Lithium-ion batteries, while approaching their theoretical energy density limits, struggle to meet the growing demands for large-scale energy storage and long-lasting devices. Li-O₂ batteries, with their significantly higher theoretical energy density (3500 Wh kg⁻¹ compared to ~387 Wh kg⁻¹ for Li-ion), offer a promising alternative. However, their commercialization faces significant hurdles. The open architecture required for oxygen transport in Li-O₂ batteries leads to liquid electrolyte leakage and volatilization, resulting in poor cycling performance. Efforts to use low-volatility electrolytes, such as sulfamide and sulfonamide-based electrolytes and solvate ionic liquids, have yielded limited success, as even low-volatility electrolytes eventually deplete. Furthermore, lithium dendrite formation during cycling can puncture the separator, causing short circuits and thermal runaway. Materials with non-Newtonian fluid properties, exhibiting both shear-thinning and shear-thickening behavior, offer a potential solution. Shear-thinning can alleviate stress from dendrite growth, while shear-thickening can provide mechanical reinforcement under high current conditions. Previous research has demonstrated the potential of non-Newtonian fluids in lithium metal anodes and zinc metal batteries to mitigate dendrite formation and improve cycle life. This work aims to leverage these properties to develop a multifunctional quasi-solid electrolyte for Li-O₂ batteries.

The existing literature highlights the challenges of developing high-performance Li-O₂ batteries, focusing on the issues of electrolyte volatility and lithium dendrite growth. Researchers have explored various strategies to improve electrolyte stability, including the use of low-volatility solvents and ionic liquids. However, these approaches have not fully addressed the problem of electrolyte consumption and the associated degradation of battery performance. Moreover, the use of porous separators in Li-O₂ batteries remains vulnerable to dendrite penetration, leading to safety concerns. The use of non-Newtonian fluids in battery electrolytes is a relatively novel concept. Recent studies have demonstrated the potential of shear-thinning and shear-thickening fluids to improve the stability and performance of lithium and zinc metal batteries by mitigating dendrite growth and adapting to volume changes during cycling. These promising results provided the foundation for the current study investigating the application of a non-Newtonian quasi-solid electrolyte in Li-O₂ batteries.

The researchers designed a non-Newtonian fluid quasi-solid electrolyte (NNFQSE) composed of sulfonated silica nanoparticles (SiO₂-SO₃Li) and poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP). The SiO₂-SO₃Li nanoparticles were synthesized through a three-step process involving the synthesis of porous SiO₂ nanoparticles, followed by sulfonation with H₂SO₄, and finally, lithiation with LiOH. The NNFQSE was prepared via a tape casting method, where PVDF-HFP was dissolved in acetone and ethyl alcohol, and then the SiO₂-SO₃Li nanoparticles were added to create a composite mixture. This mixture was cast onto a Teflon mold and dried to create the quasi-solid electrolyte membrane. The resulting NNFQSE was characterized using various techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), rheometry, and nuclear magnetic resonance (NMR) spectroscopy. Electrolyte uptake and retention were determined by measuring the weight change of the electrolyte membrane after immersion in the liquid electrolyte. Electrochemical impedance spectroscopy (EIS) was used to measure the ionic conductivity, and linear sweep voltammetry (LSV) was employed to determine the electrochemical window. Lithium symmetrical cells and Li-O₂ cells were assembled using the NNFQSE to evaluate its performance in real-world battery applications. Long-term cycling tests were conducted at various current densities to assess the lifespan and stability of the batteries. Furthermore, computational simulations using COMSOL Multiphysics were used to model the mechanical behavior of the electrolyte under different stress conditions and to examine its effect on dendrite growth.

The synthesized NNFQSE SiO₂-SO₃Li/PVDF-HFP exhibited a unique honeycomb morphology, enabling efficient Li⁺ transport and high electrolyte retention. The electrolyte uptake was significantly higher for the NNFQSE (380 wt%) compared to PVDF-HFP (270 wt%) and SiO₂/PVDF-HFP (300 wt%). Importantly, the NNFQSE maintained a high electrolyte retention ratio (90.5%) even after 360 h in flowing air. The NNFQSE also showed excellent thermal stability, with minimal heat shrinkage (1%) after heating at 180 °C for 1 h. NMR spectroscopy confirmed strong interactions between the SiO₂-SO₃Li nanoparticles, PVDF-HFP, and the liquid electrolyte. The NNFQSE exhibited a high ionic conductivity (5.4 × 10⁻³ S cm⁻¹ at 20 °C) and a wide electrochemical window (up to 4.85 V). Rheological measurements revealed both shear-thinning and shear-thickening properties. At low strain rates, the electrolyte behaved as a shear-thinning fluid, accommodating stress from Li plating/stripping. At high strain rates, it exhibited shear-thickening, mechanically stiffening to inhibit dendrite penetration. In lithium symmetrical cells, the NNFQSE enabled stable cycling for over 2000 h at 1 mA cm⁻² with a low polarization voltage (~50 mV). In Li-O₂ cells, the NNFQSE resulted in a remarkable lifespan exceeding 5000 h at 100 mA g⁻¹, significantly outperforming cells with conventional separators and other electrolytes. Post-mortem analysis showed that the NNFQSE maintained its structural integrity and electrolyte content even after 5000 h of cycling. Nano-indentation tests and COMSOL simulations further confirmed the shear-thinning and shear-thickening behavior of the NNFQSE, demonstrating its ability to suppress dendrite growth.

The findings demonstrate the effectiveness of the non-Newtonian fluid quasi-solid electrolyte in addressing the key challenges associated with Li-O₂ batteries. The high electrolyte retention and unique rheological properties of the NNFQSE effectively prevent electrolyte depletion and dendrite formation, resulting in significantly improved battery lifespan and safety. The combination of experimental results and computational simulations provides strong evidence for the multifunctional nature of the NNFQSE. The shear-thinning behavior under normal operation conditions helps to accommodate volume changes and reduce stress on the lithium anode, preventing dendrite formation. The shear-thickening behavior under high current or stress conditions enhances the mechanical strength of the electrolyte, effectively suppressing dendrite penetration. This study establishes a new paradigm for designing high-performance electrolytes for Li metal batteries, highlighting the potential of non-Newtonian fluids to overcome limitations of conventional electrolytes.

This research successfully designed and demonstrated a non-Newtonian fluid quasi-solid electrolyte (NNFQSE) that significantly enhances the lifespan and safety of Li-O₂ batteries. The NNFQSE's unique combination of high electrolyte retention, superior ionic conductivity, wide electrochemical window, and shear-responsive properties effectively mitigated lithium dendrite growth and electrolyte volatilization. Future work could focus on optimizing the composition and structure of the NNFQSE to further improve its performance and explore its application in other types of lithium metal batteries.

While this study demonstrates significant improvements in Li-O₂ battery performance, further research is needed to fully assess the long-term stability and scalability of the NNFQSE. The impact of different operating conditions, such as temperature variations and extended cycling at high current densities, on the electrolyte's performance needs to be investigated. The synthesis process of the SiO₂-SO₃Li nanoparticles could be further optimized to improve the efficiency and reduce the cost. Finally, a comprehensive life cycle assessment should be conducted to evaluate the environmental impact of the proposed electrolyte.