Characteristics of Boron Doped Mesophase Pitch-Based Carbon Fibers as Anode Materials for Lithium Secondary Cells
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Experimental Samp 1 e
T Mesophase Pitch-based Carbon Fibers(MCF), which we used were "MELBLON M, short carbon fibers manufactured by PETOCA, LTD. MCF were carbonized at 65
If C, then milled. The milled MCF's average particle diameter is 18gm. Boron was doped to the fiber 0--10 wt % by adding B4C(10Um) to the pre-carbonized milled fibers and then heat-treated at several temperatures up to 30000 C in Ar, to obtain various grades of crystallized MCF.
Measurement The characteristics of boron doped MCF were examined by XRD and electrochemical methods. Structural parameters of the boron doped MCF were 569
Mat. Res. Soc. Symp. Proc. Vol. 496 01998 Materials Research Society
measured by XRD with Cu-K a radiation. Electrochemical measurements 1. Anode sheet Milled MCF is mixed with 7 wt% of PVDF binder and NMP for solvent, and then the mixed paste is spread on copper foil. The sheet of MCF is pressed at 1.5 ton/cm 2, and dried at 150°C under vacuum for 3 hours. 2. Three-electrodes cell A sheet of electrode (20mm in square and about 80,um in thickness) is set in a three-electrodes cell with metallic lithium foil electrodes as the counter and the reference electrodes in excess electrolyte. 3. Electrolyte 1 Mol/l solution of LiCIO4 in mixtures of ethylene carbonate (EC) and dimethyle carbonate (DMC) (volume ratio = 1/1) is used as the electrolyte. 4. Charge and discharge condition •-5th cycle Charge condition : I mA/cm2 constant current density to 0.01V vs Li/Li+ potential and keep constant voltage at 0.01V total charge time 8 hours. Discharge condition : 1 mA/cm constant current density discharge to 1.5V. 6--10th cycle Charge condition : 2 mA/cm 2 constant current density to 0.01V and keep constant voltage at 0.01V total charge time 5 hours. Discharge condition : 2 mA/cm 2 constant current density discharge to 1.5V. 1 --15th cycle Charge condition : 6 mA/cm constant current density to 0.01V and keep constant voltage at 0.01V total charge time 3 hours. Discharge condition : 6 mA/cm 2 constant current density discharge to 1.5V. 5. Cyclic voltammograms (CV) The potential sweep rate: 5,u V/sec, The potential sweep range: 350'-O mV(vs Li/Li+ potential) RESULTS Fig. 1 shows the relationship between various B4C addition MCF's d002 value and heat treated temperature. It shows that B4C addition group b-MCF are highly graphitized over 2, 2W C by boron doping. Boron is the graphitization catalyst. Boron is the only metal that can replace a carbon atom in the crystal structure [1]. When the boron doping , it cut the carbon sp3 bonding and accelerate the graphitization of MCF. 570
0.343 0.342 0.341
-M- B4C-Owt% ,-0,A
.. A.-
0.340
0q
;I 0
B4 C-3wt% B 4C-5wt% B4 C1Owt%
O ..
0.339 0.338
"
0.337
-
0.3365 0.334
.... i ...., 2000
2200
2400
2600
2800
i
3000
3200
H.T.T (°C) Fig. I Relationships between various B 4C addition MCF
d 002 value and heat treated temperature Fig. 2 shows the relationship between B4C addition and MCF's peak intensity of 1 101/1100 , doo2 value heat treated at 3,000C. I 101/I100 peak intensit
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