S battery

S battery

Electrochemistry Communications 22 (2012) 177–180 Contents lists available at SciVerse ScienceDirect Electrochemistry Communications journal homepag...

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Electrochemistry Communications 22 (2012) 177–180

Contents lists available at SciVerse ScienceDirect

Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom

Communication

Fabrication of favorable interface between sulfide solid electrolyte and Li metal electrode for bulk-type solid-state Li/S battery Motohiro Nagao, Akitoshi Hayashi, Masahiro Tatsumisago ⁎ Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1‐1, Gakuen-cho, Naka-ku, Sakai, Osaka, 599‐8531, Japan

a r t i c l e

i n f o

Article history: Received 11 May 2012 Received in revised form 11 June 2012 Accepted 13 June 2012 Available online 19 June 2012 Keywords: Lithium metal secondary battery All-solid-state battery Li/S battery Solid electrolyte Interface

a b s t r a c t Formation of favorable interface between a lithium metal electrode and a solid electrolyte by inserting a lithium thin film contributes to reproducible lithium dissolution and deposition in rechargeable bulk-type allsolid-state cells. A lithium symmetrical cell with a Li2S-P2S5 solid electrolyte was galvanostatically cycled at room temperature. A bulk-type solid-state Li/S cell exhibited a good cycle performance with a capacity higher than 900 mAh g− 1 at 0.064 mA cm − 2 (0.03 C). © 2012 Elsevier B.V. All rights reserved.

1. Introduction Lithium metal has 10 times greater theoretical specific capacity than graphite and shows the lowest reaction potential among negative electrodes. From these aspects, rechargeable lithium metal batteries are expected to be an ultimate energy storage system [1]. Highly repetitive electrochemical deposition and dissolution during charge and discharge reaction are desired to realize the batteries with a lithium metal electrode. However, the rechargeable lithium metal batteries using a liquid electrolyte have some concerns about its safety and cyclability [2]. This is because an undesirable morphology of lithium electrodeposited during cycling induces a dendritic growth of lithium metal [3], leading to an internal short circuit of the battery and resulting in its fire and explosion. Replacement of flammable organic liquid electrolytes with nonflammable inorganic solid electrolytes is effective in improving safety of lithium secondary batteries. Two types of all-solid-state batteries using inorganic solid electrolytes (SEs) have been studied; one is a thin-film battery and the other is a bulk-type battery which is composed of compressed powder layers. Thin-film lithium metal batteries using a lithium phosphorus oxynitride glass electrolyte exhibited long cycle life with little capacity degradation over 40,000 cycles [4,5]. Therefore, inorganic SEs are key materials for rechargeable lithium metal batteries. On the other hand, bulk-type batteries also have potential to serve as lithium metal batteries with high capacities. Several improvements are needed to realize ultimate bulk-type rechargeable lithium metal batteries; the first improvement is lithium ion conductivity of SE,

⁎ Corresponding author. Tel./fax: + 81 72 254 9331. E-mail address: [email protected] (M. Tatsumisago). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2012.06.015

the second one is chemical stability of SEs against lithium metal and the third one is control of interface between SEs and lithium metal electrodes. Sulfide-based SEs have high Li + ion conductivity equivalent to liquid electrolytes [6–9]. Above all, we have developed Li2S-P2S5 SE with lithium ion conductivity higher than 5 × 10 − 3 S cm − 1 at room temperature [9]. Additionally, we have studied the time dependence of resistance at room temperature for the symmetric cell Li/Li2S-P2S5 SE/Li to investigate the stability of the Li2S-P2S5 SE against lithium. This result suggests that the Li2S-P2S5 SE is basically stable in contact with lithium [10]. However, charge–discharge properties in rechargeable lithium metal batteries with sulfide-based SE have hardly been investigated. We focused on the formation of a favorable interface between electrode and electrolyte such as increment in the contact area between them to realize reversible lithium deposition and dissolution in the bulk-type batteries. A vacuum-evaporation of lithium was thus applied to form a favorable interface between the Li2S-P2S5 SE and the lithium metal negative electrode. A sulfur active material with high theoretical capacity of 1672 mAh g − 1 was used as a positive electrode. Here we show a reversible electrochemical performance of bulk-type solid-state Li/S cells using the lithium thin film for the first time. The use of the lithium thin film achieved high capacity of over 900 mAh g − 1 in solid-state Li/S cells. In this study, a lithium thin film was deposited by vacuumevaporation to increase contact area between the lithium electrode and the Li2S-P2S5 SE. The effects of the insertion of the lithium thin film on reversibility of lithium dissolution and deposition of allsolid-state cells were investigated. Furthermore, the electrochemical performance of the bulk-type solid-state Li/S cells was evaluated.

