Successfully commercialized poly(ethylene oxide) (PEO)based solid polymer batteries (SPBs) are expected to continuously play a key role in the next generation of high-energy density lithium-ion battery technologies. However, the introduction of high-voltage cathodes, accompanied by safety concerns such as PEO decomposition and the associated gas release, is worthy of more attention. This study employs in situ DEMS to study the gassing behavior of LiCoO 2 |PEO-LiTFSI|Li SPBs. The experiments, together with theory calculations, reveal that a surface catalytic effect of LiCoO 2 is the root cause of the unexpected H 2 gas release of PEO-based SPBs at 4.2 V. The surface coating of LiCoO 2 with a stable solid electrolyte Li 1.4 Al 0.4 Ti 1.6 (PO 4 ) 3 (LATP) can mitigate such a surface catalytic effect and therefore extend the stable working voltage to >4.5 V. The crossover effect of HTFSI, which is generated at the cathode side due to oxidation/dehydration of PEO and reacts with lithium at the anode side, is proposed to explain the H 2 generation behavior.

Solid state lithium batteries are widely accepted as promising candidates for next generation of various energy storage devices with the probability to realize improved energy density and superior safety performances. However, the interface between electrode and solid electrolyte remain a key issue that hinders practical development of solid state lithium batteries. In this review, we specifically focus on the interface between solid electrolytes and prevailing cathodes. The basic principles of interface layer formation are summarized and three kinds of interface layers can be categorized. For typical solid state lithium batteries, a most common and daunting challenge is to achieve and sustain intimate solid-solid contact. Meanwhile, different specific issues occur on various types of solid electrolytes, depending on the intrinsic properties of adjacent solid components. Our discussion mostly involves following electrolytes, including solid polymer electrolyte, inorganic solid oxide and sulfide electrolytes as well as composite electrolytes. The effective strategies to overcome the interface instabilities are also summarized. In order to clarify interfacial behaviors fundamentally, advanced characterization techniques with time, and atomic-scale resolution are required to gain more insights from different perspectives. And recent progresses achieved from advanced characterization are also reviewed here. We highlight that the cooperative characterization of diverse advanced characterization techniques is necessary to gain the final clarification of interface behavior, and stress that the combination of diverse interfacial modification strategies is required to build up decent cathode-electrolyte interface for superior solid state lithium batteries.