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Electrodeposition of lithium film under dynamic conditions and its application in all-solid-state rechargeable lithium battery
Solid State Ionics 176 (2005) 1051 – 1055 www.elsevier.com/locate/ssi
Electrodeposition of lithium film under dynamic conditions and its application in all-solid-state rechargeable lithium battery Xuelin Yang, Zhaoyin WenT, Xiujian Zhu, Shahua Huang Shanghai Institute of Ceramics, Chinese Academy of Sciences, Graduate School, Chinese Academy of Sciences, 1295 Ding xi Road, Shanghai 200050, PR China Received 28 March 2003; received in revised form 27 January 2005; accepted 6 February 2005
Abstract The morphology of lithium electrodeposited under dynamic conditions from LiPF6/(EC+DMC) solution was observed. The dendritic lithium was effectively suppressed at the current density as high as 2.0 mA cm2 under dynamic conditions compared to the electrodeposition at 0.5 mA cm2 under static conditions. Impedance measurements were employed to monitor the formation and breaking of surface layers during electrodeposition. The experimental results showed that the interface stability of Li/P(EO)20Li(CF3SO2)2N–10 wt.% gLiAlO2 using electrodeposited lithium film was much better than that using commercial lithium foil, which demonstrated that the electrodeposited lithium was a preferable candidate as anode for all-solid-state lithium battery. D 2005 Elsevier B.V. All rights reserved. PACS: 81.15.P; 84.60.D Keywords: Electrodeposition; Dynamic; All-solid-state; Lithium; SEI
1. Introduction All-solid-state rechargeable lithium battery has been considered as the most favorable candidate for use in large-scale battery systems [1]. Lithium metal anode is usually used with a thickness corresponding to three to four times overcapacity [2], and commercial lithium is not suitable for such kind of battery because it is difficult to obtain lithium foil with thickness in the order of few microns; in addition, the redundant lithium will also pose a threat to battery security. Several studies have been reported on all-solid-state rechargeable batteries with lithium foil deposited by rf magnetron sputtering [3] or vacuum evaporation in pressure below 2.0105 Pa [4], but it is unpractical to fabricate all-solid-state lithium rechargeable batteries on a large scale with such kind of lithium. Electrodeposition has numerous advantages over the above physical deposition, such as low cost, low operating temperature and strong adherence to the substrate. The T Corresponding author. Tel.: +86 21 52411704; fax: +86 21 52413903. E-mail address: [email protected] (Z. Wen). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.02.009
commercial success of lithium secondary battery using metal lithium metal anode and liquid electrolyte is still not realized until now because of possible formation of dendritic lithium, which causes capacity loss and short-circuiting during a charge/discharge cycling process. It is significant to obtain dendrite-free electrodeposited lithium and to improve its surface performance. Over the past several decades, many attempts have been contributed, typically adding inorganic compounds such as HF [5], CO2 [6] or organic compounds such as vinylene carbonate [7], PEO [8] and trimethylsilylacetylene [9] to the electrolyte to modify the lithium surface or agitating molten lithium in hydrocarbon oil mixture with 0.3 wt.% anhydrous CO2 to slow down the reaction with moisture [10]. The surface film of the electrodeposited lithium works as an ionically conductive interface and is therefore defined as solid electrolyte interface (SEI). In the electrodeposition of lithium, the presence of SEI influences significantly the morphology of lithium. R. Mogi et al. revealed that the formation of SEI accelerated at elevated temperatures and the passivation of lithium surface is very rapid with the resulted surface film easily self-repairable when being
1052
X. Yang et al. / Solid State Ionics 176 (2005) 1051–1055
damaged during cycling tests [11]. It was noticeable that the agglomeration of lithium was observed at the electrodeposition current density above 1.0 mA cm2 and was intensified at increased current density even some agents had been added into the electrolyte [12]. In this work, electrodeposition of dendrite-free lithium under dynamic conditions has been developed without any additive. SEI formation and stability of the SEI in a symmetric Li/P(EO)20Li(CF3SO2)2N–10 wt.% g-LiAlO2/ Li cell were also investigated.
