http://pubs.acs.org/journal/aelccp
Solvent-Engineered Design of Argyrodite Li6PS5X (X = Cl, Br, I) Solid Electrolytes with High Ionic Conductivity Laidong Zhou,† Kern-Ho Park,† Xiaoqi Sun,† Fabien Lalère,† Torben Adermann,‡ Pascal Hartmann,‡ and Linda F. Nazar*,† ACS Energy Lett. 2019.4:265-270. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/12/19. For personal use only.
†
Department of Chemistry and the Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada ‡ BASF SE, Ludwigshafen 67063, Germany S Supporting Information *
ABSTRACT: Argyrodites, Li6PS5X (X = Cl, Br), are considered to be one of the most promising solid-state electrolytes for solid-state batteries. However, while traditional ball-mill approaches to prepare these materials do not promote scale-up, solution-based preparative methods have resulted in poor ionic conductivity. Herein, we report a solution-engineered, scalable approach to these materials, including the new argyrodite solid solution phase Li6−yPS5−yCl1+y (y = 0−0.5), that shows very high ionic conductivities (up to 3.9 mS·cm−1) and negligible electronic conductivities. These properties are almost the same as their analogues prepared by solid-state methods, owing to a lack of amorphous contributions and low impurity contents ranging from 3 to 10%. Electrochemical performance is demonstrated for Li6PS5Cl in a prototype solid-state battery and compared to that of the same solid electrolyte derived from classic ball-milling processing. Young’s modulus more than 5−10-fold lower.12,13 They also provide high ionic conductivities up to 25 mS·cm−1 at room temperature, competitive with those of commercial liquid electrolytes. Prominent examples include the thio-LISICON phase Li3.25Ge0.25P0.75S4 (2.2 mS·cm−1, Ea= 0.21 eV);14 Li10 GeP 2 S12 (12 mS·cm −1 , Ea = 0.25 eV), 15 and its derivatives,16,17 such as Li9.54Si1.74P1.44S11.7Cl0.3 (25 mS·cm−1, Ea = 0.24 eV),18 Li7P3S11 (17 mS·cm−1, Ea = 0.18 eV),19−21 as well as the Li-argyrodite phases Li6PS5X (X = Cl, Br) (∼1 mS· cm−1, Ea = 0.3−0.4 eV).22−24 Among these, the latter have amongst the best stability with respect to lithium metal because an interphase composed of Li2S, Li3P and LiX (X = Cl, Br) forms at a very slow rate in contact with Li that acts as an in situ protective passivating layer.25 A few pioneering ASSB full cells have used Li-argyrodite as the solid electrolyte with different cathode and anode combinations (either Li/In or Li foil).26−33 Nevertheless, most Li-argyrodite electrolytes reported to date have been prepared by mechanical milling of the precursors followed by heat treatment. Ball-milling consumes much energy and makes the synthesis very difficult to scale up
R
echargeable all-solid-state batteries offer great potential to meet the growing demand for high energy density and safe energy storage systems.1 The key component of such systems is a highly ionically conductive solid-state electrolyte (SSE), which removes safety concerns inherent with flammable organic electrolytes in Li-ion batteries.2−4 SSEs also allow more efficient cell packaging, thereby improving energy density.5 Control of the interphases formed at the electrolyte interface with both the positive electrode (cathode)6,7 and the negative electrode (anode)8,9 are important considerations. Ductile solid electrolytes with a suitable Young’s modulus can, in principle, help suppress lithium dendrite growth in the case of a lithium anode and enable better interfacial contact with the cathode material upon cycling. However, establishing scalable routes to such materials is a significant problem that, if not overcome, may curtail the development of the solid-state battery field due to simple economics. Here, we address this challenge with a report of argyrodite solid electrolytes formed by a scalable solution-engineered approach that exhibits high ionic conductivities and shows excellent performance in a prototype solid-state cell. Many superionic materials have been investigated as candidates for lithium solid electrolytes.10,11 Whereas oxides or phosphates are brittle and rigid, lithium thiophosphates are softer and much more easily processed and densified, with a © 2018 American Chemical Society
Received: October 18, 2018 Accepted: November 20, 2018 Published: November 20, 2018 265
DOI: 10.1021/acsenergylett.8b01997 ACS Energy Lett. 2019, 4, 265−270
Letter
Cite This: ACS Energy Lett. 2019, 4, 265−270
Letter
ACS Energy Letters
Figure 1. XRD patterns and SEM images (insets) of (a) Li6PS5Cl, (b) Li6PS5Br, and (c) Li6PS5I from solution synthesis (all reflections correspond to the respective argyrodite phase except for the impurities as marked); Rietveld refinement of XRD patterns of (d) Li6PS5Cl and (e) Li6PS5Br. Black circles−experimental data; red lines−fitted data; blue lines−difference curve between observed and calculated data; ticks−Bragg peak positions of Li6PS5X (green), Li3PO4 (cyan), LiX (magenta), Li2S (burgundy), and Si (orange, 10 wt % addition).
