- Email: [email protected]
The Electrochemical Properties of LiF-Ni Nanocomposite Thin Film
ACTA PHYSICO-CHIMICA SINICA Volume 22, Issue 9, September 2006 Online English edition of the Chinese language journal Cite this article as: Acta Phys. -Chim. Sin., 2006, 22(9), 1111−1115.
RESEARCH PAPER
The Electrochemical Properties of LiF-Ni Nanocomposite Thin Film Yongning Zhou,
Changliang Wu,
Hua Zhang,
Xiaojing Wu,
Zhengwen Fu*
Department of Chemistry and Laser Chemistry Institute, Department of Materials Science, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai 200433, P. R. China
Abstract:
Nanocomposite of LiF-Ni with a highly heterogeneous mixture was fabricated using pulsed laser deposition (PLD)
method, and it exhibited remarkable electrochemical activity. The electrochemical reaction of LiF-Ni nanocomposite thin-film electrode with Li was first investigated by the discharge/charge and cyclic voltammetry. The initial charge capacity of the LiF-Ni/Li cell was found to be 107 mAh·g−1. The process of releasing Li from the as-deposited LiF-Ni nanocomposite thin films was confirmed by the ex-situ high-resolution transmission electron microscopy and the selected area electron diffraction measurements. These results provided a direct experimental evidence to support the electrochemical decomposition of LiF driven by the transition metal Ni in the potential range from 1.0 V to 4.0 V. LiF-Ni nanocomposite electrodes could be a novel and appropriate candidate for Li-storage materials. Key Words:
LiF-Ni; Nanocomposite thin film; Pulsed Laser deposition; Electrochemical reaction
With the growing global demands for energy storage, it is necessary to search for and design advanced anode materials as a substitute for the presently used carbonaceous negative electrodes in view of the high storage densities of rechargeable lithium-ion batteries. Nowadays, considerable attention has been devoted for designing novel classes of Li insertion/deinsertion compounds or Li-alloying compounds and Li electrochemically driven reversible reaction compounds such as transition metal oxides, nitrides, and fluorides[1−10]. Previous results showed that the highly reversible electrochemical reactivity of 3d-metal oxides with Li at room temperature led to the interesting conclusion that the reversible formation and decomposition of Li2O could be driven by nanosized transition metals. The reversible electrochemical reaction of nanosized transition metal fluorides with Li was also found in the potential vs Li/Li+ range from 0.01 V to 4.5 V[1,3], which was similar to that of the reaction of metal oxides with Li. In fact, LiF and Li2O have been reported to be inactive in lithium-ion batteries[9], and previous reports[1−3] indicated that it
should be an essential condition for a heterogeneous mixture of transition metallic M (M= Fe, Co, Ni) and LiF or Li2O on an atomic or nanometer scale to establish their reactions. It is very difficult to fabricate the reactant system involving metallic M dispersed into LiF or Li2O in this rigorous condition. Tarascon et al.[1,2] and Maier et al.[9] made attempts to electrochemically decompose LiF and Li2O powder when mechanically milled with Ti and Co powders with a very low Li-extraction capacity. It is generally recognized that both LiF and transition metals are inactive electrochemical materials. Thus, until now, there has been no study on the decomposition of LiF or Li2O driven by transition metals at room temperature. Although it seems to be an effective way to electrochemically insert lithium into transition metal fluorides or oxides to fabricate LiF-M or Li2O-M systems, the physical and chemical states of LiF and M in LiF-M system could not be controlled, and only the nature of electrochemical reaction of LiF with M could be understood. There is no available report on the use of the nanocompo-
Received: March 3, 2006; Revised: April 26, 2006. * Corresponding author. Email: [email protected]; Tel: +8621-65642522. The project was supported by the National Natural Science Foundation of China (20203006). Copyright © 2006, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn
ZHOU Yongning et al. / Acta Physico-Chimica Sinica, 2006, 22(9): 1111−1115
sites consisting of metal M and LiF as Li electrode storage materials. Both LiF and transition metal M have higher chemical stability than metallic lithium. So, it has the special advantage of storing Li by using LiF-M nanocomposites. The synthesis of a novel Li-storage material using the nanocomposites consisting of metal cobalt and LiF as electrodes that have promising electrochemical activity has been reported in this study, and electrochemical behaviors of LiF-Ni nanocomposites also provide a direct evidence on the electrochemical decomposition of LiF by transition metals.
