A new lithium-ion conducting glass ceramic in the composition of 75Li2S · 5P2S3 · 20P2S5 (mol%)

Solid State Ionics 262 (2014) 733–737

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A new lithium-ion conducting glass ceramic in the composition of 75Li2S · 5P2S3 · 20P2S5 (mol%) Yuji Ooura a, Nobuya Machida a,⁎, Takahiro Uehara a, Shunji Kinoshita a, Muneyuki Naito a, Toshihiko Shigematsu a, Shigeo Kondo b a b

Department of Chemistry, Konan University, Okamoto 8-9-1, Higashinada-ku, Kobe 658-8501, Japan Department of Chemistry for Materials, Mie University, Kurimamachiya, Tsu, Mie 514-8507, Japan

a r t i c l e

i n f o

Article history: Received 13 May 2013 Received in revised form 7 August 2013 Accepted 18 August 2013 Available online 18 September 2013 Keywords: Solid electrolytes Lithium ion Glass ceramics Lithium battery All-solid-state battery

a b s t r a c t Lithium-ion conducting glass-ceramic electrolyte in the composition 75Li2S · 5P2S3 · 20P2S5 (mol%) was prepared by heat treatment of an amorphous precursor. The amorphous precursor was obtained by a high-energy ball-milling process. The glass ceramic showed high ion conductivity 2.0 × 10−3 Scm−1 at room temperature. Raman scattering measurements suggested that the glass ceramic essentially composed of thio-phosphate anand P2S4− ions, PS3− 4 6 , as structural units. A laboratory-scale all-solid-state battery was also assembled by use of the glass ceramic as solid electrolytes and LiNi1/3Mn1/3Co1/3O2 powders as positive electrode materials. The all-solid-state battery showed good charge–discharge cycle performance at room temperature under a constant current density of 0.1 mAcm−2. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Toward the goal of realizing sustainable and greener energy scenario, economic and efficient energy storage forms a key challenge. In this context, rechargeable lithium-ion batteries have become a frontrunner device empowering small-scale portable electronic devices to largescale electric vehicles [1,2]. The solidification of electrolytes is useful for improving the safety and reliability of the lithium-ion secondary batteries. Non-flammable lithium-ion conducting solid electrolytes are an indispensable material for producing all-solid-state lithium batteries with high safety performance [3–12]. The solid electrolytes are required to have high lithium-ion conductivity and good electrochemical stability over a wide potential range for the all-solid-state lithium batteries. Recently, Kamaya et al.[13] have reported a lithium super ionic conductor, Li10GeP2S12, exhibits extremely high ion conductivity 1.2 × 10−2 Scm−1 at room temperature. Kanno et al. have also reported a series of sulfide crystals as solid solution in the system Li3PS4-Li4GeS4 and named those crystals thio-LISICON [14–17]. The thio-LISICON, Li3.25Ge0.25P0.75S4, exhibits high ion conductivity 2.2 × 10−3 Scm−1 at room temperature [14–17]. On the other hand, Hayashi et al. have found two types of highly ion conductive glass ceramics, 80Li2S · 20P2S5 and 70Li2S · 30P2S5 (mol%). The 80Li2S · 20P2S5 and 70Li2S · 30P2S5 glass ceramics were prepared by the heat treatment of amorphous precursors at around 513 K and 623 K, respectively [18–21]. In the 80Li2S · 20P2S5 glass ceramic, thio-LISICON analogous crystalline phase was precipitated ⁎ Corresponding author. Tel.: +81 78 435 2563; fax: +81 78 435 2539. E-mail address: [email protected] (N. Machida). 0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2013.08.023

