A Study of Li_3.8Ge_0.9S_0.1O₄ Solid Electrolyte Stability Relative to Electrode Materials of Lithium Power Sources

Abstract

The stability of Li_3.8Ge_0.9S_0.1O₄ lithium-conducting solid electrolyte versus lithium metal and Li–V bronze Li_1.3V₃O₈ is studied in the present research. Isothermal heat treatment and thermal analysis of the mixtures of Li_1.3V₃O₈ and Li_3.8Ge_0.9S_0.1O₄ powders indicate that there is no interaction between them below 300–350 °C. Moreover, Li_3.8Ge_0.9S_0.1O₄ solid electrolyte is stable versus lithium at 100 °C for 240 h. A model of a lithium-ion power source with a Li_1.3V₃O₈-based cathode and a lithium metal anode is assembled and tested. The data obtained show that Li_3.8Ge_0.9S_0.1O₄ can be used in all-solid-state medium-temperature lithium and lithium-ion batteries.

1. Introduction

High-energy Li and Li-ion powersources have established themselves among the key power sources for consumer electronics due to high values of specific energy, largest voltage, low self-discharge [1,2,3]. However, despite these advantages, the devices in question share a common drawback. The overwhelming majority of Li and Li-ion batteries nowadays use electrolytes containing organic liquids, which creates problems associated with the probable electrolyte leakage or overheating and boiling of the solvent during operation. In addition, in many cases, organic solvents are flammable, and the use of such power sources under certain conditions may result in combustion and explosion [3,4,5]. One possible way of addressing this issue is a transition to all-solid-state lithium batteries [6,7], in which both electrode materials and the electrolyte are solids. Thus, designing all-solid-sate lithium and lithium-ion power sources is among crucial tasks of solid state electrochemistry. This is confirmed by a constantly growing number of publications on this topic, e.g., [8,9].

While essentiallythe same materials can be used as electrodes in traditional lithium batteries with a liquid electrolyte and in all-solid-state power sources, designing all-solid-state power sources involves the usage of solid electrolytes that should meet certain requirements that considerably limit the number of suitable compounds [10,11,12,13]. The main requirement is a high ionic conductivity. In the case of Li and Li-ion power sources, these should have alithium–cation conductivity not less than 10⁻⁶ S∙cm⁻¹ at ambient temperature. Moreover, the electronic conductivity should be as low as possible in order to prevent self-discharge.

Various solid electrolytes satisfying such requirements have been reported, e.g., electrolytes with a garnet-type structure [14,15,16], NASICON-like [17,18,19], perovskite [20,21], glass–ceramic Li_1.6Al_0.6Ge_1.4(PO₄)₃ electrolytes [21], and also LISICON-structured phases [22]. The latter type of solid electrolytes has long been known and is well-investigated [21,23,24]. The type includes a lot of varieties, but derivatives of lithium orthosilicate and orthogermanate (Li₄SiO₄ and Li₄GeO₄, respectively) offer the highest conductivity and are, therefore, the most suitable option for all-solid-state power sources. Solid solutions of Li₄SiO₄–Li₃PO₄ system have until recently been the most common LISICON-structured solid electrolytes for Li batteries [25,26,27,28], since they have a rather high lithium–cation conductivity (about 10⁻⁶ S∙cm⁻¹) with a satisfactory stability versus metallic Li at temperatures close to ambient [29]. However, nowadays, we also find works reporting on solid solutions based on lithium orthogermanate in a Li₄GeO₄–Li₂SO₄ system that are used as solid electrolytes for lithium power sources [30,31]. Solid solutions in a Li₄GeO₄–Li₂SO₄ system have a slightly higher conductivity than the solid electrolytes of a Li₄SiO₄–Li₃PO₄ system [32], which should compensate for the higher cost of germanium compared to silicon. In addition, when subjected to a special treatment, Li_4-2xGe_1−xS_xO₄ solid electrolytes form a glass–ceramic material offering a high lithium–cation conductivity and also formability, which makes it possible to decrease the contact resistance at the electrode–electrolyte interface [30].