2. Experimental 2.1. Electrodeposition of lithium Before the electrodeposition, the copper substrate with diameter of 1.4 cm and thickness of 0.5 Am was polished to a mirror surface, washed with distilled water and acetone in an ultrasonic bath, and was dried for 6 h under vacuum at room temperature. Lithium foil 1.5 cm2.0 cm was used as counterelectrode. The electrolyte was ethylene carbonate (EC)+dimethyl carbonate (DMC) solvent (volume ratio of 1:1) containing 1 mol dm3 LiPF6 as the solute. The water content in the electrolytes was analyzed to be less than 20 ppm by Karl-Fischer titration. A galvanostatic method was used for the electrodeposition of lithium under magnetic stirring at current densities of 0.5, 1.0, 1.5 and 2.0 mA cm2, respectively. After deposition, the lithium film was washed with pure propylene carbonate (PC) to remove residual electrolyte and dried in an argon dry box at room temperature. The morphology of electrodeposited lithium was observed with a field emission scanning electron microscopy (JSM-6700F JEOL). The impedance spectra before and after electrodeposition were monitored by means of SI Solartron 1260 Impedance Analyzer in the frequency range from 0.1 Hz to 100 kHz within an alternating amplitude of 10 mV. 2.2. Deposition/dissolution cycling test A deposition and dissolution cycling of lithium was performed under the same conditions as those for the electrodeposition as described above. A well-defined amount of lithium was initially deposited on the working electrode at 0.5 and 1.0 mA cm2; subsequently, it was dissolved at the same current density until the dissolution potential reached +1 V vs. the counterelectrode. The efficiency of a cycle was defined as the quotient of the dissolved and deposited amounts of lithium Eeff ¼
Qdissolved idissolved tdissolved ¼ Qdeposited ideposited tdeposited
where i dissolved is the current density during dissolution and i deposited is that during deposition; t dissolved and t deposited are the corresponding dissolution and deposition times.
2.3. Evolution of SEI between Li and solid composite polymer electrolyte The composite polymer electrolyte of P(EO)20Li(CF3SO2)2N–10 wt.% g-LiAlO2 was prepared in an argon-filled glove box as described in [13]. The mixture of PEO (M w=9105) and Li(CF3SO2)2N was dissolved in acetonitrile with g-LiAlO2 powders added. The suspension of the mixture was cast onto a Teflon sheet and then heated to evaporate solvent at 50 8C. After evaporation, the electrolyte film was dried under vacuum at 80 8C for 10 h to remove residual solvent. The time dependence of the impedance spectra of two symmetric cells, Li/P(EO)20Li(CF3SO2)2N–10 wt.% gLiAlO2/Li, respectively, using electrodeposited lithium and commercial lithium foil, was measured to evaluate stability of SEI between lithium and the composite polymer electrolyte.
3. Results and discussion 3.1. Morphology of electrodeposited lithium When lithium is electrodeposited on a copper substrate, chemical reactions between the lithium surface and electrolyte components would happen and new kind of surface film (SEI) would be formed. However, the impurities in the electrolyte, such as H2O, CO2, HF, etc., could also greatly accelerate the chemical reaction. So, the competition between the electrodeposition rate, which is controlled by current density and the chemical reaction rate accelerated by impurities, would determine the morphology of electrodeposited lithium. If the chemical reactions are too slow to modify the lithium surface, lithium deposits in a dendrite form. But if the chemical reactions proceed more quickly than the electrodeposition of lithium, then lithium film with a uniform morphology will be obtained. Since the concentration of impurities in the electrolyte will decrease with an increase of deposited lithium, the formation of lithium dendrite is incident to the late stage of the electrodeposition even at a current density as low as of 0.5 mA cm2. Therefore, we normally suppress dendrite formation by adding some additives in the electrolyte; however, we have to maintain a constant concentration of any additive in the electrolyte. Fig. 1(a) and (b) show the SEI micrography of lithium deposited in 1 mol cm3 LiPF6/EC+DMC (1:1) at a current density of 0.5 mA cm2 under static conditions. As shown, the film was made up of agglomerates of dendritic lithium. It was reasonable to presume that overgrown agglomerates of lithium dendrite would desquamate from the substrate after long period of deposition. Suppression of lithium dendrite was successfully realized by a simple magnetic stirring during the electrodeposition. Fig. 1c shows the morphology of lithium film deposited at the current density as high as 2.0 mA cm2. As shown, homogeneous lithium particles with
X. Yang et al. / Solid State Ionics 176 (2005) 1051–1055
1053
250 Static Dynamic
Ri / Ohm
200 150 100 50 0 0
5
10
15
20
25
Q / C cm-2 Fig. 2. Interface resistance vs. the quantity of electricity in 1.0 mol dm3 LiPF6/(EC+DMC) at 1.0 mA cm2.
