Comprehensive Insights into the Surface Chemistry and Reactivity of Na+ - Based Electrolytes
Authors: Eshetu Gebrekidan Gebresilassie , Grugeon Sylvie , Diemant Thomas , Laruelle Stéphane , Jürgen Behm Rolf , Armand Michel , Passerini Stefano 
Published: February 2020

Amid post – lithium energy storage devices, the sodium - ion battery (NIB) appears to be potentially attractive alternatives and viable technology. Unlike lithium (Li), sodium (Na) mineral deposits are unlimited, attainable at low cost, and evenly distributed. If the practical deployment of NIBs could be realized, there will be approximately a 3 orders of magnitude relaxation in the constraints on resources, accompanied by sustainability, improved environmental benignity and cost reduction.[1–4] As in any electrochemical storage devices, the scientific community has been active in the development of electrode materials for NIBs, however, studies dealing with Na+ - electrolytes, interphases and their reactivity at the vicinity of the electrified surface with (de-) sodiated electrodes are much scarcer. Yet, the electrolyte should be equally considered as it is largely responsible for the life - time and the credibly possible performances, in terms of practically accessible capacity, chemical/thermal stress (safety), rate capability etc. Moreover, though the experience and know - how accumulated from LIBs could help in the development of electrolytes and understanding of interfaces for NIB, it does not give guarantee that electrolyte solvents and salts suitable for LIB are also suitable for NIB. Thus, one should not assume a simple transition of knowledge from Li+ - to Na+ - chemistries. Additional  intriguing question could be whether replacing Li+ by Na+ would result in a similar SEI layer, e.g., nature, composition, and reactivity, or not? Though one would initially assume similar trends, some of the peculiar characteristics, such as the larger size of Na+, higher solubility of Na - based SEI components/degradation products, lower Lewis acidity and higher reduction potential of Na+, etc., could lead to a quite different SEI layer, and preferential reactivity of solvent molecules.

In recognition of the urgent need for detailed investigation of Na - electrolytes, an in - depth study on diverse electrolytes comprising of various NaX (X = PF6, ClO4, TFSI, FSI, and FTFSI) salts, dissolved in organic carbonates  (EC/DMC, EC/DEC, and EC/PC) and ionic liquids - based solvents was carried out using CV, DSC, GC/MS, FTIR, and XPS. The electrolytes were evaluated based on their responses towards Al current collector corrosion, formation of soluble degradation products, reactivity towards fully sodiated hard carbon (Nax - HC), their detailed surface (SEI) chemistry, and SEI layer cracking characteristics.

The reactivity of the Na - based electrolytes depends on the nature of the salt anion, solvent mixture and chemistry (Li vs. Na). For a fixed Na - salt, the reactivity towards Nax - HC is in the order of increasing: EC/PC < EC/DEC < EC/DMC.  For a defined solvent mixture, the stability is NaFSI > NaTFSI > NaPF6 > NaFTFSI > NaClO4 and NaFSI > NaTFSI > NaClO4 > NaPF6 > NaFTFSI, in terms of the decreasing onset exothermic temperature and increasing the total heat generated respectively. GC/MS analysis revealed that the generation of soluble SEI telltales heavily depends on the linear carbonates (DMC vs. DEC) and battery chemistry (Li+
vs. Na+).

FTIR and depth - profiling XPS analysis on both cycled and uncycled binder free half - cells using the above-mentioned Na - based electrolytes provided deeper insight into the surface chemistry (SEI composition, thickness, evolution etc.), enlisting the role of salt anions, solvents and battery chemistry.

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