High ionic conductivity of multivalent cation doped Li₆PS₅Cl solid electrolytes synthesized by mechanical milling

Abstract

The performances of next generation all-solid-state batteries might be improved by using multi-valent cation doped Li₆PS₅Cl solid electrolytes. This study provided solid electrolytes at room temperature using planetary ball milling without heat treatment. Li₆PS₅Cl was doped with a variety of multivalent cations, where an electrolyte comprising 98% Li₆PS₅Cl with 2% YCl₃ doping exhibited an ionic conductivity (13 mS cm⁻¹) five times higher than pure Li₆PS₅Cl (2.6 mS cm⁻¹) at 50 °C. However, this difference in ionic conductivity at room temperature was slight. No peak shifts were observed, including in the synchrotron XRD measurements, and the electron diffraction patterns of the nano-crystallites (ca. 10–30 nm) detected using TEM exhibited neither peak shifts nor new peaks. The doping element remained at the grain boundary, likely lowering the grain boundary resistance. These results are expected to offer insights for the development of other lithium-ion conductors for use in all-solid-state batteries.

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Introduction

All-solid-state batteries with solid electrolytes show promise for use in next-generation batteries.¹ Sulfide solid electrolytes have become promising candidates due to the strong polarizability of sulfur anions, which has led to high ionic conductivity and superior mechanical properties in comparison with oxide solid electrolytes.^2,3

P₂S₅-based solid electrolytes have been extensively investigated, where studies have found that thio-LISICONs,^4–6 Li₇P₃S₁₁ glass ceramics,^7,8 Li₁₀GP₂S₁₂ (LGPS) super Li ion conductors,⁹ and Li_9.54Si_1.74P_1.44S_11.7Cl_0.3 exhibited a high ionic conductivity of 25 mS cm⁻¹ at room temperature.¹⁰ Li₆PS₅X (X = Cl, Br, I) with an argyrodite-type structure has a high ionic conductivity of more than 1 mS cm⁻¹ at room temperature (about 25 °C), wide electrochemical window, and moderate mechanical properties. A higher ionic conductivity is expected in Li₆PS₅X (X = Cl, Br, I) solid electrolytes when used as a cathode composite.

The ionic conductivities of Li₆PS₅X (where X = Cl, Br, I) range widely from 10⁻⁶ S cm⁻¹ for Li₆PS₅I to 10⁻³ S cm⁻¹ for Li₆PS₅Cl, with Li₆PS₅Br exhibiting intermediate conductivity.¹¹ Thus, Li₆PS₅Cl has been chosen as the focus of this study. The ionic conductivity of Li₆PS₅X (X = Cl, Br, I) with an argyrodite-type structure has been enhanced using numerous approaches, including aliovalent substitution of S²⁻ and/or P⁵⁺ by other cations. Aliovalent substitution of the S²⁻ anion has been explored in several studies, revealing that the ionic conductivity of Li₆PS_5−xSe_xI is higher (0.28 mS cm⁻¹) than the unmodified material (0.0025 mS cm⁻¹) at room temperature.^12,13 Furthermore, a Te-doped Li_6.25PTe_0.125S_5.125Cl_0.75 solid electrolyte exhibited a relatively high ionic conductivity of 4.5 mS cm⁻¹ at room temperature,¹⁴ while another study found that Cl⁻ substituted at the S²⁻ site of Li₆PS₅Cl improved ionic conductivity to 9.4 mS cm⁻¹ at Li_5.5PS_4.5Cl_1.5 (ref. 15) and 6.4 mS cm⁻¹ at Li_5.7PS_4.7Cl_1.3.¹⁶

Aliovalent substitution of the P⁵⁺ cation with other cations has been investigated. Si-, Ge-, and Sn-substituted Li₆PS₅I argyrodite solid electrolytes had very high ionic conductivities of 2.0, 5.4, and 0.1 mS cm⁻¹, respectively.^17,18 The Sn substitution of P in Li₆PS₅I argyrodite solid electrolytes has also been reported, where Li_6.24P_0.823Sn_0.177S_4.58I_0.9 exhibited a conductivity of 0.35 mS cm⁻¹.¹⁹ Introducing a multivalent cation to a argyrodite-type Li₆PS₅Cl solid electrolyte for aliovalent substitution of Li⁺ site was recently reported,²⁰ where the ionic conductivity of Li_5.4Al_0.2PS₅Br almost tripled (2.4 mS cm⁻¹) and the substitution of Al³⁺ to the Li⁺ sites was confirmed by Rietveld refinements.²⁰

