Single‐crystalline Ni‐rich cathode (SC‐NCM) has attracted increasing interest owing to its greater capacity retention in advanced solid‐state lithium batteries (SSLBs), while suffers from severe interfacial instability during cycling. Here, via atomic layer deposition, Li3PO4 is introduced to coat SC‐NCM (L‐NCM), to suppress undesired side reaction and enhance interfacial stability. The dynamic degradation and surface regulation of SC‐NCM are investigated inside a working SSLB by in situ atomic force microscopy (AFM). We directly observe the uneven cathode electrolyte interphase (CEI) and surface defects on pristine SC‐NCM particle. Remarkably, the formed amorphous LiF‐rich CEI on L‐NCM maintains its initial structure upon cycling, and thus endows the battery with improved cycling stability and excellent rate capability. Such on‐site tracking provides deep insights into surface mechanism and structure–reactivity correlation of SC‐NCM, and thus benefits the optimizations of SSLBs. Insights into the surface mechanism on the single‐crystalline LiNi0.5Co0.2Mn0.3O2 (SC‐NCM) cathode are disclosed by in situ atomic force microscopy in solid‐state batteries. Via atomic layer deposition, the Li3PO4 is introduced to coat SC‐NCM, leading to the uniform formation of LiF‐rich cathode electrolyte interphase and suppression of undesired side reaction, which endows batteries with enhanced interfacial stability, durability and dynamics.