Background
All-solid-state batteries (ASSBs) are considered ideal for next-generation energy storage systems due to their high energy density and safety. Among them, sulfide solid electrolytes (such as Li6PS5Cl, LPSCl) with sulfur silver germanium ore structure have become key candidate materials for ASSBs due to their high ionic conductivity, low electronic conductivity and good ductility. However, its side reaction with moisture in the air can produce toxicH2Sgas, leading to structural decomposition, decreased ionic conductivity, and increased electronic conductivity, severely restricting the commercialization of ASSBs. Although stability can be improved by elemental substitution (e.g., replacing P5+ with soft acids) or oxygen doping, existing studies have ignored the changes in electron conductivity and its effect on lithium dendrite growth. In this study, fluorine-rich LPSCl (F-LPSCl) with core-shell structure was prepared by chemical vapor deposition fluorine treatment and annealing, aiming to improve air stability while maintaining its electrochemical properties, and provide a new strategy for the practical application of ASSBs.
Job introduction
The team of Professor Taeseup Song of Hanyang University in South Korea reported a method for the preparation of sulfide-based SEs: a simple fluorine treatment process after annealing to construct sulfides composed of Li6PS5The core-shell structure consists of a Cl(LPSCl) core and a fluorine-rich LPSCl shell to improve air stability. Most importantly, thanks to the fluorine-rich LPSCl shell, the prepared SE maintains low electronic conductivity after exposure to an atmosphere at 25°C and 20% relative humidity, thereby improving electrochemical performance without short circuits. The experimental results showed that the fluorine-rich LPSCl shell effectively inhibited the side reaction with water and reduced the degree of irreversible side reaction. Combined with the first-principles density functional theory (DFT) model, the mechanism of air stability enhancement of F-LPSCl is deeply understood. The whole battery assembled with atmospheric exposure F-LPSCl has an initial discharge capacity of 168.5 mAh g-1 at 0.05 C and good cycling stability and rate performance after 500 cycles at 0.3 C.
Content introduction
In this study, a method synthesized by Li6PS5 by simple fluorine treatment followed by annealing is describedThe method of SE particles (hereinafter referred to as "F-LPSCl") composed of Cl(LPSCl) nuclei and fluorine-rich LPSCl shells to improve the atmospheric stability of LPSCl (Figure 1). This F-LPSCl core-shell structure offers several advantages. First, fluorine-rich shells enhance hydrophobicity due to their low surface energy, making them highly air-stable to moisture. The fluorine-rich LPSCl-encased nucleus maintains its original crystal structure and high ionic conductivity. Compared with exposed LPSCl, the synthesized F-LPSCl had significantly reduced H2Sgas formation and good ionic conductivity after exposure to air.
Figure 1. Schematic diagram of the core-shell structure of F-LPSCl synthesized by fluorine treatment.
The crystal structure of exposed LPSCl and F-LPSCl was studied using X-ray diffraction (XRD), as shown in Figure 2a. The diffraction spectrum of the exposed LPSCl contains the structural peak of the sulfur-silver-germanium ore with a space group of F-43m, indicating that the LPSCl phase has been successfully synthesized. After fluorination treatment by chemical vapor deposition (CVD), F-LPSCl showed no significant phase changes and structural collapse of LPSCl. Figure 2b shows that both the exposed LPSCl and F-LPSCl have main peaks associated with PS43-ion vibrations at 426 cm-1, at about 200 and 264 cm-1There is a secondary peak everywhere. All peaks are consistent with the Raman spectra of the previously reported sulfur-silver-germanium structure. X-ray photoelectron spectroscopy (XPS) of exposed LPSCl and F-LPSCl reveals surface chemistry, as shown in Figure 2c. F-LPSCl has a distinct F 1s peak at 685.0 eV, corresponding to a lithium-fluorine bond. The binding energy of the peak at 686.6 eV corresponds to PS43-interaction with F. The coating thickness of F-LPSCl was also characterized by high-resolution transmission electron microscopy (HR-TEM) images, as shown in Figure 2d. HR-TEM images of F-LPSCl show a fluorine-rich LPSCl layer on the surface with an average thickness of 6.6 nm and a standard deviation of ±2.8 nm, in contrast to the clean surface observed in bare LPSCl. A uniform distribution of phosphorus, sulfur, chlorine, and fluorine was visible in the EDX map, confirming the presence of fluorine in F-LPSCl (Figure 2e). These results indicate that bare LPSCl and F-LPSCl have similar deformability and structural integrity. To further investigate the distribution of F⁻ and LiF⁻, time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth analysis was performed. As shown in Fig. 2f, the strong peaks corresponding to F⁻ and LiF⁻ in F-LPSCl appear in the surface region, but gradually weaken in the bulk phase. TOF-SIMS results confirmed fluorination on the LPSCl surface. To determine the hydrophobicity of the exposed LPSCl and F-LPSCl membranes, static contact angles (SCAs) were measured. As shown in Fig. 2g, h, the exposed LPSCl film exhibits hydrophobicity with an SCA of 77.0° due to the addition of PTFE binder. The SCA of the F-LPSCl membrane increased to 117.2°, which also showed hydrophobicity. This confirms that the fluorine-rich shell enhances hydrophobicity. The F-LPSCl membrane remains stable after 30 minutes, while the exposed LPSCl membrane decomposes when exposed to water.
Figure 2. (a) X-ray diffraction spectra of bare LPSCl and F-LPSCl, (b) Raman spectra of bare LPSCl and F-LPSl, (c) X-ray photoelectron spectra of F1s in bare LPSCl and F-LPSCl, (d) high-resolution transmission electron microscopy images of F-LPSCl, (e) scanning electron microscopy-energy dispersive X-ray spectra of F-LPSCl, (f) Time-of-flight secondary ion mass spectrometry depth profile of F− and LiF− fragments in bare LPSCl and F-LPSCl, (g,h) contact angle of 5 μl water droplets on the bare LPSCl and F-LPSCl membranes.
Fig.3 Focusing on the air stability and mechanism of F-LPSCl, its performance advantages are verified by experimental and theoretical analysis: Comparison of H2Sproduction: Under 20% humidity and 25°C, the H2S production amount of F-LPSCl(1.0 cm) 3/g) was significantly lower than that of exposed LPSCl (1.4 cm3/g), indicating that it had stronger hydrolysis resistance. Changes in ionic conductivity: The ionic conductivity of the two was similar (about 1.5×10-3 S/cm) before exposure, and the decrease in F-LPSCl (5.96×10-4 S/cm) after exposure was less than that of exposed LPSCl (2.76×10). -4 S/cm), and the structural stability is better. Comparison of electronic conductivity: The electronic conductivity increased to 2.74×10-8 S/cm after exposure to exposed LPSCl, while F-LPSCl remained at 9.85×10-10 S/cm, effectively inhibiting electron conduction. DFT theory support: The adsorption energy of the LiF shell for H2O(1.52 eV) is lower than that of LPSCl (2.06 eV), indicating that the surface of F-LPSCl is more difficult to be eroded by water, which explains the reason for the stability improvement from the theoretical level.
Figure 3. (a) concentration of hydrogen sulfide gas in exposed LPSCl and F-LPSCl at 20.0% relative humidity and 25.0°C; (b) Ionic conductivity of exposed LPSCl and F-LPSCl before and after air exposure; (c) Electronic conductivity of exposed LPSCl and F-LPSCl before and after air exposure; (d) The adsorption configuration of water on the surfaces of LPSCl (111) and lithium fluoride (001) was studied by DFT analysis. (e) The water adsorption energy of the surfaces of LPSCl (111) and lithium fluoride (001) was calculated.
