Electrochemical Performance of Natural Graphite by Surface Modification Using Aluminum

Abstract

We investigated electrochemical performance of natural graphite surface-treated with aluminum compared to that of untreated graphite. Aluminum triethoxide was used as the treatment source. In the treatment process, the drying temperature has critical influence on electrochemical performance of the treated samples. The surface property of treated samples was investigated by scanning electron microscopy and Raman spectroscopy. The E 2g vibration mode of the treated sample was sharpened and shifted upward, whereas no remarkable changes were found in A 1g mode. The electrochemical performance improved markedly with Al treatment. The impedance measurement shows that the Al treated sample had much smaller charge-transfer resistance during cycling.Rechargeable lithium-ion batteries, using a carbon material as anode and transition metal oxide, such as LiCoO 2 , LiMn 2 O 4 , or LiNiO 2 , as cathode, have been developed. Recently, there has been a lot of research to improve battery performance, usually carried out on various types of carbonaceous material ranging from graphite to disordered carbon by control of various properties such as structure, 1-3 surface modification. 4-6 Among the several types of carbonaceous materials, natural or synthetic graphitic carbons are commonly found in most commercial products in the market today due to their merits such as flat and low working voltage with respect to lithium metal and relatively high coulombic efficiency. Specifically, natural graphite can be thought of as a potential material for anodes in lithium-ion batteries due to cost considerations. 7 However, it is known that it is difficult to control the key parameters of natural graphite that affect their characteristics for use as anodes because carbon materials have large variations in their electrical properties. Among the various parameters of natural graphite, surface property is one of the critical factors that affect electrochemical performance, such as reversible specific capacity, cycle life, and rate capability. 7 In this paper, we report on an investigation of the influence of surface and/or structure modification of natural graphite by introducing aluminum compound for electrochemical performance of anodes in lithium-ion batteries. ExperimentalAluminum treated samples were prepared by aluminum triethoxide (Al͑OC 2 H 5 ͒ 3 ), Soekawa Chemicals͒ treatment on NG2 ͑Kansai chemicals͒. The NG2 graphite was soaked in ethanol solution containing 10 wt % aluminum triethoxide, followed by ultrasonic treatment for 3 h, filtration, and drying at various temperatures for 1 day to remove residual alcohol. Untreated samples were also prepared by alcohol soaking without Al triethoxide, ultrasonic treatment, and heat-treatment for comparison with Al-treated one. The infrared ͑IR͒ spectra of the samples were measured with a Jasco FT/IR 350 by KBr method. An X-ray diffractometry ͑XRD, Rigaku RINT2500͒ with Cu K␣ radiation was used to measure Bragg reflections from untreated and treated NG2. Raman scattering measurements were carried...

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