While the aliovalent substitution of cations and/or anions has been widely explored, mixed cation effects have also been found to improve conductivity in glass materials²¹ and polymer electrolytes.^22,23 Furthermore, grain boundary engineering has been used to decrease grain boundary resistance at ionic conductors.^24,25 However, the effect of introducing other cation elements into the grain boundary of argyrodite-type Li₆PS₅Cl solid electrolytes to improve ionic conductivity has not yet been investigated.

This study aimed to use a mechanical milling method at room temperature (no heat treatment) to introduce another cation element without causing substitution of the Li₆PS₅Cl crystal lattice. The effect of introducing another cation element without solid solution formation on the structure and ionic conductivity of the argyrodite-type Li₆PS₅Cl solid electrolyte was evaluated. The multivalent cation drastically improved the ionic conductivity at higher temperatures (above 50 °C), where the 0.98Li₆PS₅Cl–0.02YCl₃ sample exhibited an ionic conductivity five times higher (13 mS cm⁻¹) than Li₆PS₅Cl (2.6 mS cm⁻¹) at 50 °C. However, this difference in ionic conductivity was slight at room temperature. Synchrotron XRD measurements and TEM observations revealed that the doping element remained at the grain boundary and lowered the grain boundary resistance.

Experimental

Synthesis

Samples were prepared using a mechanical milling synthesis method. Li₂S (Mitsuwa Chemical, 99.9%), P₂S₅ (Merck Group, 99%), LiCl (WAKO FUJIFILM, 99.9%), CaS (Kojundo Laboratory, 99.99%), BaS (Aldrich, 99.99%), SrS (Aldrich, 99.99%), YCl₃ (Aldrich, 99.99%), AlCl₃ (Aldrich, 99.99%), and ZnCl₂ (Aldrich, 99.99%) were used without further purification. A typical batch was prepared by weighing and mixing an appropriate amount of each starting material using agar and a mortar for ca. 15 min. The mixture was transferred to a zirconia pot (45 ml) with 15 zirconia balls (diameter = 10 mm). The pot was rotated at 600 rpm for 20 h using a Pulverisette 7 micro ball mill (Fritsch Co., Ltd.). The samples were recovered and characterized without a further heat treatment step. All synthesis experiments were performed in dry Ar atmosphere.

Characterization

The crystal structures of the various doped powders were characterized using X-ray diffraction (XRD; Ultima IV, Rigaku Co., Ltd.). The samples were sealed in specialized holders equipped with a Be window (Rigaku Co., Ltd.) in an Ar-filled glove box to prevent exposure to air humidity. The morphology of the solid electrolyte was evaluated using field-emission scanning electron microscopy (FE-SEM) with energy dispersive X-ray spectroscopy (EDS) (JSM-7800F, JEOL Ltd.). Synchrotron X-ray diffraction measurements were conducted at the BL02B2 beamline at SPring-8 facility using a high-flux synchrotron X-ray source with a wavelength of 0.6199 Å.²⁶ The sample was sealed under vacuum in a quartz capillary tube (diameter = ca. 0.3 mm) and images were acquired using a Debye–Scherrer diffraction camera. Field-emission transmission electron microscopy (FE-TEM) observations were conducted at an accelerating voltage of 200 kV (at room temperature JEM-2100F, JEOL Ltd.).²⁷ Samples were mounted on an amorphous carbon film supported using a Cu grid for TEM observations in a glove box filled with inert Ar gas. Additionally, air exposure was avoided using a vacuum-transfer TEM holder (Gatan Model 648). The microstructure of the multivalent cation doped Li₆PS₅Cl samples was evaluated based on bright-field TEM images, hollow-cone dark-field (HCDF) images, and corresponding electron diffraction (ED) patterns. The incident electron beam was tilted at a fixed angle and was automatically rotated around the optical axis of the objective lens.^28,29 Therefore, the diffraction spots in the ED pattern were accurately captured in the HCDF image. The precipitated crystalline phase was identified using the “ProcessDiffraction” software package. The ED patterns were converted to one-dimensional intensity profiles for comparison with a powder XRD pattern database.^30,31