1
Experimental
Nanosized LiF-Ni thin films were prepared on the stainless steel substrates by pulsed laser deposition (PLD). The apparatus used for PLD has been described by Ref.[11]. A 355-nm laser beam from Q-Switched Nd:YAG laser (Spectra Physics GCR-150) with a pulsed repetition rate of 10 Hz and a pulsed width of 5 ns was focused onto the surface of the target. The incident angle between the laser beam and the target surface normal was 45º. The laser intensity was about 2 J·cm−2. The distance between target and substrate was 4 cm. The pressure of Ar ambient gas was controlled at 10 Pa using a needle valve during the deposition. Composite targets consisting LiF and Ni were obtained as targets using cold-pressed LiF powder (99%, Aldrich) and nickel metal powder of high purity (99.99%) with the molar ratio of 2:1. The substrate was kept at the room temperature. To avoid the oxidation of nanosized metal Ni in air as well as the oxidation of LiF-Ni nanocomposite solution in liquid electrolyte, a solid-state electrolyte of lithium phosphorous oxynitride (Lipon) thin film has been prepared by r.f. sputtering to coat onto the surface of the as-deposited LiF-Ni nanocomposite as a separator layer. Its thickness is less than 1 μm with a high Li-ion conductivity close to 1×10−6 S·cm−1. The weights of thin films were determined using electrobalance (BP 211D, Sartorius). High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) measurements were carried out by a Phillips CM200-FEG TEM at 160 kV accelerating voltage. For the electrochemical measurements, the cells were constructed using the as-deposited LiF-Ni nanocomposite thin film as a working electrode and two lithium sheets as a counter electrode and a reference electrode, respectively. The electrolyte consisted of 1 mol·L−1 LiPF6 in a nonaqueous solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 (Merck). The cells were assembled in an Ar-filled glove box. Charge-discharge measurements were performed at room temperature using a Land CT2001A battery test system. The cells were cycled between 1.0 and 4.0 V vs Li/Li+ at a current density of 28 μA·cm−2. The cyclic voltammogram (CV) tests were performed with a scanning rate of 0.5 mV·s−1 between 1.0 and 4.0 V on a CHI660A electro-
chemical working station (CHI Instruments, TN).
2
Results and discussion
Fig.1 shows the charging and discharging profiles of LiF-Ni nanocomposite used as thin-film electrode in the Li cells. The profiles exhibit its electrochemical activity. A voltage plateau from 2.0 V to 3.0 V is observed in the first charging process. This plateau can be ascribed to the decomposition of LiF and the fluorization/amorphization of Ni metal. The initial charge capacity of LiF-Ni cell is found to be 107 mAh·g−1and is associated with the release of Li. The initial discharge capacity is 613 mAh·g−1. The second charge shows highly reversible electrochemical reactivity. The second charge capacity is 362 mAh·g−1, and the discharge capacity is 313 mAh·g−1. Subsequently, charge/discharge processes still maintain similar discharge/charge curves as the second, the cell exhibits good stability, and the capacity fading is less than 0.5% per cycle. However, no charging takes place for pure LiF and Ni thin films fabricated by the same PLD method. The results show that LiF-Ni nanocomposite thin films exhibit high electrochemical activity with high specific capacities and good cycle performance. Fig.2 shows the first three cyclic voltammograms for the as-deposited film electrode of LiF-Ni between 1.0 and 4.0 V measured at a scan rate of 0.5 mV·s−1. There is one obscure anode peak at 2.0 V with respect to the initial charging process and one clear