In addition toa high ionic conductivity, solid electrolytes for all-solid-state batteries should be characterized by stability in contact with electrode materials during operation. The interaction of most solid electrolytes with lithium metal is the major challenge presented by lithium power sources. Since there are very few compounds thermodynamically stable versus Li, the solid electrolytes should at least possess kinetic stability at the duration and temperatures of the supposed power source operation. The work presented in [31] studies the power source with a Li_3.6Ge_0.8S_0.2O₄ solid electrolyte, lithium anode and LiNi_1/3Mn_1/3Co_1/3O₂ used as cathode; however, since in the above-mentioned work lithium was separated from the electrolyte using a polyethylene oxide film, one cannot draw conclusions about the reaction of Li_3.6Ge_0.8S_0.2O₄ with metallic lithium on the basis of the data from this paper.

Vanadium oxides and lithium vanadates, in particular, Li_1+xV₃O₈ lithium–vanadium bronzes, are currently viewed as promising cathode materials for lithium and lithium-ion power sources [33,34,35]. Lithium–vanadium oxide LiV₃O₈ was previously shown not to react with Li_3.4Si_0.4P_0.6O₄ phosphate–silicate solid electrolyte below 300 °C [29]. There are no data on the chemical interaction of lithium–vanadium bronzes with Li₄GeO₄-based solid solutions in the available literature.

The purpose of this work is studying the stability of lithium-conducting solid electrolyte based on lithium orthogermanate of a Li₄GeO₄–Li₂SO₄ system in contact with Li metal and Li_1+xV₃O₈ vanadium bronze.

2. Materials and Methods

2.1. Preparation of Samples

Highly conductive composition in a Li₄GeO₄–Li₂SO₄ system, containing 90 mol.% Li₄GeO₄ and 10 mol.% Li₂SO₄, which can also be written as Li_3.8Ge_1.9S_0.1O₄, was chosen as the solid electrolyte. Li₂CO₃ (reagent grade), GeO₂ (extra pure grade) and Li₂SO₄ (reagent grade) obtained from VEKTON, Russia were used as the starting reagents for the preparation of the solid electrolyte. The components were dried at 300 °C, and the required amounts were weighed within the accuracy of ± 10⁻⁴ g (FX-40CJ analytical balance, Tokyo, Japan, BMI Surplus), mixed in a jasper mortar and sintered in alundum crucibles as follows: 750 °C/4 h→900 °C/8 h→1000 °C/8 h→1000 °C/8 h. After each heat treatment stage, the reaction mixture was reground and sieved to 0.1 mm mesh size. The material thus obtained was then ground to fine powder (particle size below 0.05 mm) and pressed into round tablets measuring ~ 10 mm in diameter and 1–1.5 mm height and bars~5 mm width, 6 mm thickness, 2–3 mm heightin a stainless steel die at 2–3 t/cm². The samples were sintered on a platinum sheet for 8 h at 1080 °C, the density of the tablets thus produced was ~95% of the theoretical.

Lithium–vanadium oxide of Li_1.3V₃O₈ composition was used as cathode material. The oxide was produced by the reaction of NH₄VO₃ (analytical grade)and Li₂CO₃ (reagent grade) taken in a 6:1.3 mole ratio in a distilled water solution and subsequent heat treatment at 400 °C for 4 h [29].

2.2. Samples Characterization

The completeness of the LiV₃O₈ and Li_3.8Ge_0.9S_0.1O₄ synthesis, as well as the phase composition of the samples after the tests, were controlled through X-ray powder diffraction (XRD).

Study of the chemical stability of Li_3.8Ge_0.9S_0.1O₄ in contact with the Li_1.3V₃O₈ bronze was performed. To do this, a mixture of bronze and solid electrolyte powders was prepared in a weight ratio of 1:2. The mixture was kept for 5 h at 300 °C and then was studied by XRD. In addition, the thermal behavior of these mixtures was studied by the DSC in the range of 25–600 °C.

XRD was performed using a Rigaku DMAX-2200 diffractometer (RIGAKU, Tokyo, Japan) in filtered CuК_α-radiation (λ = 1.54178 Å) 2θ = 15–70° generated at 40 kW, 30 mA in step mode with the step of 0.02° and 0.3 s counting time. The unit cell parameters were calculated using the Jade 6.5 software. The error in determining the cell parameters did not exceed 0.02%.