Electrochemical measurements

The relationship between temperature and total conductivity was investigated using alternating-current impedance spectroscopy (SI 1260, Solartron Analytical) from 10⁶ to 10 Hz under a dry Ar flow. Sample pellets (weight = ca. 100 mg; diameter = ca. 10 mm; thickness = ca. 1 mm) were prepared via uniaxial pressing at a pressure of 500 MPa at room temperature. The pellets were placed in a poly ether ether ketone (PEEK) holder with two pieces of carbon paper as blocking electrodes. The cell was measured under an Ar-stream in a glass tube and the temperature was gradually increased from room temperature to 110 °C and held at each testing temperature for 1 h before measuring the impedance value. The conductivity (σ) was calculated using the following formula:
where l represents the length between electrodes, S denotes the surface area, and R represents the resistance value. The pellet thickness was used as the length between electrodes (l), and the surface area (S) was calculated using the diameter. Moreover, the resistance value (R) was determined at the impedance plots.

Results and discussion

Cation doping level

The element doping levels were screened based on calcium ion (Ca²⁺) as one of examples of multivalent cations. The XRD patterns of the (1 − x)Li₆PS₅Cl–xCaS (where 0 ≤ x ≤ 0.2) doped samples synthesized using the ball milling method without heat treatment were evaluated (Fig. 1). All peaks were attributed to Li₆PS₅Cl (ICSD #259205), indicating that the argyrodite-type phase was formed in all samples. No CaS peaks were detected at doping levels below an x value of 0.1, while a CaS peak was observed at 22° when x was 0.2 (assigned using ICSD #28902 as a reference). The argyrodite-type Li₆PS₅Cl structure was maintained despite the formation of CaS. The ionic radius of Ca²⁺ (1.00 Å) is larger than Li⁺ (0.59 Å). However, no peak shifts were observed in the (1 − x)Li₆PS₅Cl–xCaS samples within the investigated degree of substitution.³² This indicated that Ca²⁺ was not incorporated into the Li₆PS₅Cl crystal lattice and a solid solution, such as Li_6−2xCa_xPS₅Cl, did not form during planetary ball milling without heat treatment. The lattice parameters of pure Li₆PS₅Cl (i.e. x = 0) was 9.862(3) Å, which was similar to reported values of 9.839(4) Å,³³ confirming the formation of argyrodite Li₆PS₅Cl solid electrolytes during room temperature synthesis.

Fig. 1 X-ray diffraction patterns of (1 − x)Li₆PS₅Cl–xCaS (0 ≤ x ≤ 0.2) synthesized by ball milling method.

The relationship between temperature and ionic conductivity of the (1 − x)Li₆PS₅Cl–xCaS (0 ≤ x ≤ 0.2) doped samples was based on conductivity values calculated from the impedance plots (Fig. 2a). The impedance plots were also illustrated in Fig. S1.† These curves were characteristic of pure ionic conductors and included a spike attributed to the bulk, grain boundary, and electrode resistances of the solid electrolytes. The resistance values corresponded to the intercept of the Nyquist plot on the x-axis (e.g., when the imaginary part of the impedance is equal to zero). The ionic conductivity was calculated as the sum of the grain-boundary and bulk resistances. The ionic conductivity of the pure Li₆PS₅Cl solid electrolyte was 1.3 mS cm⁻¹ at 25 °C, which was similar to a reported values of 1.33 mS cm⁻¹.³³ Meanwhile, the activation energy of 0.17 eV was lower than the previous reports of 0.30 eV.³³ The ionic conductivity of Ca²⁺ doped Li₆PS₅Cl at room temperature (26 °C), where the x = 0.01, 0.02, 0.03, 0.05, and 0.1 samples, were slightly higher of 1.6, 1.5, 1.6, 1.4, and 1.4 mS cm⁻¹, respectively. These results did not correspond with the findings a study on Li_5.4Al_0.2PS₅Br solid electrolytes synthesized by mechanical milling methods with heat treatment.²⁰ The Ca²⁺ doped Li₆PS₅Cl samples in which x = 0.01, 0.02, 0.03, 0.05, and 0.1 exhibited higher ionic conductivity than the undoped sample above 50 °C. Notably, the ionic conductivity of x = 0.02 was drastically improved. The x = 0.03 sample exhibited the highest ionic conductivity of 12.9 mS cm⁻¹ at 70 °C, which was four times the conductivity of Li₆PS₅Cl (3.4 mS cm⁻¹). The activation energy was calculated above 50 °C because the ionic conductivity at room temperature was relatively low along with the low linearity of the fitted curves. The activation energy slightly increased in the Ca²⁺ doped Li₆PS₅Cl compared with the pure Li₆PS₅Cl solid electrolyte, where the x = 0.01, 0.02, 0.03, 0.05, and 0.1 samples were 0.21, 0.23, 0.28, 0.28, and 0.27 eV, respectively. These activation energies for Ca-doped Li₆PS₅Cl (0.17–0.28 eV) were similar to other superionic conductors (typically, 0.2–0.3 eV).^7–11

