Advancements and Challenges in Organic–Inorganic Composite Solid Electrolytes for All-Solid-State Lithium Batteries

Abstract

To address the limitations of contemporary lithium-ion batteries, particularly their low energy density and safety concerns, all-solid-state lithium batteries equipped with solid-state electrolytes have been identified as an up-and-coming alternative. Among the various SEs, organic–inorganic composite solid electrolytes (OICSEs) that combine the advantages of both polymer and inorganic materials demonstrate promising potential for large-scale applications. However, OICSEs still face many challenges in practical applications, such as low ionic conductivity and poor interfacial stability, which severely limit their applications. This review provides a comprehensive overview of recent research advancements in OICSEs. Specifically, the influence of inorganic fillers on the main functional parameters of OICSEs, including ionic conductivity, Li+ transfer number, mechanical strength, electrochemical stability, electronic conductivity, and thermal stability are systematically discussed. The lithium-ion conduction mechanism of OICSE is thoroughly analyzed and concluded from the microscopic perspective. Besides, the classic inorganic filler types, including both inert and active fillers, are categorized with special emphasis on the relationship between inorganic filler structure design and the electrochemical performance of OICSEs. Finally, the advanced characterization techniques relevant to OICSEs are summarized, and the challenges and perspectives on the future development of OICSEs are also highlighted for constructing superior ASSLBs.

1. Introduction

Rechargeable lithium-ion batteries (LIBs) are associated with significant safety concerns due to flammable and volatile organic liquid electrolytes, especially in large-scale energy storage applications such as electric vehicles and electronic devices. In addition, the energy density of commercial lithium-ion batteries with liquid electrolyte and carbon-based anodes has reached 260 Wh kg−1, which is close to their theoretical limitation. All-solid-state lithium metal batteries (ASSLBs), with super-high theoretical energy density (>300 Wh kg−1) and excellent safety, have been widely recognized as one of the most promising next-generation battery technologies. Solid-state electrolytes (SEs), as an important component of ASSLBs, have presented a rapidly increasing trend of investigations on SEs research in recent years.

The physicochemical properties of the SEs, including interfacial reaction kinetics, safety, and durability, are critical to ASSLBs. SEs can be divided into inorganic solid electrolytes (ISEs) and organic solid electrolytes (OSEs). ISEs exhibit high ionic conductivity (10−4–10−3 S cm−1), Li+ transference number (~1), excellent thermal stability, and ultra-high mechanical strength. However, the inherent fragility and high hardness often result in poor interfacial wettability with both the cathode and anode and significantly increased processing challenges. Therefore, the practical application of ISEs still faces uncertainty. By contrast, OSEs show higher feasibility with excellent elasticity, well flexibility, superior interface adhesion, and relatively high compatibility. However, the polymer matrix with high crystallinity at room temperature (RT) always results in low ionic conductivity (10−7–10−5 S cm−1), which is unfavorable for achieving high power density. Furthermore, the thermodynamic instability (oxidation potential less than 4 V vs. Li+/Li) restricts the matching with high-voltage cathode materials, while relatively inferior mechanical properties struggle to inhibit the lithium dendrite formation and growth. In this situation, numerous strategies have been employed to enhance the overall performance of OSEs, such as block/cross-linked copolymerization, incorporation of inorganic fillers, and development of composite solid electrolytes.