synthesis and characterization of new solid electrolyte

SYNTHESIS AND CHARACTERIZATION OF NEW SOLID ELECTROLYTE LAYER (LI2O)2(P2O5)Y* EVVY KARTINI, WAGIYO HONGGOWIRANTO, SUPARDI, HERI JODI, AZIS K.JAHYA, WAHYUDIANINGSIH Center for Science and Technology of Advanced Materials,National Nuclear Energy Agency, Puspiptek Area, Serpong, South Tangerang 15314, Indonesia

The energy storage materials, such as lithium ion battery is very important for the application in the electric vehicles, electronic devices or home appliances. The battery contains electrodes (anode and cathode), separator and the electrolyte materials. The existing organic liquid electrolyte is a flammable material, when contact with open atmosphere. The use of a separator in between electrodes is also un-safety, due to low melting point and easily to get short. In an effort to overcome these problems, many researches have been focused to find new solid electrolytes for replacing the liquid electrolyte. In order to overcome this problem, mixtures of solid electrolytes (Li2O)x(P2O5)y with the ratio of x:y = 1:1, 2:1, 3:1, 4:1 and 5:1 has been synthesized by melt quenching method. The conductivities are increasing with various Lithium contents from 10-8 S/cm to 10-5 S/cm. The final products show that the conductivity maximum was observed for (Li2O)2(P2O5). Furthermore, in order to obtain the solid electrolyte layer that is applicable for solid state battery, the addition of some polymer binders such as PVDF is recommended. The powders of 95% (Li2O)2(P2O5) and 5% PVDF were mixed and dissolved in NMP. The new solid electrolyte layer based lithium phosphate oxide with the thickness of less than 0.5 mm was obtained and applied as a solid electrolyte in the solid state rechargeable battery. It is expected, that this layer will replace the existing separator and liquid polymer electrolyte, and it is promising for applications in high powers and energy density batteries with higher safety, stability, and reliability. Keywords: Li3PO4, LiPO3, (Li2O)x(P2O5)y, solid electrolyte; electrochemical properties

*

This work is supported by research Grant SINAS Consortium of Lithium Battery, Ministry Research and Technology, Indonesia. 1

Proceedings of the 14th Asian Conference on Solid State Ionics (ACSSI 2014) Copyright © 2014 ACSSI 2014 Organizers. All rights reserved. ISBN: 978-981-09-1137-9 148 doi:10.3850/978-981-09-1137-9 147

Proceedings of the 14th Asian Conference on Solid State Ionics (ACSSI 2014)

1. Introduction 1.1. Scientific Background The energy storage materials, such as lithium ion batteries are very important for the application in the electric vehicles, electronic devices or home appliances. A rechargeable lithium-ion battery (LIB) consists of cathode, anode, electrolyte and separator, which are very important in producing very high energy density of battery [1]. There are many challenges to improve the existing lithium ion battery materials, in order to increase their life time, cyclic ability and also their stability and safety. Improvements for better electrode materials such as LiCoO2, LiMn2O4 and LiFePO4, and their corresponding systems are very crucial [2,3]. On the other hand, another important aspect of safety and friendly environment are still in the highest list of demand for the electrical energy storage. It has been tremendous issues, that the use of organic liquid electrolyte will give potential hazard on using current Lithium ion battery. A combustion of the electronic devices or cars, due to explosive materials in the separator or liquid-electrolyte is necessary to be resolved. In an effort to improve the safety of next-generation batteries, the research has been focused on finding new solid electrolytes with high ionic conductivity and electrochemical stability [4,5]. Superionic conducting glasses, such as lithium phosphate glasses LiPO3, is very important not only in the application as lithium-ion battery component, but also in the scientific point of view [5,6]. However, the ionic conductivity of this system is still low around 10-8 to 10-7 S/cm at room temperature. The lithium solid electrolyte Li3PO4 has become more interesting in the application of all solid state battery, and extremely suitable for the all thin film solid state battery, though the ionic conductivity of Li3PO4 is still low at 5.1×10−7 S cm-1 [7]. Due to this reason, our research focused on developing lithium phosphorous oxide superionic conductor, with the aim to obtain better solid electrolyte with higher ionic conductivity and better thermal and mechanical stability. A previous work on Li2O-P2O5 was performed by P.Dabas and K.Hariharan [8], but they focused on increasing the number glass former, P2O5. The former Li2O acts as the glass modifier, while the later P2O5 as the glass former. So far there are no studies on Li2O-P2O5, with highly containing lithium oxides, Li2O. In order to improve performance of existing lithium phosphate solid electrolytes LiPO3 and Li3PO4, our group developed new series of lithium-phosphorous 149


