Rapid and Economic Synthesis of a Li7PS6 Solid Electrolyte from a

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Rapid and Economic Synthesis of Li7PS6 Solid Electrolyte from Liquid Approach Dominika A. Ziolkowska, William Arnold, Thad Druffel, Mahendra K. Sunkara, and Hui Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19181 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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ACS Applied Materials & Interfaces

Rapid and Economic Synthesis of Li7PS6 Solid Electrolyte from Liquid Approach Dominika A. Ziolkowska 1,4, William Arnold 2, Thad Druffel 1, Mahendra Sunkara 1,3, and Hui Wang 1,2,*

1

Conn Center for Renewable Energy Research, University of Louisville, Louisville, KY,

40292, USA 2

Mechanical Engineering Department, University of Louisville, Louisville, KY, 40292,

USA 3

Chemical Engineering Department, University of Louisville, Louisville, KY, 40292, USA

4

Chemistry Department, University of Warsaw, Poland

KEYWORDS: Lithium argyrodites, Liquid synthesis, Solid electrolytes, Lithium thiophosphate, Solidstate Li batteries

ACS Paragon Plus Environment

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ABSTRACT

Solid electrolytes are the key to realize future solid-state batteries that show the advantages of high energy density and intrinsic safety. However, most solid electrolytes require long time and energy consuming synthesis conditions of either extended ball milling or high temperature solid-state reactions, impeding practical applications of solid electrolytes for large-scale systems. Here we report a new and rapid liquid-based synthetic method for high purity Li7PS6 solid electrolyte through the stoichemical reaction of Li3PS4 and Li2S. This method relies on facile and low cost solution-based soft chemistry to complete chemical reaction in extensively short time (2 hours). The prepared Li7PS6 solid electrolyte shows a high phase purity, an impressive ionic conductivity (0.11 mS cm-1), and a reasonable electrochemical stability with metallic lithium anode. Our results highlight using an economic and nontoxic solvent to quickly synthesize Li7PS6 solid electrolyte, which would promote the development of solid-state batteries for next-generation energy storage systems.


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Introduction

The growing interest from the electric vehicle industry signals a strong need for rapid development of safer and higher energy density portable energy storage. All-solidstate batteries (ASSBs) are considered to be one of the most promising next-generation lithium (Li) battery systems to meet these requirements and have made remarkable progress in recent years.1-4 ASSBs are a realistic alternative to the conventional liquid electrolyte-based Li-ion batteries due to their improved safety, stability and energy density. The solid electrolyte is an indispensable component of ASSBs and plays a


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crucial role in the solid-state battery performance. An ideal solid electrolyte should exhibit an ionic conductivity above 10-4 Scm-1 at room temperature, a large electrochemical stability window and stability against electrodes, especially metallic Li anode. Furthermore, the production cost is also an important factor that must be considered in the research and development of solid electrolyte materials.5, 6

Lithium argyrodites are relatively new and very promising class of sulfide-based Li ion superconductors.7 They originated from the mineral argyrodite (Ag8GeS6),8 which is the first representative of that group of solids and characterized by its high ionic conductivity (~10-3 Scm-1)9 and fast Ag+ ion mobility. Pure lithium argyrodite (Li7PS6) was reported to have a cubic phase at high-temperature (HT) or an orthorhombic phase at low-temperature (LT). The cubic HT-phase shows higher ionic conductivities and can be stabilized by the replacement of sulfur by halogen anions (0.7-1.0×10-3 Scm-1).10 The study of argyrodite materials has yielded a basic understanding of the temperaturedependent diffusion paths of ions based on their structural properties. Strong interest in lithium argyrodites has grown due to the important advantages of: (1) very high intrinsic


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Li-ion conductivities (10-4 to 10-3 S cm-1), (2) stability within a large electrochemical window (~ 5V - suitable even for high voltage cathode materials), (3) and very high flexibility in both anion and cation doping (giving opportunity to a broad variety of composition).11

