Interface-Driven High Ionic Conductivities in Hydridoborate/Oxide Nanocomposite Electrolytes: A Pathway to High-Capacity All-Solid-State Batteries

Abstract

In the pursuit of renewable energy storage, batteries are expected to play a major role. All-solid-state batteries (ASSBs) employing solid electrolytes (SEs) as ion conductors have the potential to improve the energy density and overall safety of batteries compared to traditional batteries using organic liquid electrolytes. Hydridoborates, combinations of complex anions with the general formula [BxHy]z− and charge carrier cations such as Li+, Na+, K+, and Mg2+, have been perceived as promising SEs in recent times due to properties such as softness and suitable electrochemical stability against Li and Na metal. In pristine form, the ionic conductivities of hydridoborates are insufficient for battery applications at low to moderate temperatures. However, several methods have been developed to obtain highly conductive hydridoborate materials at room temperature. In this Account, interface engineering, a highly effective approach to obtain conductive and electrochemically stable hydridoborate/oxide nanocomposites, is elaborated. Remarkable ionic conductivities, up to three to four orders of magnitude higher compared to their pristine form, are observed upon confining hydridoborates in oxidic host materials. Generally, the nanocomposite materials are synthesized using either a mechanochemical treatment strategy or by melt infiltrating the hydridoborate in porous oxides. In the pioneering works, LiBH4 nanoconfined in SiO2 or Al2O3 was researched extensively, whereby highly mobile Li+ was observed at moderate temperatures (40 °C). Interfacial regions between LiBH4 and the oxides were affirmed to be crucial for the enhanced electrochemical properties. Various spectroscopic techniques, such as solid-state nuclear magnetic resonance, indicate chemically distinct interface species, where the hydroxyl groups of oxides are involved in interphase formation. A multitude of oxides were confirmed to be able to create highly conductive LiBH4-based nanocomposites. Similar to LiBH4 counterparts, NaBH4 was three orders of magnitude more conductive upon nanocomposite formation. From the boron K-edge electronic structure investigation, the formation of B–O-like bonds within the hydridoborate/oxide interphase was indicated. More conclusive evidence of the nature of the interface species came from X-ray Raman spectroscopy, where the weakening of the M–BH4 bond was observed, and the formation of trigonal boron moieties was determined to be key to the improved ionic conductivity. Importantly, interphase-driven enhanced electrochemical properties within hydridoborate/oxide SEs have been demonstrated in several ASSBs with promising capacity retention at room temperature. In most cases, the addition of oxides is beneficial for the electrochemical stability of the SEs This is especially true for the [B12H12]2– compounds, which show enhanced oxidative stability compared to [BH4]−, hence enabling utilization in ASSBs based on state-of-the-art high-voltage cathodes.