cathodic peak at 1.2 V with respect to the initial discharging process. The oxidizing peak at 2.0 V provides an evidence that the as-deposited LiF-Ni thin film can be charged to release Li. In the second cycle, the anodic peak moves to 2.2 V, and the cathodic peak moves to 1.6 V, which remain unchanged at subsequent cycles. This result is in good agreement with the discharge/charge measurements. The question lies in how to release Li from the as-deposited LiF-Ni nanocomposite thin film during the charging process. Evidences for the nature of charging process to release Li from the as-deposited LiF-Ni nanocomposite thin film could
Fig.1 The charge and discharge profiles of LiF-Ni thin films
ZHOU Yongning et al. / Acta Physico-Chimica Sinica, 2006, 22(9): 1111−1115
Fig.2 The first three CV profiles of LiF-Ni thin films
be found out by the ex-situ HRTEM and SAED measurements. For the ex-situ measurements, to avoid the exposure to oxygen or water, care must be taken in the handling of thin-film electrode at different stages including the as-deposited, charging to 4.0 V, and discharging to 1.0 V as they were carried out. The thin-film electrode after charging to 4.0 V or discharging to 1.0 V was disassembled in an Ar-filled dry box and was rinsed in anhydrous DMC to eliminate residual salts. The formation of LiF-Ni nanocomposite is confirmed by the ex-situ high-resolution TEM image. A typical TEM image from the as-deposited LiF-Ni nanocomposite thin film is shown in Fig.3(a), in which many crystalline particles that are few nanometres in size are observed. Clear lattice stripes can be seen in the HRTEM image. The SAED pattern in this phase shown in Fig.3(b) exhibits clear rings made up of discrete spots, indicating the nanosized polycrystalline nature of the as-deposited thin film. All d-spacings derived from SAED spectra could be assigned to LiF, Ni, and NiF2, and are shown in Table 1, indicating that the as-deposited thin film mainly consists of nanosized LiF, metallic Ni, and NiF2. During the fabrication of the as-deposited LiF-Ni nanocomposite thin film, part of lithium in laser plasma escapes due to light atomic weight and high vapour pressure; the reactive collision
Fig.3 Ex-situ high-resolution TEM image (a) and SAED (b) of as-deposited LiF-Ni thin film
between laser ablated cobalt metal and radical fluorine atoms or ions may occur and result in the formation of small quantity of transition metal halides. When the thin film electrode is charged to 4.0 V, it can also be characterized by HRTEM and SAED measurements. Many Moire stripes are observed in the HRTEM image shown in Fig.4(a). This means that the orientation of the crystal particles in the charged film is much different, and the crystal lattice is blurry, implying that the film is not well crystallized. SAED spectra in the region (Fig.4(b)) for the thin film charged to 4.0 V differ significantly from the as-deposited thin
Table 1 The comparison between experimental values and JCPDS standards of d-spacings (0.1 nm) As-depositeda
First discharging to 4.0 Vb
First discharging to 1.0 Vc
This workd
LiF-Fm3m
This work
Ni-P63/mmc
This work
NiF2-P42/mnm
This work
NiF2-P44/mnm
This work
Ni-P63/mmc
2.022
2.013 (200)
2.332
2.301 (100)
3.269
3.288 (110)
2.589
2.570 (101)
2.306
2.301 (101)
1.425
1.424 (220)
1.089
1.084 (004)
2.598
2.570 (101)
2.347
2.325 (200)
1.994
2.033 (101)
1.619
1.644 (220)
1.765
1.724 (211)
1.165
1.110 (201)
a=4.04±0.01
a=4.03
a=2.69
a=2.65
a=4.60±0.03
a=4.65
1.605
1.644 (220)
c=4.36
c=4.34
c=3.11±0.01
c=3.08
1.356
1.385 (301)
a=4.62±0.05
a=4.65
a=2.66±0.05
a=2.65
c=3.06±0.05
c=3.08
c=4.32±0.04
c=4.34
