By using first-principles calculations, we investigate the interfacial failure between a lithiated Sn electrode and a Cu current collector for lithium-ion batteries. The results show that segregation of Li ions at the interfacial region leads to the transformation of interfacial bonding from Sn-Cu to Li-Cu bonds, and weak Li-Sn bonds near the interface region. This leads to the weakened interfacial strength upon lithiation and failure of most lithiated Sn/Cu interfaces within Li(x)n slabs. In addition, the formation of CuxSn alloys at interface could increase the interface strength and enhance the deformation resistance of electrode materials, which explains why CuxSn containing composite electrodes exhibits the excellent electrochemical performance.