The rapid advancement of energy storage technologies has positioned solid-state batteries as a cornerstone for next-generation applications, offering unparalleled advantages in energy density, thermal resilience, and operational safety. Among the critical components of solid-state batteries, sulfide-based electrolytes such as Li₁₀GeP₁₂S₂ (LGPS) have garnered significant attention due to their ultrahigh lithium-ion conductivity (~10⁻³ S cm⁻¹). However, the thermal stability of LGPS in high-energy-density configurations, particularly when interfaced with advanced cathode materials like LiNi₀.₉₂Co₀.₀₄Mn₀.₀₄O₂ (NCM92) and anode materials such as silicon carbide (SiC), remains underexplored. This study systematically investigates the thermal degradation mechanisms of LGPS-based solid-state battery materials, employing a multidisciplinary approach to unravel the interplay between heat generation, gas evolution, and interfacial reactions.
1. Introduction
Solid-state batteries represent a paradigm shift from conventional lithium-ion systems, eliminating flammable liquid electrolytes and enabling higher energy densities. Despite their promise, thermal instability at material interfaces—especially under high-temperature conditions—poses a critical challenge. Sulfide electrolytes like LGPS exhibit exceptional ionic conductivity but may undergo exothermic reactions with high-nickel cathodes, triggering thermal runaway. This work addresses these challenges by evaluating the thermal stability of LGPS in combination with NCM92 and SiC, aiming to establish design principles for safer solid-state battery architectures.
2. Materials and Methods
2.1 Materials Preparation
- LGPS Electrolyte: Sourced from Ganfeng Lithium Co., with a nominal composition of Li₁₀GeP₁₂S₂.
- NCM92 Cathode: Extracted from commercial 1 Ah pouch cells (100% state of charge, SOC).
- SiC Anode: Processed via solvent immersion (DMC) and drying to remove residual electrolytes.
2.2 Thermal Analysis Techniques
- Differential Scanning Calorimetry (DSC): NETZSCH DSC 25, 50–500°C, 10°C/min.
- Simultaneous Thermal Analysis-Mass Spectrometry (STA-MS): STA449F5-QMS403D, monitoring O₂ (m/z=32) and SO₂ (m/z=64).
- Microstructural Characterization: SEM-EDS (MERLIN Compact) and XRD (D8 Advance).
- Surface Chemistry Analysis: XPS (ESCALAB Xi+) with Ar⁺ sputtering.
2.3 Experimental Design
- Sample Ratios: Cathode/electrolyte and anode/electrolyte mixtures at 2:1 (wt%).
- Temperature Protocols: Isothermal holds at 200°C, 310°C, and 410°C for phase evolution studies.
3. Results and Discussion
3.1 Thermal Behavior of LGPS and Electrodes
DSC Analysis:
- LGPS Alone: No significant heat flow observed below 500°C, confirming intrinsic thermal stability (Table 1).
- NCM92 Cathode: Exothermic peak at 232.8°C (ΔH = 123.3 J/g), attributed to phase transition (layered → spinel) with O₂ release:LiNi0.92Co0.04Mn0.04O2→LixNiO2+O2↑LiNi0.92Co0.04Mn0.04O2→LixNiO2+O2↑
- LGPS + NCM92 Mixture: Two-stage exothermicity—initial peak at 232.3°C (ΔH = 106.8 J/g) and secondary reaction at 310°C (ΔH = 252 J/g).
STA-MS Gas Evolution:
- O₂ Release: Dominant at 200–250°C, correlating with NCM92 decomposition.
- SO₂ Detection: Trace signals at 220°C, suggesting minor sulfide oxidation:P2Sx+O2→SO2↑+P2OyP2Sx+O2→SO2↑+P2Oy
Table 1. Thermal Properties of Solid-State Battery Components
| Material | Exothermic Onset (°C) | Peak Temperature (°C) | ΔH (J/g) |
|---|---|---|---|
| LGPS | — | — | — |
| NCM92 | 213 | 232.8 | 123.3 |
| LGPS + NCM92 | 213 | 232.3 / 310 | 358.8 |
| SiC | 200 | 300 | 85.4 |
| LGPS + SiC | 200 | 300 | 72.1 |
3.2 Microstructural and Chemical Evolution
SEM-EDS Observations:
- 260°C: NCM92 particles retain layered morphology; LGPS remains intact with minimal S/O diffusion.
- 410°C: Severe agglomeration, S migration to Ni-rich regions, and P-O bonding (Figure 1).
XRD and XPS Findings:
- 260°C: LGPS retains crystallinity; surface sulfides (P₂Sₓ) and sulfates (SO₄²⁻) detected.
- 410°C: Formation of NiS, NiO, and Li₃PO₄, confirming redox-driven decomposition:LGPS+NiO2→NiS+Li3PO4+HeatLGPS+NiO2→NiS+Li3PO4+Heat
4. Mechanisms and Implications for Solid-State Batteries
The thermal degradation of LGPS-based solid-state batteries follows a dual-path mechanism:
- Low-Temperature Phase Transition (200–250°C): Cathode structural collapse releases reactive O₂, initiating localized sulfide oxidation.
- High-Temperature Redox (310–500°C): Bulk exothermic reactions between LGPS and cathode decomposition products (e.g., NiO₂) generate metal sulfides, oxides, and phosphates.
Critical Design Considerations:
- Cathode Selection: Low-nickel or phosphate-based cathodes (e.g., LiFePO₄) mitigate O₂ release.
- Interface Engineering: Coatings (e.g., Li₃PO₄) to suppress S-O interactions.
- Thermal Management: Active cooling systems targeting 200–300°C thresholds.
5. Conclusion
This study elucidates the thermal stability limits of LGPS-based solid-state battery materials, highlighting the critical role of cathode-electrolyte interactions in triggering thermal runaway. While LGPS exhibits robust standalone stability, its reactivity with high-nickel cathodes necessitates material-level optimizations to enhance safety. Future work will focus on hybrid electrolyte systems and advanced interfacial coatings to realize the full potential of solid-state batteries in high-energy-density applications.