Solid electrolytes represent a critical advancement in battery technology, promising improved safety and energy density compared to conventional liquid electrolytes. However, their application in high-voltage systems (≥4.5V) faces significant challenges due to electrochemical instability. Understanding the limits of their stability, decomposition mechanisms, and methods to enhance their performance is essential for developing next-generation solid-state batteries.

The electrochemical window of a solid electrolyte defines the voltage range within which it remains stable without decomposing. For high-voltage applications, this window must withstand potentials above 4.5V versus Li/Li+. Common solid electrolytes, such as oxide-based (e.g., LLZO, Li7La3Zr2O12) and sulfide-based (e.g., Li10GeP2S12) materials, exhibit varying stability limits. Oxide electrolytes typically demonstrate wider electrochemical windows, often exceeding 5V, while sulfide-based materials tend to decompose at lower voltages, around 2.5V to 3V. The electrochemical window is typically measured using linear sweep voltammetry or cyclic voltammetry, where the current response indicates the onset of decomposition. For instance, LLZO shows minimal current below 5V, confirming its stability, whereas Li3PS4 exhibits significant current increase above 2.7V, signaling decomposition.

Decomposition pathways in solid electrolytes under high voltage are influenced by their chemical composition and structure. In oxide electrolytes, oxygen evolution can occur at high potentials, leading to the formation of oxygen vacancies and phase transitions. For example, LLZO may undergo partial reduction of Zr4+ to Zr3+ at the anode interface while experiencing oxygen loss at the cathode interface. Sulfide electrolytes, on the other hand, are prone to sulfur oxidation, forming polysulfides or elemental sulfur, which degrade ionic conductivity. Thiophosphate-based materials like Li6PS5Cl decompose into Li2S and P2S5 at voltages above 2.8V, limiting their utility in high-voltage cells. Polymer electrolytes, such as PEO-based systems, face oxidative degradation at potentials exceeding 4V, resulting in chain scission and cross-linking reactions.

To extend the high-voltage stability of solid electrolytes, doping and multilayer designs have emerged as effective strategies. Doping involves introducing foreign elements into the electrolyte lattice to modify its electronic and ionic properties. In oxide electrolytes, Al or Ta doping in LLZO enhances its stability by reducing electronic conductivity and suppressing oxygen loss. For sulfide electrolytes, halogen doping (e.g., Cl or Br in Li6PS5X) increases oxidation resistance by strengthening the PS4 tetrahedra. Doping with elements like Ge or Sn in thiophosphates also shifts the decomposition onset to higher voltages by stabilizing the sulfur framework.

Multilayer designs employ a combination of different electrolytes to leverage their individual strengths while mitigating weaknesses. A common approach involves pairing a high-voltage-stable oxide layer (e.g., LLZO) with a high-conductivity sulfide layer (e.g., Li10GeP2S12). The oxide layer interfaces with the high-voltage cathode, providing stability, while the sulfide layer ensures efficient ion transport. Another design incorporates a thin interfacial buffer layer, such as Li3PO4, between the cathode and electrolyte to prevent direct contact and reduce side reactions. These multilayer systems must maintain strong interfacial adhesion and minimal resistance to avoid performance degradation.

Interfacial engineering is another critical factor in achieving high-voltage stability. The solid electrolyte must form a stable interface with both the cathode and anode to prevent parasitic reactions. Cathode-electrolyte interphases often form due to electrochemical decomposition, leading to increased impedance. Strategies such as surface coating the cathode with LiNbO3 or Al2O3 can suppress interfacial reactions by acting as a barrier layer. Similarly, anode-side stability is improved by using lithium alloys or composite anodes that reduce dendrite formation and interfacial degradation.

Mechanical properties also play a role in high-voltage stability. Solid electrolytes must withstand volumetric changes during cycling without cracking or delaminating. Materials with high fracture toughness, such as composite electrolytes incorporating polymers or flexible binders, can accommodate strain better than brittle ceramics. For instance, adding a polymer matrix to LLZO improves its mechanical resilience while maintaining ionic conductivity.

The ionic conductivity of solid electrolytes must remain high even under high-voltage conditions. Some materials experience conductivity drops due to phase transitions or decomposition products blocking ion pathways. Optimizing grain boundaries through sintering aids or nanostructuring can mitigate this issue. For example, hot-pressing LLZO reduces grain boundary resistance, enhancing overall conductivity. Similarly, sulfide electrolytes benefit from controlled crystallization to minimize amorphous phases that may hinder ion transport.

In summary, achieving high-voltage stability in solid electrolytes requires a multifaceted approach. Electrochemical window measurements provide insight into stability limits, while understanding decomposition pathways guides material design. Doping and multilayer architectures offer viable solutions to extend operational voltages, and interfacial engineering ensures long-term performance. Mechanical robustness and maintained ionic conductivity further contribute to the viability of these electrolytes in high-voltage applications. Continued research into these areas will be crucial for realizing the full potential of solid-state batteries in demanding energy storage systems.