Despite many important findings of lithium argyrodites being reported,10-Error! Reference source not found.Error! Reference source not found.Error! Reference source not found.14

the stringent synthesis

requirements of either melt-quenching or high-energy ball milling are prohibitive for its scalable manufacturing at low costs. The preparation conditions for the melt-quenching method is harsh with difficulties in controlling the reactant concentrations resulting in impurities during the cooling process, which makes its applicability very unlikely.14 The ball milling fabrication process is relatively time consuming and it is hard to obtain uniform products; although the conductivity values using this methods seems to be the highest.15,16 Recently, liquid-based synthesis method has attracted intense interests due to the great flexibility for the material preparation and manufacturing simplicity. Many efforts have been devoted to other solid conductors in Li2S-P2S5 family such as β-Li3PS4


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and Li7P3S11. Liang’s group reported the wet-chemical synthesis to obtain β-Li3PS4 crystallites by using tetrahydrofuran (THF)17 and acetonitrile (ACN),18,19 achieving an ionic conductivity up to 1.6 × 10-4 S cm-1 at room temperature (RT). 17-19 Other groups have used ethyl propionate,20 or ethyl acetate21 with similar results. Later, Wang et al. made Li7P3S11 using acetonitrile and obtained an ionic conductivity of 8.7 × 10−4 S cm−1 at RT.16 Similarly, Ito et al. mixed Li2S and P2S5 precursors in 1,2-dimethoxyethane (DME) solvent to acquire Li7P3S11, reporting an ionic conductivity above 10-4 S cm-1.22 Compared with these materials, the synthesis of Li7PS6 solid electrolyte via a solution phase is still under developed. Besides, the reported liquid synthesis approach typically involves toxic and expensive solvents such as ACN, THF, and DME. For large scale synthesis, inexpensive and less toxic solvents would be more ideal.

In addition to scalability, non-toxic solvent based liquid synthesis would also be an efficient approach to prepare homogeneous composite cathodes for ASSBs. Most cathode materials have poor conductivities and need to mix with carbon/solid electrolyte to enhance their electronic/ionic conductivities. Dry mixing or ball milling usually results


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in the aggregation of each component. In contrast, Kim et al. reported a liquid-based scalable fabrication protocol for their Li6PS5Cl-infiltrated LiCoO2 and graphite electrodes, and achieved high reversible capacities (141 and 364 mA h g–1) at 0.14 mA cm–2 and 30 °C.23 Therefore, finding an optimal solid electrolyte synthetic method is significant for the success of ASSBs at scales large enough to serve the mobile electric market. The liquid synthesis approach and understanding the solid electrolyte formation is the best way to achieve this technological goal.

In this work, we report for the first time the liquid synthesis of Li7PS6 using inexpensive and nontoxic anhydrous ethanol solvent through the reaction of Li3PS4 and Li2S. This rapid synthetic approach can be accomplished in 2 hours at low sintering temperatures (200 °C). The synthesized Li7PS6 has high phase purity and excellent room temperature ionic conductivity of 0.11 mS cm-1, the highest value compared with other liquid phase synthesis to date. The electrochemical test indicates that the Li7PS6 exhibits a reasonable stability with metallic Li to 5 V without evidence of side reactions. This liquid synthesis opens new possibilities for a higher purity and homogeneity material through


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a simpler and scalable manufacturing process, which may have significant importance on the next generation battery technology.

Experimental Section

Materials Synthesis:

Synthesis of Li3PS4 precursor: Li3PS4 was prepared using method describe by Wang et al.Error! Reference source not found. In brief the Li2S and P2S5 were dissolve in acetonitrile (ACN), stirred for 8h at RT and then filtrated (Figure S1). The obtained white powder was then dried at 80°C under vacuum (Li3PS4·(ACN)2) followed by a heat treatment in 200°C (βLi3PS4).

Synthesis of Li7PS6 electrolyte: A stoichiometric mixture (2:1 molar) of Li2S and β-Li3PS4 was dissolved in a small quantity of anhydrous ethanol (25 ml) in argon atmosphere. Next, the mixture was heated to 90°C under vacuum to evaporate the solvent yielding a white precipitate (not longer than 1 hour), and then treated at 200°C for 1 hour to obtain a final product (Li7PS6).


