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A new advanced lithium ion battery: Combination of high performance amorphous columnar silicon thin film anode, 5 V LiNi0.5Mn1.5O4 spinel cathode and fluoroethylene carbonate-based electrolyte solution
Electrochemistry Communications 33 (2013) 31–34
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
A new advanced lithium ion battery: Combination of high performance amorphous columnar silicon thin film anode, 5 V LiNi0.5Mn1.5O4 spinel cathode and fluoroethylene carbonate-based electrolyte solution K. Fridman a, R. Sharabi a, R. Elazari a, G. Gershinsky a, E. Markevich a,⁎, G. Salitra a,⁎, D. Aurbach a, A. Garsuch b, J. Lampert b a b
Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel BASF SE, Ludwigshafen 67056, Germany
a r t i c l e
i n f o
Article history: Received 14 March 2013 Received in revised form 10 April 2013 Accepted 11 April 2013 Available online 18 April 2013
a b s t r a c t A new advanced Li-ion battery comprising a high performance amorphous columnar silicon thin film anode, a high voltage LiNi0.5Mn1.5O4 spinel composite cathode and fluoroethylene carbonate (FEC)-based electrolyte solution (FEC/DMC 1:4 with 1 M LiPF6) is reported. This advanced battery demonstrated hundreds of cycles, excellent charge–discharge efficiency and rate capability. © 2013 Elsevier B.V. All rights reserved.
Keywords: Silicon anode LiNi0.5Mn1.5O4 5 V lithium-ion battery Fluoroethylene carbonate
1. Introduction Research and development of advanced, high energy density Li ion batteries is one of the most important and “hot” topics in modern electrochemistry. High voltage Li-ion batteries are potential power sources for the electric vehicles due to the highest energy density of all the commercialized rechargeable batteries. LiNi0.5Mn1.5O4 with spinel structure possesses high operating voltage (red-ox potential of 4.7 V vs. Li/Li +), theoretical capacity of about 147 mA h g −1 and high rate capability. However, LiNi0.5Mn1.5O4/graphite cells exhibit severe capacity fading [1]. To avoid a rapid capacity loss Lee et al. proposed additives, namely, succinic anhydride (SA) and propane sultone (PS), which formed SEI on the graphite anode surface [1]. These additives improved the capacity retention of the full cells LiNi0.5Mn1.5O4/graphite cells compared to the additive free electrolyte solution, but yet capacity fading was pronounced. Combination of LiNi0.5Mn1.5O4 cathode with TiO2 [2] and Li4Ti5O12 [3,4] anodes decreases the cell voltage. Very promising anode materials for the high voltage Li-ion cells are lithium alloys, which deliver the highest specific capacity in Li batteries. High voltage Li ion battery formed by combining LiNi0.5Mn1.5O4 spinel cathode with nanostructured tin-carbon anode with good performance was demonstrated in [5,6]. Silicon is another element which forms with lithium an alloy in the same potential range of 0–1 V vs. Li/Li+. This ⁎ Corresponding authors. Tel.: +972 3 531 8832; fax: +972 3 738 4053. E-mail addresses: [email protected] (E. Markevich), [email protected] (G. Salitra). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.04.010
alloy possesses huge specific capacity of 3580 mA h g −1 [7,8] which is one order of magnitude higher than that of graphite (372 mA h g −1), the most commonly used anode material for Li ion batteries. An effort to combine silicon composite anode prepared from silicon nanoparticles with LiNi0.5Mn1.5O4 spinel cathode resulted in high voltage cell which exhibited marked capacity fading during 30 cycles [9]. In this paper we present Li ion battery based on 5 V LiNi0.5Mn1.5O4 spinel cathode and columnar silicon thin film anode with excellent performance when the electrolyte solution was 1 M LiPF6 in fluoroethylene carbonate (FEC)/dimethyl carbonate (DMC) 1:4 w/w.
