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

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

Sulfide-based solid electrolytes are critical components for the development of next-generation all-solid-state batteries (ASSBs), offering enhanced safety and higher energy density. This review examines recent advancements in sulfide solid electrolytes, with a focus on improving their ionic conductivities through a deeper understanding of their crystal structures. By analyzing current research trends and future outlooks, the review aims to outline a pathway for the creation of more robust and efficient sulfide-based solid electrolytes, contributing to the realization of safer and higher-performance ASSBs.

Introduction

The transition to a sustainable and energy-efficient future heavily relies on advancements in lithium-ion battery (LIB) technology, particularly for electric vehicles (EVs) and energy storage systems (ESSs). As these technologies play a pivotal role in achieving Net Zero Emissions by 2050, many regions are planning to phase out internal combustion engine vehicles in favor of EVs. However, current EVs and ESSs face two significant challenges: energy density and safety. Presently, EVs typically offer a driving range of 150-300 miles on a single charge, a critical factor for their commercial viability. To extend driving distances while considering space and weight constraints, batteries with higher energy densities are essential.

Currently, LIBs have energy densities at the cell level of 260-295 Wh/kg and 650-730 Wh/L, nearing their theoretical limits. However, targets set by the U.S. Department of Energy for advanced batteries in EVs are 350 Wh/kg and 750 Wh/L, indicating that current LIB technologies still have room for improvement. The commercialization of portable devices, EVs, and ESSs using LIBs has also raised significant safety concerns, highlighted by incidents such as the Samsung Galaxy Note 7 explosions and Boeing 787 Dreamliner battery fires. These incidents primarily stem from the use of low-flash-point organic solvents in conventional LIBs, underscoring the need for systemic improvements in the materials used for electrodes, electrolytes, and separators.

Developing ASSBs that replace liquid electrolytes with solid alternatives presents a promising solution to these challenges. The solid nature of these electrolytes allows for a broader operational temperature range and reduces ignition risks compared to flammable organic liquids. Additionally, solid electrolytes can facilitate the use of high-energy-density lithium metal anodes by preventing dendrite growth—a significant issue in liquid electrolytes that can lead to short circuits. The absence of electrolyte leakage in solid-state batteries also allows for bipolar stacking of battery modules, enhancing volumetric and gravimetric energy densities.

Ionic Conductivity of Solid Electrolytes

To match or exceed the performance of current LIBs, the ionic conductivity of solid electrolytes must be comparable to that of liquid electrolytes, which typically range from 1-10 mS/cm at room temperature. Recent breakthroughs in solid electrolyte technologies have led to the discovery of room-temperature ionic conductors, including garnet-type structures, Li10GeP2S12 (LGPS), and thio-Lithium Ion Superionic Conductors (thio-LISICONs), exhibiting lithium ionic conductivities from 0.1 mS/cm to 10 mS/cm.

Sulfides, known for their high polarizability, have produced well-known superionic conductors with conductivities reaching up to 32 mS/cm at room temperature among various lithium ionic conductors. Their softness allows for reduced grain boundaries through cold pressing, simplifying the manufacturing process by omitting the sintering typically required for oxide-based solid electrolytes.

Structural Innovations in Sulfide Solid Electrolytes

Sulfide solid electrolytes (SSEs) can be classified into two categories: crystalline structures and glass. Crystalline structures include the argyrodite family and LGPS, while glass materials typically consist of precursors like Li2S and MxSy. The key distinction between these two is that glasses have less-ordered lithium ion diffusion pathways, leading to relatively high conductivity.

The ionic conductivity of crystals is influenced by several factors: the concentration of carrier ions or vacancies, the dimensions of mobile ion diffusion channels, and the polarization of framework ions. Optimizing these factors is crucial for enhancing ionic conductivity. Various SSE systems, including glassy sulfides and thio-LISICONs, have shown promise in improving lithium-ion conduction.

Conclusion and Outlook

The exploration of novel solid electrolyte structures, particularly those utilizing sulfide-based materials, is essential for the advancement of ASSBs. While high ionic conductivity is vital, achieving robust chemical and electrochemical stability remains a significant challenge. Future efforts should focus on interfacial engineering, hybrid assembly of ASSBs, and microstructure modification to enhance the performance and safety of these battery systems.

Continued research on sulfide-based solid electrolytes will further our understanding of ion conduction mechanisms and enable the discovery of new crystal structures and chemistries, ultimately leading to more efficient and reliable all-solid-state batteries.