DSC 204 F1 Phoenix unit (NETZSCH, Germany) was used for thermal analysis. The measurement was carried out in the temperature range of 30–600 °C in Pt crucibles in air with the heating rate of 10 °C/min.The results obtained were processed using the NETZSCH Proteus software.

The Raman spectra were obtained using U-1000 microscope-spectrometer (RENISHAW, Stonehouse, UK) (Ar⁺-radiation, λ = 514 nm) at 100–4000 cm⁻¹.

The morphology of the synthesized material and the shape and distribution of its particles were studied using a MIRA 3 LMU scanning electron microscope (TESCAN, Brno, Czech Republic).

The EPR spectrum of the Li_1.3V₃O₈ bronze was recorded at room temperature on an X-band Adani CMS 8400 spectrometer.

2.3. Conductivity Measurement

Sintered round tablets were used to measure the conductivity of Li_3.8Ge_0.9S_0.1O₄ solid electrolyte and to study its stability against Li.

The total conductivity was measured using platinum electrodes painted on the both sides of the bars through vacuum evaporation. The electrical conductivity was measured by impedance spectroscopy on a P-40X potentiostat-galvanostat (Elins, Zelenograd, Russia) with FRA-24M module for electrochemical impedance measurements in the frequency range 1–500 kHz. In addition, the electronic conductivity was determined using DC with Pt electrodes at 20–40 mV. The sample with Pt electrodes was sandwiched between the silver current leads using springs, which provided good electrical contact. Stainless steel screens served to reduce thermal convective flows and contributed to better isothermality of the cell. The measuring cell was placed in a quartz tube and together with the latter in an electric furnace. The experiments involved several parallel measurements both during heating and cooling. The heating and cooling rate was 2 °C/min. The measurement temperature was controlled with a Pt-Pt/Rh thermocouple to an accuracy of ±0.5 °C. Isothermal exposure was carried out at each measurement temperature. The values of electrical conductivity obtained during cooling and heating practically coincided.

A Li│Li_3.8Ge_0.9S_0.1O₄│Li electrochemical cell was assembled to study the stability of Li_3.8Ge_0.9S_0.1O₄ against metallic Li. In the process of isothermal exposure, the electrical resistance of the cell was measured. A chemical reaction of the electrolyte and lithium is expected to change the resistance due to the formation of reaction products at the Li|electrolyte interface. The conductivity measurements were carried out by impedance spectroscopy in the AC frequency range of 1–500 kHz and galvanostatic pulse technique.

A Li|Li_3.8Ge_0.9S_0.1O₄|Li_1.3V₃O₈ electrochemical cell was assembled to evaluate the suitability of lithium–vanadium oxide Li_1.3V₃O₈ as a cathode of an all-solid-state battery. A layer of Li_1.3V₃O₈ was painted on the pellet of Li_3.8Ge_0.9S_0.1O₄ solid electrolyte by rubbing it into the surface of the pellet; the coating quality was controlled using an optical microscope. The geometrical dimensions of the electrochemical cell were: height 1 mm, diameter 10 mm. The mass of the Li_1.3V₃O₈ on the pellet surface was 0.0016 g. The electrochemical characteristics of the all-solid-state battery were determined using potentiostat–galvanostat P-40X.Impedance spectra were taken in the frequency range 1–500 kHz. Cyclic voltammetry (CV) was carried out in the range of ± 3 V from the initial potential with a scanning speed of 1mV/s. The cell charge/discharge was performed in a galvanostatic mode at a current density of 13 μA/cm². Galvanostatic pulse technique was performed with two pairs of pulses of ±100 µA at a base value of 0 µA with time resolution of 2ms.

All experiments using lithium metal were performed in an argon atmosphere.

3. Results and Discussion

3.1. Characterization of Li_3.8Ge_0.9S_0.1O₄

The XRD pattern of Li_3.8Ge_0.9S_0.1O₄ sample is presented in Figure 1. All the diffraction peaks observed practically coincide with the reflections given in the literature for the sample close in composition, i.e., Li_3.6Ge_0.8S_0.2O₄ (PDF-2, No. 44-0337) and correspond to the orthorhombic cell, Pnma, with parameters: a = 10.7987 Å, b = 6.2221 Å, c = 5.1282 Å.

