A Novel Strategy Enhances Ionic Conductivity and Stability in Sodium Halide Solid Electrolytes for Advanced Solid-State Sodium-Ion Batteries.
Researchers develop a path to improve ion transport in sodium solid electrolytes, promising safer and longer-lasting batteries.
Designing halide solid electrolytes with high ionic conductivity and good electrochemical stability is essential for the advancement of all-solid-state sodium-ion batteries (ASSIBs). However, most sodium-based halide solid electrolytes suffer from limited ionic conductivities due to their structural characteristics, leading to challenges in creating efficient energy storage solutions. In a recent study, a team of researchers unveiled a new strategy that significantly boosts the conductivities of sodium halides by regulating vacancy and charge carrier concentrations through a straightforward method of Na- and Cl-deficient compositions.
“This study provides a versatile pathway for creating inorganic ion conductors with high conductivity and long-term cyclability,” said the authors of the article. By optimizing the structure, the researchers achieved several-fold enhancements in ionic conductivity of series sodium halides, which is a critical factor in the efficacy of ASSIBs.
Historically, sodium-ion batteries have been favored for large-scale applications due to the abundant and cost-effective raw materials. However, the successful implementation of ASSIBs hinges on the development of solid electrolytes (SEs) that exhibit high room-temperature ionic stability and good interfacial compatibility with electrodes. Previous studies have demonstrated that conventional polymer-based SEs often face conductivity challenges, while oxides require high sintering temperatures that limit their practical use. On the other hand, sodium halides present unique opportunities—with their potential for high conductivity—yet often fail to match the performance of their lithium counterparts.
To rectify these deficiencies, the authors of the study focused on improving the ionic conductivity of sodium halide solid electrolytes (HNISEs) by considering critical thresholds for carrier and vacancy concentrations. “Achieving a rational equilibrium between vacancy concentrations and mobile charge carrier numbers within HNISE structures is pivotal for ion migration efficiency,” the authors noted. Their innovative approach involved carefully tuning the compositions of Na2+xMxZr1-xCl6-type (where M = Yb or Er) sodium halides without adding external dopants.
As a result of this meticulous design, several compositions demonstrated significantly improved ionic conductivity, achieving values exceeding previously reported metrics for sodium halides. The team also employed a fluorination-induced amorphization protocol, enhancing both electrochemical stability and interfacial compatibility while avoiding detrimental impacts on conductivity. The alterations in the structural properties of the fluorinated sample primarily lead to increased local structural disorder and enhanced coordination of sodium ions, further facilitating ion transport.
This new class of solid electrolytes proved to be promising in practical applications. When paired with various electrodes, the optimized Na0.5ZrCl4F0.5 catholyte enabled ASSIBs to maintain 94.4% of its initial discharge capacity even after 300 cycles at room temperature, demonstrating not only high conductivity but also impressive stability over time.
The development of high-conductivity solid electrolytes can radically change the landscape for ASSIBs, positioning sodium-ion batteries as viable options for a range of applications, from grid-scale storage to portable electronics. The findings from this research contribute a significant advance in the field and pave the way for future innovation in battery technologies, with the potential for further studies to refine and improve these sodium halide electrolytes.
Overall, the study showcases a promising direction in solid-state battery development, combining functional performance with economic viability—critical factors needed for the scaling and practical adoption of sodium-ion batteries in a world increasingly dependent on sustainable energy solutions.
