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7Li-NMR study of lithium charged phosphorus-doped disordered carbon
ELSEVIER
SyntheticMetals 103(1999) 2521-2522
‘I&NMR
study of lithium
charged phosphorus-doped
disordered
carbon
S. Wang, H. Matsui and Y. Matsumura Fundamental Research Laboratories, Osaka Gas Co., Ltd., Osaka, Japan, Abstract 7Li high resolution nuclear magnetic resonance (NMR) measurements at various temperatures in fully-charged phosphorusdoped disordered carbon material have been per&med. It was fbund that a broad line at 69.5 ppmand a relatively sharp line in near 15.0 ppm were observed at room temperature and fbur lines at 20.8, 74.6, 144.6 and 199.0 ppm were observed at low temperature. The mechanism ofLi storage in P-doped disordered carbon is discussed. Keywords:
Rechargeable batteries, Disordered carbon materials, NMR Mechanism of Li storage
Introduction Worldwide research is in progress to develop Li ion rechargeable batteries(LIB) using carbon as the anode[l]. Much of effort is focused on developing new carbon anodes with high discharge capacity. Disordered carbon materials have been highlighted as an anode material of the LIB because these materials could store rmch mOre lithiumwith a discharge capacity surpassing the theoretical capacity of a graphite anode[2,3]. Recently, it was Ibund that phosphorus or borondoped(P or B-doped) disordered carbon exhibited high reversible capacity conpared to the normal disordered carbon[4]. In order to develop a new carbon anode, it is necessary to understand the mechanism of Li storage in P or B-doped disordered carbon. Our previous work has shown that 7Li-NMR spectroscopy can provide meaningtil inforrration on the nature of the lithiumspecies in a fully-charged carbon anode and especially Ibr equilibrium arrangensents of the lithium species at low tenperature[5]. In this work, the Li-NMR measurements at various temperatures for the fully-charged P-doped disordered carbon (PDC) were perforrred. Based on the NMR results, a model ofLi storage in the PDC has been proposed.
electrochemical and NMR measurement conditions have been described elsewhere[6]. Results and Discussion The relationship between the P atom content in the disordered carbon and the capacity is shown in Fig.1. When P
,,t,""'.,""',",,"""'.'.,l 0 1
3
4
5
6
P atan content(Wt%)
Experimental Fig. 1 The relatkmhip
PDC material was prepared by the pyrolysis of a mixture of coal-tar pitch and PzOs at 1100 “C. This material has a small crystallite size (the c-axis dimension, Lc = 12 A) and a large interlayer distance between graphite layers (do02= 3.8 A). A random crystallite orientation was observed using the pole figure technique. The PDC material was mixed with 7 weight % polymer binder to make an electrode. A three-electrode electrochemical cell was asset&led with the PDC electrode as the working electrode, and a Li sheet and a Li chip were used as the counter and reference electrodes, respectively. A solution of 1M propylene carbonate/LiC104 was used as the electrolyte. The filly-charged PDC with a capacity of 667 tnAhg-‘, which corresponds to an insertion ofabout C3 3Li was obtained by the charging process at constant current and voltage. 7Li high resolution NMR measurements of the tilly-charged PDC at various temperatures have been perfimd. Details of the
2
betweenPatun con&H and capacity
content increases,the capacity becomeslinearly large. This meansthat the capacity of the PDC hasdirect relation with P atoms.In orderto study the mechanismofLi storagein PDC, the 7Li-NMR spectra of the fully-charged PDC with a reversiblecapacity of553 r&h/g at various temperatureswere measuredasshown in Fig. 2. At roomtemperature,the ti~llychargedPDC showsa broad line at 69.5 ppmand a relatively sharpline at 15.0 ppm With decreasingtemperature,the line width becamebroaderand somenew bandsappeared.When the temperaturewas at -100 “C, the Li-NMR spectra clearly showedfbur diErent signals.One band with strong strength appearedat 20.8 ppm and three broad bandsappearedat 74.6, 144.6 and 199.0ppm Theseresults indicate that the rate of exchangeor thermalmtion ofthe Li speciesslowed down on the NMR time scaleand the equilibrium arrangements of the lithium specieswereobservedat low temperature.Four kinds