The Raman spectrum for Li_3.8Ge_0.9S_0.1O₄ correlates with the literature data [30] for this type of compounds (Figure 2). The band at 762 cm⁻¹ and two bands at 990 and 1100 cm⁻¹(Figure 2(1)) can be attributedto the vibrations of the isolated tetrahedra of GeO₄⁴⁻ [36] and SO₄²⁻ [37], respectively. The bands’ characteristic of Li₂CO₃ and other possible products of the reaction between the sample and air humidity and CO₂, which are registered in the range of high wavenumbers [38], were not found on the Raman spectrum (Figure 2(2)).

3.2. Characterization of Li_1.3V₃O₈ and Determination of V⁴⁺ Content

The XRD pattern of the lithium vanadate produced in this work corresponds to the line diagram of powder pattern for LiV₃O₈ standard in PDF-2 database, No. 72-1193 (Figure 3a). Li_1.3V₃O₈ has a monoclinic cell with parameters: a = 6.68 Å, b = 3.6 Å, c = 12.03 Å, β = 107.83 °, Vol = 275.40 ų, Z = 2 and space group P21/m (11) [39]. The microstructure of Li_1.3V₃O₈ prepared throughsolid state reaction is shown in Figure 3b. One can see that the grain size of the resulting powder is about 100 nm.

Part of vanadium atoms in Li_1.3V₃O₈ bronze exist in +4 oxidation state [40]; therefore, its formula can be written as Li_1+x V_x⁴⁺V_3−x⁵⁺O₈. The electronic conductivity of the bronze depends on the ratio of vanadium in different oxidation states. The presence of uncompensated ion spins in the material can be established using electron paramagnetic resonance (EPR).

An asymmetric singlet with g_||=1.94, g_┴= 1.97 was registered on the EPR spectrum (Figure 4). Such a signal is characteristic of V⁴⁺ ions [41]. Integral intensities of both spectra were calculated in order to determine the number of paramagnetic centers. The integral intensity is proportional to the number of unpaired spins. The number of paramagnetic centers was found by comparing with the reference sample—non-aqueous CrCl₃. The calculation indicated that the amount of V⁴⁺ in the Li_1.3V₃O₈ sample is 4%. Thus, one can see that Li_1.3V₃O₈ contains mixed-valence vanadium ions, which results in mixed electronic-ionic conductivity, which is necessary for the cathode material to operate efficiently.

3.3. The Conductivity of Li_3.8Ge_0.9S_0.1O₄

The conductivity of Li_3.8Ge_0.9S_0.1O₄ was determined using impedance spectroscopy in the temperature range of 20–350 °C. Some of the impedance plots are shown in Figure 5. They consist of a semicircular arc with a considerably shifted center point and a low-frequency tail. The total resistance of the sample was found as the interception point of the semicircular arc and the real axis. Above 150 °C, the plots contain only the electrode tail (Figure 5, insert); in this case, the resistance was found through the extrapolation of the low-frequency tail onto the Z’axis.

The temperature dependences of electronic and total conductivity for Li_3.8Ge_0.9S_0.1O₄ are shown in Figure 6. At 25 °C, conductivity of Li_3.8Ge_0.9S_0.1O₄ is 5 × 10 ⁻⁶ S/cm. Activation energy is ~50 kJ/mol. The total conductivity value is close to the one given in [32]. The electronic component of conductivity for Li_3.8Ge_0.9S_0.1O₄ was found to be less than 0.1% across the whole temperature range studied (Figure 6, curve 2).

The values of the specific electrical conductivity of Li_3.8Ge_0.9S_0.1O₄ solid electrolyte for a number of temperatures are presented in

Table 1. Specific electrical conductivity of a solid electrolyte Li_3.8Ge_0.9S_0.1O₄.

Li3.8Ge0.9S0.1O4
t, °C	σ, S/cm
25	5 × 10 −6
60	6 × 10 −5
170	4 × 10 −3
390	4 × 10 −1

.