a) d-spacings of as-deposited film, b) d-spacings of the film first charging to 4.0 V, c) d-spacings of first discharging to 1.0 V, d) d-spacings derived from SAED analysis
ZHOU Yongning et al. / Acta Physico-Chimica Sinica, 2006, 22(9): 1111−1115
film. The rings associated with d-spacings agree well with those of NiF2, in which the lattice parameter data shown in Table 1 can be assigned to P42/mnm NiF2. In all the regions, SAED spectra from LiF and Ni metal were not found, indicating that NiF2 should be the charging product and nanosized Ni and LiF should take part in electrochemical reaction to release Li. The HRTEM image and SAED spectra in the region after thin-film electrode discharging to 1.0 V are shown in Fig.5(a, b), respectively. Two-dimensional lattice stripes can be observed clearly in the HRTEM image, indicating that the film after discharging is well crystallized. According to SAED analysis shown in Table 1, regular spots in the SAED pattern could be attributed to the diffractions from a single-crystal P63/mmc Ni metal. Analysis of the SAED pattern does not reveal any interplanar spacing assignable to LiF, indicating that LiF must be very highly dispersed. The dimensions of clusters of LiF below 1 nm would not be observable by SAED. As previously reported on the electrochemical reaction of transition metal fluorides[3], the electrochemical reaction of CoF2 with Li will be decomposed into Co metal after discharging. From Fig.1 and the result shown in Table 1, LiF is not observed after the first cycle. It could be highly dispersed in the
lattice of metal Ni. Obviously, the structures and chemical compositions of the films after the first cycle are different from as-deposited films. It may be used to explain the phenomenon that the plateau of subsequent charging and discharging profiles is different from that of the initial cycle. Combined with cyclic voltammograms shown in Fig.2, oxidation and reduction peaks at 2.2 V and 1.6 V corresponding to the reversible decomposition and formation of LiF. The electrochemical performance of CoF2 thin films fabricated by pulsed laser deposition has been reported[3] and a low reversible capacity of 250 mAh·g−1 and poor cycle may be due to its lower electrochemical activity, in which the reversible conversion reaction of LiF with Co may involve the decomposition and formation of CoF instead of CoF2. Compared with these previous samples, the as-deposited LiF-Ni nanocomposite thin film exhibits a higher specific capacity and good cycle performance. SAED data confirm the electrochemical conversion reaction of nanosized LiF and Ni into NiF2. So a heterogeneous mixture of transition metallic Ni and LiF as well as NiF2 on an atomic or nanometer scale may be a key to enhance their electrochemical activity for excellent electrochemical performance.
Fig.4 Ex-situ high-resolution TEM image (a) and SAED (b) of LiF-Ni thin film charging to 4.0 V
Fig.5 Ex-situ high-resolution TEM image (a) and SAED (b) of LiF-Ni thin film discharging to 1.0 V
ZHOU Yongning et al. / Acta Physico-Chimica Sinica, 2006, 22(9): 1111−1115
3
Conclusions
A nanocomposite LiF-Ni thin film exhibits electrochemical activity, and its electrochemical reaction mechanism with lithium is studied. The results have provided direct evidence to support the decomposition of LiF driven by transition metals. The microstructure and composition of the LiF-Ni nanocomposite with a highly heterogeneous mixture have been controlled by the PLD method on nanometer scale, and the LiF-Ni thin film presents an active electrochemical behavior. LiF-Ni thin films indicating electrochemical activity and good electrochemical performance would reopen new opportunities in this fascinating field of storing Li for Li-ion batteries.