2. Experimental Silicon thin film electrodes were prepared by DC magnetron sputtering [10,11]. The surface density of the obtained a-Si film was 1.3 mg/cm 2 (~ 6 μm thick). LiNi0.5Mn1.5O4 powder was obtained from BASF SE. Composite electrodes comprised 90 wt.% of LiNi0.5Mn1.5O4, 5 wt.% of carbon black (SuperP, Superior graphite, USA) and 5 wt.% of PVdF (Aldrich). The cathode sheets were fabricated by spreading slurry (suspension of LiNi0.5Mn1.5O4 powder and carbon black in a PVdF/N-methylpyrrolidon solution) on an aluminum foil current collectors with a doctor blade device. Typically, the electrodes contained 4.2 ± 0.2 mg of active mass. The electrolyte solutions were 1 M of LiPF6 in an EC + DMC 1:1 (EC-based) and 1 M of LiPF6 in an FEC + DMC 1:4 (FEC-based) mixtures (both Li-battery grade from Merck, KGaA).
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K. Fridman et al. / Electrochemistry Communications 33 (2013) 31–34
A 2000
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Fig. 1. Results of the galvanostatic cycling of Si/Li cells. (A) Specific charge capacity and voltage at the end of charge vs. cycle number in EC- (red curves) and FEC-based (black curves) electrolyte solutions. The inset shows the voltage profile of the cell cycled in the FEC-based electrolyte solution (B, D) and (C, E) SEM images of Si electrodes after 500 cycles in the EC-based electrolyte solution and FEC-based electrolyte solution, respectively. Surface density of Si electrodes is 1.3 mg cm−2. Current rate 1C with respect to limiting capacity of 600 mA h g−1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
a-Si anodes were tested galvanostatically in coin-type cells (2523, NRC, Canada) vs. lithium metal (Chemetall Foote Corporation, USA) at 30 °C. They were initially discharged from OCV down to 150 mV and then charged up to 1.2 V vs. Li/Li + at a current density of 120 mA/g. After that Si anodes were cycled with a discharge cut-off voltage of
10 mV vs. Li/Li + and charge cut-off capacity of 600 mA h g −1 at a current density of 600 mA/g which corresponds to the 1C current rate with respect to the limiting capacity of 600 mA h g −1. Before the use of the film Si electrodes as anodes in full cells, they were galvanostatically pre-passivated and partially pre-lithiated in
K. Fridman et al. / Electrochemistry Communications 33 (2013) 31–34
Thin film Si anodes with excellent cycling characteristics prepared by DC magnetron sputtering were totally amorphous [10,11]. Fig. 1 reports the galvanostatic charge–discharge performance of a Si/Li cells in EC- and FEC-based electrolyte solutions. The galvanostatic tests were performed with the limitation of specific capacity of Si electrode by 600 mA h g −1, which is about twice higher than that of common graphite and is reasonably high to ensure capacity balance with composite LiNi0.5Mn1.5O4 spinel cathode. In every galvanostatic cycle Si electrodes were discharged down to 10 mV vs. Li/Li+ and then charged up to the specific capacity of 600 mA h g−1. The limitation of the capacity restricts the amplitude of the repeated expansion/contraction of Si bulk and, thus, ensures long cycle life of Si electrode. As a result (Fig. 1A) we obtained a very stable cycling of Si/Li cells during more than 1500 cycles for the FEC-based electrolyte solution. Voltage profile (inset) becomes stable by about the 200th cycle and does not change over more than 1300 subsequent cycles. In the EC-based electrolyte Si electrodes demonstrated considerably shorter cycle life of about 500 cycles. For all Si/Li cells cycled in both FEC- and EC-based electrolyte solutions Coulomb efficiency exceeded 98%. SEM images of Si electrodes cycled for 500 cycles in EC- and FEC-based electrolyte solutions are shown in Fig. 1B and C, respectively. It is clearly seen that in both cases cycling of Si electrodes leads to rifting apart of a uniform surface of the pristine Si film into separate islands of about 10–30 μm in diameter. In the case of the FEC-based electrolyte these islands are coated with very homogeneous and relatively thin surface films. For the electrode cycled in the EC-based electrolyte very thick irregular coating layers cover the islands and fully close the gaps between them in several locations. Obviously, the growth of these layers results in overexpansion of the islands which leads to their rearing and separation from copper foil current collector (Fig. 