3.4. Stability of Li_3.8Ge_0.9S_0.1O₄in Contact with Lithium-Vanadium Oxide Li_1.3V₃O₈

The available literature reports on the application of solid electrolytes of Li₄GeO₄–Li₂SO₄ system in all-solid-state power sources operating at ambient temperature and at elevated temperatures. Thus, in [31], it is shown that a Li│Li_3,6Ge_0,8S_0,2O₄│LiNi_1/3Mn_1/3Co_1/3O₂ power source delivers good performance at 60 °C. In view of this, a tentative study into the chemical resistance of the chosen solid electrolyte in relation to Li_1.3V₃O₈ cathode material was undertaken.For this purpose, the thermal behavior of a mixture of Li_1.3V₃O₈ and Li_3.8Ge_0.9S_0.1O₄ powders was analyzedwith DSC across the temperature range of 20–600 °C. No anomalies were found on the TG and DSC curves below 355 °C (Figure 7). In the range of 350–410 °C, a slight weight loss is registered on the TG curve, while the DSC curve contains a diffuse endothermic heat effect of low intensity at the same temperatures. At 530–560 °C, an intense exothermic peak can be seen on the DSC curve, while the XRD pattern for the mixture of Li_1.3V₃O₈ and Li_3.8Ge_0.9S_0.1O₄ powders held at 560 °C for 5 h contains lines of Li₃VO₄ (indicated *) and Li₂Ge₄O₉(indicated +)alongside the reflections of the solid electrolyte (Figure 8 XRD pattern 4), which indicates that Li_3.8Ge_0.9S_0.1O₄ and the bronze enter into a chemical reaction at ~530 °C. At the same time, the XRD pattern for the mixture of these materials collected after a five-hour heat treatment at 300 °C contains only the lines of Li_1.3V₃O₈ and Li_3.8Ge_0.9S_0.1O₄ (Figure 8.XRD pattern 1).Thus, both the thermal analysis for the mixture of solid electrolyte and lithium–vanadium oxide powders, and the isothermal exposure demonstrate that no reaction of Li_1.3V₃O₈ and Li_3.8Ge_0.9S_0.1O₄ can be observed below 300–350 °C.

3.5. Stability of Li_3.8Ge_0.9S_0.1O₄in Contact with Metallic Lithium

The interaction between Li_3.8Ge_0.9S_0.1O₄ solid electrolyte and metallic lithium was evaluated based on determining the change in the resistance of Li│Li_3.8Ge_0.9S_0.1O₄│Li symmetrical cell. The cell was held at 100 °C for 240 h. During the isothermal exposure, the cell was periodically cycled (−4 V… +4 V, 20 mV/s). Measurements of the electrical resistance of the cell performed by the pulse method as well as by the impedance spectroscopy method after isothermal exposure (Figure 9) showed that the conductivity of Li|Li_3.8Ge_0.9S_0.1O₄|Li cell did not change within the uncertainty of the experiment. After the experiment, the solid electrolyte sample was weighed and examined under the microscope. The weight and the color of the sample did not change. The results of the conductivity measurement suggest that no observable interaction between Li_3.8Ge_0.9S_0.1O₄ and metallic lithium takes place under the conditions of the experiment.

3.6. Testing the Li|Li_3.8Ge_0.9S_0.1O₄|Li_1.3V₃O₈ Cell Model

A Li|Li_3.8Ge_0.9S_0.1O₄|Li_1.3V₃O₈ electrochemical cell with Li anode and lithium–vanadium bronze as cathode was assembled. Lithium–vanadium oxide has good adhesion to the solid electrolyte and has high electronic conductivity; therefore, binders and additives that increase the electrical conductivity of the active mass of the cathode were not used in this work. The cell is schematically shown in Figure 10a. The height of the cell model was 1 mm and its diameter was 10 mm. As one can see from the micrograph of the fractured surface of the cell (Figure 10b), the average thickness of the cathodic layer was 25 μm. The mass of the cathodic material on the surface of the electrolyte tablet was 0.0016 g.

The resistance of the solid state cell was measured using galvanostatic pulse technique and impedance spectroscopy across the temperature range of 20–100 °C. The impedance plots presented in Figure 11. The resistance values determined from the impedance plots coincided with the values produced by the pulse method.