References
Trascon, J. M. J. Power Sources, 1999, 81: 79 3 Fu, Z. W.; Li, C. L.; Liu, W. Y.; Ma, J.; Wang, Y.; Qin, Q. Z. J. Electrochem. Soc., 2005, 152: E50 4 Wang, Y.; Liu, W. Y.; Fu, Z. W. Acta Phys. -Chim. Sin., 2006, 22(1): 65 5 Badway, F.; Pereira, N.; Cosandey, F.; Amatucci, G. G. J. Electrochem. Soc., 2003, 150: A1209 6 Badway, F.; Pereira, N.; Cosandey, F.; Amatucci, G. G. J. Electrochem. Soc., 2003, 150: A1318 7 Obrovac, M. N.; Dunlap, R. A.; Sanderson, R. J.; Dahn, J. R. J. Electrochem. Soc., 2001, 148: A576 8 Obrovac, M. N.; Dahn, J. R. Electrochem. Solid-State Lett., 2002, 5(4): A70 9 Li, H.; Richter, G.; Maier, J. Adv. Mater., 2003, 15: 736 10 Leroux, F.; Ouvrard, G. R.; Power, W. P.; Nazar, L. F. Electro-
1 Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nature, 2000, 407: 496 2 Denis, S.; Baudrin, E.; Orsini, F.; Ouvrard, G.; Touboul, M.;
chem. Solid-State Lett., 1998, 1: 523 11 Xue, M. Z.; Fu, Z. W. Acta Phys. -Chim. Sin., 2005, 21(7): 707
RESEARCH PAPER
The Electrochemical Properties of LiF-Ni Nanocomposite Thin Film Yongning Zhou,
Changliang Wu,
Hua Zhang,
Xiaojing Wu,
Zhengwen Fu*
Department of Chemistry and Laser Chemistry Institute, Department of Materials Science, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai 200433, P. R. China
Abstract:
Nanocomposite of LiF-Ni with a highly heterogeneous mixture was fabricated using pulsed laser deposition (PLD)
method, and it exhibited remarkable electrochemical activity. The electrochemical reaction of LiF-Ni nanocomposite thin-film electrode with Li was first investigated by the discharge/charge and cyclic voltammetry. The initial charge capacity of the LiF-Ni/Li cell was found to be 107 mAh·g−1. The process of releasing Li from the as-deposited LiF-Ni nanocomposite thin films was confirmed by the ex-situ high-resolution transmission electron microscopy and the selected area electron diffraction measurements. These results provided a direct experimental evidence to support the electrochemical decomposition of LiF driven by the transition metal Ni in the potential range from 1.0 V to 4.0 V. LiF-Ni nanocomposite electrodes could be a novel and appropriate candidate for Li-storage materials. Key Words:
LiF-Ni; Nanocomposite thin film; Pulsed Laser deposition; Electrochemical reaction
With the growing global demands for energy storage, it is necessary to search for and design advanced anode materials as a substitute for the presently used carbonaceous negative electrodes in view of the high storage densities of rechargeable lithium-ion batteries. Nowadays, considerable attention has been devoted for designing novel classes of Li insertion/deinsertion compounds or Li-alloying compounds and Li electrochemically driven reversible reaction compounds such as transition metal oxides, nitrides, and fluorides[1−10]. Previous results showed that the highly reversible electrochemical reactivity of 3d-metal oxides with Li at room temperature led to the interesting conclusion that the reversible formation and decomposition of Li2O could be driven by nanosized transition metals. The reversible electrochemical reaction of nanosized transition metal fluorides with Li was also found in the potential vs Li/Li+ range from 0.01 V to 4.5 V[1,3], which was similar to that of the reaction of metal oxides with Li. In fact, LiF and Li2O have been reported to be inactive in lithium-ion batteries[9], and previous reports[1−3] indicated that it
should be an essential condition for a heterogeneous mixture of transition metallic M (M= Fe, Co, Ni) and LiF or Li2O on an atomic or nanometer scale to establish their reactions. It is very difficult to fabricate the reactant system involving metallic M dispersed into LiF or Li2O in this rigorous condition. Tarascon et al.[1,2] and Maier et al.[9] made attempts to electrochemically decompose LiF and Li2O powder when mechanically milled with Ti and Co powders with a very low Li-extraction capacity. It is generally recognized that both LiF and transition metals are inactive electrochemical materials. Thus, until now, there has been no study on the decomposition of LiF or Li2O driven by transition metals at room temperature. Although it seems to be an effective way to electrochemically insert lithium into transition metal fluorides or oxides to fabricate LiF-M or Li2O-M systems, the physical and chemical states of LiF and M in LiF-M system could not be controlled, and only the nature of electrochemical reaction of LiF with M could be understood. There is no available report on the use of the nanocompo-
Received: March 3, 2006; Revised: April 26, 2006. * Corresponding author. Email: [email protected]; Tel: +8621-65642522. The project was supported by the National Natural Science Foundation of China (20203006). Copyright © 2006, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn
ZHOU Yongning et al. / Acta Physico-Chimica Sinica, 2006, 22(9): 1111−1115
sites consisting of metal M and LiF as Li electrode storage materials. Both LiF and transition metal M have higher chemical stability than metallic lithium. So, it has the special advantage of storing Li by using LiF-M nanocomposites. The synthesis of a novel Li-storage material using the nanocomposites consisting of metal cobalt and LiF as electrodes that have promising electrochemical activity has been reported in this study, and electrochemical behaviors of LiF-Ni nanocomposites also provide a direct evidence on the electrochemical decomposition of LiF by transition metals.
