Cu2-xS as one of the first tested battery anode material [1] has attracted large amount of attentions due to its high specific theoretical capacity (560 mAh g-1 for CuS and 337 mAh g-1 for Cu2S) [2], high energy density and good electronic conductivity for fast reaction kinetic (10-3 Ω-1 cm-1 to 140 Ω-1 cm-1 [3], [4]) . However, the reaction mechanism of Cu2-xS versus Li has not been elucidated. Also, as the technology advances, Cu2-xS nanoparticle can be synthesized with great control over size, morphology. Whether this will benefit the battery performance of Cu2-xS remain a question. In this thesis, we studied the structural change of Cu2¬-xS (three different phase particles, anilite Cu1.75S, roxbyite Cu1.81S and djurleite Cu1.94S) during discharge versus the lithium foil. Several advanced characterization tools (X-ray diffractometer, high-resolution transmission electron microscope) and electrochemical testing (galvanostatic charge-discharge) are used to observe the structural and electrochemical behavior. As a result, we proposed a common reaction mechanism for Cu2-xS versus Li-ion. Moreover, given the ability to synthesis Cu2-xS nanoparticle with excellent size, morphology and dispersity control [5], we studied the size-dependent electrochemical properties of Cu2-xS. We found as the size of the particle decrease, the electrochemical curve resembles more like a supercapacitor while as the particle becomes bigger, it shows discharge plateau which is typical to battery material. With Electron Energy Loss Spectroscopy and X-ray Photoelectron Spectroscopy, we studied the electronic structure of the material which helps us to explain this interesting behavior.