Phase-unifying mirrors used as output couplers of laser resonators allow to improve the output laser beam divergence. In this paper, a phase-unifying unstable cavity has for the first time been applied to a high-power, commercially available XeF laser (Lamda Physik 210 i), and a laser output beam of 280 mJ, of 30 ns duration (FWHM), and with a divergence close to the diffraction limited value has been obtained at λ=351 nm.