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Structural and morphological investigation

The phase composition and crystal structure were analyzed using X-ray diffraction (Bruker D8 Discover) with nickel-filtered Cu-Kα radiation (λ = 1.5418 Å). The Scherrer equation was used to estimate the crystallite size of the obtained materials. The chemical and structural data were obtained from the Raman spectroscopy, which was measured using Renishaw in Via Raman/PL Microscope and a 632.8 nm emission line of a HeNe laser. General morphologies of all samples were investigated using TESCAN Vega3 scanning electron microscope (SEM).

Conductivity and Electrochemical stability

Electrochemical impedance spectroscopy (EIS) was carried out to measure the ionic conductivities of produced samples in the frequency range from 1 MHz to 100 mHz with an amplitude of 100 mV using Bio-Logic VSP300. For measurements, dense pellets (1/2” diameter) were prepared by cold pressing the powder with C/Al as blocking electrodes at each side and placed in Swagelok cells. As expected for pure ionic conductor, EIS spectra present a semi-arc at high frequencies and a straight line at


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lower frequencies. The intercept of straight line at the axis is employed to determine the total ionic conductivity of the material. In addition, the temperature dependent spectra where recorded from RT to 90 °C to obtain the Arrhenius plot. Swagelok cells were also used to complete cyclic voltammetry (CV) and cycling performance measurements. For CV test, Li/SE/Pt cells were scanned at 50 mV s-1 rate between -0.5 and 5V vs. Li/Li+ at room temperature using Bio-Logic VSP 300 potentiostat. For symmetric cell cycling, the Li/SE/Li symmetric cells were assembled and cycled on a battery system (Bio-Logic VSP) under different current densities of 20, 50, 100 μA cm-2.

Results and Discussion

Straightforward chemical reaction to produce Li7PS6 from liquid approach

Conventionally, Li7PS6 has been synthesized through the solid-state reaction of Li2S with P2S5 at 550 ºC for several hours or even days.7,11 A liquid synthesis for producing Li7PS6 is very challenging due to the stability of precursors in a solvent, for example, P2S5 reacts with the ethanol to form dialkyldithiophosphoric acid.24 In this work, Li7PS6 solid electrolyte was synthesized from a simple wet chemical method by reacting Li3PS4 and


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Li2S in an anhydrous ethanol (Figure 1), and subsequent heat treatment at 200 °C. This reaction (Eq.1) benefits from the facts that both of Li3PS4 and Li2S could dissolve in ethanol.

𝐿𝑖3𝑃𝑆4 + 𝐿𝑖2𝑆

𝐸𝑡𝑂𝐻

𝐿𝑖7𝑃𝑆6

The as-synthesized powder was characterized by X-ray powder diffraction (XRD) in Figure 2a. Interestingly, this powder displays several sharp peaks at 2θ=25.5, 30, 31.2º, corresponding to (220), (311) and (222) planes in cubic HT-phase of Li7PS6 (space group F-43m). These characteristic diffraction peaks are in good agreement with the pure phase of cubic Li7PS6 in previous literatures.25,26 The stabilization of Li7PS6 cubic phase at room temperature from the liquid-based synthesis is important because the cubic phase represents faster ion-transport framework than the LT-orthorhombic structure

[11].

Similar phenomenon was reported previously for the wet chemical

synthesis of Li3PS4 which stabilize the β-phase at room temperature instead of γ-phase [17,18].

The stabilization process of cubic phase is contributed by the interaction between

ethanol solvent and precursors (Li2S and Li3PS4). For solid-state reaction, it needs a


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high temperature and long time to allowing the reorganization of Li-ions and crystal nucleation. Whereas in liquid-based synthesis, the solvent medium provides more flexibility for the movement of solvated ions and thus facilitates the Li7PS6 formation. The cubic structure of Li7PS6 has a unit cell parameter of 9.88 Å, which is in good agreement with previous reports.6,11. The crystal size is estimated at 34 nm based on Scherrer equation.

