Heat-Treatment of Synthetic Graphite under Argon and Effect on Li-Ion Electrochemistry

Abstract

The novel use of heat-treatment in flowing Ar to systematically affect particle and crystallite dimensions of synthetic graphite LK-702 is reported. Samples were characterized by percent mass loss, X-ray powder diffraction, scanning electron microscopy, Brunauer-Emmett-Teller ͑BET͒ surface area, and methylene blue adsorption surface area measurements. The treated graphites were studied as possible electrode materials for Li-ion cells. The BET and methylene blue surface areas increased for treatment times up to 48 h, then remained constant. For times up to 48 h, Ar treatment was demonstrated to break apart larger graphite particles, narrowing the particle size distribution. The irreversible capacity of the graphite decreased substantially with increased Ar treatment time, while the reversible capacity remained unchanged.The chemistry of lithium ion ͑Li-ion͒ cells has been a subject of much recent research interest. 1,2 While graphites are widely used in commercial Li-ion cells, fully understanding their complex electrochemistry has remained a continuing challenge. 3-8 Graphite particle size 9,10 and shape [11][12][13][14][15][16] have been shown to have important impact on Li-ion electrochemistry, and have been studied by several authors. The relationship between graphite crystallite characteristics and Liion electrochemistry has also been investigated. [17][18][19][20][21][22] While the studies above have demonstrated the potential electrochemical significance of graphite particle size and shape, they either focus on physical methods of particle size separation such as mechanical sieving, or rely on the use of starting graphites with inherently different sizes and shapes. We present here an investigation of heat-treatment in flowing Ar as an alternative method for affecting graphite particle and crystallite size and shape. While heat-treatment under Ar has been previously used as a means of graphite surface cleaning, 23-26 to our knowledge this is the first use of Ar heattreatment to systematically affect graphite particle and crystallite characteristics. Small samples of synthetic graphite LK-702 were heat-treated under Ar for gradually increasing times at 1000°C. Using this Ar treatment, a series of LK-702 samples with varying particle size, particle shape, surface area, and crystallite size characteristics was systematically generated. Treated samples were then characterized using scanning electron microscopy ͑SEM͒, BrunauerEmmett-Teller ͑BET͒ surface area, methylene blue adsorption surface area, differential thermal ͑DT͒ analysis and X-ray powder diffraction ͑XRD͒ measurements. Finally, coin-sized cells were used to study the treated graphite electrochemistries vs. lithium metal. ExperimentalGraphite treatment.-Before all treatments, the synthetic flake graphite LK-702 ͑Nippon Carbon Company͒ was dried at 150°C under vacuum for at least 24 h. All samples were heat-treated in a Lindberg Blue M TF55035A tube furnace. For each treatment, ϳ2.5 g of graphite was added to a ceramic boat, which was then placed in ce...

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