The Arrhenius plot for the reciprocal of the internal specific resistance of Li|Li_3.8Ge_0.9S_0.1O₄|Li_1.3V₃O₈ cell is linear (Figure 12). However, its value is slightly lower compared to Li_3.8Ge_0.9S_0.1O₄ due to the additional contribution of the contact resistance at the electrode|solid electrolyte interface. The effective activation energy is 53 kJ/mol, which is close to the value for the solid state electrolyte (50 kJ/mol).

The voltametric study of the Li|Li_3.8Ge_0.9S_0.1O₄|Li_1.3V₃O₈ cell was conducted across the 1.5–4.5 V potential range at the scanning rate of 1 mV/s at 100 °C. The current–voltage curve for the 3rd cycle of potential sweep is presented in Figure 13.

The cathode part of the current–voltage curves for the lithium power sources with a liquid electrolyte and Li_1+xV₃O₈ as cathode material contains several peaks at the potential of 3.17–3.57 V. The peaks are related to the insertion of lithium cations into different sites of the LiV₃O₈ lattice. In our case, these peaks do not separate, which is the result of large polarization, and the I-E curve contains one broad peak with the maximum at ~3.5 V. In the anodic part of the curve, the peaks corresponding to the extraction of Li⁺ cations from different sites of Li₁V₃O₈ crystal lattice do not separate either (Figure 13).

The charge and discharge cycling of the solid state cell was performed galvanostatically with a two-electrode setup at the current of ±10 μA for 2 h. Figure 14a contains charge/discharge curves of Li|Li_3.8Ge_0.9S_0.1O₄ Li_1.3V₃O₈ cell for 40 cycles. In the 40th cycle, the charge capacity and discharge capacity were found to be 22 mAh/g and 19 mAh/g, respectively. There is no capacity loss during 40 charge/discharge cycles, and, consequently, there is no degradation of the electrode/electrolyte interface. At the temperature of 100 °C, the cell voltage is about 3.6 and 1.5 V in the charged and discharged state, respectively. The relatively low value for the capacity of the power source must be chiefly related to the high contact resistance at the electrode/electrolyte boundary and should be minimized in future experiments in order to enhance the characteristics. Moreover, 99.4 % of Coulombic efficiency was maintained after 40 cycles (Figure 14b).

4. Conclusions

Lithium–cation solid electrolyte Li_3.8Ge_0.9S_0.1O₄ with LISICON-type structure and lithium–vanadium oxide Li_1.3V₃O₈ containing 4% vanadium ions in 4+ oxidation state have been synthesized. The stability of Li_3.8Ge_0.9S_0.1O₄ in relation to Li_1.3V₃O₈ was studied using DSC/TG and isothermal heat treatment followed by XRD. The isothermal heat treatment and thermalanalysis of the mixture of Li_1.3V₃O₈ and Li_3.8Ge_0.9S_0.1O₄ powders demonstrated that no reaction between them takes place below 300–350 °C. A 240-h isothermal exposure of Li│Li_3.8Ge_0.9S_0.1O₄│Li cell did not reveal any observable interaction between the solid state electrolyte and lithium. A model of a lithium-ion power source with a Li_1.3V₃O₈-based cathode and a Li anode was assembled and tested. The all-solid-state Li|Li_3.8Ge_0.9S_0.1O₄|Li_1.3V₃O₈ electrochemical cell is safe and relatively simple to produce. At the temperature of 100 °C, the cell voltage was found to be about 3.6 and 1.5 V in the charged and discharged state, respectively. There is no capacity loss during 40 charge/discharge cycles, and, consequently, there is no degradation of the electrode/electrolyte boundary. In the 40th cycle, the charge capacity and discharge capacity were found to be 22 mAh/g and 19 mAh/g, respectively. The relatively low value of the capacity of the power source must be chiefly related to the high contact resistance at the interface between the electrode and electrolyte, which should be minimized in future experiments in order to enhance the characteristics.

Thus, the present study confirms the promising outlook of the Li_3.8Ge_0.9S_0.1O₄ solid electrolyte and lithium–vanadium bronze Li_1.3V₃O₈-based cathode for all-solid-state medium-temperature batteries with a Li anode.