1B). Thus, growth of the surface resistance and disconnection of the separate islands from the current collector are the most probable reasons for worse cycling behavior of Si/Li cells in the EC-based electrolyte solution compared to the FECbased one. This conclusion is in complete agreement with the results of Oumellal et al. [12] which shows that the main cause of capacity fade of Si-based negative electrodes is degradation of the liquid electrolyte with the formation of a blocking layer on the active mass, which inhibits lithium diffusion and leads to rise of the electrode polarization. Another parameter which is of crucial importance for the cycling performance of thin film columnar Si electrodes is the potential range of the reversible Li alloying with Si. Indeed, the utilization of only about one-sixth of the full capacity of Si electrode involves the possibility to choose the working potential range related to the depth of lithiation of silicon. We found, that Si/Li cells which were cycled at a lower potential range, namely with discharge cut-off potential equal to 10 mV vs. Li/Li + with the subsequent charge limited to 600 mA h g −1 (“charge capacity limited procedure”), exhibited much more stable cycling than that observed for the cells cycled at a higher potential range with the same limiting specific capacity (discharge limited to 600 mA h g −1, followed by charge up to 1.2 V
A Capacity (mAh g-1)
3. Results and discussion
vs. Li/Li +, “discharge capacity limited procedure”) [11]. It is clear that “charge capacity limited cycling procedure” corresponds better to the real conditions of cycling of anodes in complete cells and provides their higher voltage. Fortunately, this procedure resulted in much better cycling response of Si electrodes, most probably, due to the formation of more effective protective surface films which are formed in the course of repeated charging of the electrodes at lower potentials. Based on these findings we performed partial lithiation of Si anodes down to 10 mV vs. Li/Li+ prior to the assembling of LiNi0.5Mn1.5O4/Si full cells. Fig. 2A shows cycling results of LiNi0.5Mn1.5O4/Si cells in two electrolyte solutions. For the cells cycled in the EC-based electrolyte solution (black curve), a stable cycling was observed typically during 20–30 initial cycles. After that a sudden increase in the irreversible capacity with a drastic growth of the charge capacity value and a decrease in discharge capacity was observed. Obviously, at this point the rate of parasitic side reactions on the electrodes becomes comparable or even higher than that of Faradaic process and, finally, this situations leads to the failure of the cells. At the same time, the cells cycled in the FEC-based electrolyte (red curve) demonstrated very stable cycling with charge–discharge efficiency approaching 100%. Cycling of LiNi0.5Mn1.5O4/Si cells in the EC-based electrolyte with higher current rates (Fig. 2B, black curve) makes it possible to perform more charge–discharge cycles. This is not surprising, since in this case the portion of the charge, which relates to the side reactions on the electrodes, is relatively smaller than that of coulombic charge associated with the reversible Li doping–undoping into the structure of the electrodes. However, to the contrast to the cells cycled with FEC-based electrolyte (red curve), one can observe the typical behavior of the cells
300 250 EC-based electrolyte, charge
200 FEC-based electrolyte
150 100
EC-based electrolyte, discharge
50 0 0
50
100
150
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B 300
Capacity (mAh g-1)
two electrode coin type cells containing Li counter electrodes. The cells comprising silicon electrodes, PE separator (Setela Tonen, Japan), an electrolyte solution, and Li counter electrodes were assembled in an argon glove box (M.Braun). After that we performed five galvanostatic cycles of Si electrodes at 30 °C with the voltage cut-off limits of 10 mV and 1.2 V and current density of 120 mA/g in the first cycle and 600 mA/g in four subsequent cycles. Finally, the Si electrodes were discharged galvanostatically down to 10 mV vs. Li/Li +, withdrawn from Si/Li cells and used for the preparation of the complete cells with LiNi0.5Mn1.5O4 cathodes. Galvanostatic cycling of Si/Li and LiNi0.5Mn1.5O4/Si cells was carried out using a BT2000 battery tester (Arbin Instruments, USA). SEM images were obtained by FEI Inspec S.