pressed into pellets and sintered at 550 °C under vacuum in a carbon-coated quartz tube (see the Supporting Information (SI), for details). Figure 1a−c shows the XRD patterns of the products after heat treatment. The argyrodite phase (F-43m) is present as the major crystalline phase for the Cl and Br argyrodites, while the remainder is comprised of minor Li3PO4, LiCl/LiBr, and Li2S impurities (see below for detailed analysis). We note that previously reported solid-statesynthesized Li6PS5Cl and Li6PS5Br also exhibit a certain amount of Li3PO4 and LiX impurities (i.e, a total phase fraction of around 10 wt % for Li6PS5Cl),42 which is possibly caused by residual moisture in the quartz tubes. The final products from the solution-engineered synthesis show somewhat higher impurity levels, which is possibly caused by some reaction between PS43− units and ethanol, as previously reported,43 or a trace of moisture in the ethanol. A similar solution synthesis procedure was also utilized to synthesize almost-phase-pure Li6PS5I with only a trace of Li3PO4 impurity (Figure 1c). Despite the relatively low ionic conductivity of Li6PS5I (Figure S2), in agreement with previous reports,44 crystallization of the iodide nonetheless demonstrates broad application of this solution synthesis approach to the Liargyrodite family. A very important factor for solid electrolytes is their degree of crystallinity. We determined the degree of crystallinity of final products and the weight percent of the crystalline argyrodite phase in the final products using Si (10 wt %) as an external standard in the Rietveld refinements (see the SI for details). Rietveld refinements45 yield lattice and atomic parameters of Li6PS5Cl and Li6PS5Br similar to previously reported values (Figure 1d−e, Tables S1 and S2).42 In the
and thus impractical for solid-state batteries. Solutionengineered synthesis not only solves these problems but also potentially reduces the subsequent heat treatment temperature and/or time. Solution-based approaches have been achieved for Li4PS4I,34 β-Li3PS4,35 Li7P2S8I,36 and Li7P3S1137 phases, with varying degrees of success (i.e., ionic conductivity and phase purity of the product). One attempt has been reported in the literature for the argyrodite Li6PS5Cl, involving a dissolution−reprecipitation process from an ethanol solution.38 However, a mechanical milling step was still required, and the ionic conductivity of the product was only 1.4 × 10−2 mS· cm−1, two magnitude orders lower than that of materials obtained from a conventional all-solid-state process. A direct and effective “all-solution” synthesis of the argyrodite Li6PS5X materials remains a challenge. Herein, we demonstrate a direct solution synthesis approach to argyrodite Li6PS5X (X = Cl, Br, I) and Li6−yPS5−yCl1+y solid electrolytes with high Li ion conductivities of ∼2.4 mS·cm−1 at room temperature for the Cl and Br phases and higher for the mixed Cl/Br and Cl-rich phases, up to ∼4 mS·cm−1. The Li3PS4·3THF, Li2S, and LiX (X = Cl, Br, I) precursors were dissolved in a tetrahydrofuran (THF) and ethanol mixture. THF was used as the solvent for the synthesis of the Li3PS4/ solvent complex as it enables a shorter reaction time (hours) than solvents such as dimethoxyethane (DME; 10 days) or acetonitrile (ACN; 2 days), and it also exhibits a lower boiling point, which facilitates its removal.34,35,39−41 Ethanol with a moderately high dielectric constant helps to dissolve Liargyrodite to form a pale yellow solution. After evaporation of the solvents, the thoroughly mixed reactants were poorly crystallized, as shown in Figure S1. The powder was then 266
DOI: 10.1021/acsenergylett.8b01997 ACS Energy Lett. 2019, 4, 265−270
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ACS Energy Letters Li6PS5Cl and Li6PS5Br products, the final products were fully (100%) crystallized, and the weight percentages of crystalline argyrodite were 77(5) and 91(6)%, respectively, with crystalline Li3PO4, LiX (Cl, Br respectively), and Li2S impurities accounting for the remainder (Tables 1 and 2).