The result of electrodeposition under static conditions is also given in Fig. 2 for comparison with the dynamic one. The interface resistance experienced the original two stages similar to that under dynamic conditions; however, a higher SEI resistance was obtained for the static one. A rapid decrease of the interface resistance induced by the breaking of the SEI layer after the total charge above 7 C cm2 was found, which could be attributed to the cracks of SEI from which lithium dendrite can grow. The effect of magnetic stirring on the morphology of deposited lithium may be ascribed to the formation of a compact SEI and the suppression of preferred orientation of lithium. 3.2. Lithium cycling efficiency It is well known that SEI is derived from the reactions between lithium surface and electrolyte components, especially the impurities, and the chemical and physical properties of such a film govern the deposition/dissolution performance of the electrodeposited lithium [14]. Fig. 3 Fig. 1. Morphology of lithium films electrodeposited (10 C cm2) on copper substrate in the solution of 1.0 mol dm3 LiPF6/(EC+DMC) under the conditions: (a, b) 0.5 mA cm2, static; (c) 2.0 mA cm2, dynamic.
100
90
Efficiency / %
hemispherical shape were obtained. No any dendritic lithium was found. The evolution of interface resistance (R i) during the electrodeposition over the quantity of electricity under dynamic conditions was shown in Fig. 2. Three main regions can be distinguished within the whole deposition range. As seen, the R i decreased at the original stage from 0 to 5C cm2, then increased within 5 to 13 C cm2 and remained constant afterwards. The first region was ascribed to the reactions between electrolyte and native film of counterelectrode, whereas the second one was to the SEI growth of working electrode. After the quantity of electricity approached 13 C cm2, the deposited lithium would be separated completely from the electrolyte by SEI, which remained stable with an increase of deposited charge.
80 0.5mA / cm2 1.0mA / cm2
70
60
50 10
20
30
40
50
Cycle Number Fig. 3. Deposition–dissolution cycling efficiency of lithium in LiPF6/ (EC+DMC) at 1 mA cm2 under dynamic conditions.
X. Yang et al. / Solid State Ionics 176 (2005) 1051–1055
shows the deposition/dissolution cycling efficiencies of electrodeposited lithium on the copper substrate in LiPF6/ EC+DMC (1:1) at 0.5 and 1.0 mA cm2 with magnetic stirring. The cycling efficiency at 0.5 mA cm2 was as high as 93.5% since the 38th cycle except for an initial poor cycling efficiency due to the formation of SEI. When current density was further improved to 1.0 mA cm2, cycling efficiency decreases slightly, but it is still much higher than that cycled under static conditions. In the present systems, one may presume that magnetic stirring has an impact influence on the formation of SEI within the initial cycles, which would effectively separate deposited lithium from electrolyte and ensure an improved performance of the subsequent cycling.
