Direct View on the Origin of High Li+ Transfer Impedance in All‐Solid‐State Battery

Abstract Large interfacial resistance plays a dominant role in the performance of all‐solid‐state lithium‐ion batteries. However, the mechanism of interfacial resistance has been under debate. Here, the Li + transport at the interfacial region is investigated to reveal the origin of the high Li + transfer impedance in a LiCoO 2 (LCO)/LiPON/Pt all‐solid‐state battery. Both an unexpected nanocrystalline layer and a structurally disordered transition layer are discovered to be inherent to the LCO/LiPON interface. Under electrochemical conditions, the nanocrystalline layer with insufficient electrochemical stability leads to the introduction of voids during electrochemical cycles, which is the origin of the high Li + transfer impedance at solid electrolyte‐electrode interfaces. In addition, at relatively low temperatures, the oxygen vacancies migration in the transition layer results in the formation of Co 3 O 4 nanocrystalline layer with nanovoids, which contributes to the high Li + transfer impedance. This work sheds light on the mechanism for the high interfacial resistance and promotes overcoming the interfacial issues in all‐solid‐state batteries.