Solid electrolytes based on polyethylene oxide (PEO) have been studied for decades, owing to their facile and low-cost processing, good electrochemical stability, and excellent complexation with alkali metal salts. Complexes of PEO with appropriate sodium salts are well known for ionic conduction. Here, pristine NaPF6:P(EO)16 and a composite solid electrolyte containing TiO2 nanowires were investigated as candidates for rechargeable solid-state sodium batteries. Comprehensive electrochemical characterizations were carried out, including ionic conductivity, transference number, and structural stability. At elevated temperatures, the specific capacity of an all-solid-state Na3Ti2(PO4)3 (Na/NTP) sodium battery was 110 mAh g−1, higher than room-temperature cells with liquid electrolyte solutions. We attribute this behavior to increased conductivity of the polymer electrolyte, induced by the ceramic nanofiller, combined with enhanced electronic conductivity of the NTP cathode.

The transference number for cations, t+, is one of the most important parameters for characterizing polymeric and/or composite solid electrolytes. It expresses the contribution of the positive charge carriers to the total conductivity, which in turn reflects the degree of polarization due to the negative carriers in the electrolyte systems. Four electrochemical methods based on different equations commonly used for obtaining t+ are compared. A series of experiments were conducted with solid polymer electrolytes based on polyethylene oxide with and without TiO2 ceramic additive. Interestingly, the oldest method developed and presented four decades ago, emerges as the most simple, reliable, sensitive, repeatable, and stable option for determining t+ values over time.

High electrolyte-electrode interface stability is essential for solid state batteries to avoid side reactions that form interphases and voids, leading to loss of contact and increased impedance. Such detrimental situations increase overvoltage, reduce cycling efficiency, and shorten battery cycle life. While composite solid electrolytes were studied extensively, the effect of inorganic additives in the polymer matrix on the electrolyte-anode interface remains unclear. Here, solid electrolyte was studied for batteries with sodium metal anode based on polyethylene oxide (PEO) polymeric matrix containing ceramic additive. Extensive electrochemical analyses under both AC and DC conditions were performed, and chemical reactions between sodium metal and the PEO matrix, which produce interphases at the electrode-electrolyte interface, were investigated. Addition of sodium beta aluminate in the matrix appears to mitigate these reactions, removing a major obstacle on the way to effective all-solid-state rechargeable sodium batteries.

Solid-state batteries have received renewed attention in recent years. The present study compares all-solid-state sodium batteries containing polyethylene oxide (PEO) polymer electrolyte (PE) with two salts, NaPF6 and NaClO4. Electrochemical properties were determined by means of both AC and DC measurements. Battery prototypes with PEO:NaClO4 have a better specific capacity; however, a composite electrolyte system containing TiO2 nanoparticles shows greater influence in PEO:NaPF6. This is probably due to the titania particles acting as a scavenger of HF, an inevitable contaminant in electrolyte systems containing PF6- anions.