The evolution of the impedance spectra of two cells, which were formed by sandwiching the polymer electrolyte between symmetric lithium electrodes respectively using the electrodeposited lithium film and commercial lithium foil, was shown respectively in Fig. 4a and b. The fit of
-250 1day 2days 3days 4days 5days 6days 7days
Ri Zw
-200
Z'' / Ohm
Cd
-150 -100 -50 0 0
50
100
150
200
250
300
Z' / Ohm
b
-500 1day 2days 3days 4days 5days 6days 7days 8days
Z'' / Ohm
-400 -300 -200
0
100
200
300
400
500
400 300 200 100 0
0
1
2
3
4
5
6
7
8
Time / days
impedance data was performed with the help of an appropriate equivalent electronic circuit as shown in the inset of Fig. 4a in which the total interface impedance is consisted of a typical Warburg impedance element (Z w) and a parallel combination of an interfacial resistance (R i) and double-layer capacitance (C d) [15]. Fig. 5 demonstrated the dependence of R i value on the aging time of both the cells. As seen, in a cell with electrodeposited lithium film as electrode, R i increased somewhat and turned to be stable in 3 days, which indicated that the reactions between composite polymer electrolyte and electrodeposited lithium under dynamic conditions reached an equilibrium state quickly. While the cell using commercial lithium foil displayed comparable initial resistance as that with electrodeposited lithium film, but it increased rapidly and did not reach equilibrium in 6 days; meanwhile, the final R i of it was much greater than that of electrodeposited lithium. Therefore, lithium film electrodeposited under dynamic conditions would be a preferable candidate as anode for all-solid-state lithium/polymer battery because of its favorable effect to suppress reactions between lithium and composite polymer electrolyte.
4. Conclusions
-100 0
Deposited Lithium Commercial Lithium
500
Fig. 5. Time dependence of the interfacial resistance derived from impedance analysis of the cells using electrodeposited lithium film and commercial lithium foil as electrodes.
3.3. Interface stability of lithium with solid electrolyte
a
600
Ri / ohm
1054
600
Z' / Ohm Fig. 4. Evolution of impedance response of Li/P(EO)20Li(CF3SO2)2N–10 wt.% g-LiAlO2/Li cells at 50 8C: (a) electrodeposited lithium and (b) commercial lithium.
An electrodeposition of lithium film under dynamic conditions was performed to modify the lithium surface. The dendrite formation of lithium was suppressed at a current density as high as 2.0 mA cm2 and deposition/ dissolution cycling efficiency was also improved under dynamic conditions. The electrodeposited lithium film was introduced to Li/P(EO)20Li(CF3SO2)2N–10 wt.% g-LiAlO2/ Li symmetric cell to investigate interface stability; the impedance spectra based on the electrodeposited lithium and a composite polymer electrolyte showed a small and stable interface resistance.
X. Yang et al. / Solid State Ionics 176 (2005) 1051–1055
Acknowledgments This work was financially supported by key project of Natural Science Foundation of China (NSFC) No. 20333040.
References [1] T. Iwahori, Y. Ozaki, A. Funahashi, H. Momose, I. Mitsuishi, S. Shiraga, S. Yoshitake, H. Awata, Journal of Power Sources 81–82 (1999) 872. [2] J.B. Bates, N.J. Dudney, B. Neudecker, A. Ueda, C.D. Evans, Solid State Ionics 135 (2000) 33. [3] S.D. Jones, J.R. Akridge, Solid State Ionics 86–88 (1996) 1291. [4] Y. Furuyama, K. Ito, S. Dohi, A. Taniike, A. Kitamura, Journal of Nuclear Materials 313–316 (2003) 288. [5] K. Kanamura, S. Shiraishi, Z. Takehara, Journal of Fluorine Chemistry 87 (1998) 235.