0379-6779/99/$ - seefrontmatter0 1999ElsevierScience S.A. All rightsreserved. PII: SO379-6779(98)01088- 1
2522
S. Wang et al. I Synthetic
Metals
ofLi species with dierent electronic densities exist in the fully-charged PDC 69.5 ppm
72 wm
JL i”:, 23 wm
23 “C
0.0 “c
300
20
74ppm,
Lu’~““~“~‘,““‘iUC~‘~”
300
200
100
wm
0.0
-100
200
100
cl0
103 (1999)
2521-2522
Based on the NMR and ESCA results, a model of Li storage in the PDC is presented in Fig.3. Since the PDC has the almost the same amorphous structure and its carbons electronic states were not significantly a&ted by P-doping, it can be considered that the lithium species in the PDC are located between the graphitic layers (20.8 ppm), on the surhce of a crystallite and at the edge ofthe graphitic layers(74.6 and
-1eQ -200
20.8 ppm 74.6 ppm fl
300
-2m
200
DO
00
-1p
-200
Fig.2 NMR spectra of the fully-charged PDC at various temperatures
l
Li located between the graphitic layers
0
Li located bn the surface of-a crystallite
@ Li located at the edge of the graphitic layers In a previous study[5], we observed three kinds of interactions between Li species and carbon for the IIrllycharged disordered carbon by Li-NMR at low temperature. According to theoretical results, the band at 18.7 ppm is assigned to the lithium species located between the graphitic layers and the bands at 71.5 and 148.5 ppmmay correspond to the lithiumspecies located on the surfnce ofa crystallite and at the edge ofthe graphitic layers, respectively. Comparison with the Li-NMR results ofthe disordered carbon, we can find that the PDC has almost the same three bands at 20.8, 74.6 and 144.6 ppm, and a new band at 199.0 ppm The new band may play an important role in storing rmch mre Li species so that the PDC has higher capacity than that of the disordered carbon, ESCA spectra ofthe PDC and the disordered carbon were examined. The results are shown in the Table 1. It shows that the Ct, binding energy ofthe PDC is 284.1 eV, which is Table 1 ESCA results of the carbon materials P (wt%)
Binding Cls
Energy (eV) P2s
P-doped disordered carbon
4.8
284.1 129.2 130.8 132.3
Disordered carbon
0.0
284.0
similar to that of the disordered carbon. Ihe Pb binding energy in the PDC is in the range of 129.2-132.3 eV. The result shows that the carbon electronic states were not significantly a&ted by the P-doping. The phosphorus in the PDC are mixture states consisting of the (C6H5)xP and (C6H5)3PO types. At the present stage, we can not distinguish which kind of phosphorus states play the important role for Li storage in the PDC.
c] Li located in the near P atom Fig. 3 a model of Li storage in the P-doped disordered carbon 144.6 ppm). On the other hand, we assume that P species may make some cross-linking structures between neighboring crystallites or exist at the edge of the graphitic layers. P species tray store trore Li species so that a new band at 199.0 ppm was observed at low terrperature. This may be the &in reason why the PDC can have a higher capacity than that of the disordered carbon. Based on this discussion, we can conclude that P species in the filly-charged PDC plays an important role in the storage Li species, so that it can have a higher capacity than that ofthe disordered carbon. At room temperature, because the rate of exchange or thermal motion ofthe Li species is too tit, we could not observe the equilibriumarrangetints ofthe lithium species. At low temperature, the rate ofexchange or thermel motion ofthe Li species become slow so that four interactions among the Li species, carbon and P species can be &und. Therecre, in order to develop a new anode material with a high capacity, it is very important to increase the Li species storage sites. References 1. B. Scrosati, Nature 373, 557( 1995). 2. Y. Matsumura, S. Wang and J. Mondori, Carbon 33, 1457 (1995). 3. J. R. Danh, T. Zheng, Y. Liu, J.S. Xue, Science 270, 590 (1995). 4. A. Komaru, H. Azuma, Y. Nishi, Japan patent, No. H.5-74457. 5. S. Wang, H. Matsui and Y. Matsumura, MRS Fall Procedings, Boston, 1997. 6. Y. Nakagawa, S. Wang, Y. Matsumura and C. Yamaguchi, Synth. Met. 85, 1363 (1994).
