Ionic Conductivity and Stability of the Lithium

ECS Transactions, 72 (8) 139-146 (2016) 10.1149/07208.0139ecst ©The Electrochemical Society

Ionic Conductivity and Stability of the Lithium Aluminum Germanium Phosphate Y. Cui, M. Rohde, T. Reichmann, M. Mahmoud, C. Ziebert, H. J. Seifert Institute for Applied Materials - Applied Materials Physics (IAM-AWP), Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany

Lithium aluminum germanium phosphate (LAGP) solid state electrolytes with different compositions were synthesized and then heat-treated at different temperatures. The ionic conductivity of the prepared samples was studied. Lithium ion cells using LAGP as solid state electrolyte were assembled and tested. The stability of the prepared LAGP samples was investigated.

Introduction Nowadays lithium ion batteries (LIB) are the most important portable power sources for electronic equipment of different kinds. They have a number of advantages such as high energy density, no memory effect and low self-discharge. However, safety concerns still exist. The liquid electrolyte used in LIB is inflammable and becomes instable at elevated temperature. Furthermore, the separator in LIB is also vulnerable due to the growing of Li dendrites. The lithium aluminum germanium phosphate (LAGP) solid-state electrolytes are thermally more stable, non-flammable with higher melting points compared to the commercially used liquid and polymer based electrolytes [1]. Since LAGP is a dense ceramic material, it is also expected to be able to withstand the dendrite growth, which will allow the application of Li metal anode. Lithium aluminum germanium phosphate (LAGP: Li1+xAlxGe2-x(PO4)3) have a NASICON (Sodium(Na) Super Ionic Conductor) structure of its crystalline phase. The crystal of LAGP is built up by GeO6-octahedra and PO4-tetrahedra which are linked to each other by their corners forming a 3D-skeleton. They are oriented along the c-axis through which the diffusing Li-ions can pass [2–4]. Doping of the parent structure LiGe2(PO4)3 by Al3+ ions will introduce additional charge compensating Li+ ions and also increase the size of the bottle neck in Li diffusion paths. Consequently, it will lead to higher ionic conductivity with lower activation energy [5, 6]. In this study LAGP samples with different compositions (different Li content) were produced and characterized. To test the functionality of the LAGP as solid electrolyte, Liion cells were assembled and tested. The stability of prepared LAGP materials was investigated. Experimental work LAGP glasses were prepared using the melt-quenching-route. Similar procedures were also employed in several studies [3, 7]. Li2CO3 (Fluka, 99.0%), Al2O3 (SigmaAldrich, 98.5%), P2O5 (Analar Normapur, 99.1%) and GeO2 (Alfa Aesar, 99.98%) were used as starting materials. The mixture of these powders was heated up to 1450 °C using

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ECS Transactions, 72 (8) 139-146 (2016)

