Successfully commercialized polyethylene 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 safety concerns such as PEO decomposition and the associated gas release is worthy for more attention. This study employs in-situ DEMS to study the gassing behavior of LiCoO2|PEO-LiTFSI|Li SPBs. The experiments, together with theory calculations, reveal that a surface catalytic effect of LiCoO2 is the root cause for the unexpected H2 gas release of PEO-based SPBs at 4.2 V. The surface coating of LiCoO2 with a stable solid-electrolyte Li1.4Al0.4Ti1.6(PO4)3 (LATP) can mitigate such surface catalytic effect and therefore extend the stable working voltage to over 4.5 V. The crossover effect of HTFSI, which is generated at cathode side due to oxidation/dehydration of PEO and reacts with lithium at anode side, is proposed to explain the H2 generation behavior.