SyntheticMetals 103(1999) 2521-2522
‘I&NMR
study of lithium
charged phosphorus-doped
disordered
carbon
S. Wang, H. Matsui and Y. Matsumura Fundamental Research Laboratories, Osaka Gas Co., Ltd., Osaka, Japan, Abstract 7Li high resolution nuclear magnetic resonance (NMR) measurements at various temperatures in fully-charged phosphorusdoped disordered carbon material have been per&med. It was fbund that a broad line at 69.5 ppmand a relatively sharp line in near 15.0 ppm were observed at room temperature and fbur lines at 20.8, 74.6, 144.6 and 199.0 ppm were observed at low temperature. The mechanism ofLi storage in P-doped disordered carbon is discussed. Keywords:
Rechargeable batteries, Disordered carbon materials, NMR Mechanism of Li storage
Introduction Worldwide research is in progress to develop Li ion rechargeable batteries(LIB) using carbon as the anode[l]. Much of effort is focused on developing new carbon anodes with high discharge capacity. Disordered carbon materials have been highlighted as an anode material of the LIB because these materials could store rmch mOre lithiumwith a discharge capacity surpassing the theoretical capacity of a graphite anode[2,3]. Recently, it was Ibund that phosphorus or borondoped(P or B-doped) disordered carbon exhibited high reversible capacity conpared to the normal disordered carbon[4]. In order to develop a new carbon anode, it is necessary to understand the mechanism of Li storage in P or B-doped disordered carbon. Our previous work has shown that 7Li-NMR spectroscopy can provide meaningtil inforrration on the nature of the lithiumspecies in a fully-charged carbon anode and especially Ibr equilibrium arrangensents of the lithium species at low tenperature[5]. In this work, the Li-NMR measurements at various temperatures for the fully-charged P-doped disordered carbon (PDC) were perforrred. Based on the NMR results, a model ofLi storage in the PDC has been proposed.
electrochemical and NMR measurement conditions have been described elsewhere[6]. Results and Discussion The relationship between the P atom content in the disordered carbon and the capacity is shown in Fig.1. When P
,,t,""'.,""',",,"""'.'.,l 0 1
3
4
5
6
P atan content(Wt%)
Experimental Fig. 1 The relatkmhip
PDC material was prepared by the pyrolysis of a mixture of coal-tar pitch and PzOs at 1100 “C. This material has a small crystallite size (the c-axis dimension, Lc = 12 A) and a large interlayer distance between graphite layers (do02= 3.8 A). A random crystallite orientation was observed using the pole figure technique. The PDC material was mixed with 7 weight % polymer binder to make an electrode. A three-electrode electrochemical cell was asset&led with the PDC electrode as the working electrode, and a Li sheet and a Li chip were used as the counter and reference electrodes, respectively. A solution of 1M propylene carbonate/LiC104 was used as the electrolyte. The filly-charged PDC with a capacity of 667 tnAhg-‘, which corresponds to an insertion ofabout C3 3Li was obtained by the charging process at constant current and voltage. 7Li high resolution NMR measurements of the tilly-charged PDC at various temperatures have been perfimd. Details of the
2
betweenPatun con&H and capacity
content increases,the capacity becomeslinearly large. This meansthat the capacity of the PDC hasdirect relation with P atoms.In orderto study the mechanismofLi storagein PDC, the 7Li-NMR spectra of the fully-charged PDC with a reversiblecapacity of553 r&h/g at various temperatureswere measuredasshown in Fig. 2. At roomtemperature,the ti~llychargedPDC showsa broad line at 69.5 ppmand a relatively sharpline at 15.0 ppm With decreasingtemperature,the line width becamebroaderand somenew bandsappeared.When the temperaturewas at -100 “C, the Li-NMR spectra clearly showedfbur diErent signals.One band with strong strength appearedat 20.8 ppm and three broad bandsappearedat 74.6, 144.6 and 199.0ppm Theseresults indicate that the rate of exchangeor thermalmtion ofthe Li speciesslowed down on the NMR time scaleand the equilibrium arrangements of the lithium specieswereobservedat low temperature.Four kinds