1
Experimental
Nanosized LiF-Ni thin films were prepared on the stainless steel substrates by pulsed laser deposition (PLD). The apparatus used for PLD has been described by Ref.[11]. A 355-nm laser beam from Q-Switched Nd:YAG laser (Spectra Physics GCR-150) with a pulsed repetition rate of 10 Hz and a pulsed width of 5 ns was focused onto the surface of the target. The incident angle between the laser beam and the target surface normal was 45º. The laser intensity was about 2 J·cm−2. The distance between target and substrate was 4 cm. The pressure of Ar ambient gas was controlled at 10 Pa using a needle valve during the deposition. Composite targets consisting LiF and Ni were obtained as targets using cold-pressed LiF powder (99%, Aldrich) and nickel metal powder of high purity (99.99%) with the molar ratio of 2:1. The substrate was kept at the room temperature. To avoid the oxidation of nanosized metal Ni in air as well as the oxidation of LiF-Ni nanocomposite solution in liquid electrolyte, a solid-state electrolyte of lithium phosphorous oxynitride (Lipon) thin film has been prepared by r.f. sputtering to coat onto the surface of the as-deposited LiF-Ni nanocomposite as a separator layer. Its thickness is less than 1 μm with a high Li-ion conductivity close to 1×10−6 S·cm−1. The weights of thin films were determined using electrobalance (BP 211D, Sartorius). High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) measurements were carried out by a Phillips CM200-FEG TEM at 160 kV accelerating voltage. For the electrochemical measurements, the cells were constructed using the as-deposited LiF-Ni nanocomposite thin film as a working electrode and two lithium sheets as a counter electrode and a reference electrode, respectively. The electrolyte consisted of 1 mol·L−1 LiPF6 in a nonaqueous solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 (Merck). The cells were assembled in an Ar-filled glove box. Charge-discharge measurements were performed at room temperature using a Land CT2001A battery test system. The cells were cycled between 1.0 and 4.0 V vs Li/Li+ at a current density of 28 μA·cm−2. The cyclic voltammogram (CV) tests were performed with a scanning rate of 0.5 mV·s−1 between 1.0 and 4.0 V on a CHI660A electro-
chemical working station (CHI Instruments, TN).
