Li-Ion Conductivity in Metal Hydride-Based Nanocomposite Electrolytes: The Effect of Nanoscaffold Porosity and Surface Chemistry

Abstract

The development of energy storage technologies, such as rechargeable batteries, is crucial for the transition to a sustainable energy supply. Lithium-ion batteries are an effective means of energy storage, which is demonstrated by their wide application ranging from mobile phones to laptops and electric vehicles. Unfortunately, Li-ion batteries suffer from safety issues arising from their combustible organic electrolytes. All-solid-state batteries (ASSBs), in which the common liquid organic electrolyte is replaced by a solid-state electrolyte (SSE), could potentially lead to safer batteries with increased energy density.[1] Recently, metal hydrides (e.g. LiBH4 and LiCB11H12) have gained attention as promising solid electrolytes due to their electrochemical and thermal stability, low density and high ionic conductivity albeit at elevated temperatures. However, for successful incorporation of metal hydride SSEs in ASSBs, sufficient ionic conductivity at room temperature is a prerequisite. Therefore, the development of strategies that enhance the room temperature conductivity in complex hydrides (10-8 S cm-1 for LiBH4) is of major importance.In this contribution, we combined two promising strategies to enhance ion mobility in metal hydrides, namely, partial ionic substitution[2] and nanoconfinement[3], which led to highly conductive metal hydride-based nanocomposites (Figure 1). Specifically, via partial ion substitution with LiNH2, followed by nanoconfinement in a mesoporous oxide scaffold, LiBH4-LiNH2/metal oxide nanocomposites with conductivities reaching 5⸱10-4 S cm-1 at 30 °C were obtained[4], compared to 2⸱10-8 S cm-1 for pure LiBH4.Interestingly, the conductivity of the LiBH4-LiNH2/metal oxide nanocomposites is strongly influenced by the chemical and physical nature of the mesoporous metal oxide. We systematically studied the influence of the scaffold properties on the conductivity of nanoconfined LiNH2-substituted LiBH4 using mesoporous silica scaffolds (SBA-15) with varying surface chemistry and pore structure. The conductivity varied over three orders of magnitude when tuning both the porosity and surface chemistry of the metal oxide scaffold.[5] Our study reveals that the LiBH4-LiNH2/metal oxide conductivity is affected by the chemical nature of the scaffold, similar to LiBH4/metal oxide nanocomposites. A conductivity improvement of a factor of two is achieved by changing the SiO2 (SBA-15) surface chemistry through alumination (Figure 2a). On the other hand, different from nanoconfined LiBH4, the conductivity of LiBH4-LiNH2/metal oxide nanocomposites is largely dictated by the scaffold pore volume (Figure 2b). Notably, the conductivity can be varied from 4⸱10-7 S cm-1 to 5⸱10-4 S cm-1 by increasing the scaffold pore volume from 0.51 to 1.00 cm3 g-1.Our work demonstrates that the origin of the conductivity enhancement in anion-substituted complex hydride-based solid electrolytes is different from other nanoconfined complex hydrides, e.g. LiBH4. In particular, for nanoconfined LiBH4-LiNH2, the conductivity improvement is attributed to stabilization of a highly conductive phase inside the scaffold pores, rather than the formation of a conductive interfacial layer at the hydride/oxide interface as observed for nanoconfined LiBH4. Thus, it is clear that the conductivity of metal hydride-based nanocomposite ion conductors is closely linked to the properties of scaffold material. These fundamental insights on the influence of scaffold properties on ion mobility in nanocomposite materials could be applicable to other cation- and anion substituted ion conductors as well. Thereby, this work provides useful insights for the design novel solid-state electrolytes with excellent ionic conductivity, which are crucial for the development of next generation batteries. References Janek, Jürgen, and Zeier, Wolfgang G. "A solid future for battery development." Nature Energy 1.9 (2016): 1-4.Maekawa, Hideki, et al. "Halide-stabilized LiBH4, a room-temperature lithium fast-ion conductor." Journal of the American Chemical Society 131.3 (2009): 894-895.Blanchard, Didier, et al. "Nanoconfined LiBH4 as a fast lithium ion conductor." Advanced Functional Materials 25.2 (2015): 184-192Zettl, Roman, et al. "Combined Effects of Anion Substitution and Nanoconfinement on the Ionic Conductivity of Li-Based Complex Hydrides." The Journal of Physical Chemistry C 124.5 (2020): 2806-2816.de Kort, Laura M., et al. "The effect of nanoscaffold porosity and surface chemistry on the Li-ion conductivity of LiBH4–LiNH2/metal oxide nanocomposites." Journal of Materials Chemistry A 8.39 (2020): 20687-20697. Figure 1