Electrolyte Salts for Sodium-Ion Batteries: NaPF6 or NaClO4?

NaClO4 and NaPF6, the most universally adopted electrolyte salts in commercial sodium-ion batteries (SIBs), have a decisive influence on the interfacial chemistry, which is closely related to electrochemical performance. The complicated and ambiguous interior mechanism of microscopic interfacial chemistry has prevented reaching a consensus regarding the most suitable sodium salt for high-performance SIB electrolytes. Herein, we reveal that the solvation structure induced by different sodium salt anions determines the Na+ desolvation kinetics and interfacial film evolution process. Specifically, the weak interaction between Na+ and PF6- promoted sodium desolvation and storage kinetics. The solvation structure involving PF6- induced the anion's preferential decomposition, generating a thin, inorganic compound-rich cathode-electrolyte interphase that ensured interface stability and inhibited solvent decomposition, thereby guaranteeing electrode stability and promoting the charge transfer kinetics. This study provides clear evidence that NaPF6 is not only more compatible with industrial processes but also more conducive to battery performance. Commercial electrolyte design employing NaPF6 will undoubtedly promote the industrialization of SIBs.

Increase of Solid Polymer Electrolyte Ionic Conductivity Using Nano-SiO2 Synthesized from Sugarcane Bagasse as Filler

The synthesize of solid polymer electrolyte (SPE) based on polyethylene oxide (PEO), NaClO₄ and nano-SiO₂ was carried out by solution cast technique. Nano-SiO₂ was synthesized from sugarcane bagasse using sol-gel method. FTIR analysis was carried out to investigate the bonding between nano-SiO₂ and PEO/NaClO₄. The morphology of the SPE was characterized using SEM. XRD and DSC analysis showed that SPE crystallinity decreased as nano-SiO₂ concentration was increased. Mechanical analyses were conducted to characterize the SPE tensile strength and elongation at break. EIS analysis was conducted to measure SPE ionic conductivity. The PEO/NaClO₄ SPE with the addition of 5% nano-SiO₂ from sugarcane bagasse at 60 °C produced SPE with the highest ionic conductivity, 1.18 × 10^-6 S/cm. It was concluded that the addition of nano-SiO₂ increased ionic conductivity and interface stability at the solid polymer electrolyte-PEO/NaClO₄.

The Unusual Conductivity of Na+ in PEO-Based Statistical Copolymer Solid Electrolytes: When Less Means More

The low conductivity of Na⁺ electrolytes in solid polymer electrolytes (SPEs) curtails the development of Na polymer batteries. In this study, NaClO₄ (3-24 wt %, 90-9:1 O:Na) is dissolved in statistical copolymers of ethylene oxide (EO) and propylene oxide (PO) (0-20 mol %). Remarkably, the conductivity of these SPEs increases as the concentration of Na⁺ decreases, thus departing from the usual Nernstian behavior. Using a combination of calorimetric measurements and molecular dynamic simulations, this unusual phenomenon is attributed to the presence of physical cross-links generated by Na⁺ . As a result, polymers containing a low salt concentration (3 wt %) display a drastically enhanced ionic conductivity (up to 0.2 10^-4  S cm^-1 at 25 °C), thus paving the way for the design of all-solid-state PEO-based sodium batteries operational at room temperature.