superionic conductor, (Li2O)x(P2O5)y by increasing a number of lithium contents [9]. Various (Li2O)x(P2O5)y glasses containing large amounts of Li2O were synthesized by melt quenching method [9]. The conductivities are increasing with various Lithium contents from 10-8 S/cm to 10-5 S/cm. The final products show that the conductivity maximum was observed for (Li2O)2(P2O5). Furthermore, in order to obtain the solid electrolyte layer (Li2O)2(P2O5) that is applicable for solid state battery, the addition of some polymer binders such as PVDF is recommended. It is expected, that this layer will replace the existing separator and liquid polymer electrolyte, and it is promising for applications in high powers and energy density batteries with higher safety, stability, and reliability 2. Experimental Methods 2.1.Synthesis Mixtures of solid electrolytes (Li2O)x(P2O5)y with the ratio of x:y = 1:1, 2:1, 3:1,4:1 and 5:1 were prepared by melt quenching methods. The appropriate amounts of lithium carbonate, Li2CO3 (Aldrich, 99.99%) and ammoniumdihydrogen-phosphate, NH4H2PO4 (Merck, 99.99%) were mixed and heated gradually up to 800oC. The molten mixture was then quenched in the liquid nitrogen. Detail preparation has been described elsewhere [9]. The transparent bulk glass of LiPO3 was obtained for the composition x:y=1:1.. A product of (Li2O)2(P2O5) showed opaque bulk glass. A product of (Li2O)3(P2O5) resulted a white granular powder corresponds to crystalline Li3PO4. Another mixture containing large number of Li2O resulted white granular powder. All bulk glasses and granular powder were finely crushed and grinded by using an agate mortar and used for different characterizations. Furthermore, in order to obtain the solid electrolyte layer that is applicable for solid state battery, the addition of some polymer binders such as poly vinylidene fluoride (PVDF, Aldrich) is recommended. The powders of 95% (Li2O)2(P2O5) and 5% PVDF were mixed and dissolved in N-methyl pyrrolidinone (NMP, 99% Alrich). The electrolyte slurry was coated on the separator and dried in the vacuum. A new solid electrolyte layer (Li2O)2(P2O5) with the thickness of less than 0.5 mm was obtained and applied as a solid electrolyte in the solid state rechargeable battery 150


2.2. Characterizations In order to complete these features, several characterizations have been conducted of these materials. The crystalline phases present in the crystallized samples were identified by X-ray diffraction (XRD) analyses (CuKa radiation) at room temperature. The microstructures of these samples were measured by a Scanning Electron Microscope (SEM). Electrical conductivities of the glasses and glass–ceramics, were measured by an alternating current (AC) impedance method using an impedance analyzer (LCR meter Hioki) with the oscillation level of_1 V in the frequency range of 1 Hz–1kHz 2.3. Electrochemical Measurements The electrochemical measurements were carried out using a CR-2032 type coin cell with a Celgard membrane separator (MTI), and 1 M LiPF6-ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 in volume, Merck) as electrolyte. The composite cathode electrode was composed of 80 wt.% active material, 10.% conductive carbon black (MTI) and 10 wt.% PVDF, which were mixed in N-methyl pyrrolidinone (NMP) solvent to dissolve the binder and to obtain viscous slurry. The slurry was then coated on to an etched aluminum foil (20 μm thickness) using doctor Blade method. Cells were assembled in Argon-filled glove box (VYGOR, USA). Li-metal foil with thickness of 0.2 mm (MTI, USA) was used as counter (anode) and reference electrode. Galvanostatic cycling between 3.2-4.0 Volt vs Li at room temperature was carried out using 8 Channel Battery Analyzer (0.002-1mA, up to 5 V) with software program for Battery testing (Model BST8-WA, MTI USA). 3.Results and Discussions 3.1. X-ray Diffraction The X-ray diffraction data for the composition 1:1 is showing only a halo pattern. This result confirms that (Li2O)(P2O5) an amorphous phase. The broad peak position is at 2Θ~28o, corresponding to the peak of crystalline LiPO3. The composition of 2:1 is partial-crystalline phase. The diffraction contains background from the crystalline LiPO3 and other unknown crystalline phase.