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foil was attached on the lithium thin film. The four-layered pellet was sandwiched by two stainless steel disks as a current collector. Electrochemical tests were conducted at 25 °C in Ar atmosphere using charge–discharge measuring devices (BTS-2004, Nagano Co.).

Cell potential vs. Li / V

3 2.5 2

3. Results and discussion

1.5 1

25 oC 0.064 mA cm-2

0.5 0

0

40

80

120

160

200

Capacity / mAh g-1 (Li4Ti5O12) Fig. 1. Charge–discharge curves of an all-solid-state Li/Li2S-P2S5 solid electrolyte/ Li4Ti5O12 cell using a lithium foil at 0.064 mA cm− 2 at 25 °C.

2. Experimental The 80Li2S 20P2S5 (mol%) glass-ceramic SE was prepared using mechanical milling and subsequent heat treatment [6]. In order to investigate electrochemical properties of lithium dissolution and deposition, a symmetrical cell with Li/80Li2S 20P2S5/Li configuration was fabricated. A lithium thin film was formed by depositing lithium by vacuum-evaporation on both sides of the SE layer. For comparison of reversibility of its dissolution and deposition, the symmetrical cells with or without insertion of lithium thin film between lithium foil and the solid electrolyte were constructed and were evaluated by galvanostatic cycling. The interface between lithium thin film and the SE was observed by a scanning electron microscope (SEM, JSM-6610A, JEOL). A lithium titanate composite electrode was prepared by grinding lithium titanate (Li4Ti5O12, Titankogyo Co.), 80Li2S20P2S5 SE and acetylene black (AB) with a mortar. The weight ratio of Li4Ti5O12:SE:AB was 36:55:9. A sulfur composite electrode was prepared by mechanical milling the mixture of sulfur (Aldrich, 99.998%), the SE and AB with a planetary ball-mill apparatus. The weight ratio of S:SE:AB was 35:30:35. Laboratory-scale solid-state cells were fabricated as follows. The Li4Ti5O12 or sulfur composite electrode and the SE were uniaxially pressed. Lithium was vacuum-evaporated on the SE surface of the opposite side of the composite electrode and then the lithium

The initial charge–discharge curves for an all-solid-state Li/ 80Li2S 20P2S5/Li4Ti5O12 cell at 0.064 mA cm− 2 are shown in Fig. 1. The cells were galvanostatically discharged to 1.1 V and then charged to 2 V at 25 °C. The cell shows the discharge capacity of about 120 mAh g − 1 and the discharge plateau of 1.55 V vs. Li, which is the same as that of the cell using conventional liquid electrolyte [11]. However, an abrupt potential change is observed at the end of the charge process. It was reported that this behavior was due to an internal short-circuit in lithium metal batteries using a polymer electrolyte [12]. We measured a resistance of this cell by the ac impedance method before and after this charge–discharge test as shown in Fig. 1. The resistance was 150 Ω before the test and drastically decreased to less than 10 Ω after the test. This would imply that the solid-state cell results in internal short circuit as observed in the polymer lithium batteries. We have been investigating the details about this behavior, and will report these results in a forthcoming paper. Further enhancement of the reversibility of lithium dissolution and deposition is required. Kotobuki et al. reported that the increment in the contact area between SE and lithium metal electrode would be a solution for improvement of a reversible capacity in bulk-type cells with oxide SEs such as Li7La3Zr2O12 [13]. To obtain favorable contacts between the Li2S-P2S5 SE and the lithium metal electrode, a lithium thin film was formed on the SE by vacuum evaporation. Fig. 2 shows the SEM image for cross-section of the interface between the Li2S-P2S5 SE and the lithium thin film. The electrode/electrolyte interface having intimate contact is obtained, and the thickness of lithium thin film is about 1 μm. The galvanostatic cycling performances for symmetric cells with Li/80Li2S 20P2S5 SE/Li using a lithium foil (a) without and (b) with the lithium thin film are depicted in Fig. 3. The total current flow time in both the cells is shown in the x-axis in Fig. 3, and the lithium deposition and dissolution were switched every 3 h at a constant current density of 0.064 mA cm − 2. The resistance of the cells was measured by the ac impedance method after each lithium deposition and dissolution reaction. In Fig. 3(a), the cell voltage drops significantly with an increase in the current flow time. Decrease in the resistance was also observed, and the resistance became about 3 Ω after

Lithium thin film

SE layer

Fig. 2. SEM image of cross-section of a solid electrolyte (SE) layer with a lithium thin film.