33
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EC-basedelectrolyte
200 C/8
150
C/8 C/4
C/2
1C
100
2C
50
FEC-based electrolyte
0 0
20
40
60
Cycle number Fig. 2. Typical curves of charge (full dots) and discharge (hollow dots) capacity vs. cycle number obtained upon galvanostatic cycling of LiNi0.5Mn1.5O4/Si cells at a current rate of (A) C/8, referred to the cathode, and (B) at different current rates, as indicated, in EC- (black curves) and FEC-based (red curves) electrolyte solutions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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K. Fridman et al. / Electrochemistry Communications 33 (2013) 31–34
failure after the return to the C/8 current rate. Thus, FEC-based electrolyte is a critically important component of LiNi0.5Mn1.5O4/Si full cells. Although in the literature FEC in the electrolyte solutions for Li batteries is known to be a highly efficient component for passivation of the surface of anodes [11,13–15], recently we have shown [16] that FEC-based electrolyte ensures to be more effective than EC-based electrolyte passivation of high voltage LiCoPO4 cathode operating at the same voltage range as LiNi0.5Mn1.5O4 spinel. Thus, the reasons for such different behavior of LiNi0.5Mn1.5O4/Si full cells in two electrolyte solutions may be both better passivation of the Si anode surface and more effective passivation of the surface of high voltage cathode in the FEC-based electrolyte. Fig. 3 demonstrates cycling results of complete LiNi0.5Mn1.5O4/Si cells obtained with FEC-based electrolyte solution at 0.5C current rate. The capacity retention of the cell comprised 92.2% after 200 cycles, 88.5% after 300 cycles and 74.2% after 500 cycles. To our knowledge, a lithium ion battery with combination of Si anode and LiNi0.5Mn1.5O4 spinel cathode demonstrating stable cycling over hundreds of cycles has so far never been reported. 4. Conclusions We have demonstrated superior cycling characteristics of the monolithic columnar amorphous silicon thin film electrode in FEC-based electrolyte solution. We have shown that this thin film amorphous Si electrode may be successfully combined with LiNi0.5Mn1.5O4 spinel cathode in complete high voltage Li-ion cells. Excellent performance of these cells was achieved by replacing the conventional 1 M LiPF6 EC/DMC electrolyte solution with 1 M LiPF6 FEC/DMC electrolyte.
Discharge capacity (mAh g-1)
A 250 200 150 100 50 0 0
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Specific capacity (mAhg-1) Fig. 3. Typical curve of discharge capacity vs. cycle number (A) and voltage profile (B) obtained upon galvanostatic cycling of LiNi0.5Mn1.5O4/Si cells at 0.5C rate (10 initial cycles at C/8 rate) in the FEC-based electrolyte solution.
References [1] H. Lee, Seungdon Choi, Sanghoon Choi, H.-J. Kim, Y. Choi, S. Yoon, J.-J. Cho, Electrochemistry Communications 9 (2007) 801–806. [2] G. Armstrong, A.R. Armstrong, P.G. Bruce, P. Reale, B. Scrosati, Advanced Materials 18 (2006) 2597–2600. [3] H.M. Wu, I. Belharouak, H. Deng, A. Abouimrane, Y.-K. Sun, K. Amine, Journal of the Electrochemical Society 156 (2009) A1047–A1050. [4] H.-G. Jung, M.W. Jang, J. Hassoun, Y.-K. Sun, B. Scrosati, Nature Communications 2 (2011) 516, http://dx.doi.org/10.1038/ncomms1527. [5] G. Derrien, J. Hassoun, S. Panero, B. Scrosati, Advanced Materials 19 (2007) 2336–2340. [6] J. Hassoun, P. Reale, S. Panero, B. Scrosati, Advanced Materials 21 (2009) 4807–4810. [7] T.D. Hatchard, J.R. Dahn, Journal of the Electrochemical Society 151 (2004) A838–A842. [8] B. Key, M. Morcrette, J.M. Tarascon, C.P. Grey, Journal of the American Chemical Society 133 (2011) 503–512. [9] J. Arrebola, A. Caballero, J.L. Gomes-Camer, L. Hernan, J. Morales, L. Sanchez, Electrochemistry Communications 11 (2009) 1061–1064. [10] R. Elazari, G. Salitra, G. Gershinsky, A. Garsuch, A. Panchenko, D. Aurbach, Electrochemistry Communications 14 (2012) 21–24. [11] R. Elazari, G. Salitra, G. Gershinsky, A. Garsuch, A. Panchenko, D. Aurbach, Journal of the Electrochemical Society 159 (2012) A1440–A1445. [12] Y. Oumellal, N. Delpuech, D. Mazouzi, N. Dupre, J. Gaubicher, P. Moreau, P. Soudan, B. Lestriez, D. Guyomard, Journal of Materials Chemistry 21 (2011) 6201–6208. [13] S.