Research on sodium-ion batteries has recently been rediscovered and is currently mainly focused on finding suitable electrode materials that enable cell reactions of high energy densities combined with low cost. Naturally, an assessment of potential electrode materials requires a rational comparison with the analogue reaction in lithium-ion batteries. In this paper, we systematically discuss the broad range of different conversion reactions for sodium-ion batteries based on their basic thermodynamic properties and compare them with their lithium analogues. Capacities, voltages, energy densities and volume expansions are summarized to sketch out the scope for future studies in this research field. We show that for a given conversion electrode material, replacing lithium by sodium leads to a constant shift in cell potential ΔE°(Li-Na) depending on the material class. For chlorides ΔE°(Li-Na) equals nearly zero. The theoretical energy densities of conversion reactions of sodium with fluorides or chlorides as positive electrode materials typically reach values between 700 W h kg(-1) and 1000 W h kg(-1). Next to the thermodynamic assessment, results on several conversion reactions between copper compounds (CuS, CuO, CuCl, CuCl2) and sodium are being discussed. Reactions with CuS and CuO were chosen because these compounds are frequently studied for conversion reactions with lithium. Chlorides are interesting because of ΔE°(Li-Na)≈ 0 V. As a result of chloride solubility in the electrolyte, the conversion process proceeds at defined potentials under rather small kinetic limitations.
The present study compares the physico-chemical properties of non-aqueous liquid electrolytes based on NaPF6, NaClO4 and NaCF3SO3 salts in the binary mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The ionic conductivity of the electrolytes is determined as a function of salt concentration and temperature. It is found that the electrolytes containing NaClO4 and NaPF6 exhibit ionic conductivities ranging from 5 mS cm(-1) to 7 mS cm(-1) at ambient temperature. The electrochemical stability window of the different electrolytes is studied by linear sweep voltammetry (LSV) and cyclic voltammetry (CV) measurements with respect to a variety of working electrodes (WE) such as glassy carbon (GC), graphite and a carbon gas diffusion layer (GDL). Electrolytes containing NaPF6 and NaClO4 are found to be electrochemically stable with respect to GC and GDL electrodes up to 4.5 V vs. Na/Na(+), with some side reactions starting from around 3.0 V for the latter salt. The results further show that aluminium is preferred over different steels as a cathode current collector. Copper is stable up to a potential of 3.5 V vs. Na/Na(+). In view of practical Na-ion battery systems, the electrolytes are electrochemically tested with Na0.7CoO2 as a positive electrode. It is inferred that the electrolyte NaPF6-EC : DMC is favorable for the formation of a stable surface film and the reversibility of the above cathode material.
The achievement of high ionic conductivity in new "polymeric ionic liquids" (PILs) is of great interest as it refers to the solid state electrolytes and their applications in electrochemistry. Four ionic monomers, including two new ones, namely N-[(2-methacryloyloxy)-propyl]-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (M1), 1-[2-(methacryloyloxy)propyl]-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (M2), N-methyl-N-ethylpyrrolidinium(3-sulfopropyl) methacrylate (M3), and 1-methyl-3-ethylimidazolium(3-sulfopropyl) methacrylate (M4), were prepared and used both for the synthesis of linear PILs and ionic networks with poly(ethylene glycol) dimethacrylate (PEGDM). The optimal polymerization conditions for obtaining high molar mass PILs (M sD up to 1.24 Â 10 6 g/mol) were identified. The copolymerization of oppositely charged monomers was studied as well. Polycations, polyanions, and their random ionic copolymers were compared in terms of their physical properties. The examined properties were found to depend mainly on the nature of the counterion. It became obvious that the bulk ionic conductivity of hydrophilic polyanions is greatly affected by the humidity. It increases up to 220 times upon transferring from dry air to 20% relative humidity. Ionic conductivity increased in random ionic copolymers synthesized from oppositely charged monomers feasibly, suggesting that the ion transport was improved by the partial formation of mobile ionic liquid within the polymer.
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