Hydro-photo-synergy unlocks deep and reversible chemistry of solid-state lithium–oxygen batteries

Harnessing light to drive high-energy batteries is an intriguing goal, yet photoactivity often fails for reasons that have remained elusive, which is starkly exemplified in photo-assisted solid-state lithium-oxygen batteries. Here, we identify photo-shielding by opaque, ionically resistive discharge products as the critical, previously overlooked failure mechanism. This "catalyst blinding" effect not only blocks light but also passivates the electrochemical interface, causing a catastrophic kinetic shutdown. We overcome this limitation with a "hydro-photo-synergy" strategy, using controlled water vapor to shift the solid-state electrochemistry from a 2e^- to an efficient 4e^- pathway and transform the product into highly transparent, ionically conductive products. This dual optical and electrochemical enhancement sustains photocatalysis for deep and reversible reactions, unlocking a 2-fold increase in accessible capacity, a low overpotential (0.3 V), and outstanding stability (>170 cycles for 34 mA h cm^-2). This work establishes <i>in situ</i> product engineering as a new paradigm to unlock high-energy solid-state photo-electrochemical devices.