-K. Jeong, M. Inaba, R. Mogi, Y. Iriyama, T. Abe, Z. Ogumi, Langmuir 17 (2001) 8281–8286. [14] N.-S. Choi, K.H. Yew, K.Y. Lee, M. Sung, H. Kim, S.-S. Kim, Journal of Power Sources 161 (2006) 1254–1259. [15] H. Nakai, T. Kubota, A. Kita, A. Kawashima, Journal of the Electrochemical Society 158 (2011) A798–A801. [16] R. Sharabi, E. Markevich, K. Fridman, G. Gershinsky, G. Salitra, D. Aurbach, G. Semrau, M.A. Schmidt, Electrochemistry Communications 28 (2013) 20–23.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
A new advanced lithium ion battery: Combination of high performance amorphous columnar silicon thin film anode, 5 V LiNi0.5Mn1.5O4 spinel cathode and fluoroethylene carbonate-based electrolyte solution K. Fridman a, R. Sharabi a, R. Elazari a, G. Gershinsky a, E. Markevich a,⁎, G. Salitra a,⁎, D. Aurbach a, A. Garsuch b, J. Lampert b a b
Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel BASF SE, Ludwigshafen 67056, Germany
a r t i c l e
i n f o
Article history: Received 14 March 2013 Received in revised form 10 April 2013 Accepted 11 April 2013 Available online 18 April 2013
a b s t r a c t A new advanced Li-ion battery comprising a high performance amorphous columnar silicon thin film anode, a high voltage LiNi0.5Mn1.5O4 spinel composite cathode and fluoroethylene carbonate (FEC)-based electrolyte solution (FEC/DMC 1:4 with 1 M LiPF6) is reported. This advanced battery demonstrated hundreds of cycles, excellent charge–discharge efficiency and rate capability. © 2013 Elsevier B.V. All rights reserved.
Keywords: Silicon anode LiNi0.5Mn1.5O4 5 V lithium-ion battery Fluoroethylene carbonate
1. Introduction Research and development of advanced, high energy density Li ion batteries is one of the most important and “hot” topics in modern electrochemistry. High voltage Li-ion batteries are potential power sources for the electric vehicles due to the highest energy density of all the commercialized rechargeable batteries. LiNi0.5Mn1.5O4 with spinel structure possesses high operating voltage (red-ox potential of 4.7 V vs. Li/Li +), theoretical capacity of about 147 mA h g −1 and high rate capability. However, LiNi0.5Mn1.5O4/graphite cells exhibit severe capacity fading [1]. To avoid a rapid capacity loss Lee et al. proposed additives, namely, succinic anhydride (SA) and propane sultone (PS), which formed SEI on the graphite anode surface [1]. These additives improved the capacity retention of the full cells LiNi0.5Mn1.5O4/graphite cells compared to the additive free electrolyte solution, but yet capacity fading was pronounced. Combination of LiNi0.5Mn1.5O4 cathode with TiO2 [2] and Li4Ti5O12 [3,4] anodes decreases the cell voltage. Very promising anode materials for the high voltage Li-ion cells are lithium alloys, which deliver the highest specific capacity in Li batteries. High voltage Li ion battery formed by combining LiNi0.5Mn1.5O4 spinel cathode with nanostructured tin-carbon anode with good performance was demonstrated in [5,6]. Silicon is another element which forms with lithium an alloy in the same potential range of 0–1 V vs. Li/Li+. This ⁎ Corresponding authors. Tel.: +972 3 531 8832; fax: +972 3 738 4053. E-mail addresses: [email protected] (E. Markevich), [email protected] (G. Salitra). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.04.010
alloy possesses huge specific capacity of 3580 mA h g −1 [7,8] which is one order of magnitude higher than that of graphite (372 mA h g −1), the most commonly used anode material for Li ion batteries. An effort to combine silicon composite anode prepared from silicon nanoparticles with LiNi0.5Mn1.5O4 spinel cathode resulted in high voltage cell which exhibited marked capacity fading during 30 cycles [9]. In this paper we present Li ion battery based on 5 V LiNi0.5Mn1.5O4 spinel cathode and columnar silicon thin film anode with excellent performance when the electrolyte solution was 1 M LiPF6 in fluoroethylene carbonate (FEC)/dimethyl carbonate (DMC) 1:4 w/w.