1055
[6] T. Osaka, T. Momma, Y. Matsumoto, Y. Uchido, Journal of the Electrochemical Society 144 (1997) 1709. [7] H. Ota, K. Shima, M. Ue, J. Yamaki, Electrochimica Acta 49 (2004) 565. [8] C. Liebenow, K. Luhder, Journal of Applied Electrochemistry 26 (1996) 689. [9] J.S. Skamoto, F. Wudl, B. Dunn, Solid State Ionics 144 (2001) 295. [10] U.S. Patent 5,776,369. 1998. [11] R. Mogi, M. Inaba, Y. Iriyama, T. Abe, Z. Ogumi, Journal of Power Sources 108 (2002) 163. [12] K. Kanamura, S. Shiraishi, Z. Takehara, Journal of the Electrochemical Society 143 (1997) 2187. [13] X.J. Zhu, Z.Y. Wen, Z.H. Gu, Z.X. Lin, Journal of Power Sources 139 (2005) 269. [14] M. Ishikawa, Y. Takaki, M. Morita, Y. Matsuda, Journal of the Electrochemical Society 144 (1997) L91. [15] G.B. Appetecchi, S. Passerini, Journal of the Electrochemical Society 149 (2002) A891.
Electrodeposition of lithium film under dynamic conditions and its application in all-solid-state rechargeable lithium battery Xuelin Yang, Zhaoyin WenT, Xiujian Zhu, Shahua Huang Shanghai Institute of Ceramics, Chinese Academy of Sciences, Graduate School, Chinese Academy of Sciences, 1295 Ding xi Road, Shanghai 200050, PR China Received 28 March 2003; received in revised form 27 January 2005; accepted 6 February 2005
Abstract The morphology of lithium electrodeposited under dynamic conditions from LiPF6/(EC+DMC) solution was observed. The dendritic lithium was effectively suppressed at the current density as high as 2.0 mA cm2 under dynamic conditions compared to the electrodeposition at 0.5 mA cm2 under static conditions. Impedance measurements were employed to monitor the formation and breaking of surface layers during electrodeposition. The experimental results showed that the interface stability of Li/P(EO)20Li(CF3SO2)2N–10 wt.% gLiAlO2 using electrodeposited lithium film was much better than that using commercial lithium foil, which demonstrated that the electrodeposited lithium was a preferable candidate as anode for all-solid-state lithium battery. D 2005 Elsevier B.V. All rights reserved. PACS: 81.15.P; 84.60.D Keywords: Electrodeposition; Dynamic; All-solid-state; Lithium; SEI
1. Introduction All-solid-state rechargeable lithium battery has been considered as the most favorable candidate for use in large-scale battery systems [1]. Lithium metal anode is usually used with a thickness corresponding to three to four times overcapacity [2], and commercial lithium is not suitable for such kind of battery because it is difficult to obtain lithium foil with thickness in the order of few microns; in addition, the redundant lithium will also pose a threat to battery security. Several studies have been reported on all-solid-state rechargeable batteries with lithium foil deposited by rf magnetron sputtering [3] or vacuum evaporation in pressure below 2.0105 Pa [4], but it is unpractical to fabricate all-solid-state lithium rechargeable batteries on a large scale with such kind of lithium. Electrodeposition has numerous advantages over the above physical deposition, such as low cost, low operating temperature and strong adherence to the substrate. The T Corresponding author. Tel.: +86 21 52411704; fax: +86 21 52413903. E-mail address: [email protected] (Z. Wen). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.02.009
commercial success of lithium secondary battery using metal lithium metal anode and liquid electrolyte is still not realized until now because of possible formation of dendritic lithium, which causes capacity loss and short-circuiting during a charge/discharge cycling process. It is significant to obtain dendrite-free electrodeposited lithium and to improve its surface performance. Over the past several decades, many attempts have been contributed, typically adding inorganic compounds such as HF [5], CO2 [6] or organic compounds such as vinylene carbonate [7], PEO [8] and trimethylsilylacetylene [9] to the electrolyte to modify the lithium surface or agitating molten lithium in hydrocarbon oil mixture with 0.3 wt.% anhydrous CO2 to slow down the reaction with moisture [10]. The surface film of the electrodeposited lithium works as an ionically conductive interface and is therefore defined as solid electrolyte interface (SEI). In the electrodeposition of lithium, the presence of SEI influences significantly the morphology of lithium. R. Mogi et al. revealed that the formation of SEI accelerated at elevated temperatures and the passivation of lithium surface is very rapid with the resulted surface film easily self-repairable when being
1052
X. Yang et al. / Solid State Ionics 176 (2005) 1051–1055
damaged during cycling tests [11]. It was noticeable that the agglomeration of lithium was observed at the electrodeposition current density above 1.0 mA cm2 and was intensified at increased current density even some agents had been added into the electrolyte [12]. In this work, electrodeposition of dendrite-free lithium under dynamic conditions has been developed without any additive. SEI formation and stability of the SEI in a symmetric Li/P(EO)20Li(CF3SO2)2N–10 wt.% g-LiAlO2/ Li cell were also investigated.