a heating rate of 5 °C/min in an Al2O3 crucible and held at that temperature for 10 min. The melt was then quenched on a steel plate at room temperature and pressed with another steel plate immediately to form a thin glass disc. The glass disc breaks into pieces due to the thermal stress. The glass pieces were heat-treated to be transformed into the crystalline materials with better ionic conductivity. The chemical composition of the prepared glass was analyzed using inductively coupled plasma-optical emission spectrometry (ICP-OES, Optima 4300 DV, PerkinElmer). The oxygen content was calculated from the amount of the other elements and their oxidation number. The ionic conductivity of the LAGP samples was measured using impedance spectroscopy (Sourcetronic 2826). The principles of these measurements were explained elsewhere [8, 9]. The ionic conductivity was measured from room temperature up to 240 °C. To test the functionality of LAGP as solid electrolyte in lithium ion secondary battery, test cell was assembled where LiCoO2 was used as cathode and Li metal as anode. A thin(0.4 mm thick, 10 mm diameter) cylindrical LAGP tablets were sintered from LAGP glass powder which was milled from the prepared LAGP glass using the process reported in [10]. LiCoO2 was sputtered on this polished LAGP tablet using radio frequency magnetron sputtering. Li metal foil was pressed onto the other side of the LAGP tablet. The cell was tested with the Arbin BT2000 battery cycler at 100°C. Cyclic voltammetry (CV) was carried out on LAGP vs. Li metal to test its stability. A Biologic multifunctional tester(BCS-815) was used to register the CV diagram. Results and discussion Ionic conductivity of LAGP The overall chemical formulas of 3 different LAGP batches are shown in Table 1. They are named as high Li, medium Li and low Li, respectively. In previous study the optimized heat treatment temperature was found at 800°C [10]. All the prepared samples were heat treated at 800°C for 6 hours. The ionic conductivities of these samples are shown in figure 1. From the figure, it is clear that the low Li batch has a lower ionic conductivity than the other two compositions at all temperature. The medium Li sample and the high Li sample have similar conductivity values at room temperature (around 2∙10-4 S cm-1), which is in the same range as reported by other authors [11, 7]. On the other hand, when the temperature increases above 50°C, the high Li sample shows a higher ionic conductivity than the medium Li sample. The similar conductivity values of these two compositions at room temperature could be attributed to the space charge effect of the AlPO4 minor phase, which was also reported by Kumar et.al. The space charge effect decreases the ionic conductivity at lower temperature [12]. At elevated temperature the space charge effect has weakened and the higher Li content increases the ionic conductivity. At 100°C the ionic conductivity reaches a level close to 10-2 S cm-1, which is already comparable with the values of liquid electrolytes. Stability of LAGP LIB cells using LAGP were assembled and then charged and discharged. However, the test shows that the capacity has a strong decay within the first cycles (Figure 2). After the cycling test the cell was disassembled and investigated where a black substance was found at the interface between the LAGP solid electrolyte and the Li metal anode. Figure 3 shows the cross section of LAGP with the interface next to Li anode. It shows that the

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reaction zone induced the generation of cracks in the LAGP surface where the volume of LAGP has increased. This indicates that the black substance should not be simply formed from deposited material (e. g. lithium) on the LAGP surface. In order to exclude the influence of the LiCoO2 or any other impurities, Li metal foil was simply pressed on polished LAGP pieces (from the high Li batch) and the tablets were held in Argon filled glovebox for 2 weeks. Figure 4 shows the light microscope and SEM picture of LAGP with the surface, which has reacted with Li metal. It shows that a black reaction phase has formed on LAGP. The SEM picture shows that cracks appear in the LAGP surface due to volume changes in the reaction zone. Cyclic voltammetry was carried out to investigate when this reaction starts on LAGP. The cell with the specific structure shown in figure 5 was assembled where the LAGP tablet works as positive electrode at the side sputtered with gold and as electrolyte at the side pressed against Li foil. When reaction takes place on LAGP, electrons could flow through the external circle and the gold current collector to the LAGP. The Li+ ions could migrate across the LAGP/Li interface and through the LAGP tablet to the reaction zone near the gold current collector. The voltage applied on this assembled cell varies with the rate of 5 mV/s. The CV diagram is shown in figure 6. The voltage increases to 4.0 V at first, decreases down to 0 V and then increases to 4.0 V for multiple cycles. As the voltage got lower than 0.9 V, the reduction current started to increase. It shows that the LAGP began to be reduced at a voltage lower than 0.9 V vs Li. As the voltage further decreased till 0.2 V, the reduction current increased gradually. After that the current began to increase drastically, which could be attributed to the reduction of Li+ ions. While the voltage increases, an oxidation peak appeared at 0.6 V. This peak is due to the oxidation of Li, which was formed during the reduction process. A broad oxidation peak appeared at 1.1 V. This peak could be attributed to the oxidation of the reduction product from LAGP. Another series of CV scans were carried out between 0.3 and 4.0 V. Since the voltage stayed higher than 0.3 V, Li+ ions were not reduced. The oxidation peak at 0.6 V has also not appeared again. The reduction current between 0.3 and 0.9 V and the oxidation peak at 1.1 V remained the same as in the previous scans. The cyclic voltammetry has shown that the LAGP was reduced when the potential vs Li is lower than 0.9 V. The reduction product from LAGP was oxidized at around 1.1 V. For the application of LAGP as solid electrolyte in LIB, this observed stability problem need to be further investigated and solved. In the work of Peña et. al and Yoon et.al, The CV diagram of GeO2 was measured vs Li metal [13, 14]. The GeO2 shows several reduction peaks at the voltage lower than 1 V, which was attributed to the reduction of Ge4+ ions and the reduction of Li+ ions. An oxidation peak at around 0.5 V due to the oxidation of Li was also observed. At around 1.2 V an oxidation peak also appears which was attributed to the oxidation of the reduced Germanium ions. The similarity of both CV measurements indicates that the reaction on LAGP could also be due to reduction of Ge4+ ions. Since no GeO2 phase was detected or found in the LAGP samples investigated in this study, it is most likely that Ge4+ ions in LAGP were reduced. However, it is not quite clear what kind of substance was formed. The reaction product was measured with XRD but no phase other than LAGP, AlPO4 could be identified. This could be due to amorphous phase formation, but it could be also possible that the crystalline phase was too small to be detected with XRD. In the work of Peña et.