0379-6779/99/$ - seefrontmatter0 1999ElsevierScience S.A. All rightsreserved. PII: SO379-6779(98)01088- 1
2522
S. Wang et al. I Synthetic
Metals
ofLi species with dierent electronic densities exist in the fully-charged PDC 69.5 ppm
72 wm
JL i”:, 23 wm
23 “C
0.0 “c
300
20
74ppm,
Lu’~““~“~‘,““‘iUC~‘~”
300
200
100
wm
0.0
-100
200
100
cl0
103 (1999)
2521-2522
Based on the NMR and ESCA results, a model of Li storage in the PDC is presented in Fig.3. Since the PDC has the almost the same amorphous structure and its carbons electronic states were not significantly a&ted by P-doping, it can be considered that the lithium species in the PDC are located between the graphitic layers (20.8 ppm), on the surhce of a crystallite and at the edge ofthe graphitic layers(74.6 and
-1eQ -200
20.8 ppm 74.6 ppm fl
300
-2m
200
DO
00
-1p
-200
Fig.2 NMR spectra of the fully-charged PDC at various temperatures
l
Li located between the graphitic layers
0
Li located bn the surface of-a crystallite
@ Li located at the edge of the graphitic layers In a previous study[5], we observed three kinds of interactions between Li species and carbon for the IIrllycharged disordered carbon by Li-NMR at low temperature. According to theoretical results, the band at 18.7 ppm is assigned to the lithium species located between the graphitic layers and the bands at 71.5 and 148.5 ppmmay correspond to the lithiumspecies located on the surfnce ofa crystallite and at the edge ofthe graphitic layers, respectively. Comparison with the Li-NMR results ofthe disordered carbon, we can find that the PDC has almost the same three bands at 20.8, 74.6 and 144.6 ppm, and a new band at 199.0 ppm The new band may play an important role in storing rmch mre Li species so that the PDC has higher capacity than that of the disordered carbon, ESCA spectra ofthe PDC and the disordered carbon were examined. The results are shown in the Table 1. It shows that the Ct, binding energy ofthe PDC is 284.1 eV, which is Table 1 ESCA results of the carbon materials P (wt%)
Binding Cls
Energy (eV) P2s
P-doped disordered carbon
4.8
284.1 129.2 130.8 132.3
Disordered carbon
0.0
284.0
similar to that of the disordered carbon. Ihe Pb binding energy in the PDC is in the range of 129.2-132.3 eV. The result shows that the carbon electronic states were not significantly a&ted by the P-doping. The phosphorus in the PDC are mixture states consisting of the (C6H5)xP and (C6H5)3PO types. At the present stage, we can not distinguish which kind of phosphorus states play the important role for Li storage in the PDC.
c] Li located in the near P atom Fig. 3 a model of Li storage in the P-doped disordered carbon 144.6 ppm). On the other hand, we assume that P species may make some cross-linking structures between neighboring crystallites or exist at the edge of the graphitic layers. P species tray store trore Li species so that a new band at 199.0 ppm was observed at low terrperature. This may be the &in reason why the PDC can have a higher capacity than that of the disordered carbon. Based on this discussion, we can conclude that P species in the filly-charged PDC plays an important role in the storage Li species, so that it can have a higher capacity than that ofthe disordered carbon. At room temperature, because the rate of exchange or thermal motion ofthe Li species is too tit, we could not observe the equilibriumarrangetints ofthe lithium species. At low temperature, the rate ofexchange or thermel motion ofthe Li species become slow so that four interactions among the Li species, carbon and P species can be &und. Therecre, in order to develop a new anode material with a high capacity, it is very important to increase the Li species storage sites. References 1. B. Scrosati, Nature 373, 557( 1995). 2. Y. Matsumura, S. Wang and J. Mondori, Carbon 33, 1457 (1995). 3. J. R. Danh, T. Zheng, Y. Liu, J.S. Xue, Science 270, 590 (1995). 4. A. Komaru, H. Azuma, Y. Nishi, Japan patent, No. H.5-74457. 5. S. Wang, H. Matsui and Y. Matsumura, MRS Fall Procedings, Boston, 1997. 6. Y. Nakagawa, S. Wang, Y. Matsumura and C. Yamaguchi, Synth. Met. 85, 1363 (1994).