Solid electrolytes represent a critical advancement in battery technology, offering improved safety and energy density compared to liquid electrolytes. A key challenge in their implementation lies in their electrochemical stability window—the voltage range within which they remain stable against oxidation and reduction. This article examines the stability of oxide, sulfide, and halide solid electrolytes against common electrodes like NMC (LiNiMnCoO₂) and lithium metal, focusing on thermodynamic predictions from density functional theory (DFT) and experimental validation through cyclic voltammetry. The discussion extends to decomposition mechanisms and strategies to enhance stability through doping and interface engineering.

The electrochemical stability window of a solid electrolyte defines its operational limits. Oxidation occurs at high potentials, where the electrolyte loses electrons, while reduction occurs at low potentials, where it gains electrons. Beyond these limits, decomposition reactions degrade performance. DFT calculations provide a theoretical foundation for predicting these limits by computing the formation energies of decomposition products. Experimental techniques like cyclic voltammetry then validate these predictions by measuring current responses under applied potentials.

Oxide-based solid electrolytes, such as LLZO (Li₇La₃Zr₂O₁₂), exhibit high oxidative stability but limited reduction stability. DFT studies indicate that LLZO remains stable up to approximately 4.5 V vs. Li/Li⁺ before oxygen loss initiates decomposition. However, at potentials below 0.5 V vs. Li/Li⁺, reduction leads to lithium plating and zirconium reduction. Cyclic voltammetry confirms these limits, showing negligible current within the predicted window but sharp increases beyond it. The decomposition products include La₂O₃, Li₂O, and reduced zirconium species. To enhance stability, doping with Ta or Nb increases oxidative resistance by strengthening Zr-O bonds, while interfacial coatings like Al₂O₃ mitigate reduction by blocking electron transfer.

Sulfide electrolytes, such as Li₃PS₄, display broader stability windows but face challenges at both high and low potentials. DFT predicts a stability range of 1.7–2.5 V vs. Li/Li⁺, with oxidation forming sulfur species (S₈, SO₄²⁻) and reduction producing Li₂S and P. Cyclic voltammetry experiments align with these predictions, showing decomposition onset near 2.6 V and 1.5 V. Sulfides are particularly susceptible to reactions with lithium metal, forming resistive interphases that increase impedance. Strategies to improve stability include halogen doping (e.g., Li₃PS₄₋ₓClₓ), which elevates the oxidation limit by stabilizing sulfur anions, and artificial SEI layers (e.g., LiI) that suppress reduction.

Halide solid electrolytes, like Li₃YCl₆, offer intermediate stability with distinct advantages. DFT calculations suggest a window of 2.0–4.0 V vs. Li/Li⁺, where oxidation releases chlorine and reduction forms LiCl and Y. Experimental data corroborates this, with minimal decomposition within the window but significant degradation outside it. Halides exhibit better compatibility with high-voltage cathodes like NMC than sulfides but still require stabilization at low potentials. Doping with Br or I expands the stability window by modifying the anion framework, while thin polymer interlayers reduce interfacial reactivity.

Comparative analysis reveals trade-offs among these material classes. Oxides excel in oxidative environments but require modifications for low-potential operation. Sulfides offer higher ionic conductivity but narrower stability windows. Halides balance conductivity and stability but face synthesis challenges. The choice of electrolyte depends on the electrode pairing—oxides for high-voltage cathodes, sulfides for intermediate applications, and halides for balanced performance.

Decomposition products play a critical role in long-term stability. For oxides, La₂O₃ and Li₂O form insulating layers that increase resistance. Sulfides generate Li₂S and P, which reduce ionic transport. Halides produce LiCl, which can partially conduct ions but degrades overall performance. Mitigation strategies focus on preventing these reactions through doping, coatings, or hybrid electrolyte designs.

Interface engineering is equally crucial. For lithium metal anodes, thin protective layers (e.g., Li₃N or polymer films) block electron transfer and prevent reduction. For NMC cathodes, oxide coatings (e.g., LiNbO₃) suppress oxidative decomposition. Hybrid systems combining multiple electrolytes (e.g., oxide-sulfide bilayers) leverage the strengths of each material while minimizing weaknesses.

Thermodynamic calculations and experimental validation must align to ensure accurate stability assessments. Discrepancies often arise from kinetic effects, impurities, or interfacial reactions not captured by DFT. Advanced characterization techniques, such as X-ray photoelectron spectroscopy (XPS) and impedance spectroscopy, help bridge this gap by identifying decomposition products and interfacial phases.

Future directions include developing multi-scale models that integrate DFT with mesoscale simulations to predict long-term degradation. Machine learning approaches may accelerate material discovery by screening dopants and interfaces for optimal stability. Experimental efforts should focus on in situ and operando techniques to monitor decomposition in real time.

In summary, the electrochemical stability of solid electrolytes is a complex interplay of material chemistry, electrode interactions, and interfacial phenomena. Oxide, sulfide, and halide electrolytes each present unique advantages and challenges, necessitating tailored stabilization strategies. Combining thermodynamic insights with experimental validation enables the design of robust systems capable of meeting the demands of next-generation batteries. Continued advancements in computational and experimental methods will further refine these materials, pushing the boundaries of voltage stability and performance.