2
Results and discussion
Fig.1 shows the charging and discharging profiles of LiF-Ni nanocomposite used as thin-film electrode in the Li cells. The profiles exhibit its electrochemical activity. A voltage plateau from 2.0 V to 3.0 V is observed in the first charging process. This plateau can be ascribed to the decomposition of LiF and the fluorization/amorphization of Ni metal. The initial charge capacity of LiF-Ni cell is found to be 107 mAh·g−1and is associated with the release of Li. The initial discharge capacity is 613 mAh·g−1. The second charge shows highly reversible electrochemical reactivity. The second charge capacity is 362 mAh·g−1, and the discharge capacity is 313 mAh·g−1. Subsequently, charge/discharge processes still maintain similar discharge/charge curves as the second, the cell exhibits good stability, and the capacity fading is less than 0.5% per cycle. However, no charging takes place for pure LiF and Ni thin films fabricated by the same PLD method. The results show that LiF-Ni nanocomposite thin films exhibit high electrochemical activity with high specific capacities and good cycle performance. Fig.2 shows the first three cyclic voltammograms for the as-deposited film electrode of LiF-Ni between 1.0 and 4.0 V measured at a scan rate of 0.5 mV·s−1. There is one obscure anode peak at 2.0 V with respect to the initial charging process and one clear cathodic peak at 1.2 V with respect to the initial discharging process. The oxidizing peak at 2.0 V provides an evidence that the as-deposited LiF-Ni thin film can be charged to release Li. In the second cycle, the anodic peak moves to 2.2 V, and the cathodic peak moves to 1.6 V, which remain unchanged at subsequent cycles. This result is in good agreement with the discharge/charge measurements. The question lies in how to release Li from the as-deposited LiF-Ni nanocomposite thin film during the charging process. Evidences for the nature of charging process to release Li from the as-deposited LiF-Ni nanocomposite thin film could
Fig.1 The charge and discharge profiles of LiF-Ni thin films
ZHOU Yongning et al. / Acta Physico-Chimica Sinica, 2006, 22(9): 1111−1115
Fig.2 The first three CV profiles of LiF-Ni thin films
be found out by the ex-situ HRTEM and SAED measurements. For the ex-situ measurements, to avoid the exposure to oxygen or water, care must be taken in the handling of thin-film electrode at different stages including the as-deposited, charging to 4.0 V, and discharging to 1.0 V as they were carried out. The thin-film electrode after charging to 4.0 V or discharging to 1.0 V was disassembled in an Ar-filled dry box and was rinsed in anhydrous DMC to eliminate residual salts. The formation of LiF-Ni nanocomposite is confirmed by the ex-situ high-resolution TEM image. A typical TEM image from the as-deposited LiF-Ni nanocomposite thin film is shown in Fig.3(a), in which many crystalline particles that are few nanometres in size are observed. Clear lattice stripes can be seen in the HRTEM image. The SAED pattern in this phase shown in Fig.3(b) exhibits clear rings made up of discrete spots, indicating the nanosized polycrystalline nature of the as-deposited thin film. All d-spacings derived from SAED spectra could be assigned to LiF, Ni, and NiF2, and are shown in Table 1, indicating that the as-deposited thin film mainly consists of nanosized LiF, metallic Ni, and NiF2. During the fabrication of the as-deposited LiF-Ni nanocomposite thin film, part of lithium in laser plasma escapes due to light atomic weight and high vapour pressure; the reactive collision
Fig.3 Ex-situ high-resolution TEM image (a) and SAED (b) of as-deposited LiF-Ni thin film
between laser ablated cobalt metal and radical fluorine atoms or ions may occur and result in the formation of small quantity of transition metal halides. When the thin film electrode is charged to 4.0 V, it can also be characterized by HRTEM and SAED measurements. Many Moire stripes are observed in the HRTEM image shown in Fig.4(a). This means that the orientation of the crystal particles in the charged film is much different, and the crystal lattice is blurry, implying that the film is not well crystallized. SAED spectra in the region (Fig.4(b)) for the thin film charged to 4.0 V differ significantly from the as-deposited thin
Table 1 The comparison between experimental values and JCPDS standards of d-spacings (0.1 nm) As-depositeda