2. Experimental Silicon thin film electrodes were prepared by DC magnetron sputtering [10,11]. The surface density of the obtained a-Si film was 1.3 mg/cm 2 (~ 6 μm thick). LiNi0.5Mn1.5O4 powder was obtained from BASF SE. Composite electrodes comprised 90 wt.% of LiNi0.5Mn1.5O4, 5 wt.% of carbon black (SuperP, Superior graphite, USA) and 5 wt.% of PVdF (Aldrich). The cathode sheets were fabricated by spreading slurry (suspension of LiNi0.5Mn1.5O4 powder and carbon black in a PVdF/N-methylpyrrolidon solution) on an aluminum foil current collectors with a doctor blade device. Typically, the electrodes contained 4.2 ± 0.2 mg of active mass. The electrolyte solutions were 1 M of LiPF6 in an EC + DMC 1:1 (EC-based) and 1 M of LiPF6 in an FEC + DMC 1:4 (FEC-based) mixtures (both Li-battery grade from Merck, KGaA).
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K. Fridman et al. / Electrochemistry Communications 33 (2013) 31–34
A 2000
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Fig. 1. Results of the galvanostatic cycling of Si/Li cells. (A) Specific charge capacity and voltage at the end of charge vs. cycle number in EC- (red curves) and FEC-based (black curves) electrolyte solutions. The inset shows the voltage profile of the cell cycled in the FEC-based electrolyte solution (B, D) and (C, E) SEM images of Si electrodes after 500 cycles in the EC-based electrolyte solution and FEC-based electrolyte solution, respectively. Surface density of Si electrodes is 1.3 mg cm−2. Current rate 1C with respect to limiting capacity of 600 mA h g−1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
a-Si anodes were tested galvanostatically in coin-type cells (2523, NRC, Canada) vs. lithium metal (Chemetall Foote Corporation, USA) at 30 °C. They were initially discharged from OCV down to 150 mV and then charged up to 1.2 V vs. Li/Li + at a current density of 120 mA/g. After that Si anodes were cycled with a discharge cut-off voltage of
10 mV vs. Li/Li + and charge cut-off capacity of 600 mA h g −1 at a current density of 600 mA/g which corresponds to the 1C current rate with respect to the limiting capacity of 600 mA h g −1. Before the use of the film Si electrodes as anodes in full cells, they were galvanostatically pre-passivated and partially pre-lithiated in
K. Fridman et al. / Electrochemistry Communications 33 (2013) 31–34
Thin film Si anodes with excellent cycling characteristics prepared by DC magnetron sputtering were totally amorphous [10,11]. Fig. 1 reports the galvanostatic charge–discharge performance of a Si/Li cells in EC- and FEC-based electrolyte solutions. The galvanostatic tests were performed with the limitation of specific capacity of Si electrode by 600 mA h g −1, which is about twice higher than that of common graphite and is reasonably high to ensure capacity balance with composite LiNi0.5Mn1.5O4 spinel cathode. In every galvanostatic cycle Si electrodes were discharged down to 10 mV vs. Li/Li+ and then charged up to the specific capacity of 600 mA h g−1. The limitation of the capacity restricts the amplitude of the repeated expansion/contraction of Si bulk and, thus, ensures long cycle life of Si electrode. As a result (Fig. 1A) we obtained a very stable cycling of Si/Li cells during more than 1500 cycles for the FEC-based electrolyte solution. Voltage profile (inset) becomes stable by about the 200th cycle and does not change over more than 1300 subsequent cycles. In the EC-based electrolyte Si electrodes demonstrated considerably shorter cycle life of about 500 cycles. For all Si/Li cells cycled in both FEC- and EC-based electrolyte solutions Coulomb efficiency exceeded 98%. SEM images of Si electrodes cycled for 500 cycles in EC- and FEC-based electrolyte solutions are shown in Fig. 1B and C, respectively. It is clearly seen that in both cases cycling of Si electrodes leads to rifting apart