Novel interface-engineered sodium-based nanocomposites as solid-state-electrolytes

Abstract

To meet worldwide increasing energy demands and minimize CO2 emissions, energy production from renewable sources such as wind and solar should be utilized on a large scale. However, these sources are intermittently harvested. Therefore, sufficient large-scale energy storage is required. Sodium-based rechargeable batteries are attractive for large-scale energy storage due to the high worldwide abundance, low cost and suitable redox potential of sodium. Current high capacity Na-S batteries operate at 300◦C because of the poor interfacial contact with the ceramic solid-state-electrolyte (SSE). Complex (boro)hydrides are proposed as SSEs but suffer from low ionic conductivity at moderate temperatures. Therefore, there is a need for improved electrolytes for applications in all-solid-state sodium-batteries. In this work, melt infiltration of low melting point sodium salts in mesoporous Al2O3 and SiO2 scaffolds is performed to enhance the conductivity by interface engineering. Using this approach, oxide-nanocomposites of NaBH4, NaNH2, NaNO2, NaNO3, NaClO3 and NaAlCl4 were prepared. NaBH4@Al2O3 and NaNH2@SiO2 composites with 130% pore filling show a thousandfold higher conductivity compared to crystalline bulk NaBH4 and NaNH2. The obtained conductivity at 297K was 2.7×10−6 for NaBH4@Al2O3 and 3.1×10−7 Scm−1 for NaNH2@SiO2. The composites showed an activation energy of 0.46 and 0.69 eV respectively, lower than the crystalline bulk. The origin of the conductivity enhancement is investigated. NaBH4- and NaNH2-oxide composites show a loss of long-range crystallinity induced by confinement in the pores. A reaction between the support surface hydroxyl groups and these sodium salts is found. In NaBH4@Al2O3, B-O bonds, Na2B12H12 species and mobile Na-ions are observed at the interface. The observed enhanced conductivity is attributed to these interfacial effects. This work has investigated the melt infiltration approach to design novel sodium-based solid-state-electrolytes that show conductivity enhancement by three orders of magnitude. These materials show potential for applications in next generation all-solid-state sodium batteries.