Unraveling the formation mechanism of solid-liquid electrolyte interphases on LiPON thin films.

Most commercial lithium-ion batteries and other types of batteries rely on liquid electrolytes, which are preferred due to their high ionic conductivity and facilitate fast charge-transfer kinetics at the electrodes. On the other hand, hybrid battery concepts that combine solid and liquid electrolytes might be needed to suppress unwanted shuttle effects in liquid electrolyte-only systems, in particular if mobile redox systems are involved in the cell chemistry. However, at the then newly introduced interface between liquid and solid electrolyte, a solid-liquid electrolyte interphase forms. In this study, we analyze the formation of such an interphase between the solid electrolyte lithium phosphorous oxide nitride (LixPOyNz , "LiPON") and various liquid electrolytes using in situ neutron reflectometry, quartz crystal microbalance, and atomic force microscopy measurements. Our results show that the interphase consists of two layers: a non-conducting layer directly in contact with "LiPON", and a lithium-rich outer layer. Initially, a fast growth of the solid-liquid electrolyte interphase is observed, which slows down significantly afterward, resulting in a thickness of about 20 nm eventually. Here, a formation mechanism is proposed, which describes the solid-liquid electrolyte interphase growth as fast deposition of a mostly-covering film with only a little degree of remaining porosity. The residual void space is then slowly filled, thus blocking the remaining channels for ionic conduction, which leads to increasing resistance of the interphase. The results obtained imply that hybrid battery concepts with liquid electrolyte and solid electrolyte can be hampered by highly resistive interphases, whose formation cannot be simply slowed down or suppressed. Further research is required regarding possible countermeasures.