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The composition of 3:1 gives the results of (Li2O)3(P2O5) corresponding to the crystalline structure of Li3PO4. This result was confirmed by quick analysis using a fool proof refinement method on the X-ray data, that those majority of Bragg peaks are belong to the γ and ȕ phases of Li3PO4. Further increasing Li2O, for (Li2O)4(P2O5) with composition 4:1, the samples show similar crystal structure of earlier phase Li3PO4 with unknown phase. But, more detail refinement is still needed. A different crystal structure was obtained for (Li2O)5(P2O5) with the composition of 5:1.

d

Figure 1. X-ray diffraction of the (Li2O)x(P2O5)y with various compositions ( x:y=1:1, 2:1,3:1, 4:1 and 5:1) [9]

3.2. Conductivity Figure 2 shows the ionic conductivity of the series sample (Li2O)x(P2O5)y. The ionic conductivity is of LiPO3 was around 10-8 S/cm, as previously reported [6]. The total conductivity consists of DC conductivity, which is frequency independent, ıDC, and the AC conductivity ı(Ȧ). The frequency dependent of conductivity data were fitted with the power law equation ı(Ȧ) = A f(Ȧ)n , where 152


A is the pre-exponential factor and n is the exponent power. Table 1 shows the DC conductivity, A and n values for (Li2O)x(P2O5)y with various compositions.

ŽŶĚƵĐƚŝǀŝƚǇŽĨůĞĐƚƌŽůǇƚĞ>ŝ(L 2O) x(P ϮKͲW ϮK ϱ 2O5)y ϭ͘ͲϬϰ

ȋȀȌ

2:1 ϭ͘ͲϬϱ

5:1 ϭ͘ͲϬϲ

4:1

ϭ͘ͲϬϳ

3:1 ϭ͘ͲϬϴ ͳǤΪͲͳ

ͳǤΪͲʹ

ͳǤΪͲ͵

ͳǤΪͲͶ

ͳǤΪͲͷ

ȋȌ

Figure 2. Conductivity of (Li2O)x(P2O5)y with ( x:y=2:1,3:1,4:1 and 5:1) [9] The conductivities are rather similar for (Li2O)4(P2O5) and (Li2O)3(P2O5). namely 10-7 S/cm. However, for (Li2O)2(P2O5) and (Li2O)5(P2O5)., the conductivities are 100 times higher than those compositions, about 10-5 S/cm. The final products showed that the highest conductivity was about 6.78 10-6 S/cm for (Li2O)2(P2O5). It is reflecting that this material is good for battery applications. Table 1. Conductivity measurement for (Li2O)x(P2O5)y with various compositions. No.

Ratio x:y

σDC (S/cm)

A

n

1.

2:1

6.7881 E-06

1.3019 E-09

0.55597

2.

3:1

1.1004 E-07

3.6307 E-11

0.7531

3.

4:1

2.2163 E-07

4.6341 E-12

0.9421

4.

5:1

6.8903 E-06

9.5558 E-08

0.4375

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3.3. Scanning Electron Microscope (SEM) The microstructures of bulk (Li2O)2(P2O5) and (Li2O)3(P2O5) are shown in the figure 3(a) and 3(b), respectively. (a

(b

(c

(d

Figure 3. SEM photograph of (a) bulk (Li2O)2(P2O5) ; (b) bulk (Li2O)3(P2O5) ; (c) layer. (Li2O)2(P2O5) and (d) layer (Li2O)3(P2O5)

The mixture of (Li2O)2(P2O5) produced a partially crystalline sample, as shown in Fig.3(a). This is due to over the solubility limit of that LiPO3 glass with the addition of Li2O. However, the base glass LiPO3 is still a major fraction of this sample, as confirmed by X-ray data. Further increasing amount of Li2O, the sample crystalized with different crystal structures, as shown in Figures 3(b) for (Li2O)3(P2O5) The results showed various shapes of crystallized samples with the grain boundaries around 10-100 μm corresponds to a crystalline Li3PO4, a wellknown solid electrolyte material for solid state battery. The layers of (Li2O)2(P2O5) and (Li2O)3(P2O5) are shown in the figure 3(c) and 3(d), respectively. After mixing with polymer binder, both solid electrolyte layers showed more homogenous structures with less grain boundary and clusters. It is important to obtain a homogenous surface layer in order to have a better contact and to avoid the short circuit during application in the battery cell.

