Improved cycling behavior of Li -doped natural graphite anode for lithium secondary batteries

Abstract

Graphitic materials have been widely used as negative electrodes for lithium secondary batteries. In spite of its success as a anode material, graphitic anode still suffer from serious problems, including the electrolyte decomposition and subsequent surface film formation, which leads to irreversible capacity [1-2]. 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 [3-4]. It was observed that, out of various forms of carbon materials, less crystallized graphtic materials, such as natural graphite, tended to give a high discharge capacity compared with highly crystallized materials [5]. 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. A major problem in natural graphite is the large irreversible capacity during the first lithium intercalation-deintercalation process, poor cyclability during cycling and rate capability. In this research, we demonstrate that Li-doping of the natural graphite can greatly improve the cycling performance, which can be applied to other graphitic materials. XRD patterns of the samples of pristine natural graphite and Li 0.14 wt.% -doped natural graphite prepared by ball-milling method are shown in Figure 1. Fig. 1(a) is a typical X-ray diffraction profile of the pristine natural graphite (China). The peaks between 42o and 47o belong to the rhombohedral phase (ABCABC……) of natural graphite. Fig. 1(b) is the XRD profile of natural graphite ball-milled in the ethanol for 20 hours. The peaks between 42o and 47o are observed, but the relative intensity of the (00l) peaks to the (hkl) peaks reflection is greater. This means that the milling process has reoriented the crystals to lie preferentially on the basal planes [6]. Fig. 1(c) is the XRD profile of the sample of Li 0.14 wt.% -doped natural graphite prepared by calcination after ball-milling treatment. The XRD profile of Li 0.14 wt.% -doped natural graphite shows amorphous shape. The diffuse XRD profile of the sample shows the formation of very fine crystals, so-called disordered graphite [7]. Figure 2 shows the cycling behavior (specific discharge capacity vs. cycle number) for the pristine and various Li (x = 0.14 wt.% and 0.34 wt.%)-doped contents of graphite electrode. The graphite electrodes were galvanostatically cycled between 0.01 V and 1.5 V vs. Li/Li+ at room temperature (30 oC). The charge and discharge current density was 65 mA/g (0.2 C-rate). The graph clearly shows the superior cyclability of the Li-doped graphite electrodes. Natural graphite doped Li 0.14 wt.% 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. Although initial discharge capacity (356 mAh/g) of pristine natural graphite is similar to that of Li 0.14 wt.% -doped natural graphite, the pristine natural graphite gradually lost its discharge capacity and exhibited poor capacity retention of < 85 % after 50 cycles. The improved electrochemical performance was attributed to presence of Li atoms along the edges of the graphite structure minimizing the electrolyte breakdown at the anode surface and migration of solvated Li ions.