The Structural Effect of a Composite Solid Electrolyte on Electrochemical Performance and Fire Safety

Area of Science:

• Materials Science
• Electrochemistry
• Solid-state Batteries

Background:

• Composite solid electrolytes are crucial for advanced battery technologies.
• Lithium-ion batteries require electrolytes with high ionic conductivity, stability, and safety.
• Al-doped Li₇La₃Zr₂O₁₂ (LLZO) and Poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) composites offer potential but require structural optimization.

Purpose of the Study:

• To investigate the structural impact of 0D and 1D Al-doped LLZO within a PVDF-HFP matrix on electrochemical performance and fire safety.
• To compare the properties of 1D_Al-LLZO@PVDF-HFP with 0D_Al-LLZO@PVDF-HFP and pure PVDF-HFP electrolytes.
• To elucidate the relationship between filler morphology, ionic conductivity, electrochemical stability, and safety.

Main Methods:

• Synthesis and characterization of composite solid electrolytes (0D_Al-LLZO@PVDF-HFP and 1D_Al-LLZO@PVDF-HFP).
• Analysis using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, and Raman spectroscopy.
• Electrochemical testing including ionic conductivity, interfacial resistance, electrochemical stability window, and battery performance (symmetric and full cells).
• Fire safety evaluation through flammability tests.

Main Results:

• The 1D_Al-LLZO@PVDF-HFP composite exhibited a unique structure that enhanced lithium salt dissociation, leading to high ionic conductivity (1.40 × 10^-4 S/cm) and a high lithium-ion transference number (0.75).
• Optimized 1D_Al-LLZO@PVDF-HFP showed improved electrochemical stability (4.75 V vs. Li/Li⁺) and low interfacial resistance.
• Cells utilizing 1D_Al-LLZO@PVDF-HFP demonstrated superior long-term stability (>2000 h in symmetric cells) and good performance in full cells (85.7% capacity retention over 200 cycles).
• All Al-LLZO@PVDF-HFP composites, regardless of structure, showed enhanced fire safety (nonflammability) compared to pure PVDF-HFP.

Conclusions:

• The 1D structure of Al-LLZO within the PVDF-HFP matrix is beneficial for regulating crystallinity and facilitating ion transport, leading to enhanced electrochemical performance.
• The presence of the inorganic Al-LLZO filler is the primary factor contributing to the nonflammability of the composite electrolytes.
• 1D_Al-LLZO@PVDF-HFP represents a promising composite solid electrolyte for safer and higher-performing lithium-ion batteries.