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3.3. Coin Battery Cell In order to check performance of the new solid electrolyte layer, several arrangements were made in the form of coin cell. The first and second cells consist of LiMn2O4/Separator+LiTf3/Li and LiMn2O4/Separator+LiPf6/Li, which can be used as references. Another two cells are using the solid electrolyte layers (Li2O)2(P2O5) and (Li2O)3(P2O5), with the arrangements LiMn2O4/ layers (Li2O)2(P2O5) +LiPf6/Li and LiMn2O4/ layers (Li2O)2(P2O5) +LiPf6/Li. In this case, a small amount (ppm) of LiPf6 was used just to start contacting the interface between the solid electrolyte and the cathodes. As the preliminary test, the conductivities of the coin cells were measured by impedance spectrometer. Those values of conductivities are shown in Table 3. It is clear that the ionic conductivity of the cell using solid electrolyte layer have a higher conductivity compared with the other cells that using only the separator. The highest conductivity was achieved for the cell with the solid electrolyte layer (Li2O)2(P2O5) which is about 2.318 10-2 S/cm, while the cell with commercial solid electrolyte layer Li3PO4 is around 8.974 10-3 S/cm. Table 3. Conductivity of coin cells with various arrangements [10] . No 1. 2. 3. 4.

Sample Cell_0 Cell_1 Cell_2 Cell_3

σDC (S/cm) 1.469 E-03 4.174 E-04 2.318 E-02 8.974 E-03

A 4.5059e-013 2.0583e-014 1.0514e-011 1.5101e-011

n 1.258 1.3878 1.2358 1.0674

3.4.Electrochemical Studies The galvanostatic charge-discharge profiles from the initial, until the 50th of the cell-2 (LiMn2O4/(Li2O)2(P2O5)/Li and between 3.2-4.0 V are displayed in As shown in the figure, this all solid state battery exhibited low specific capacity. The initial cycle charge capacity and discharge capacity were 0.015mA/mg and 0.01 mAh/mg, respectively, hence the sample weight was 0.06 mg.

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Figure 4. (a) Galvanostatic charge-discharge profiles from the initial, until the 50th of the cell-2 (LiMn2O4/(Li2O)2(P2O5)/Li and between 3.2-4.0 V, (b) Charge-discharge capacity at a current 0.01 mAh.

Figure 5. The charge and discharge process of the cell-2

The charge and discharge current vs time plot of the cell-2 from the initial until 14th is shown in figure 5. The charge and discharge process for one cycle took about 2.5 hours, and relatively stable until 14th cycle and 50th cycle. The charge and discharge capacity vs. cycle number plot of the cell-2 up to 14th cycles is shown in Fig.6 and up to 50th cycles, not shown here. The energy efficiencies up to 50th cycles of cell-2 are still stable from the initial discharge 99% to 98% and 95%, at the 14th and 50th cycles. Moreover, the charge and discharge plots stay close after the 50th cycle, which indicates a good electrochemical reversibility. The reason of the stability after the fifth cycle can be attributed to the ‘formation cycle’ of the electrode and solid electrolyte material during initial cycles. 156


.

Figure 6. The charge and discharge efficiency of the cell-2 (Please noted that the 15 th cycle was just interrupted during the cycle)

4. Conclusions The new solid electrolyte based on lithium phosphorous oxide glass, (Li2O)x(P2O5)y have been synthesized and characterized. The maximum conductivity of about 10-5 S/cm was observed for the solid electrolyte with the composition of (Li2O)2(P2O5) . The solid electrolyte layer of (Li2O)2(P2O5) with the thickness of 0.5mm was made by mixing with polymer binder and used for a battery. A coin battery cell with the arrangement of LiMn2O4/ (Li2O)2(P2O5)/Li showed a better ionic conductivity of about 2.3 10-2 S/cm in comparison with the conductivity of cell-3 with commercial Li3PO4 (8.9 10-3 S/cm). Though the capacity of the battery is still low, at 25 mAh/g, but charge-discharge efficiencies are relatively stable even after 50th cycles. Further improvement of solid electrolyte is very important in order to obtain all solid state battery. Acknowledgments The Ministry Research and Technology, Indonesia, for funding though the Research Grant of SINAS Consortium on Lithium Ion Batteries. References 1. Shi-Xi

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