Relation Between Heat-Treatment Temperature and Characteristics of Polyparaphenylene(PPP)-Based Carbon Materials for Lit
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Relation Between Heat-Treatment Temperature and Characteristics of Polyparaphenylene(PPP)-Based Carbon Materials for Lithium Ion Secondary Batteries Kozo Osawa, Tatsuo Nakazawa, Kyoich Oshida, Morinobu Endo1 and Mildred S. Dresselhaus2 Nagano National College of Technology, 716 Tokuma , Nagano-shi, 381-8550, JAPAN. 1 Shinshu University, 4-17-1 Wakasato, Nagano-shi, 380-8553, JAPAN. 2 Masatussets Institute of Technology, Cambridge, MA 02139, USA. ABSTRACT Polyparaphenylene (PPP)-based carbon powder is expected for materials of the negative electrode of the lithium ion secondary batteries. In this study, effects of heat treatment temperature on the PPP materials are investigated by means of electrical and structural measurement. The layer structure of the powder is analyzed in detail by the transmission electron microscope observation combined with digital image processing. Charge-discharge current characteristics, electrical resistivity under pressure and layer structure of the PPP-based carbon powder are measured and discussed. INTRODUCTION Disordered structure carbons are useful materials for many applications such as a negative electrode of lithium (Li) ion secondary batteries, since a large amount of Li ion can be held in gaps between crystallites of the carbon. An interesting relation between crystalline thickness (Lc(002)) and charge capacity of the Li ion secondary batteries was reported [1]. Well ordered graphite and low crystalline materials have a large capacity, though intermediate crystalline size materials show a small capacity. Polyparaphenylene (PPP)-based carbon is a kind of disordered carbons and has the very small crystallite size. So, it is expected as the material contributing to the improvement of Li ion secondary batteries [1, 2]. Two major synthesizing methods are known for PPP materials, Yamamoto method and Kovacic method [3]. Though the material prepared by Yamamoto method tends to have crystalline structure, the materials prepared by Kovacic method have a disordered random structure. So, the latter material has a possibility of larger ion adsorption and is prefer to use as the negative electrode of the Li ion secondary battery. The purpose of our project is to establish the suitable preparation process for PPP-based carbon used as negative electrode of Li ion secondary batteries. In this study, the properties of the PPP-based carbon powders prepared at the heat treatment temperature (HTT) between 650 and 750C is investigated. The discussion is focused on two points; why the electrical resistivity of PPP powders are strongly depended on HTTs, and what changes are occurred in the structure of samples by heat treatment. EXPERIMENTALS PPP powders synthesized by Kovacic method were used as the starting materials. The powder was heat treated in pure argon flow at a temperature between 650 and 800C in 1 hour. Figure 1 shows the apparatus for the charge-discharge capacity measurement used in this study. A small volume glass container with four electrode inlets (Li electrode (x2), carbon electrode an
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