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Abstract Photo‐assisted Li–O2 batteries are introduced as a promising strategy for reducing severe overpotential by directly employing photocathodes. Herein, a series of size‐controlled single‐element boron photocatalysts are prepared by the meticulous liquid phase thinning methods by combining probe and water bath sonication, and their bifunctional photocathodes in the photo‐assisted Li–O2 batteries are systematically investigated. The boron‐based Li–O2 batteries have shown incremental round‐trip efficiencies as the sized reduction of boron under illumination. It is noteworthy that the completely amorphous boron nanosheets (B4) photocathode not only delivers an optimizing round‐trip efficiency of 190% on the basis of the ultra‐high discharge voltage (3.55 V) and ultra‐low charge voltage (1.87 V) but also gives a high rate performance and ultralong durability with a round‐trip efficiency of 133% after 100 cycles (200 h) compared with the other‐sized boron photocathodes. This remarkable photoelectric performance of the B4 sample can be attracted to the synergistic effect on the suitable semiconductor property, high conductivity, and strengthened catalytic ability of boron nanosheets coated with ultrathin amorphous boron‐oxides overlayer. This research can open a new avenue to facilitate the rapid development of high‐efficiency photo‐assisted Li–O2 batteries.