during the heating process, and the 80Li2S · 20P2S5 glass ceramic showed high ion conductivity 1 × 10−3 Scm−1 at 25 °C [19,20]. The other glass ceramic of the composition 70Li2S · 30P2S5 was made up of the thermodynamically meta-stable Li7P3S11 crystalline phase [21,22]. The Li7P3S11 crystal exhibited high-lithium ion conductivity 3 × 10−3 Scm−1 at 298 K. Those results indicate that the approach through the preparation of glass ceramics should be very attractive to make new solid electrolytes with high ion conductivity. Recently, we have reported that amorphous solid electrolytes in the systems Li2S-P2S3 [23] and Li2S-P2S3-P2S5 [24] have been obtained by a high-energy ball-milling process, and the solid electrolytes have good electrochemical stability for the all-solid-state lithium batteries. In those systems, an amorphous 75Li2S · 5P2S3 · 20P2S5 sample shows high lithium-ion conductivity 6.2 × 10−4 Scm−1 at 25 °C and has prominent electrochemical stability more than amorphous 66.7Li2S · 33.3P2S5 solid electrolyte. The amorphous 75Li2S · 5P2S3 · 20P2S5 sample has es4− 3− sentially consisted of PS3− 4 and P2S6 thio-phosphate anions; PS4 is mo4− nomeric tetrahedral ortho-thio-phosphate anion, and P2S6 is the P-P bonded hexa-thio-hypophosphate anion. On the other hand, the amorphous 66.7Li2S · 33.3P2S5 sample has consisted of P2S4− anions; i.e., 7 the P-S-P bonded hepta-thio-phosphate anion. The prominent electrochemical stability of the amorphous 75Li2S · 5P2S3 · 20P2S5 solid electrolyte should be attributed to the structural features such as the containing thio-phosphate anions. Mercier et al. [25] reported that the P2S4− anion was thermodynamically unstable at a temperature above 7 450 °C and decomposed into P2S4− 6 anion and elemental sulfur according to the reaction: P2S4− → P2S4− + S. Then, we speculate that the P2S4− 7 6 7 unit would be also electrochemically unstable even at room temperature.

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We suppose that the glass ceramic essentially composed of PS3− 4 and 4− P2S4− 6 units, without P2S7 unit, should be electrochemically stable and that the glass ceramic should have high ion conductivity more than the amorphous Li2S-P2S3-P2S5 solid electrolytes. In this study, we attempted to prepare glass ceramic solid electrolytes in the compositions of 75Li2S · xP2S3 · (25 − x)P2S5 (mol%) (0 b x b 10) and investigated lithium-ion conducting properties of the obtained samples. 2. Experimental The system studied can be designed as 75Li2S · xP2S3 · (25 − x) P2S5 (mol%) (0 b x b 10). In the notation of the chemical composition, x denotes the P2S3 content in mole percent. Throughout the study, composition will be given based on mole percent. The 75Li2S · xP2S3 · (25 − x)P2S5 amorphous precursors were prepared by high-energy ball-milling process using a planetary ball-milling apparatus (Fritsch, P-7) from the starting materials Li2S(Nihon kagakukogyo, 99%), P2S3(Aldrich, 98%) and P2S5(Aldrich, 99%). The mixture of desired amounts of the starting materials was placed in a stainless steel (SUS-316) vial with tetragonal zirconia balls (7 balls of 10 mm diameter) and the vial was sealed with an O-ring. The total mass of the starting materials was 1.5 g. The ball-milling rotation speed was 380 rpm and the milling time was 35 h. The glass-transition temperature and crystallization temperatures of the obtained amorphous precursors were evaluated with differential scanning calorimetry (DSC) at a heating rate of 10 K min−1 using a Rigaku Thermoplus 2. Glass-ceramic samples were prepared by heat treatment of the obtained amorphous precursors at temperatures higher than the crystallization temperatures for 2 h. A typical heat treatment temperature was in the range of 523 to 563 K. All the processes were carried out in a dry Ar filled grove box. The obtained glass-ceramic samples were characterized by X-ray diffraction measurements with Cu-Kα radiation. For the measurements, the samples were sealed in an airtight container with beryllium windows and the container was mounted on an X-ray diffractometer (Rigaku, MultiFlex). Raman spectra of the samples were measured by use of a Raman spectrometer (Nihonbunko, NRS3000) using the 532 nm line of a semiconductor laser. For the measurements, the samples were sealed in an airtight container with a quartz glass window, and the container was mounted on the spectrometer. For ac impedance measurements, the glass-ceramic sample was pressed into a pellet of 10 mm diameter under a pressure 380 MPa, and indium foils were attached on both sides of the pellet to serve the electrodes. The ac impedance of the pellet was measured in a dry argon atmosphere from 2 MHz to 20 Hz over the temperature range of 280 to 380 K with a precision LCR meter (Agilent, E4980A). The total electrical conductivity of the sample was determined by use of complex impedance analysis. We also attempted to fabricate laboratory-scale all-solid-state batteries employing the obtained samples. The positive electrode of the test cells was a mixture of LiTi2P3O12-coated LiNi1/3Mn1/3Co1/3O2 (59 mass%), obtained solid electrolytes (39 mass%) and acetylene black (2 mass%) powders. In some cases, a high-interfacial resistance between the positive electrode materials and sulfide-based solid electrolytes has been observed for all-solid-state cells. To reduce the interfacial resistance, a coating of lithium-ion conductive oxides such as Li4Ti5O12, Li2SiO3, Li2ZrO3 and LiTi2P3O12 on the positive electrode materials is reported to be effective [9,11,12,26]. Thus, the LiTi2P3O12-coated NMC is used as positive electrode materials for the test cells. The negative electrode of the test cells was Li4.4Si alloy. The Li4.4Si alloy is a meta-stable phase, which is prepared by a mechanical milling process and shows good charge–discharge reversibility [27,28]. The meta-stable alloy has a wide solid-solution range of Li3.8Si to Li5.2Si. Thus, the meta-stable Li4.4Si alloy is available for the negative electrode materials for both types of all-solid-state batteries of which the charge–discharge cycles are started with the charge process (lithium insertion into the alloy) and/or with the discharge process (lithium extraction from the alloy).

