Innovations in Structural Engineering for Sulfide-Based Solid Electrolytes in All-Solid-State Batteries

Towards Practical All-Solid-State Batteries: Structural Engineering Innovations for Sulfide-Based Solid Electrolytes

Sulfide-based solid electrolytes are critical for the development of next-generation all-solid-state batteries (ASSBs) due to their safety features and higher energy density. This paper reviews recent innovations in sulfide-based solid electrolytes, emphasizing improvements in ionic conductivity through a deeper understanding of their crystal structures. By analyzing current research trends and future directions, this review aims to establish a roadmap for developing more robust and efficient sulfide-based solid electrolytes, thereby contributing to the advancement of safer and higher-performance ASSBs.

Introduction

The transition to a sustainable and energy-efficient future heavily depends on advancements in lithium-ion battery (LIB) technology, especially for electric vehicles (EVs) and energy storage systems (ESSs). As EVs and ESSs are pivotal in achieving net-zero emissions by 2050, many regions have announced plans to phase out internal combustion engine vehicles within the next 10 to 30 years in favor of EVs. However, these technologies currently face two major challenges: energy density and safety. Presently, EVs can only travel between 150-300 miles on a single charge, a limitation that hinders their commercial viability. Given the constraints of space and weight in EVs, batteries with higher energy densities are essential for longer driving ranges. Currently, LIBs have energy densities of 260-295 Wh/kg and 650-730 Wh/L at the cell level, which are nearing their theoretical limits. The U.S. Department of Energy and the U.S. Advanced Battery Consortium set ambitious targets of 350 Wh/kg and 750 Wh/L for advanced battery technologies, suggesting that current LIB technologies still have a way to go.

Moreover, the rapid commercialization of portable devices, EVs, and ESSs using LIBs has raised significant safety concerns, highlighted by incidents such as the Samsung Galaxy Note 7 explosions and Boeing 787 Dreamliner battery fires. The primary culprit behind these safety issues is the use of low-flash-point organic solvents in conventional LIB liquid electrolytes, which necessitates comprehensive improvements across the battery system, particularly in the materials used for the cathode, anode, electrolyte, and separator.

One potential solution to these challenges is the development of ASSBs that replace traditional liquid electrolytes with solid electrolytes. Solid electrolytes provide broader operational temperature ranges and reduce the risk of ignition associated with flammable organic liquids. Additionally, solid electrolytes can allow for the use of high-energy-density lithium metal anodes by mitigating dendrite growth, which is a critical issue in liquid electrolytes that can lead to short circuits. The absence of electrolyte leakage in solid-state batteries also permits the bipolar stacking of battery modules, a shift from the monopolar design typically used in current LIBs. Furthermore, the dry-electrode processing design offers the potential for higher energy densities through the adoption of high-loading composite electrodes. Unlike liquid-based LIBs, which face limitations in transport and wettability, ASSBs can enhance both the volumetric and gravimetric energy densities of the battery system, offering improved safety and performance.

Current State of Solid Electrolytes

Current liquid electrolytes in LIBs typically consist of LiPF6 in mixtures such as ethylene carbonate with dimethyl carbonate or propylene carbonate, exhibiting ionic conductivities of 1-10 mS/cm at room temperature. To match or exceed the performance of existing LIBs, the ionic conductivity of solid electrolytes must be comparable to that of liquid electrolytes, which necessitates significant advancements in this field. Major breakthroughs include the discovery of room-temperature ionic conductors such as garnet-type structures, Li10GeP2S12 (LGPS), and thio-Lithium Ion Superionic Conductors (thio-LISICONs), which exhibit lithium ionic conductivities ranging from 0.1 mS/cm to 10 mS/cm.

Sulfides, known for their high polarizability, include many notable superionic conductors, with conductivities reaching up to 32 mS/cm at room temperature. Due to their inherent softness, the grain boundaries within the particles can be significantly reduced through cold pressing, allowing the sintering process—typically required to minimize grain boundaries in oxide-based solid electrolytes—to be omitted after cell assembly. This simplification enhances the practicality and scalability of ASSB fabrication.

Structural Aspects of Sulfide Solid Electrolytes

Sulfide solid electrolytes can be classified into crystalline structures and glass. Crystalline structures, such as the argyrodite family and LGPS, exhibit higher ionic conductivities compared to glassy materials. The arrangement of polyanion building blocks within these structures significantly influences ionic conductivity. Recent studies suggest that the connectivity of lithium-ion pathways within these frameworks is crucial for enhancing ionic conduction.

Conclusion and Outlook

This review highlights the significant relationship between crystal structure and ionic conductivity in sulfide-based solid electrolytes, particularly those with the LGPS-type structure, which has shown the highest ionic conductivities reported to date. As we move forward, continued research is essential to deepen our understanding of ion conduction mechanisms within crystalline structures and to discover new crystal structures and chemistries that can further enhance performance.

Authors’ Contributions

• Conceived the manuscript: Roh, J.; Do, N.
• Wrote the manuscript: Roh, J.; Do N.
• Reviewed the manuscript: Hong, S. T.; Chae, M. S.
• Contributed to the discussion: Roh, J.; Do, N.; Lee, H.; Lee, S.; Pyun, J.; Hong S. T.; Chae, M. S.

Acknowledgments

This work was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MSIT).

Conflicts of Interest

All authors declare no conflicts of interest.

Copyright

© The Author(s) 2025. This article is licensed under a Creative Commons Attribution 4.0 International License, which allows unrestricted use, sharing, adaptation, distribution, and reproduction in any medium, provided appropriate credit is given to the original authors and source.