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al it was also reported that an amorphous phase will be formed when GeO2 and Li react together [13]. In the study of Hartmann et.al the lithiation of LAGP was also investigated [15]. Their results have shown that fully reduced Ge and intermediate reduced Ge ions (between Ge4+ and Ge0) can be formed which was confirmed using XPS measurements. Conclusion In this work the ionic conductivity of LAGP samples with three different Li contents were investigated. At elevated temperature (higher than 50°C) the ionic conductivity shows dependence on the Li content. The ionic conductivity increases with increasing Li content. LIB cells with LAGP as solid electrolyte were assembled. However, the LAGP showed a reaction with Li. At a voltage below 0.9 V vs Li the LAGP was reduced. Ge4+ ions in the LAGP were reduced to lower oxidation number. The LAGP can be used as solid electrolyte in lithium ion batteries only when the stability problem is investigated and solved.

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Ionic conductivity T (°C) 250

200

150

100

50

0.1

(Scm-1)

0.01

1E-3

High Li Medium Li Low Li

1E-4

1E-5 2.0

2.5

3.0

3.5

1000/T (1/K)

Figure 1. The ionic conductivity of the LAGP samples with different composition. The logarithmic value of the ionic conductivity was plotted against 1000/T, where T is the measuring temperature. 4.5

Voltage(V)

Current(mA)

0.00

3.5

Currnet (mA)

0.01

4.0

Voltage (V)

0.02

-0.01

3.0 -0.02

0

20000

40000

Test time (s)

Figure 2. Voltage and current of the test cell using LAGP solid electrolyte.

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Figure. 3 Cross section of the LAGP solid electrolyte tested in the LIB cell. The reaction zone is in the circle.

Figure. 4 Light microscope and SEM picture of the LAGP reacted with Li metal.

Figure. 5 Setup of the cell for cyclic voltammetry. Gold was sputtered on one side of the LAGP tablet as positive current collector. Li foil was pressed on the other side of LAGP as negative electrode.

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0.02

1st-3rd cycle 0V - 4V 4th-6th cycle 0.3V - 4V Li reduction and oxidation

Current (mA)

0.01

0.00

-0.01

reaction between LAGP and Li

-0.02

-0.03 0

1

2

3

4

Voltage vs Li (V)

Figure 6. Cyclic voltammetry diagram of the cell shown in figure 5

TABLE I. The overall chemical formula of different LAGP batches LAGP Chemical formula High Li Li1.72Al0.54Ge1.37P3O11.9 Medium Li Li1.52Al0.52Ge1.36P3O11.7 Low Li Li1.27Al0.41Ge1.48P3O11.7

Acknowledgments We gratefully acknowledge the funding from the Helmholtz Association in Germany within the framework of the Helmholtz Energy Alliance “Stationary Electrochemical Storages and Converters” (HA-E-0002).

1. 2. 3. 4. 5. 6. 7. 8.

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