New 7 Li-NMR Evidence for Lithium Insertion in Disordered Carbon
- PDF / 387,470 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 66 Downloads / 191 Views
=3.7 A). 545 Mat. Res. Soc. Symp. Proc. Vol. 496 © 1998 Materials Research Society
For electrochemical Li doping and undoping," a mixture of disordered carbon material and polymer binder was pressed into a sheet-shaped electrode. The size of the sheet-shaped electrode was 2.0 cm x 10.0 cm with a thickness of 110 [tm. The electrode was dried at 180 'C under vacuum for 4h. It was used as the anode and a Li sheet and a Li chip were used as the counter and reference electrodes, respectively. A solution of 1M propylene carbonate/LiCiO4 was used as the electrolyte. A solution of 1M EC / DEC / LiCIO4 was used as the electrolyte for the graphite carbon. In order to obtain a sample of the discharged carbon electrode, the cell is charged from its rest voltage (usually in the range of 2.5- 3.0 V vs. Li / Li÷) to lmV vs Li / Li+ at a constant current, and kept at lmV until a steady-state current was reached (10s - 10- A), and then discharged to 2.0 V (Li / Li÷) under a constant current and kept at 2.0 V(Li / Li+) until a steady-state current was reached (10-5 - 10-6 A) by employing a charge/discharge unit (Hokuto HJ-201B). Charge and
discharge current densities were both 1.0 mA/cm 2 . The charged and discharged carbon electrodes were rinsed with propylene carbonate to remove electrolyte, dried in a vacuum and then put into a solid NMR sample tube under an argon atmosphere. The NMR measurements were made using a Bruker ASX 200 spectrometer with a multinuclear MAS solid-state NMR probe. iU solid-state NMR spectra were recorded at a frequency of 77.7 MHz. The actual measurements were carried out by adopting a magic angle spinning of 5000 rpm. The 7Li-NMR shift was calibrated in ppm relative to IM LiCI (-1.19 ppm) as the external reference standard. RESULTS AND DISCUSSION Knight shift at room temperature The 7Li-NMR spectra at room temperature for the fully-charged disordered carbon and discharged disordered carbon are shown in Fig. 1(a) and Fig. 1(b), respectively. From the 7LiNMR spectrum shown in Fig. l(a), one can see that there are two types of broad NMR lines. One broad resonance is shifted to low field at 81 ppm with respect to that of the LiCl reference. Another resonance shift appears at -1.40 ppm. The former is plausibly explained in terms of the Fermi contact interaction between a Li nucleus and conduction electrons, that is, the Knight shift of the Li nucleus. The latter is observed at the almost 81.0 ppm -1.40 ppm same position with LiC1 (- 1.19 ppm) reference
without the Knight shift. These results indicate that
(b)
(a)
two different electronic
states surrounding the Li nucleus exist in the fully- • . .• , , . . , , I r, nuclreu eistrieted full200 100 0 -100 -200
200
,
,•
100
,
,
0
,,
I ,
-100
,
-200
charged disordered carbon.
Knightshift(ppm) Knightshift(pprm) In comparison with Fig. 1 7 (a) and Fig. l(b), we Fig. 1 Li-NMR spectra of(a) the fully-charged disordered carbon carbon suggest that the broad aid (b) discharged disordered 81 ppm at shift resonance
546
corresponds to reversible capac
Data Loading...