The test cell was prepared by successively pressing the positive electrode mixture, 75Li2S · 5P2S3 · 20P2S5 glass-ceramic solid electrolyte powder, and the Li4.4Si alloy powder at 380 MPa into a pellet of 10 mm diameter. Galvanostatic charge–discharge experiments for the cells were performed at 25 °C under a constant current density of 0.1 mAcm−2 between 4.3 and 2.5 V at room temperature by use of a battery tester (Nagano, BTS-2004H). 3. Results and discussion 3.1. Electrical conductivity of the heat-treated glass ceramics The 75Li2S · xP2S3 · (25 − x)P2S5 (mol%) amorphous precursors can be obtained in the composition range of 0 b x b 5 by use of the high-energy ball-milling process. For the ball-milled samples with the compositional parameter x larger than 6.25, X-ray diffraction measurements suggested that a slight amount of Li2S crystalline phase (starting material) remained in the samples. The 75Li2S · xP2S3 · (25 − x)P2S5 glass-ceramic samples were obtained by heat treatment at a temperature in the range of 523 K to 563 K for 2 h. Fig. 1 shows the temperature dependences of the electrical conductivities of the amorphous precursor and the glass ceramic heattreated at 523 K for 2 h. The chemical composition of those samples is 75Li2S · 5P2S3 · 20P2S5(mol%). The conductivities were determined by use of usual impedance analysis about frequency-depending impedance data of the samples. The obtained conductivities follow the Arrhenius equation σ · T = A · exp(−Ea/RT). The conductivity at 298 K, σ298K, and the activation energy for conduction, Ea, of the samples were estimated by use of the Arrhenius equation. The conductivity at 298 K, σ298K, and the activation energy for conduction, Ea, of the 75Li2S · 5P2S3 · 20P2S5 glass ceramic are 2.0 × 10−3 Scm−1 and 32 kJ mol−1, respectively. On the other hand, those of the amorphous precursor are, respectively, 6.2 × 10−4 Scm−1 and 35 kJ mol−1. The high ion conductivity of the glass ceramic is about 3 to 4 times larger than that of the amorphous precursor. The compositional dependence of the σ298K and Ea of the heattreated glass ceramics in the composition 75Li2S · xP2S3 · (25 − x) P2S5 are shown in Fig. 2. In Fig. 2, open and closed circles denote σ298K and Ea, respectively. The lines in Fig. 2 are only a guide for eyes. The left side vertical axis shows the conductivity at 298 K, and the right side axis shows the activation energy. The σ298K increases with an increase in the compositional parameter x and attains the maximum value at a compositional parameter x = 5. Further increase in x leads to a decrease in σ298K of the glass ceramics. The σ298K of the glassceramic 75Li2S · 5P2S3 · 20P2S5 sample (x = 5) is 2.0 × 10−3 Scm−1.

