Improvement of electrochemical performance in lithium-doped graphite synthesized by ball-milling method

Abstract

Graphitic materials have been widely used as negative electrodes for lithium secondary batteries. In spite of its success as an anode material, graphitic anode still suffer from serious problems, including the electrolyte decomposition and subsequent surface film formation, which leads to irreversible capacity. The irreversible reaction not only consumes a certain amount of cathode material and electrolyte, but also causes detrimental effects such as high internal pressure and lower cycling efficiency. It was observed that, out of various forms of carbon materials, less crystallized graphitic materials, such as natural graphite, tended to give a high discharge capacity compared with highly crystallized materials. In fact, most commercial carbon anodes today consist of synthetic graphite materials which contain various degrees of structural defects. Commercial carbon anodes, such as MCMB (mesocarbon microbead) and MCF (mesocarbon fiber), for lithium secondary batteries have relatively higher cost and lower discharge capacity. In this point of view, natural graphite is considered as another promising potential anode material for lithium secondary batteries because of its high reversible capacity, appropriate potential profile, and low cost. Major problems in natural graphite are the large irreversible capacity during the first lithium intercalation-deintercalation process, poor cyclability during cycling and rate capability. In this work, Lithium-doped natural graphite powders have been synthesized by ball-milling method. Lithium-doped natural graphite powders were characterized by X-ray diffraction, charge/discharge cycling, scanning electron microscope, impedance analyzer, transmission electron microscopy and X-ray photoelectron spectroscopy. Lithium-doped natural graphite powders synthesized by ball-milling method exhibited high capacity of 358 mAh/g (close to theoretical value of 372 mAh/g based on LiC6) and excellent capacity retention of > 99 % after 50 cycles. Whereas, the discharge capacity of pristine natural graphite gradually decreased, showing poor capacity retention of < 85 % after 50 cycles. Also, the irreversible capacity for Li-doped graphite electrode at 1st charge-discharge process was 5 %, whereas the pristine graphite electrode exhibited the irreversible capacity of 13 % at first cycle.