Lithium–nitrogen (Li–N₂) battery represents an emerging electrochemical technology for energy storage and N₂ fixation. However, its practical implementation is hindered by the inherent electrochemical inertness of N₂ and the inferior catalytic activity of conventional cathode materials. In this study, we report a defective cobalt phosphide (CoP_v) photoelectrocatalytic cathode featuring high-spin states, engineered through strategic introduction of phosphorus vacancies. The operational mechanism involves a sophisticated synergy, where P vacancies serve as N₂ adsorption sites, while spin-polarized electrons from high-spin-state Co sites facilitate electron transfer to adsorbed N₂. Under illumination, photoexcited electrons are directly injected into the π^* antibonding orbital of N2via the P vacancies, significantly enhancing nitrogen activation and accelerating the nitrogen reduction reaction (NRR) kinetics during discharge. Furthermore, light also facilitates the formation of a uniform film-like discharge product, enhancing nitrogen evolution reaction (NER) kinetics. Thus, the photoassisted Li–N₂ battery demonstrates exceptional performance metrics: a high discharge specific capacity of 2.71 mAh cm^–2, excellent cyclic stability (∼900 h), and an ultralow overpotential of 1.3 V, the lowest overpotential reported to date. Significant nitrogen fixation is achieved in the photoassisted process, offering crucial mechanistic insights into photoassisted Li–N₂ batteries and suggesting an innovative approach to enhance Li–N₂ battery technology toward energy storage applications.