2. Experimental 2.1. Electrodeposition of lithium Before the electrodeposition, the copper substrate with diameter of 1.4 cm and thickness of 0.5 Am was polished to a mirror surface, washed with distilled water and acetone in an ultrasonic bath, and was dried for 6 h under vacuum at room temperature. Lithium foil 1.5 cm2.0 cm was used as counterelectrode. The electrolyte was ethylene carbonate (EC)+dimethyl carbonate (DMC) solvent (volume ratio of 1:1) containing 1 mol dm3 LiPF6 as the solute. The water content in the electrolytes was analyzed to be less than 20 ppm by Karl-Fischer titration. A galvanostatic method was used for the electrodeposition of lithium under magnetic stirring at current densities of 0.5, 1.0, 1.5 and 2.0 mA cm2, respectively. After deposition, the lithium film was washed with pure propylene carbonate (PC) to remove residual electrolyte and dried in an argon dry box at room temperature. The morphology of electrodeposited lithium was observed with a field emission scanning electron microscopy (JSM-6700F JEOL). The impedance spectra before and after electrodeposition were monitored by means of SI Solartron 1260 Impedance Analyzer in the frequency range from 0.1 Hz to 100 kHz within an alternating amplitude of 10 mV. 2.2. Deposition/dissolution cycling test A deposition and dissolution cycling of lithium was performed under the same conditions as those for the electrodeposition as described above. A well-defined amount of lithium was initially deposited on the working electrode at 0.5 and 1.0 mA cm2; subsequently, it was dissolved at the same current density until the dissolution potential reached +1 V vs. the counterelectrode. The efficiency of a cycle was defined as the quotient of the dissolved and deposited amounts of lithium Eeff ¼
Qdissolved idissolved tdissolved ¼ Qdeposited ideposited tdeposited
where i dissolved is the current density during dissolution and i deposited is that during deposition; t dissolved and t deposited are the corresponding dissolution and deposition times.
2.3. Evolution of SEI between Li and solid composite polymer electrolyte The composite polymer electrolyte of P(EO)20Li(CF3SO2)2N–10 wt.% g-LiAlO2 was prepared in an argon-filled glove box as described in [13]. The mixture of PEO (M w=9105) and Li(CF3SO2)2N was dissolved in acetonitrile with g-LiAlO2 powders added. The suspension of the mixture was cast onto a Teflon sheet and then heated to evaporate solvent at 50 8C. After evaporation, the electrolyte film was dried under vacuum at 80 8C for 10 h to remove residual solvent. The time dependence of the impedance spectra of two symmetric cells, Li/P(EO)20Li(CF3SO2)2N–10 wt.% gLiAlO2/Li, respectively, using electrodeposited lithium and commercial lithium foil, was measured to evaluate stability of SEI between lithium and the composite polymer electrolyte.