of a uniform surface of the pristine Si film into separate islands of about 10–30 μm in diameter. In the case of the FEC-based electrolyte these islands are coated with very homogeneous and relatively thin surface films. For the electrode cycled in the EC-based electrolyte very thick irregular coating layers cover the islands and fully close the gaps between them in several locations. Obviously, the growth of these layers results in overexpansion of the islands which leads to their rearing and separation from copper foil current collector (Fig. 1B). Thus, growth of the surface resistance and disconnection of the separate islands from the current collector are the most probable reasons for worse cycling behavior of Si/Li cells in the EC-based electrolyte solution compared to the FECbased one. This conclusion is in complete agreement with the results of Oumellal et al. [12] which shows that the main cause of capacity fade of Si-based negative electrodes is degradation of the liquid electrolyte with the formation of a blocking layer on the active mass, which inhibits lithium diffusion and leads to rise of the electrode polarization. Another parameter which is of crucial importance for the cycling performance of thin film columnar Si electrodes is the potential range of the reversible Li alloying with Si. Indeed, the utilization of only about one-sixth of the full capacity of Si electrode involves the possibility to choose the working potential range related to the depth of lithiation of silicon. We found, that Si/Li cells which were cycled at a lower potential range, namely with discharge cut-off potential equal to 10 mV vs. Li/Li + with the subsequent charge limited to 600 mA h g −1 (“charge capacity limited procedure”), exhibited much more stable cycling than that observed for the cells cycled at a higher potential range with the same limiting specific capacity (discharge limited to 600 mA h g −1, followed by charge up to 1.2 V
A Capacity (mAh g-1)
3. Results and discussion
vs. Li/Li +, “discharge capacity limited procedure”) [11]. It is clear that “charge capacity limited cycling procedure” corresponds better to the real conditions of cycling of anodes in complete cells and provides their higher voltage. Fortunately, this procedure resulted in much better cycling response of Si electrodes, most probably, due to the formation of more effective protective surface films which are formed in the course of repeated charging of the electrodes at lower potentials. Based on these findings we performed partial lithiation of Si anodes down to 10 mV vs. Li/Li+ prior to the assembling of LiNi0.5Mn1.5O4/Si full cells. Fig. 2A shows cycling results of LiNi0.5Mn1.5O4/Si cells in two electrolyte solutions. For the cells cycled in the EC-based electrolyte solution (black curve), a stable cycling was observed typically during 20–30 initial cycles. After that a sudden increase in the irreversible capacity with a drastic growth of the charge capacity value and a decrease in discharge capacity was observed. Obviously, at this point the rate of parasitic side reactions on the electrodes becomes comparable or even higher than that of Faradaic process and, finally, this situations leads to the failure of the cells. At the same time, the cells cycled in the FEC-based electrolyte (red curve) demonstrated very stable cycling with charge–discharge efficiency approaching 100%. Cycling of LiNi0.5Mn1.5O4/Si cells in the EC-based electrolyte with higher current rates (Fig. 2B, black curve) makes it possible to perform more charge–discharge cycles. This is not surprising, since in this case the portion of the charge, which relates to the side reactions on the electrodes, is relatively smaller than that of coulombic charge associated with the reversible Li doping–undoping into the structure of the electrodes. However, to the contrast to the cells cycled with FEC-based electrolyte (red curve), one can observe the typical behavior of the cells
