Innovative Structural Engineering Approaches for Enhancing 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 crucial for advancing next-generation all-solid-state batteries due to their enhanced safety and higher energy density. This paper reviews recent innovations in sulfide-based solid electrolytes, emphasizing the enhancement of ionic conductivities through a detailed understanding of their crystal structures. By analyzing current research trends and future directions, this review aims to chart a course for developing robust and efficient sulfide-based solid electrolytes, ultimately contributing to the realization of safer and higher-performance all-solid-state batteries.

Introduction

The shift towards a more sustainable and energy-efficient future significantly hinges on advancements in lithium-ion battery (LIB) technology, especially for electric vehicles (EVs) and energy storage systems (ESSs). With EVs and ESSs playing a pivotal role in achieving Net Zero Emissions by 2050, many countries and cities have announced plans to phase out internal combustion engine vehicles within the next 10 to 30 years, favoring EVs. However, both EVs and ESSs currently confront two primary challenges: energy density and safety. Presently, the driving range of EVs is limited to 150-300 miles per charge, a critical threshold for commercial success. Given the spatial and weight constraints in EVs, higher energy density batteries are essential for longer ranges. Currently, LIBs have energy densities of 260-295 Wh kg^-1 and 650-730 Wh L^-1, nearing their theoretical limits. Nevertheless, the U.S. Department of Energy and the U.S. Advanced Battery Consortium have set ambitious targets of 350 Wh kg^-1 and 750 Wh L^-1 for advanced batteries, indicating that current LIB technologies still fall short.

The recent commercialization of portable devices, EVs, and ESSs utilizing LIBs has heightened safety concerns. Incidents such as the Samsung Galaxy Note 7 explosions, Boeing 787 Dreamliner battery fires, and frequent explosions in current EVs underscore the urgent need to address these issues. The root causes of these incidents often lie in the use of low-flash-point organic solvents in conventional liquid electrolyte LIBs, such as ethylene carbonate and dimethyl carbonate. These limitations necessitate extensive improvements across the entire battery system, particularly concerning the materials used for the cathode, anode, electrolyte, and separator.

Developing all-solid-state batteries (ASSBs) that replace conventional liquid-based electrolytes with solid electrolytes offers a promising solution to these challenges. The solid nature of these electrolytes allows for a wider operational temperature range and diminishes the risk of ignition compared to flammable organic liquid electrolytes. Additionally, solid electrolytes enable the use of high-energy-density lithium metal anodes by preventing dendrite growth, a significant issue inherent in liquid electrolytes that can lead to short circuits. Moreover, the absence of electrolyte leakage in solid-state batteries allows for the bipolar stacking of battery modules, moving away from the monopolar design typical of current LIBs. Furthermore, the dry-electrode processing design presents potential for higher energy density through the adoption of high-loading composite electrodes.

Current liquid electrolytes in LIBs usually consist of LiPF₆ in mixtures such as ethylene carbonate with dimethyl carbonate or propylene carbonate, exhibiting an ionic conductivity of 1-10 mS cm^-1 at room temperature. To match or exceed the performance of current LIBs, the ionic conductivity of solid electrolytes must be comparable to that of liquid electrolytes, necessitating significant advancements in this field. Discoveries such as room-temperature ionic conductors, including garnet-type structures, Li₁₀GeP₂S₁₂ (LGPS), argyrodite family, and thio-Lithium Ion Superionic Conductors (thio-LISICONs), which exhibit lithium ionic conductivities ranging from 0.1 mS cm^-1 to 10 mS cm^-1, exemplify this progress.

Sulfides are notable for their high polarizability and encompass many renowned superionic conductors with conductivities reaching up to 32 mS cm^-1 at room temperature among the various discovered lithium ionic conductors. Moreover, the soft nature of sulfides significantly reduces grain boundaries within the particles when the powders are cold pressed, allowing for the omission of the sintering process typically required for oxide-based solid electrolytes. This simplification makes them more viable for the scalable fabrication of ASSBs.

SULFIDE SOLID ELECTROLYTES: STRUCTURAL ASPECT

This section presents the ionic conductivity and activation energy of sulfide solid electrolytes, examining the structural characteristics of these electrolytes, including both glassy phases and various crystalline structures, to elucidate the relationship between crystal structure, ionic conductivity, and activation energy.

Glasses

Glassy sulfides, particularly the binary xLi₂S-(100-x)P₂S₅ system, have been systematically studied for their ion-conducting properties. The short-range order of the PS₄ framework exhibits different sharing modes depending on its composition. An increase in the concentration of alkali modifiers like Li⁺ leads to reduced network connectivity by forming non-bridging sulfur anions. This progressive isolation ultimately results in a dominant phase characterized by isolated PS₄ building blocks, with the highest ionic conductivity observed at x = 75, measuring 2.8 × 10^-4 S cm^-1 at room temperature.

Crystalline Materials

Inorganic glassy compounds comprising sulfides crystallize upon heating, leading to structures classified as glass-ceramics. Notable examples include Li₂P₂S₆ and Li₇P₃S₁₁, which exhibit high ionic conductivity and low activation energy. The crystal structure of Li₂P₂S₆ reveals edge-sharing PS₄ tetrahedral units, while Li₇P₃S₁₁ features corner-sharing P₂S₇ units with lithium ions occupying interstitial sites, facilitating high lithium-ion mobility.

Conclusion and Outlook

This review characterizes the relationship between crystal structure and ionic conductivity, focusing on sulfide solid electrolytes. Among various sulfide materials, those with the LGPS-type structure have demonstrated the highest ionic conductivity, reaching up to 32 mS cm^-1 at room temperature. Understanding the chemical properties of crystal structures that affect ionic conduction is essential, as is exploring unstudied chemical systems for potential new structures with high ionic conductivity.

Future research should also target interfacial engineering, hybrid assembly of ASSBs, the use of nanocomposites for anode interfaces, and microstructure modifications to enhance ionic conductivity and overall battery performance. Continued research on ionic conductors will aid in deepening our understanding of ion conduction mechanisms within crystalline structures and facilitate the discovery of new crystal structures and chemistries.