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10

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1000K / T Fig. 1. Temperature dependence of the ac conductivity of amorphous precursor (closed circles) and glass ceramic (open circles) heat-treated at 523 K for 2 h of the composition 75Li2S · 5P2S3 · 20P2S5 (mol%).

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2θ /deg. (Cu-Kα) Fig. 3. X-ray diffraction spectra of (a) the amorphous precursor and (b) the heat-treated glass-ceramic samples of the composition 75Li2S · 5P2S3 · 20P2S5 (mol%). The X-ray diffraction spectra of (c) Li3PS4 crystal, (d) Li7P3S11 crystal, and (e) 80Li2S · 20P2S5 glass ceramic in which thio-LISICON phase II has been precipitated are also shown in the figure for comparison.

x in 75Li2S xP2S3 (25-x)P2S5 (mol%) Fig. 2. Composition dependences of conductivity at 298 K, σ298K, and activation energy for conduction, Ea, of the 75Li2S · xP2S3 · (25 − x)P2S5 (mol%) glass-ceramic samples.

On the other hand, activation energy for conduction, Ea, gradually decreases with an increase in x value in the range of x = 0 to x = 5. The Ea shows a minimum at x = 5. The chemical composition showing the minimum in Ea is the same as that of the maximum in σ298K of those samples. The conductivity at 298 K, σ298K, of the 75Li2S · 5P2S3 · 20P2S5 glass ceramic is extremely high value more than 10−3 Scm−1 at room temperature. The high ion conductivity of the glass ceramic is comparable to those of the Li7P3S11 crystal and/or of the 80Li2S · 20P2S5 glass ceramic in which thio-LISICON analogous crystalline phase is precipitated. In our knowledge, those two solid electrolytes, the Li7P3S11 crystal and the 80Li2S · 20P2S5 glass ceramic, are only known to have high ion conductivity larger than 10−3 Scm−1 at 298 K in the system Li2S-P2S5.

system Li2S-P2S5 that is known to have high ion conductivity higher than 10− 3 Scm− 1. Hence, the 75Li2S · 5P2S3 · 20P2S5 glass ceramic is made up of a new crystalline phase that has high ion conductivity at room temperature. Raman spectra of the amorphous precursor and the heat-treated glass ceramic samples of the composition 75Li2S · 5P2S3 · 20P2S5 are shown in Fig. 4. The Raman spectra of Li3PS4 crystal, Li4P2S7 glass and Li4P2S6 crystal are also shown in Fig. 4 for comparison. The spectrum of Li3PS4 crystal shows the strong band at 418 cm−1 and the band is assigned to the monomeric tetrahedral PS3− anion 4 unit [29,30]. The spectrum of Li4P2S7 glass shows the Raman band at 403 cm−1, and the band is assigned to the P2S4− anion unit, i.e., the 7 dimer ions of PS4 tetrahedra with P-S-P bonds [31]. The Raman band

(a)

3.2. Precipitated crystalline phases in the glass ceramic samples

(b) Intensity / a. u.