300 250 EC-based electrolyte, charge
200 FEC-based electrolyte
150 100
EC-based electrolyte, discharge
50 0 0
50
100
150
Cycle number
B 300
Capacity (mAh g-1)
two electrode coin type cells containing Li counter electrodes. The cells comprising silicon electrodes, PE separator (Setela Tonen, Japan), an electrolyte solution, and Li counter electrodes were assembled in an argon glove box (M.Braun). After that we performed five galvanostatic cycles of Si electrodes at 30 °C with the voltage cut-off limits of 10 mV and 1.2 V and current density of 120 mA/g in the first cycle and 600 mA/g in four subsequent cycles. Finally, the Si electrodes were discharged galvanostatically down to 10 mV vs. Li/Li +, withdrawn from Si/Li cells and used for the preparation of the complete cells with LiNi0.5Mn1.5O4 cathodes. Galvanostatic cycling of Si/Li and LiNi0.5Mn1.5O4/Si cells was carried out using a BT2000 battery tester (Arbin Instruments, USA). SEM images were obtained by FEI Inspec S.
33
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C/8 C/4
C/2
1C
100
2C
50
FEC-based electrolyte
0 0
20
40
60
Cycle number Fig. 2. Typical curves of charge (full dots) and discharge (hollow dots) capacity vs. cycle number obtained upon galvanostatic cycling of LiNi0.5Mn1.5O4/Si cells at a current rate of (A) C/8, referred to the cathode, and (B) at different current rates, as indicated, in EC- (black curves) and FEC-based (red curves) electrolyte solutions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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failure after the return to the C/8 current rate. Thus, FEC-based electrolyte is a critically important component of LiNi0.5Mn1.5O4/Si full cells. Although in the literature FEC in the electrolyte solutions for Li batteries is known to be a highly efficient component for passivation of the surface of anodes [11,13–15], recently we have shown [16] that FEC-based electrolyte ensures to be more effective than EC-based electrolyte passivation of high voltage LiCoPO4 cathode operating at the same voltage range as LiNi0.5Mn1.5O4 spinel. Thus, the reasons for such different behavior of LiNi0.5Mn1.5O4/Si full cells in two electrolyte solutions may be both better passivation of the Si anode surface and more effective passivation of the surface of high voltage cathode in the FEC-based electrolyte. Fig. 3 demonstrates cycling results of complete LiNi0.5Mn1.5O4/Si cells obtained with FEC-based electrolyte solution at 0.5C current rate. The capacity retention of the cell comprised 92.2% after 200 cycles, 88.5% after 300 cycles and 74.2% after 500 cycles. To our knowledge, a lithium ion battery with combination of Si anode and LiNi0.5Mn1.5O4 spinel cathode demonstrating stable cycling over hundreds of cycles has so far never been reported. 4. Conclusions We have demonstrated superior cycling characteristics of the monolithic columnar amorphous silicon thin film electrode in FEC-based electrolyte solution. We have shown that this thin film amorphous Si electrode may be successfully combined with LiNi0.5Mn1.5O4 spinel cathode in complete high voltage Li-ion cells. Excellent performance of these cells was achieved by replacing the conventional 1 M LiPF6 EC/DMC electrolyte solution with 1 M LiPF6 FEC/DMC electrolyte.
Discharge capacity (mAh g-1)
A 250 200 150 100 50 0 0
100
200
300
400
500
Cycle number
B 6
Voltage (V)
5 4 Cycle 20
3
Cycle 100
2
Cycle 200 Cycle 300
1
Cycle 400
0 0
20
40
60
80
100
120
Specific capacity (mAhg-1) Fig. 3. Typical curve of discharge capacity vs. cycle number (A) and voltage profile (B) obtained upon galvanostatic cycling of LiNi0.5Mn1.5O4/Si cells at 0.5C rate (10 initial cycles at C/8 rate) in the FEC-based electrolyte solution.
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