Orbital-Selective Photoinduced Polaron Dynamics in Na ₂ O ₂ : Ultrafast Asymmetric Charge Transport Modulation in Sodium–Air Batteries

Understanding and controlling polaron dynamics are pivotal for improving charge transport in sodium-air batteries. Here, we employ real-time time-dependent density functional theory to investigate the ultrafast photoinduced dynamics of hole polarons (HPs) and electron polarons (EPs) in Na₂O₂ under femtosecond laser excitation. In the ground state, both polarons are highly localized and exhibit high dissociation barriers, 0.52 eV for HPs and 1.36 eV for EPs, limiting their thermally activated mobility. Upon photoexcitation, HP exhibits an increased occupation of π* antibonding orbitals, which weakens the O-O bond and drives coherent stretching oscillations. This process culminates in spontaneous, barrierless polaron dissociation with the release of mobile holes into the valence band, enabling enhanced charge delocalization and facilitating polaron hopping. In contrast, EP undergoes an additional population of σ* antibonding orbitals, further stabilizing the elongated O-O bond and increasing the dissociation barrier, thereby suppressing carrier mobility. This asymmetric photoresponse arises from an orbital-selective excitation pathway coupled to distinct bonding character at the polaron sites. These findings unveil a fundamental design principle for tuning polaronic conductivity via light and highlight the potential of optical modulation strategies for improving performance in metal-air batteries.