electrolytes have been shown to provide better contact at the grain boundaries than oxides, resulting in a more integrated solid electrolyte matrix even in cold-pressed pellets without sintering.20 The ionic conductivities of the solution-engineered Li6PS5X solid electrolytes were measured by electrochemical impedance spectroscopy (EIS) in a SS/Li6PS5X/SS (SS = stainless steel) configuration at room temperature. Resistivities of 34 and 46 Ohm were obtained for Li6PS5Cl (Figure 2a) and Li6PS5Br (Figure 2d) at room temperature, converting to total conductivities of 2.4 and 1.9 mS·cm−1, respectively. These values are in good accord with values for solid-statesynthesized Li6PS5Cl and Li6PS5Br argyrodites of ∼3 and ∼1 mS·cm−1, respectively.23,24,42 This indicates that the impurities in the solution-processed materials do not significantly lower the Li+ conduction in these highly crystalline solid electrolytes. We note that these components are indeed unlikely to affect overall ionic conductivity as the dense solid electrolyte pressed pellets facilitate excellent contact between Li6PS5X crystals and provide sufficient Li-ion percolating pathways for transport.42 The ionic conductivity of the iodide argyrodite was 2 × 10−3 mS·cm−1 (Figure S2), in accord with the typically low values reported for this phase (i.e., 4 × 10−4 mS·cm−1).44 The electronic conductivities of the two materials were measured by a DC polarization measurement of the SS/ Li6PS5X/SS symmetric cells at room temperature. Figure 2 shows the DC polarization curves of Li6PS5Cl (Figure 2b) and Li6PS5Br (Figure 2e) at three voltages. After the initial 2 s, the current decays quickly and stabilizes on the order of 10−8 A after 30 min. From a linear fit of DC voltage and stabilized current (Figure 2c,f), the DC electronic conductivities are estimated to be 5.1 × 10−6 mS·cm−1 for Li6PS5Cl and 4.4 × 10−6 mS·cm−1 for Li6PS5Br, both of which are 6 orders of magnitude lower than the ionic conductivities. This demon-
Table 1. Mass Fraction of Each Component in Li6PS5Cla component
refined mass fraction with Si (%)
normalized mass fraction (%)
Li6PS5Cl Li3PO4 LiCl Li2S Si
71(2) 9.2(9) 5.1(3) 4.8(3) 10.2(3)
77(5) 10(2) 5.6(5) 5.2(5) n/a
a With ∼10 wt % Si added as the reference standard for intensity normalization.
