Abstract: The invention relates to the use of a lyocell fiber (1) for the manufacture of a nonwoven fiber fleece (10, 100). To manufacture a thin nonwoven fiber fleece with sufficient mechanical properties, the use of a lyocell fiber is proposed, where the fiber (1) has a cross-sectional aspect ratio of at least 1.8. The invention further relates to a nonwoven fiber fleece (10, 100). In order to manufacture a thin nonwoven fiber fleece (10, 100) suitable for use as a battery separator, it is proposed, that the fiber fleece comprises at least two layers (11, 12) of fibrillated lyocell fibers (13), with the fibrillated lyocell fibers (13) having solid cores (14, 110) and fibrils (15) protruding from said cores (14), the fibers and fibrils (15) being intermingled to form the fiber fleece (10), embedding the solid cores (14) therein, whereby the solid cores (14, 110) of the fibrillated lyocell fibers (13) have an average cross-sectional aspect ratio k of at least 1.5.