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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al 4.2. Organic electrolytes: solvents Martin Karlsmo1 and Patrik Johansson1,2 1 Department of Physics, Chalmers University of Technology, 412 96 Göteborg, Sweden 2 Alistore-ERI, CNRS FR 3104, 15 Rue Baudelocque, 80039 Amiens, France Status In many ways, both R&D and the practical deployment of NIB organic electrolytes follow the development of LIB electrolytes [175–177]. This is logical, as the operating conditions and application areas are more or less the same, and thus, similar properties are needed; i.e., readily dissolved salts to create a large concentration of charge carriers offering fast ion transport with high fluidity and low viscosity. The latter also assists in the proper wetting of the separator and the porous electrodes. Additionally, high thermal and chemical stabilities are a must, to allow for usage in various climates and vs. different electrodes, as are wide ESWs matching the electrodes. Finally, all must be available at a low cost. As of today, this means linear and cyclic carbonates, such as DMC, EC and PC [174], or ethers, such as glymes, in particular, diglyme [174, 175]. While much of NIB R&D follows from earlier LIB developments, there are some distinct practical differences to be found. One is that linear carbonates, such as DMC, used to lower the melting point and viscosity of LIB electrolytes, create soluble products upon reduction in a NIB, and hence, unstable SEIs at the anode, a problem that can be overcome by smart additive usage [176]. Another is that graphite is not capable of intercalating naked Na+ ions and thus, it is not commonly used for NIBs. Therefore, PC, one of the original LIB electrolyte solvents, is frequently used for NIBs (as opposed to LIBs, as it exfoliates graphite) [174, 176, 177]. There is also a fundamental difference between the use of Na salts and Li salt: for the former, both the ion–ion and ion–solvent interactions are reduced to ca. 80% of the latter [178]—which paves the way for faster desolvation dynamics [179] which, at the cell level, manifests as improved power rates [174, 176, 177]. As with any other battery technology based on organic electrolytes, improved safety without making any departure from performance is key—and here, new NIB electrolytes, in particularm solvents, should evolve out of the LIB shadow. Another driving force should be to simplify recycling—and this, for example, means the use of F-free solvents/electrolytes [170]. Current and future challenges Even if challenged by both solid-state electrolytes (SSEs) offering performance and safety advantages and aqueous electrolytes offering lower cost and better overall sustainability, organic solvent-based electrolytes are still likely to be the main route for years to come. They can be used in their own right and/or in hybrid electrolytes, with polymers or ionic liquids, to improve performance and safety [180] or to facilitate better SSE/electrode contact. A major challenge is to create a holistic and Na+-centred (and not Li+-derived) approach to the salt(s)–solvent(s)–additive(s) recipe for a standard functional NIB electrolyte [181]. This could also reduce the complexity and offer better control and understanding of e.g., solubility issues and ion-transport advantages. Today, the latter is, at the phenomenon level, a power performance advantage compared to LIBs, which is coupled to lower solvent desolvation energies [179] and the presence of fewer contact ion-pairs [178]. However, the reduction in strong interactions renders the organic products from solvent reduction more soluble and thus less stable, and creates/results in additional inorganic SEIs. Another large challenge comes directly from the +0.3 V shift in Ered of Na+/Na vs. Li+/Li. For the same final cell voltage, there is a corresponding more stringent demand on the electrolyte/solvent Eox limit—which is not a minor issue, given the interest in high-voltage cathodes. The fact that the electrolyte must be developed not only in terms of ESW, but also to match both positive and negative electrodes, is a large challenge, as there is not yet a dominant electrochemical couple for NIBs. Improved fluidity, important for low-temperature performance as well as for the proper wetting of electrodes, must be achieved without compromising safety by using volatile solvents. Another path of research is highly concentrated electrolytes (HCEs), which improve safety, widen the ESWs, and utilise non-vehicular ion transport, which leads to improved cation transference numbers. For HCEs, Na-based systems have an earlier onset of some of these properties, compared to Li-based systems [179], which is cost-saving, as less salt is needed. However, the high viscosities of the highest concentrations of salt-based HCEs, i.e., those of very low solvent content, are a challenge for manufacturing and the cell formation stages. Advances in science and technology to meet challenges When the key performance indicators (KPIs) and the targets to be reached for NIBs by e.g., 2030 are listed, few are directly dependent on the solvent used, but very many are indirectly limited by the electrolyte. The first, most obvious KPI is the ionic conductivity, but the target, 1 mS cm−1, has already been reached. The next is the cell voltage—intimately coupled to the electrolyte/solvent Eox limit, as outlined above, while the less obviously connected KPIs are cycle-life and cost. However, for the NIB cycle life, the stability of the 41PDF Image | 2021 roadmap for sodium-ion batteries
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