Liquid Metal Mediated Heterostructure Fluoride Solid Electrolytes of High Conductivity and Air Stability for Sustainable Na Metal Batteries.
basic_science · Level V
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- Record sourced from PubMed, PMID 38319748.
- Also identified by DOI 10.1021/acsnano.3c12256.
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Abstract
Fluoride-based solid electrolytes (SEs) have emerged as a promising component for high-energy-density rechargeable solid-state batteries (SSBs) in view of their wide electrochemical window, high air stability, and interface compatibility, but they still face the challenge of low ion conductivity and the lack of a desired structure for sodium metal SSBs. Here, we report a sodium-rich heterostructure fluoride SE, Na<sub>3</sub>GaF<sub>6</sub>-Ga<sub>2</sub>O<sub>3</sub>-NaCl (NGFOC-G), synthesized via in situ oxidation of liquid metal gallium and in situ chlorination using low-melting GaCl<sub>3</sub>. The distinctive features of NGFOC-G include single-crystal Na<sub>3</sub>GaF<sub>6</sub> domains within an open-framework structure, composite interface decoration of Ga<sub>2</sub>O<sub>3</sub> and NaCl with a concentration gradient, exceptional air stability, and high electrochemical oxidation stability. By leveraging the penetration of gallium at NaF grain boundaries and the in situ self-oxidation to form Ga<sub>2</sub>O<sub>3</sub> nanodomains, the solid-phase reaction kinetics of NaF and GaF<sub>3</sub> is activated for facilitating the synthesis of main component Na<sub>3</sub>GaF<sub>6</sub>. The introduction of a small amount of a chlorine source during synthesis further softens and modifies the boundaries of Na<sub>3</sub>GaF<sub>6</sub> along with Ga<sub>2</sub>O<sub>3</sub>. Benefiting from the enhanced interface ion transport, the optimized NGFOC-G exhibits an ionic conductivity up to 10<sup>-4</sup> S/cm at 40 °C, which is the highest level reported among fluoride-based sodium-ion SEs. This SE demonstrates a "self-protection" mechanism, where the formation of a high Young's modulus transition layer rich in NaF and Na<sub>2</sub>O under electrochemical driving prevents the dendrite growth of sodium metal. The corresponding Na/Na symmetric cells show minimal voltage hysteresis and stable cycling performance for at least 1000 h. The Na/NGFOC-G/Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> cell demonstrates stable capacity release around 100 mAh/g at room temperature. The Na/NGFOC-G/FeF<sub>3</sub> cell delivers a high capacity of 461 mAh/g with an excellent stability of conversion reaction cycling.