Fig. 4 focuses on the experimental verification of F-LPSCl in inhibiting lithium dendrite growth and improving battery cycle stability, and the main contents include: Symmetrical battery test: at 0.1 and 0.5 mA/cm2 Under the current density, the symmetrical battery cycles for more than 100 hours without short circuit, while the exposed LPSCl causes lithium dendrites to grow due to the increase of electronic conductivity, causing a short circuit. Interfacial stability analysis: The LiF shell of F-LPSCl reduced the interfacial resistance and maintained a low overpotential after exposure to air, while the exposed LPSCl caused a surge in overpotential and short circuit due to the decrease of ionic conductivity and dendrite growth. SEM characterization evidence showed that lithium dendrites penetrated the electrolyte layer at 0.5 mA/cm2 current, while F-LPSCl had no obvious dendrites at 2.0 mA/cm2, confirming its excellent dendrite inhibition ability. Critical current density (CCD) comparison: The CCD (2.0 mA/cm2) of air-exposed F-LPSCl was much higher than that of exposed LPSCl (0.7 mA/cm2), attributed to its lower electron conductivity.
Figure 4. Lithium coating and stripping voltage curves on exposed LPSCl and F-LPSCl materials for symmetrical batteries under air exposure: (a) 0.1 mA cm−2, (b) 0.5 mA cm−2(c) EIS spectra of exposed LPSCl and F-LPSCl materials of symmetrical batteries exposed to air exposure, (d) lithium plating with stripped voltage curves of symmetrical cells using exposed LPSCl and F-LPSCl materials under different current densities, (e) cross-sectional SEM images of symmetrical battery SE layers using exposed exposed LPSCl and (f) air-exposed F-LPSCl SE layers.
The electrochemical performance of F-LPSCl in the whole battery was demonstrated, and its advantages were verified by comparing the exposed LPSCl: Initial discharge performance: After air exposure, the discharge capacity of the F-LPSCl whole battery reached 168.5 mAh/g at 0.05 C, which was higher than the 139.7 mAh/g of the exposed LPSCl, and the coulomb efficiency was also higher (77.14% vs 72.80%). Rate performance: The F-LPSCl battery exhibits higher discharge capacitance in the range of 0.05-1.0 C, thanks to its good ionic conductivity retention. Cycle stability: The capacity retention rate was 81.25% after 100 cycles at 0.1 C and 82.64% after 500 cycles at 0.3 C, both of which were better than bare LPSCl and there was no short circuit. Mechanistic analysis: The LiF shell of F-LPSCl improves the stability of the lithium/electrolyte interface and reduces the charge transfer resistance, thereby improving the cycle life and rate performance.
Figure 5. (a) Initial charge-discharge voltage curves of exposed LPSCl and F-LPSCl whole batteries before and after air exposure; (b) The discharge capacity of the exposed LPSCl and F-LPSCl whole cells after air exposure at different current rates (0.05, 0.1, 0.2, 0.3, 0.5, 1.0 C); (c) The performance of exposed LPSCl and F-LPSCl whole cells under 0.1 C conditions before and after air exposure for 100 cycles; (d) Performance of exposed LPSCl and F-LPSCl full cells after air exposure for 500 cycles at 0.3 C (1 C = 5.0 mA cm−2).
conclusion
The surface fluorine treatment of LPSCl by chemical vapor deposition (CVD) combined with annealing process successfully synthesized sulfur-silver germanium oxide type F-LPSCl electrolyte with core-shell structure, which significantly improved its air stability and electrochemical stability. H2 is produced when exposed to air compared to exposed LPSClThe amount of S gas is significantly reduced, and the ionic conductivity is maintained more effectively. More importantly, all-solid-state batteries (ASSBs) using F-LPSCl exhibited higher capacity, stable cycling performance, and superior rate performance compared to exposed LPSCl, which was attributed to their inhibitory effect on side reactions. Experimental and theoretical analyses show that fluorine-rich shells work through the following mechanisms: Enhanced hydrophobicity: Fluorine-rich shells with low surface energy reduce water adsorption and inhibitH2S generation and structural decomposition; Maintenance of electronic insulation: the stability of the shell structure and the low electronic conductivity of the decomposition products avoid short circuits caused by lithium dendrites; Improved interfacial stability: The LiF-rich shell improves the interfacial contact between lithium metal and the electrolyte, inhibiting dendrite growth. In summary, the application of F-LPSCl electrolytes in ASSBs provides an effective strategy for the practical application of sulfide-based solid electrolytes, which is expected to promote the commercialization of high-performance all-solid-state batteries.
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