Fig. 3 shows XRD spectra of the amorphous precursor and the heattreated glass ceramic of the composition 75Li2S · 5P2S3 · 20P2S5, which has the highest conductivity among the obtained glass ceramic samples as shown in Fig. 2. The amorphous precursor only shows hallo pattern as its XRD spectrum (Fig. 3(a)). On the other hand, in the XRD spectrum of the heat-treated glass ceramic, new diffraction lines that are marked with open circles are appeared as shown in Fig. 3(b). The XRD spectra of Li3PS4 crystal, Li7P3S11 crystal and 80Li2S · 20P2S5 glass ceramic are also shown in Fig. 3(b) for comparison. Those XRD spectra well agree with the reported data in previous works [18–22]. In the spectrum of the 80Li2S · 20P2S5 glass ceramic, diffraction lines with closed circles are assigned to the Li2S crystal, and the lines with open triangles can be assigned to the thio-LISICON region II analogous phase. The Li4-yGe1-yPyS4 thio-LISICON crystals were reported to be divided into three composition regions with different types of cation ordering: region I (0 b y b 0.6), region II(0.6 b y b 0.8), and region III (0.8 b y b 1.0) [15]. The XRD patterns in region I and III were indexed by a monoclinic super-lattice cell of a × 3b × 2c, while the patterns in region II were indexed by that of a × 3b × 3c. The XRD spectrum of the 75Li2S · 5P2S3 · 20P2S5 glass ceramic does not agree with the spectrum of any crystalline phase in the

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Fig. 4. Raman spectra of (a) the amorphous precursor and (b) the heat-treated glass ceramic of the composition 75Li2S · 5P2S3 · 20P2S5 (mol%). The Raman spectra of (c) Li3PS4 crystal, (d) Li4P2S7 glass and (e) Li4P2S6 crystal are also shown here for identification of thio-phosphate structural units.

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capacity / mAhg-1 Fig. 6. Charge-discharge curves of the cell Li4.4Si|75Li2S · xP2S3 · (25 − x)P2S5 glassceramic|LiNi1/3Mn1/3Co1/3O2 at 25 °C. The current density of the charge–discharge measurements was 0.1 mAcm−2 (0.09C in C-rate). The abscissa in the capacity that was calculated on the base of the weight of the positive electrode material LiNi1/3Mn1/3Co1/3O2.

10 mV min-1 -0.1 -1

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E/V Fig. 5. Cyclic voltammogram of the 75Li2S · xP2S3 · (25 − x)P2S5 (mol%) glass-ceramic samples. The voltammetry was performed at room temperature with a scanning rate of 10 mV · min−1.

at 383 cm−1 that is observed in the spectrum of Li4P2S6 crystal is assigned to the P2S4− anion unit, i.e., the dimer ions of PS3 tetrahedra 6 with P-P bond [25]. In the spectra of the amorphous precursor and the heat-treated glass ceramic samples of the composition 75Li2S · 5P2S3 · 20P2S5, there are two Raman bands at 418 and 383 cm−1, which are respectively assigned to the PS3− and P2S4− anion units. Those results indicate 4 6 that both the amorphous precursor and the heat-treated glass ceramic consist of the same structural units, PS3− and P2S4− 4 6 , and that the local structure of anions does not changed by the heat treatment for crystallization at 523 K. The highly ion conductive 80Li2S · 20P2S5 glass ceramic was reported to contain only PS3− units as thio-phosphate anions [19]. The other 4 highly ion conductive Li7P3S11 crystal was reported to be constructed with only PS3− and P2S4− units by Hayashi et al. [19,22]. 4 7 The new glass ceramic 75Li2S · 5P2S3 · 20P2S5 consists of PS3− 4 and P2S46 − units. Hence, the crystalline phase precipitated in the 75Li2S · 5P2S3 · 20P2S5 glass ceramic is different from those in the 80Li2S · 20P2S5 glass ceramic and/or in the Li7P3S11 crystal. 3.3. Electrochemical properties of the 75Li2S · 5P2S3 · 20P2S5 glass ceramic Electrochemical stability of the obtained 75Li2S · 5P2S3 · 20P2S5 glass ceramic sample was investigated by using cyclic voltammetry. Lithium thin plate was attached on one face of the cold-pressed pellet and used as the counter and reference electrode. A stainless steel (SUS-316) plate was also attached on the other face of the pellet and used as a working electrode for the measurement. The potential sweep was performed in the range of −0.1 to 5.0 V vs. the lithium electrode with a scanning rate of 10 mV · min−1. Fig. 5 shows the cyclic voltammogram of the 75Li2S · 5P2S3 · 20P2S5 glass ceramic sample. The measurements are examined from the rest potential to the cathodic direction and then to the anodic direction up to +5 V vs. Li/Li+ electrode. The rest potential is indicated by the small arrow in Fig. 5. The lithium deposition (Li+ + e− → Li) and dissolution (Li → Li+ + e−) reactions are observed in the potential range of −0.1 V to 0.2 V in the voltammogram. Any other electrochemical reaction is not observed in the cyclic voltammogram over whole potential range of −0.1 V to 5.0 V. Those results suggest that the 75Li2S · 5P2S3 · 20P2S5 glass ceramic sample has prominent electrochemical stabilities as solid electrolytes for all-solid-state lithium batteries.

