Abstract
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.
Highlights
Sodium-ion batteries (SIBs) are currently considered as cost-effective and more sustainable alternatives to lithium-ion batteries (LIBs).[1]
Preliminary note: Based on our previous publications,[8a,b,15] electrodes prepared with a poly(vinylidene difluoride) (PVDF) binder and cells with 2G as the solvent served as reference
PVDF and carboxymethyl cellulose (CMC) are the most common binders used in battery research
Summary
Sodium-ion batteries (SIBs) are currently considered as cost-effective and more sustainable alternatives to lithium-ion batteries (LIBs).[1]. Graphite is reduced and a specific number of solvent molecules (solv) per alkali metal ion (Aþ) is intercalated.[6] The ratio is still not completely clarified, but values of 1–2 and 15–22 for y and n are assumed.[2cÀ4,7]. Karimi et al used different types of glymes and showed a relative thickness change (expansion or shrinkage) of around 45–85% in the second to fifth cycle. These values converge with increasing cycling number.[12]. The ECD study presented here, complements the results from Zhang et al and shows the effectiveness of minimizing electrode breathing by the use of co-solvents with an in situ (operando) method. The first cycle is investigated and information about EN upon continuous cycling is gained, revealing that the pillaring effect is only temporary
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