Table 2. Mass Fraction of Each Component in the Li6PS5Bra component
refined mass fraction with Si (%)
normalized mass fraction (%)
Li6PS5Br Li3PO4 LiBr Li2S Si
78(2) 7(2) 3.0(2) 2.9(3) 9.6(3)
91(6) 8(3) 3.5(4) 3.3(4) n/a
With ∼10 wt % Si added as the reference standard for intensity normalization. a
The SEM images shown in Figure 1a,b (insets) of well-ground Li6PS5X (X = Cl, Br) materials illustrate the dense nature of the crystallite masses, which is highly beneficial when the materials are processed into ASSBs. Highly ductile sulfide solid
Figure 2. Nyquist plots for (a) Li6PS5Cl and (d) Li6PS5Br solid electrolytes (cold-pressed at 2 tons, measured at 300 K, in the frequency range from 1 MHz to 10 mHz); DC polarization curves of (b) Li6PS5Cl and (e) Li6PS5Br solid electrolytes with an applied voltage of 0.25 (black), 0.5 (red), and 0.75 V (blue). (c,f) Linear fits. 267
DOI: 10.1021/acsenergylett.8b01997 ACS Energy Lett. 2019, 4, 265−270
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ACS Energy Letters
Figure 3. XRD patterns of (a) Li6PS5Cl0.25Br0.75, (b) Li6PS5Cl0.5Br0.5, and (c) Li6PS5Cl0.75Br0.25 from solution synthesis (all reflections correspond to the respective argyrodite phase except for the impurities as marked); (d) lattice parameters from Rietveld refinements; (e) Nyquist plots for Li6PS5Cl0.25Br0.75 (blue squares), Li6PS5Cl0.5Br0.5 (red triangles), and Li6PS5Cl0.75Br0.25 (black dots) (cold-pressed at 2 tons, measured at 300 K, in the frequency range from 1 MHz to 10 mHz).
strates that the argyrodite Li6PS5Cl and Li6PS5Br solid electrolytes obtained from solution synthesis are effectively pure ionic conductors. Previously reported mixed-halide argryodites Li6PS5X (X = Cl0.75Br0.25, Cl0.5Br0.5, and Cl0.25Br0.75) exhibit higher ionic conductivities compared to the single-halide phases,42 inspiring us to also target these materials through the solutionengineered method. Figure 3a−c shows the XRD patterns of the products after heat treatment. The same results were obtained as those for the single-halide compositions, namely, almost-pure argyrodite phases are present as the major crystalline phases, with impurities of Li3PO4, Li2S, and LiX (X = Cl, Br) present. Figure 3d shows that the lattice parameters from Rietveld refinements (see Figures S3−S5) increase linearly from x = 0 to 1 in Li6PS5Cl1−xBrx, in accord with Vegard’s law for solid solutions, which indicates that the argyrodite phases consist of disordered Cl/Br ions in the structure. The ionic conductivities were measured by EIS, shown in Figure 3e. The highest ionic conductivity obtained was 3.9 mS·cm−1 for Li6PS5Cl0.5Br0.5, and extremely low electronic conductivities were again determined by DC polarization (Figure S6). Both ionic and electronic conductivities are summarized in Table 3. Previously reported DFT MD simulations suggest that Li ion conductivity can be further increased by altering the sulfur/ halide ratio of the argyrodite.46 Figure S7a shows the XRD patterns of Li5.75PS4.75Cl1.25 and Li5.5PS4.5Cl1.5 prepared via the solution-engineered synthesis method. Argyrodite phases are present as the major products, with impurities of Li3PO4 and, compared to Li6PS5X, much less Li2S (trace amount) and slightly more LiCl. The EIS results (Figure S7b) show clearly increasing ionic conductivities with increasing Cl/S ratio, namely, 3.0 mS·cm−1 for Li5.75PS4.75Cl1.25 and 3.9 mS·cm−1 for Li5.5PS4.5Cl1.5. The increased ionic conductivities and XRD
Table 3. Summary of the Ionic and Electronic Conductivities of Li-Argyrodites Prepared through the Solvent-Engineered Method material
ionic conductivity (mS·cm−1)
Li6PS5Cl Li6PS5Cl0.75Br0.25 Li6PS5Cl0.5Br0.5 Li6PS5Cl0.25Br0.75 Li6PS5Br Li5.75PS4.75Cl1.25 Li5.5PS4.5Cl1.5
2.4 3.2 3.9 3.4 1.9 3.0 3.9
electronic conductivity (mS·cm−1) 5.1 3.7 1.4 1.1 4.4 2.6 1.4
× × × × × × ×
10−6 10−6 10−5 10−5 10−6 10−5 10−5