3. Results and discussion 3.1. Morphology of electrodeposited lithium When lithium is electrodeposited on a copper substrate, chemical reactions between the lithium surface and electrolyte components would happen and new kind of surface film (SEI) would be formed. However, the impurities in the electrolyte, such as H2O, CO2, HF, etc., could also greatly accelerate the chemical reaction. So, the competition between the electrodeposition rate, which is controlled by current density and the chemical reaction rate accelerated by impurities, would determine the morphology of electrodeposited lithium. If the chemical reactions are too slow to modify the lithium surface, lithium deposits in a dendrite form. But if the chemical reactions proceed more quickly than the electrodeposition of lithium, then lithium film with a uniform morphology will be obtained. Since the concentration of impurities in the electrolyte will decrease with an increase of deposited lithium, the formation of lithium dendrite is incident to the late stage of the electrodeposition even at a current density as low as of 0.5 mA cm2. Therefore, we normally suppress dendrite formation by adding some additives in the electrolyte; however, we have to maintain a constant concentration of any additive in the electrolyte. Fig. 1(a) and (b) show the SEI micrography of lithium deposited in 1 mol cm3 LiPF6/EC+DMC (1:1) at a current density of 0.5 mA cm2 under static conditions. As shown, the film was made up of agglomerates of dendritic lithium. It was reasonable to presume that overgrown agglomerates of lithium dendrite would desquamate from the substrate after long period of deposition. Suppression of lithium dendrite was successfully realized by a simple magnetic stirring during the electrodeposition. Fig. 1c shows the morphology of lithium film deposited at the current density as high as 2.0 mA cm2. As shown, homogeneous lithium particles with
X. Yang et al. / Solid State Ionics 176 (2005) 1051–1055
1053
250 Static Dynamic
Ri / Ohm
200 150 100 50 0 0
5
10
15
20
25
Q / C cm-2 Fig. 2. Interface resistance vs. the quantity of electricity in 1.0 mol dm3 LiPF6/(EC+DMC) at 1.0 mA cm2.
The result of electrodeposition under static conditions is also given in Fig. 2 for comparison with the dynamic one. The interface resistance experienced the original two stages similar to that under dynamic conditions; however, a higher SEI resistance was obtained for the static one. A rapid decrease of the interface resistance induced by the breaking of the SEI layer after the total charge above 7 C cm2 was found, which could be attributed to the cracks of SEI from which lithium dendrite can grow. The effect of magnetic stirring on the morphology of deposited lithium may be ascribed to the formation of a compact SEI and the suppression of preferred orientation of lithium. 3.2. Lithium cycling efficiency It is well known that SEI is derived from the reactions between lithium surface and electrolyte components, especially the impurities, and the chemical and physical properties of such a film govern the deposition/dissolution performance of the electrodeposited lithium [14]. Fig. 3 Fig. 1. Morphology of lithium films electrodeposited (10 C cm2) on copper substrate in the solution of 1.0 mol dm3 LiPF6/(EC+DMC) under the conditions: (a, b) 0.5 mA cm2, static; (c) 2.0 mA cm2, dynamic.
100
90
Efficiency / %
hemispherical shape were obtained. No any dendritic lithium was found. The evolution of interface resistance (R i) during the electrodeposition over the quantity of electricity under dynamic conditions was shown in Fig. 2. Three main regions can be distinguished within the whole deposition range. As seen, the R i decreased at the original stage from 0 to 5C cm2, then increased within 5 to 13 C cm2 and remained constant afterwards. The first region was ascribed to the reactions between electrolyte and native film of counterelectrode, whereas the second one was to the SEI growth of working electrode. After the quantity of electricity approached 13 C cm2, the deposited lithium would be separated completely from the electrolyte by SEI, which remained stable with an increase of deposited charge.
80 0.5mA / cm2 1.0mA / cm2
70
60
50 10
20
30
40
50
Cycle Number Fig. 3. Deposition–dissolution cycling efficiency of lithium in LiPF6/ (EC+DMC) at 1 mA cm2 under dynamic conditions.
X. Yang et al. / Solid State Ionics 176 (2005) 1051–1055
shows the deposition/dissolution cycling efficiencies of electrodeposited lithium on the copper substrate in LiPF6/ EC+DMC (1:1) at 0.5 and 1.0 mA cm2 with magnetic stirring. The cycling efficiency at 0.5 mA cm2 was as high as 93.5% since the 38th cycle except for an initial poor cycling efficiency due to the formation of SEI. When current density was further improved to 1.0 mA cm2, cycling efficiency decreases slightly, but it is still much higher than that cycled under static conditions. In the present systems, one may presume that magnetic stirring has an impact influence on the formation of SEI within the initial cycles, which would effectively separate deposited lithium from electrolyte and ensure an improved performance of the subsequent cycling.