First discharging to 4.0 Vb
First discharging to 1.0 Vc
This workd
LiF-Fm3m
This work
Ni-P63/mmc
This work
NiF2-P42/mnm
This work
NiF2-P44/mnm
This work
Ni-P63/mmc
2.022
2.013 (200)
2.332
2.301 (100)
3.269
3.288 (110)
2.589
2.570 (101)
2.306
2.301 (101)
1.425
1.424 (220)
1.089
1.084 (004)
2.598
2.570 (101)
2.347
2.325 (200)
1.994
2.033 (101)
1.619
1.644 (220)
1.765
1.724 (211)
1.165
1.110 (201)
a=4.04±0.01
a=4.03
a=2.69
a=2.65
a=4.60±0.03
a=4.65
1.605
1.644 (220)
c=4.36
c=4.34
c=3.11±0.01
c=3.08
1.356
1.385 (301)
a=4.62±0.05
a=4.65
a=2.66±0.05
a=2.65
c=3.06±0.05
c=3.08
c=4.32±0.04
c=4.34
a) d-spacings of as-deposited film, b) d-spacings of the film first charging to 4.0 V, c) d-spacings of first discharging to 1.0 V, d) d-spacings derived from SAED analysis
ZHOU Yongning et al. / Acta Physico-Chimica Sinica, 2006, 22(9): 1111−1115
film. The rings associated with d-spacings agree well with those of NiF2, in which the lattice parameter data shown in Table 1 can be assigned to P42/mnm NiF2. In all the regions, SAED spectra from LiF and Ni metal were not found, indicating that NiF2 should be the charging product and nanosized Ni and LiF should take part in electrochemical reaction to release Li. The HRTEM image and SAED spectra in the region after thin-film electrode discharging to 1.0 V are shown in Fig.5(a, b), respectively. Two-dimensional lattice stripes can be observed clearly in the HRTEM image, indicating that the film after discharging is well crystallized. According to SAED analysis shown in Table 1, regular spots in the SAED pattern could be attributed to the diffractions from a single-crystal P63/mmc Ni metal. Analysis of the SAED pattern does not reveal any interplanar spacing assignable to LiF, indicating that LiF must be very highly dispersed. The dimensions of clusters of LiF below 1 nm would not be observable by SAED. As previously reported on the electrochemical reaction of transition metal fluorides[3], the electrochemical reaction of CoF2 with Li will be decomposed into Co metal after discharging. From Fig.1 and the result shown in Table 1, LiF is not observed after the first cycle. It could be highly dispersed in the
lattice of metal Ni. Obviously, the structures and chemical compositions of the films after the first cycle are different from as-deposited films. It may be used to explain the phenomenon that the plateau of subsequent charging and discharging profiles is different from that of the initial cycle. Combined with cyclic voltammograms shown in Fig.2, oxidation and reduction peaks at 2.2 V and 1.6 V corresponding to the reversible decomposition and formation of LiF. The electrochemical performance of CoF2 thin films fabricated by pulsed laser deposition has been reported[3] and a low reversible capacity of 250 mAh·g−1 and poor cycle may be due to its lower electrochemical activity, in which the reversible conversion reaction of LiF with Co may involve the decomposition and formation of CoF instead of CoF2. Compared with these previous samples, the as-deposited LiF-Ni nanocomposite thin film exhibits a higher specific capacity and good cycle performance. SAED data confirm the electrochemical conversion reaction of nanosized LiF and Ni into NiF2. So a heterogeneous mixture of transition metallic Ni and LiF as well as NiF2 on an atomic or nanometer scale may be a key to enhance their electrochemical activity for excellent electrochemical performance.
Fig.4 Ex-situ high-resolution TEM image (a) and SAED (b) of LiF-Ni thin film charging to 4.0 V
Fig.5 Ex-situ high-resolution TEM image (a) and SAED (b) of LiF-Ni thin film discharging to 1.0 V
ZHOU Yongning et al. / Acta Physico-Chimica Sinica, 2006, 22(9): 1111−1115
3
Conclusions
A nanocomposite LiF-Ni thin film exhibits electrochemical activity, and its electrochemical reaction mechanism with lithium is studied. The results have provided direct evidence to support the decomposition of LiF driven by transition metals. The microstructure and composition of the LiF-Ni nanocomposite with a highly heterogeneous mixture have been controlled by the PLD method on nanometer scale, and the LiF-Ni thin film presents an active electrochemical behavior. LiF-Ni thin films indicating electrochemical activity and good electrochemical performance would reopen new opportunities in this fascinating field of storing Li for Li-ion batteries.
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