As observed in Figure 2a, the synthesized Li7PS6 shows high phase purity without any impurity of resident Li2S and β-Li3PS4, which have totally different diffraction patterns. The stoichiometric amounts of reactants and a homogeneity of the liquid preparation method contribute to this high pure phase. In addition, as the reaction precursor, the source of Li3PS4 is not significant to influence this reaction as long as the purity is retained. In current work, β-Li3PS4 was prepared by reacting Li2S with P2S5 in acetonitrile following the previous reports.18,19 If the Li3PS4 precursor is made by the ballmilling methods, the produced Li7PS6 would be keep the same purity due to a complete dissolution of precursor in ethanol solvent. Furthermore, it was also found that the pure


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phase of Li7PS6 could be obtained by using Li3PS4·(ACN)2 complex as the precursor to react with Li2S in ethanol (Figure S1 and S2).

The Raman spectra of cubic Li7PS6 from liquid-based synthesis and β-Li3PS4 precursor are shown in Figure 2b. Li7PS6 shows a main peak at 421.6 cm-1 corresponding to the symmetric stretching vibration of (PS4)3- (ortho-thiophosphate) group29 and a minor peak at 497.2 cm-1 attributed to the additional (PS4)3- vibrational modes. These characteristic peaks are in good agreement with previous reports.28a,29 This observation is similar with the Raman spectrum of Li3PS4, which exhibits a strong peak at 421.1 cm-1 from vibrational mode of (PS4)3-

17,20,27

and two minor peaks at 530.2 and 568.5 cm-1

referring to the additional (PS4)3- vibrational modes29. There is no other peak coming from the Li-S interaction due to a strong ionic character of bonds between S2- and Li+ ions. Notably, a minor peak at 387.6 cm-1 is observed for Li3PS4, which corresponds to (P2S6)4- (P–P bond) vibrational mode

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due to a trace amount of Li4P2S6 (cannot detect

from XRD). However, this peak disappears in Li7PS6 after the synthetic reaction, further confirms the high purity from the liquid-based synthesis.


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Revealing the formation mechanism from structural studies

The reaction mechanism of Li7PS6 was studied. First, the intermediate product before the heat treatment at 200 C was dried to remove EtOH and then subjected to XRD for phase identification. The XRD patterns (Figure S3) showed that Li7PS6 pure phase exists in the intermediate product, indicating that this synthetic reaction in fact happens at room temperature. Although, its ionic conductivity at room temperature is very low (10-7 S cm-1). This observation also suggests that Li7PS6 is less likely to form a complex in ethanol solvent, which is different with the cases of Li3PS4 in ACN and THF

17,18.

In addition, the

interaction between Li3PS4 precursor with ethanol solvent was investigated by its dissolution and re-precipitation process in solvent medium. The Li3PS4 precursor was dissolved in ethanol and then dried at 80 ºC to collect the re-precipitated sample. The XRD results of the re-precipitated sample show unknown crystalline structure and distinct with the original Li3PS4 (Figure S5). Further heating up the re-precipitated sample to 200 ºC leads to the observation of an amorphous phase. The structural change of Li3PS4 in ethanol is further supported by the Raman spectra (Figure S6), in which the characteristic


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peaks corresponding to vibrational mode of (PS4)3- group are not observed. In contrast, Li3PS4 was dissolved in acetonitrile solvent and then re-precipitated after 80 ºC, and then subjected to 200 ºC heat treatment. The collected XRD results (Figure S4) demonstrate that Li3PS4 forms a complex of Li3PS4·(ACN)2 in acetonitrile solvent after drying at 80 ºC, and then reverts back to pure Li3PS4 phase after heating at higher temperature. This comparison reveals that the exist of Li2S and ethanol solvent are indispensable factors to successfully produce cubic phase of Li7PS6 from liquid-based synthetic approach.