M. Nagao et al. / Electrochemistry Communications 22 (2012) 177–180

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4

(a) 0.02 Cell potential vs. Li / V

Cell potential vs. Li / V

3.5 0.01

0

-0.01

3 2.5 2 1.5 3~20th

1

1st~2nd

0.5 -0.02

0 0

10

20

30

0

500

1000

1500

Capacity / mAh g-1 (Sulfur)

Time / hours

Fig. 4. Charge–discharge curves of all-solid-state Li/lithium thin film/Li2S-P2S5 SE/S cell at 25 °C at 0.013 mA cm− 2 (1st, 2nd) and 0.064 mA cm− 2 (0.03 C, 3rd–20th).

(b) Cell potential vs. Li / V

0.04

0.02

0

-0.02

-0.04 0

10

20

30

Time / hours Fig. 3. Lithium dissolution and deposition curves in the all-solid-state cells (a) lithium foil/SE/lithium foil and (b) lithium foil/lithium thin film/SE/lithium thin film/lithium foil at 0.064 mA cm− 2.

lithium dissolution and deposition for 5 cycles (about 30 h). On the other hand, by inserting the lithium thin film between the SE and the lithium foil, the cell exhibits stable voltage of +20 mV or −20 mV during repetitive lithium dissolution or deposition for 5 cycles as shown in Fig. 3(b). There was little change in the cell resistance and the resistance remained about 400 Ω for 5 cycles. In the cells using the lithium thin film, lithium dissolution and deposition take place reversibly for more than 30 h. It is revealed that the formation of the lithium thin film by evaporation onto the SE layer leads to improving reversibility of lithium dissolution and deposition in bulk-type cells. This is because an increase in contact area between the electrode and the electrolyte brings about homogeneous dissolution and deposition of lithium through the interface. Sulfur has the highest theoretical capacity among positive electrodes in lithium-ion batteries. Therefore, the rechargeable bulk-type solidstate Li/S cells are expected to show extremely high energy density. Fig. 4 shows the charge–discharge curves for the Li/S cell. In our previous work, we demonstrated that the ball-milled composite positive electrode consisting of sulfur, the Li2S-P2S5 SE and AB showed a good cyclability with high capacity in all-solid-state cells; Li–In alloy was used as a negative electrode [14]. In this study, we have used a lithium negative electrode instead of Li–In alloy to assemble all-solid-state Li/S cells. The lithium thin film was placed at the interface between the lithium foil and the SE. The cells were galvanostatically discharged to 1.3 V and then charged to 2.6 V at 25 °C. The current densities

were 0.013 mA cm − 2 at the 1st and 2nd cycles and 0.064 mA cm − 2 (0.03 C with respect to sulfur electrode) from the 3rd to 20th cycles. The obtained capacities are normalized by the weight of the sulfur active material. The reversible capacity of the cell is about 1350 mAh g − 1 at 0.013 mA cm− 2. The cell exhibits the reversible capacity of 945 mAh g− 1 at the 3rd cycle at 0.064 mA cm− 2. The cell retained the reversible capacity of 920 mAh g− 1, which is about 97% of the reversible capacity at the 3rd cycle after 20 cycles. The discharge plateau is ca. 2 V, which is coincident with the potential in Li/S cells using a liquid electrolyte [15], suggesting that lithium metal works successfully as a negative electrode in bulk-type solid-state Li/S cells. The use of the lithium thin film contributes to the achievement of the bulk-type cells with high energy density and good cycle performance because of forming an intimate contact between the lithium thin film and the SE. In this work, the cell performance was investigated only at a low current rate (0.03 C). Optimization of lithium deposition condition by vacuum evaporation is needed to tackle difficult issues such as rate performance. To the best of our knowledge, the operation of the bulk-type solid-state Li/S cells using lithium metal electrode as shown here is the first report. The use of a lithium thin film prepared by vacuum evaporation contributes to developing the fabrication of bulk-type solid-state lithium metal batteries. 4. Conclusion The effects of inserting a lithium thin film layer prepared by vacuum evaporation on the reversibility of lithium dissolution and deposition were investigated. The Li/Li2S-P2S5 SE/Li cells using the lithium thin film layer enabled lithium dissolution and deposition reversibly, and the bulk-type solid-state Li/S cells exhibited excellent cycle performance with a capacity higher than 900 mAh g − 1 at 0.064 mA cm− 2. An intimate contact between the lithium metal electrode and the SE highly contributes to the reversibility of lithium dissolution and deposition in rechargeable bulk-type solid-state cells. In order to achieve ultimate rechargeable lithium metal batteries, further studies to improve rate performance are required. Acknowledgements This work was carried out with financial support from the Japan Science and Technology Agency (JST), Core Research for Evolutional Science and Technology (CREST) Project. M.N. is grateful for a Grant-in-Aid and a fellowship from the Japan Society for the Promotion of Science (JSPS).

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