Ferroelectric BaTiO₃-induced local electric field enhances sodium-ion transport in Na₂B₁₂H₁₂ solid-state electrolytes

With the continuous growth in demand for large-scale energy storage, sodium-ion batteries have attracted considerable attention due to the abundance and low cost of sodium resources. However, traditional liquid electrolytes pose safety risks such as leakage, volatilization, and flammability, limiting their application in high-safety scenarios. Although solid-state electrolytes hold promise for improving safety and energy density, they still face key challenges, including low room-temperature ionic conductivity, high interfacial impedance, and short circuits induced by sodium dendrites. Therefore, developing solid-state electrolyte materials that combine high ionic conductivity with good stability has become a crucial scientific and engineering issue in advancing all-solid-state sodium battery technology. Among various sodium-ion conductors, the Na₂B₁₂H₁₂ electrolyte has gained attention for its low grain boundary resistance, excellent (electro)chemical reduction stability, good mechanical flexibility, and potential for high ionic conductivity. Although this material exhibits outstanding ionic conductivity at high temperatures, its room-temperature conductivity still requires improvement and currently falls short of meeting practical application needs. Current strategies for enhancing its room-temperature conductivity mainly focus on chemical modification, nanocrystallization, interface engineering, and anion doping. While these approaches have improved the ionic conductivity of Na₂B₁₂H₁₂ solid-state electrolytes to some extent, the room-temperature conductivity remains insufficient to simultaneously meet the requirements of all-solid-state sodium batteries for high power density and long-term cycling stability. In recent years, utilizing built-in electric fields in materials (such as piezoelectric and ferroelectric fields) to regulate electrochemical reaction processes and carrier transport behavior has become an important research direction in the field of energy materials. In this study, the ferroelectric effect was introduced into the Na₂B₁₂H₁₂ solid-state electrolyte, utilizing the localized polarization field generated by the ferroelectric material BaTiO₃ to guide and accelerate Na⁺ migration and broaden the electrochemical stability window. In the experiment, Na₂B₁₂H₁₂ was first synthesized, and then a BaTiO₃-Na₂B₁₂H₁₂ composite electrolyte was prepared via ball milling, followed by systematic structural characterization. Conductivity tests showed that the composite electrolyte with 10 wt.% BaTiO₃ achieved an ionic conductivity of 3.1×10–⁴ S cm–¹. Linear sweep voltammetry (LSV) analysis further confirmed the improved oxidative stability of this electrolyte, with its electrochemical stability window broadening from 3.6 V to 4.0 V. Symmetric cell cycling tests demonstrated stable sodium plating/stripping for over 1000 hours at a current density of 0.1 mA cm–². The assembled all-solid-state sodium battery exhibited a capacity retention rate of 82% after 300 cycles at a rate of 0.5 C. First-principles calculations demonstrate that the surface polarity of BaTiO₃ can effectively induce the reconstruction of the interfacial coordination environment, leading to the disengagement of Na⁺ from the confinement of the [B₁₂H₁₂]²– clusters. This reconstruction significantly reduces the migration energy barrier for Na⁺, thereby opening a two-dimensional diffusion pathway with a moderate migration barrier on the interfacial potential energy surface. These results indicate that the introduction of ferroelectric BaTiO₃ nanoparticles can effectively enhance the ionic transport properties of the electrolyte system. The ferroelectric polarization of BaTiO₃ induces a localized built-in electric field at the two-phase interface, thereby regulating the ion barrier distribution near the interface, achieving “ferroelectric-assisted” enhancement of ion transport. Additionally, the composite electrolyte exhibits a wider electrochemical stability window. This study significantly improves the ionic transport capability and electrochemical stability of solid-state electrolytes, offering new insights and a feasible pathway for the material design of high-performance all-solid-state sodium batteries.