Si1-xMnx (0 <= x <= 0.5, Delta x = 0.05) alloys were prepared by ball milling and studied as negative electrode materials in Li cells. These alloys were found to be unique among transition metals. When x <= 0.35, the alloys are essentially homogeneous with nanocrystallites of Si19Mn11 uniformly dispersed in a matrix of amorphous silicon. Nanocrystallites of SiMn and Si19Mn11 coexist in samples with x > 0.35. The formation of Li15Si4 during lithiation was totally suppressed for alloys with x >= 0.3. Cycling performance was significantly improved by Mn addition, especially for the samples in which the formation of Li15Si4 was suppressed. The addition of Mn increases the particle size, which results in reduced surface area. Capacity trends suggest that inactive Si19Mn11 and SiMn phases were formed in these alloys, consistent with XRD results. The x = 0.25 alloy achieved a volumetric capacity of 1526 Ah/L with 95% capacity retention after 50 cycles. (C) The Author(s) 2018. Published by ECS.