Band alignment and interfacial electrostatics: unraveling the dynamic space charge layer in all-solid-state batteries

All-solid-state batteries (ASSBs) are poised to transform electrochemical energy storage, yet their performance remains critically limited by high interfacial impedance. A central origin of this bottleneck is the space charge layer (SCL), an intrinsic electrostatic structure arising from electrochemical potential mismatch at solid–solid interfaces. Unlike the adaptive electric double layers in liquid electrolytes, SCLs in solids form rigid but dynamically evolving potential barriers that vary with state of charge and strongly regulate lithium-ion transport and interfacial stability. This review provides a critical and unified assessment of SCL physics in ASSBs by integrating defect chemistry, semiconductor band theory, and emerging operando characterization. We clarify SCL formation driven by Fermi level alignment, reconcile divergent views on its quantitative impact on interfacial resistance, and highlight recent experimental advances that directly visualize buried electrostatic fields. Importantly, we systematically compare two major classes of SCL regulation strategies—hierarchical band alignment engineering and interfacial field modulation—by analyzing their applicable electrolyte systems, processing complexity, scalability, and cost implications. While band alignment engineering via buffer or coating layers is particularly effective for oxide-based systems with severe electrostatic mismatch, field modulation strategies offer lower-cost and more scalable solutions for sulfide and composite electrolytes. By explicitly linking interfacial electrostatics to practical material selection and engineering constraints, this review establishes a physically grounded framework for tailoring SCL behavior and provides actionable guidance for the design of next-generation, high-performance ASSBs.