A laboratory-scale all-solid-state cell has been fabricated with the heat-treated 75Li2S · 5P2S3 · 20P2S5 glass ceramic sample, and the charge–discharge cycling tests of the cell have been carried out under a constant current density of 0.1 mA · cm−2 (0.09C in C-rate). In the test, charge–discharge criteria of voltage have been set 4.3 and 2.5 V, respectively. Fig. 6 shows the charge–discharge cycle behaviors of the solid-state cell (Li4.4Si | 75Li2S · 5P2S3 · 20P2S5 glass ceramic | LiNi1/3Mn1/3Co1/3O2) with the 75Li2S · 5P2S3 · 20P2S5 glass- ceramic as solid electrolyte and LiNi1/3Mn1/3Co1/3O2 (NMC) as positive electrode materials. In Fig. 6, the horizontal axis is the capacity calculated on the base of weight of the positive electrode material (NMC). At the first charge–discharge cycle, the charge capacity is about 183 mAhg−1 and the discharge capacity is 147 mAhg−1. The charge–discharge efficiency of the first cycle is about 80%. After second cycle, however, the test cell shows good charge–discharge cycle-ability; the charge–discharge capacities are about 143 mAhg−1 and the cycle efficiency is almost 100%. Those results as shown in Fig. 6 suggest that the 75Li2S · 5P2S3 · 20P2S5 glass ceramic sample has good electrochemical performance as solid electrolytes for all-solid-state batteries. 4. Conclusion Highly ion-conductive 75Li2S · 5P2S3 · 20P2S5 (mol%) glass ceramic solid electrolyte was prepared by heat treatment at 523 K of the amorphous precursor that was prepared by high-energy ball-milling process. The obtained glass ceramic showed high ion conductivity 2.0 × 10−3 Scm−1 at 25 °C and the low activation energy of 32 kJ mol−1 for conduction. Raman spectra revealed that the glass ceramic was composed of only PS3− and P2S4− as thio-phosphate anions. A laboratory-scale all4 6 solid-state lithium battery was also assembled by use of the glass ceramic as solid electrolyte and the battery showed good charge–discharge cycle performance at room temperature. Acknowledgment This work was supported by Japan Science and Technology Agency and by the Grand-in-Aid for Scientific Research, form the Ministry of Education, Culture, Sports and Technology of Japan. References [1] J.K. Park, Principles and Applications of Lithium Secondary Batteries, Wiley-VCH Verlag & KGaA, Germany, 2012. [2] In: C. Daniel, J.O. Besenhard (Eds.), Handbook of Battery Materials, 2nd ed., Wiley-VCH Verlag & KGaA, Germany, 2011. [3] T. Minami, Solid State Ionics for Batteries, Springer-Verlag, Tokyo, 2005. [4] K. Takada, K. Iwamoto, S. Kondo, Solid State Ionics 86–88 (1996) 877. [5] N. Machida, H. Maeda, H. Peng, T. Shigematsu, J. Electrochem. Soc. 149 (2002) A688.

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