The morphology variation from Li3PS4 precursor to Li7PS6 product was analyzed using SEM (Figure 3). It is clear that the Li3PS4 sample prepared from ACN has an interesting flake-like morphology (Figure 3a), which is similar to previous observation.14 In contrast, the Li7PS6 product shows a granular nano-sized morphology (agglomerated particles of about 100 nm size in Figure 3b). The difference in morphology of these samples is related to the solvent interaction. The dissolved and re-precipitated Li3PS4 sample from ethanol also displays a grainy shape (Figure S7). This solvent induced Li3PS4 morphology change had been observed previously.17-19 The EDX mappings of Li3PS4


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precursor and Li7PS6 product are shown in Figure 4, where the P and S atoms exhibit homogenous distributions for the materials from liquid synthesis.

Ionic conductivity and Electrochemical stability measurements

The synthesized cubic Li7PS6 powder was cold-pressed with Al/C foils at each side for form a dense pellet. The electrochemical impedance spectra (EIS) measurement was carried out to evaluate the ionic conductivity of Li7PS6 sample. As shown in Figure 4a, an ionic conductivity of 0.11 mS cm-1 at room temperature was obtained for the Li7PS6 sample synthesized in this work, which is much higher than the values of Li-P-S argyrodite from other liquid synthesis.10 The activation energy of Li7PS6 is determined to be 41.46 kJ mol-1 (0.43 eV), close to the value of Li3PS4 (35.08 kJ mol-1, 0.36 eV). In addition, this value is also consistent with the activation energies of solidstate synthesized Li7PS6 and Li6PS5Cl materials (0.3-0.48 eV),30,31 which show the trend of lower activation energies after higher temperature annealing. At elevated temperatures, cubic-Li7PS6 exhibits obviously faster Li-ion mobility a than β-Li3PS4. For instance, the ionic conductivity of Li7PS6 is 1.5×10−3 S cm−1 at 90 ºC while


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1.0×10−3 S cm−1 for Li3PS4 at the same temperature. In addition, the Nyquist plots of Li7PS6 and β-Li3PS4 show that both materials have decreased resistance as increasing temperature. (Figure S8). Figure 4b compares the conductivities and productivity for Li7PS6 prepared through different methods.12,13,32 Both synthetic approaches of solidstate reaction and mechanical milling require the reaction time longer than 40 hours and the yielded products show conductivity values of 10-5-10−4 S cm-1. In contrast, the synthetic process of Li7PS6 solid electrolyte in this work can be completed in 2 hours. Furthermore, the obtained Li7PS6 sample hold an ionic conductivity that close to the best Li-P-S argyrodite family materials prepared by solid-state methods (including glasses and glass-ceramics).12,13,32 Yubuchi et al. prepared an analogous Li6PS5Cl sample by dissolution-precipitation of the ball-milled material in solvent medium.10 They found that the conductivity value dropped 2 orders of magnitude from 1.4 × 10−3 S cm−1 at room temperature for their ball-milled sample to 10−5 S cm−1 for re-precipitated sample. This finding also suggests that reacting and nucleating Li7PS6 crystals straight from the solution is beneficial for the final ionic conductivity.


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The electrochemical stability between the liquid synthesized Li7PS6 and metallic Li was investigated by collecting cyclic voltammogram (CV) curve of a Li/Li7PS6/Pt cell, in which Li and Pt serve as the reference/courter electrode and working electrodes, respectively. The potential was scanned from -0.5 to 5.0V (vs. Li+ /Li) at a scan rate of 50 mVs-1. For cubic Li7PS6 solid electrolyte, a pair of reversible oxidation and reduction peaks is observed at around 0 V (vs. Li+ /Li) without any other side reactions (Figure 5a), which is similar with the observation for β-Li3PS4. The cathode current below 0 V is a Li deposition on working electrode (Li+ + e– —>Li), whereas the anode current above 0 V results from reversible lithium dissolution

[7].

Li7PS6 shows a bit higher anode

current than that of Li3PS4. The CV curve demonstrates that Li7PS6 from liquid-based synthesis show a good stability with Li anode over a broad electrochemical window (up to 5 V).

