A Novel Method for Obtaining a High Performance Carbon Anode for Li-Ion Secondary Batteries
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ABSTRACT Entire surfaces of the pitch-based well graphitized and poorly graphitized carbon fibers were covered with an evaporated metallic film of Au, Ag, Sn, or Zn, whose thickness was varied from 100 to 600 A. Li dope and undope characteristics as the anode of Li-ion secondary battery were compared for the samples with and without evaporated film. Not only high rate charge/discharge characteristics but cycleability have been realized to be improved remarkably for the sample whose surface was covered with a thin metal film. INTRODUCTION Recent explosive increase in the production amount of Li-ion secondary batteries is due to the dramatically rapid production expansion of cellular phones, handy personal computers and handy camcorders. These electronic appliances require of the secondary battery to be improved further in the battery characteristics of high rate charge/discharge and cycleablity as well. For the purpose of obtaining much better characteristics we have pointed out that improvement of the surface condition of active materials are one of the most important key factors in addition to the improvement of the internal structure of carbon[1]-[5]. Mild oxidation and heating in vacuo are the effective treatment, both of which have been proposed by us at first[5]-
[7]. The proposed methods, however, still allow the carbon surface to be in contact with electrolyte directly, so that the so called SEI(solid electrolyte interphase) found on the treated carbon may resemble the SEI formed on the untreated carbon surface, causing a deterioraton although it is not so serious. It is, therefore, worthwhile to explore the possibility of finding an entirely different SEI which is effective to keep the Li dope/undope reactions quite stable. In the present study the authors would like to provide a novel method to obtain very stable SEI, i.e., the entire carbon surface is coated with a film of stable soft metal which has a good affinity with lithium. In this case the SEI is formed on the surface of metal and not on the carbon surface, hence, it is expected to form a new type of SEI having a very good electrode performance. EXPERIMENTAL Materials Carbon fibers used were mesophase pitch-based carbon fiber felts(average fiber diameter of 10 Rm) prepared at different temperatures(Melblon, provided by Petoca, Ltd). Examples of SEM photographs are shown in Fig. 1. The fiber felt was heated in vacuo at 250' C for 2 hours before use. The felt sample was sliced 3mm thick and placed in a vacuum chamber for vacuum evaporation The vacuum evaporation of metal was performed by heating a pure metal rod kept 557 Mat. Res. Soc. Symp. Proc. Vol. 496 01998 Materials Research Society
in a tungsten crucible at a temperature slightly higher than the melting point in the evaporation chamber(10-s Torr). The sample was mounted on a rotating holder so that both sides of the felt specimen could be covered with the evaporated film. The metal film thickness was monitored with a quartz vibrating micro-balance mounted near the specimen in the vacuum
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