and 23 Na ex situ solid-state NMR (ss-NMR) experiments are performed to obtain insights into the storage mechanism depending on the state of charge (SOC) and the electrolyte solvent used. Distinct differences could be seen depending on the SOC, indicating a possible change of the solvation shell, differences in the mobility as well as a phase transition at the voltage plateau. Furthermore, exchange experiments reveal information on the sodium ion transport process in the graphitic lattice. The inferior cycling performance of triglyme (3G) (compared to diglyme (2G) and pentaglyme (5G)) is also reflected in the ss-NMR spectra, showing a reduced mobility and stronger interactions between sodium ions, 3G and graphite already at room temperature (RT).
Prussian white (PW) cathodes exhibit extremely fast rate kinetics for sodium ion (Na + ) insertion/de-insertion at relatively high potentials. However, one of the major bottlenecks is to pair them with appropriate anode materials having similar rate kinetics. Herein, the combination of graphite anodes and several glyme-based electrolytes as appropriate building blocks for PW cathodes to achieve high power density without compromising on energy density is reported. Low defect, Na-rich PW is synthesized, and its electrochemical behavior is studied with conventional carbonate-based electrolytes as well as with diglyme (2G), tetraglyme (4G) and a 1 : 1 mixture of 2G and 4G. The stability of the electrolytes is also monitored via in situ (operando) pressure cell measurements. Graphite j electrolyte j PW cells are then studied in both two and three electrode configurations. It was found that glymes are compatible with the graphite/PW electrode pair and the resulting cells exhibit very good cyclability and rate capability.
In situ (operando) electrochemical dilatometry (ECD) provides information on the expansion/shrinkage of an electrode during cell cycling. It is shown that the ECD signal can be used as descriptor to characterize the charge storage behavior of lithium and sodium ions in hard carbon electrodes. It is found that sodium storage in hard carbons occurs by a three‐step mechanism, namely I) insertion, II) pore filling, and III) plating. Step III can be seen from a sudden increase in electrode thickness for potentials below around 36 mV versus Na+/Na and is assigned to plating on the hard carbon surface. Interestingly, this last step is absent in the case of lithium which demonstrates that the storage behavior between both alkali metals is different. The plating mechanism is also supported by reference experiments in which bulk plating is enforced. Bulk plating on hard carbon electrodes can be detected more easily for sodium compared to lithium. It is also found that the type of binder strongly influences the dilatometry results. A comparison between the binders sodium salt of carboxymethyl cellulose and poly(vinylidene difluoride) shows that the use of the former leads to notably smaller first electrode expansion as well as a higher initial Coulomb efficiency.
Electrode materials for lithium-ion batteries (LIBs) typically show spherical particle shapes. For cathode materials, the spherical shape is obtained through the synthesis method. For graphite, the by far most popular anode material for LIBs, spherical particles are obtained through a spheroidization process. The yield of that process is quite low and limited to about 50 %, leaving substantial amounts of by-products. Using such lower quality by-products would be quite attractive for developing low-cost energy stores like sodium-ion batteries (SIBs), for which the requirements for particle sizes and shapes might be less strict as compared to high performing LIBs. Here, we study three different graphite “waste fractions” as anode material for SIBs that are obtained from the spheroidization process and how they compare to LIB battery grade material. Only negligible differences between the fractions are found when analyzing them with X-ray diffraction, Raman spectroscopy and elemental analysis. More clear differences can be seen from N2 physisorption, scanning electron microscopy and particle size analysis. For example, the surface areas of the “waste fractions” can become roughly up to twice as large as compared to the battery grade fraction and the d50 values shift by up to 11.9 µm to lower numbers. Electrochemical measurements show that the “waste fractions” can deliver the full electrode capacity and behave similar to the battery grade fraction up to 10C. However, the higher surface areas lead to more irreversible losses in the first cycle. A surprising finding is that all graphite fractions show almost identical discharge voltages, while the charging voltages differ by as much as 200 mV. This asymmetric behavior only occurs in SIBs and not in LIBs, which indicates a more complex storage behavior in case of sodium.
Gängige Methoden zur Herstellung von Schwefel‐Kohlenstoff‐Kompositen als positive Elektroden für wiederaufladbare Batterien sind die Vermahlung in der Kugelmühle und das Imprägnieren des Kohlenstoffs durch flüssigen, gelösten oder gasförmigen Schwefel. Durch den so gewährleisteten, guten Kontakt der Elemente werden verbesserte Batterieeigenschaften erhofft. Hier wird über einen unerwarteten Spillover‐Effekt des Schwefels in Kontakt mit porösem Kohlenstoff bei Umgebungsbedingungen berichtet. Nach kurzem Mischen von Schwefel und porösem Kohlenstoff im Massenverhältnis 1:1 erfolgt eine vollständige Oberflächenbedeckung des Kohlenstoffs innerhalb weniger Tage. Gleichzeitig verliert Schwefel charakteristische Eigenschaften der Volumenphase wie Kristallinität, Schmelzpunkt und Raman‐Aktivität. Der Spillover kann auch bei Gegenwart einer flüssigen Phase auftreten, und Konsequenzen des Phänomens werden am Beispiel einer Natrium‐Schwefel‐Zelle mit Festelektrolytmembran diskutiert.
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