patterns indicate successful substitution of sulfur with chlorine, which introduces Li vacancies in the argyrodite structure and further improves the ionic conductivity. However, due to the similar size and X-ray scattering factors of sulfur and chlorine, further neutron diffraction studies are essential to distinguish the degree of S/Cl disorder and Li position and occupation in the argyrodite structures. Finally, TiS2/Li11Sn6 all-solid-state prototype batteries employing Li6PS5Cl as a solid electrolyte were fabricated and cycled between 1.5 and 3.0 V vs Li/Li+ at 30 °C (see the SI and Figure S8 for details). We note that these model cells used a lithium alloy as the negative owing to its more robust mechanical properties and because the intent was simply to measure the properties of the argyrodite. Future explorations will utilize Li and a different cathode material. Figure 4 shows the first two discharge−charge voltage profiles for cells with Li6PS5Cl electrolyte prepared by a conventional solid-state route (Figure 4a) and by the solution-engineered route (Figure 4b). The cell with Li6PS5Cl obtained by the solutionengineering method exhibits theoretical capacity (239 mAh g−1) and shows no difference from the cell using the solid268
DOI: 10.1021/acsenergylett.8b01997 ACS Energy Lett. 2019, 4, 265−270
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ACKNOWLEDGMENTS
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REFERENCES
L.F.N. gratefully acknowledges the BASF International Scientific Network for Electrochemistry and Batteries for financial support. L.F.N. also thanks the NSERC for support via their Canada Research Chair and Discovery Grant programs.
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Figure 4. Comparison of all-solid-state batteries using Li6PS5Cl prepared by (a) solid-state synthesis and (b) solution-engineered synthesis: first (solid line, red) and second (dashed line, blue) are the charge−discharge voltage profiles, respectively, of TiS2/ Li6PS5Cl/Li11Sn6 cells cycled at a current density of 100 μA cm−2 (corresponding to a 0.11C rate).
state-derived electrolyte, implying that the minor impurities formed in the solution-engineered route are marginal with respect to cell performance. In summary, we developed a promising direct solutionengineered approach to argyrodite phases Li6PS5X (X = Cl, Br) using THF/ethanol mixtures as solvate complexes. The resulting solid electrolytes show high ionic conductivities up to 3.9 mS·cm−1 and negligible electronic conductivities, in excellent agreement with argyrodites obtained from all-solidstate methods. Fully crystallized Li6−yPS5−yCl1+y (y = 0−0.5), Li6PS5X (X = Cl, Br) and the mixed Cl/Br phases exhibit high Li+ conductivity. The solution synthesis present in this work is scalable and can be applied to related compounds by varying the solvent selection, i.e., DME/ethanol or simply ethanol. The all-solid-state batteries show excellent performance and exhibit no difference between cells using the argyrodite Li6PS5Cl solid electrolyte synthesized by a typical ball-milled solid-state route and the solution-engineered method we report here. These results provide new insight into the scalable preparation of existing and future thiophosphate-based solid electrolytes, which are vital to take solid-state batteries forward from a commercial perspective.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b01997. Experimental methods, electrochemical impedance spectroscopy, XRD patterns of other compositions, Rietveld refinements, and lattice parameters (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Laidong Zhou: 0000-0002-8556-3296 Linda F. Nazar: 0000-0002-3314-8197 Notes
The authors declare no competing financial interest. 269
DOI: 10.1021/acsenergylett.8b01997 ACS Energy Lett. 2019, 4, 265−270
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ACS Energy Letters
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DOI: 10.1021/acsenergylett.8b01997 ACS Energy Lett. 2019, 4, 265−270
