We report on the scalable synthesis and characterization of novel architecture three-dimensional high-capacity amorphous SiNWs-based anodes, with focus on studying their electrochemical degradation mechanisms. We achieved an unprecedented combination of remarkable performance characteristics, high loadings of 3-25 mAh/cm², a very low irreversible capacity (10% for the 3-4 mAh/cm² anodes), current efficiency greater than 99.5%, cycle stability both in half cells and a LiFePO₄ battery and fast charge–discharge rates (up to 2.7C at 20mA/cm²). These SiNWs-based binder-free 3D anodes have been cycled for over 500 cycles, exhibiting a stable cycle life. Notably, it was found that the growth of the continuous SEI layer thickness, and its concomitant increase in resistivity, represents the major reason for the observed capacity loss of the SiNWs-based anodes, as we demonstrate by cleaning and reusing cycled anodes. We also demonstrate the effects of different types of coatings on SEI and cycling stability. Our data reveal that NWs-based anodes of novel architecture are expected to meet the requirements of lithium-ion batteries for both portable and electric-vehicle applications.