A symmetric cell of Li/Li7PS6/Li was configured to demonstrate the compatibility of Li7PS6 solid electrolyte with metallic Li under various current densities (20, 50 and 100 μAcm-2) at room temperature. As shown in Figure 5b, smooth cycling performance


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was observed at lower current densities, accompanied with smaller values of over potential. When higher current density was applied, rough curves and spikes were observed (Figure S9). Under the same current density, the slight increase of overpotential values suggests the interfacial reaction between Li anode and Li7PS6 solid electrolyte. In addition, the impedance for the symmetric cell after cycling shows obviously lower value than that of the cell before cycling (Figure S10), further confirm the interfacial reactions. Nevertheless, after cycling, clear Li surface was still observed when peeling it off from Li7PS6 solid electrolyte in the symmetric cell. The XRD patterns (Figure S11) of the solid electrolyte pellet after cycling shows characteristic peaks of Li7PS6, which indicates that the interfacial reaction is not unlimited.

Conclusions

We report a rapid and economic synthesis approach for crystalline Li7PS6 solid electrolyte through the stoichiometrically chemical reaction of Li2S and Li3PS4 in ethanol medium. The synthesized Li7PS6 has the room temperature ionic conductivity of 0.11 mS cm-1 and 1.5 mS cm-1 at 90 °C, the highest value among pure materials prepared


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through liquid synthesis, and 40% higher than crystalline Li7PS6 powders from other synthesis methods (i.e. solid-state reaction and ball milling). Furthermore, it also shows good electrochemical compatibility with metallic Li anode based on symmetric cell cycling performance. This new synthetic approach leads to high purity phase of Li7PS6 material with a scalability and simple processing, which give Li7PS6 the chance as one of the most attractive electrolyte candidates in the large-scale all-solid-state battery technology.


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Figure 1. A scheme of the Li7PS6 synthesis procedure from the chemical reaction of Li2S and Li3PS4 in EtOH medium (Evaporation at 90 ºC, heat treatment at 200 ºC)


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Figure 2. (a) Comparison of XRD patterns of Li7PS6 crystalline (space group F-43m, dashed lines is index from ICCD # 00-034-0688) and β-Li3PS4 phase (space group Pnma, a=12.997 Å, b=8.081 Å, c=6.143 Å, ICCD # 01-076-0973); (b) Raman spectra of Li7PS6 and β-Li3PS4, both of them show the dominate peak from vibrational mode from (PS4)3- group.


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Figure 3. SEM images of (a) Li7PS6 product and (b) Li3PS4 precursor; and EDX maps of P and S elements in (c) (d)liquid-based synthesized Li7PS6 product and (e) (f) Li3PS4 precursor. Both samples show elemental homogeneity.


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Figure 4. (a) Arrhenius plots of Li7PS6 from ethanol and β-Li3PS4 from ACN (1.5 ×10-3 Scm-1 for Li7PS6 and 1×10-3 Scm-1 for Li3PS4 at 90 ºC); (b) comparison of conductivity values and productivity (1/synthesis time) of Li7PS6 prepared by different methods: 3×10-5 S cm-1 (solid state reaction at 650 °C for 7 days),32 8×10−5 S cm−1 (crystal powder from solid state reaction, 40 h),12,13 and this work: 1.1×10−4 S cm−1 (liquid synthesis approach, 2h).


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Figure 5. (a) CV curves of Li7PS6 and β-Li3PS4 solid electrolytes with metallic Li anode with Li/SE/Pt cell (scanning rate at 50 mV s-1 between -0.5 and 5V vs. Li/Li+ at room temperature); (b) Cycling performance of Li/ Li7PS6/Li symmetric cell (under current densities of 20, 50 μA cm-2).

ASSOCIATED CONTENT

Supporting Information. The following files are available free of charge. Detailed information about Li7PS6 prepared from Li3PS4·2(ACN), and Li3PS4 reprecipitated from EtOH, Nyquist plots of Li7PS6 and Li3PS4. (PDF)

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (H.W.)


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank the support from NSF EPSCoR Grant (Grant no: 1355438), EVSTS Grant (GB160808P1), Conn Center for Renewable Energy Research, and EVPI Internal Grant of University of Louisville.

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Rapid and Economic Synthesis of a Li7PS6 Solid Electrolyte from a

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