Abstract:

All-solid-state batteries (ASSBs) using inorganic solid electrolytes (SEs) have emerged as crucial components in energy storage applications due to their superior safety and cycle life. In recent years, due to the extensive developments of SEs with high room temperature ionic conductivity (> 10⁻³ S·cm⁻¹), the sluggish diffusion kinetics of lithium ions in SEs are no longer the primary bottleneck impeding the enhancement of ASSBs. On the contrary, the notable resistance at the cathode/SE interface has emerged as a pressing issue demanding immediate resolution. The interfacial resistances arising from various factors, including the formation of the space-charge layer, interfacial chemical reactions, and lack of intimate contact, stand as fundamental reasons for a range of performance deteriorations, such as short cycling life, low coulombic efficiency, and poor power performance. These interconnected aspects further result in differences in the orders of magnitude of the reported interfacial resistances at different fabrication temperatures and/or routes, even within the same material system. Among these factors, the solid-solid contact or chemical reaction degree is closely related to the structural and electronic properties of the selected cathode and SE materials. The observed space-charge layer effect is universal and independent of the specific components or types of ion-conductive materials. Thus, obtaining a comprehensive understanding of the physics governing the space-charge layer at the interfaces of ASSBs is pivotal for researchers to fundamentally address the high interfacial resistance stemming from it. This forms the foundation for incorporating other mechanisms (such as interfacial reactions) to more accurately quantify interfacial resistance and expedite interface research in ASSBs. In this review, we strictly derive the theoretical model of the formation of the space-charge layer caused by the inherent chemical potential difference between the cathode and SE from fundamental concepts of (electro)chemical potential and electric potential, and reveal the physical picture of its influence on interfacial resistance. Subsequently, the most recent experimental characterizations and theoretical calculations of the space-charge layer at the cathode/SE interface are comprehensively discussed. While the existence of the space-charge layer is observable through experimentation, its characterization is complicated by factors like loss of interfacial contact and interfacial reactions. Therefore, it becomes imperative to further quantify the concentration of lithium ions in the space-charge layer and its impact on interfacial resistance through theoretical calculations. However, when combining the space-charge layer model with numerical and first-principles calculations to quantitatively study interfacial resistance, accurately determining the interfacial electric potential difference at the cathode/SE interface remains challenging, resulting in several orders of magnitude difference between predicted results and experimental measurements. Consequently, grounded in the foundational physical framework of the interfacial electric potential difference, the intricate connections between this potential difference and the electronic structure of the cathode/SE interface are explored. As a result, a strategy is proposed to ascertain the interfacial electric potential difference by directly calculating the Fermi level of the cathode and SE under real bonding conditions. This endeavor is anticipated to broaden the utility of the space-charge layer model in quantitatively calculating cathode/SE interfacial resistance, offering valuable insights for optimizing the cathode/SE interface and enhancing the overall performance of ASSBs.

Key words: Cathode, Solid electrolyte, Space-charge layer, Interfacial electric-potential difference, Interfacial resistance