The evolution of the impedance spectra of two cells, which were formed by sandwiching the polymer electrolyte between symmetric lithium electrodes respectively using the electrodeposited lithium film and commercial lithium foil, was shown respectively in Fig. 4a and b. The fit of
-250 1day 2days 3days 4days 5days 6days 7days
Ri Zw
-200
Z'' / Ohm
Cd
-150 -100 -50 0 0
50
100
150
200
250
300
Z' / Ohm
b
-500 1day 2days 3days 4days 5days 6days 7days 8days
Z'' / Ohm
-400 -300 -200
0
100
200
300
400
500
400 300 200 100 0
0
1
2
3
4
5
6
7
8
Time / days
impedance data was performed with the help of an appropriate equivalent electronic circuit as shown in the inset of Fig. 4a in which the total interface impedance is consisted of a typical Warburg impedance element (Z w) and a parallel combination of an interfacial resistance (R i) and double-layer capacitance (C d) [15]. Fig. 5 demonstrated the dependence of R i value on the aging time of both the cells. As seen, in a cell with electrodeposited lithium film as electrode, R i increased somewhat and turned to be stable in 3 days, which indicated that the reactions between composite polymer electrolyte and electrodeposited lithium under dynamic conditions reached an equilibrium state quickly. While the cell using commercial lithium foil displayed comparable initial resistance as that with electrodeposited lithium film, but it increased rapidly and did not reach equilibrium in 6 days; meanwhile, the final R i of it was much greater than that of electrodeposited lithium. Therefore, lithium film electrodeposited under dynamic conditions would be a preferable candidate as anode for all-solid-state lithium/polymer battery because of its favorable effect to suppress reactions between lithium and composite polymer electrolyte.
4. Conclusions
-100 0
Deposited Lithium Commercial Lithium
500
Fig. 5. Time dependence of the interfacial resistance derived from impedance analysis of the cells using electrodeposited lithium film and commercial lithium foil as electrodes.
3.3. Interface stability of lithium with solid electrolyte
a
600
Ri / ohm
1054
600
Z' / Ohm Fig. 4. Evolution of impedance response of Li/P(EO)20Li(CF3SO2)2N–10 wt.% g-LiAlO2/Li cells at 50 8C: (a) electrodeposited lithium and (b) commercial lithium.
An electrodeposition of lithium film under dynamic conditions was performed to modify the lithium surface. The dendrite formation of lithium was suppressed at a current density as high as 2.0 mA cm2 and deposition/ dissolution cycling efficiency was also improved under dynamic conditions. The electrodeposited lithium film was introduced to Li/P(EO)20Li(CF3SO2)2N–10 wt.% g-LiAlO2/ Li symmetric cell to investigate interface stability; the impedance spectra based on the electrodeposited lithium and a composite polymer electrolyte showed a small and stable interface resistance.
X. Yang et al. / Solid State Ionics 176 (2005) 1051–1055
Acknowledgments This work was financially supported by key project of Natural Science Foundation of China (NSFC) No. 20333040.
References [1] T. Iwahori, Y. Ozaki, A. Funahashi, H. Momose, I. Mitsuishi, S. Shiraga, S. Yoshitake, H. Awata, Journal of Power Sources 81–82 (1999) 872. [2] J.B. Bates, N.J. Dudney, B. Neudecker, A. Ueda, C.D. Evans, Solid State Ionics 135 (2000) 33. [3] S.D. Jones, J.R. Akridge, Solid State Ionics 86–88 (1996) 1291. [4] Y. Furuyama, K. Ito, S. Dohi, A. Taniike, A. Kitamura, Journal of Nuclear Materials 313–316 (2003) 288. [5] K. Kanamura, S. Shiraishi, Z. Takehara, Journal of Fluorine Chemistry 87 (1998) 235.
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