Fig. 2 (a) Temperature dependence of ionic conductivity of (1 − x)Li₆PS₅Cl–xCaS (0 ≤ x ≤ 0.1) synthesized by ball milling method, (b) ionic conductivity as a function of x in (1 − x)Li₆PS₅Cl–xCaS (0 ≤ x ≤ 0.1) plotted at different temperatures.

The ionic conductivity of the pure and (1 − x)Li₆PS₅Cl–xCaS doped samples was plotted as a function of x at different temperatures (Fig. 2b). All of the samples doped with Ca²⁺ had a higher ionic conductivity than pure Li₆PS₅Cl. The samples with x = 0.02 or 0.03 exhibited the highest ionic conductivity above 50 °C and these doping levels were explored further in multivalent cation doping.

Cation doping type

The XRD patterns of the (1 − x)Li₆PS₅Cl–xMX (where x = 0.02 or 0.03 and MX = BaS, SrS, YCl₃, AlCl₃, or ZnCl₂) doped samples are given in Fig. 3. The characteristic peaks attributed to the argyrodite-type phase were observed in the patterns of all samples, confirming that the argyrodite-type Li₆PS₅Cl structure was maintained despite the introduction of various alkaline-earth-metal elements. Li₂S was confirmed at both of the doping levels based on the diffraction peak at 2θ ≈ 27° assigned by Li₂S (ICSD #196932). Although the ionic radii of Sr²⁺ (1.18 Å), Ba²⁺ (1.35 Å), Al³⁺ (0.39 Å), Zn²⁺ (0.64 Å), and Y³⁺ (0.90 Å) differ from Li⁺ (0.59 Å), no peak shifts of were observed in the BaS, SrS, YCl₃, AlCl₃, or ZnCl₂ doped samples, as was previously observed in the Ca²⁺ samples (1.00 Å).³²

Fig. 3 X-ray diffraction patterns of (1 − x)Li₆PS₅Cl–xMX (x = 0.02 or 0.03, MX = BaS, SrS, YCl₃, AlCl₃, ZnCl₂) synthesized by ball milling method.

The SEM images of the 0.98Li₆PS₅Cl–0.02YCl₃ solid electrolytes revealed particle sizes ranging from ca. 500 nm to 2 μm (Fig. 4). The distribution of Y³⁺ was investigated using EDS mapping images (Fig. 4), which indicated that Y³⁺ was well-dispersed compared with sulfur in the same regions. The SEM-EDS images of the 0.98Li₆PS₅Cl–0.02AlCl₃ solid electrolytes are given in Fig. S2† and exhibit similar sizes, morphologies, and Al distribution. These results indicated that the other cation-doped Li₆PS₅Cl would have similar particle sizes, morphologies, and distribution of doping elements.

Fig. 4 FE-SEM and EDX images of 0.98Li₆PS₅Cl–0.02YCl₃.

The temperature and ionic conductivity plots of the (1 − x)Li₆PS₅Cl–xMX doped samples indicated that the ionic conductivity of the Mg²⁺, Sr²⁺, and Ba²⁺ doped Li₆PS₅Cl was slightly higher at room temperature (Fig. 5). This difference became more substantial above 50 °C. The Sr²⁺, Ba²⁺ Al³⁺, and Zr²⁺ doped Li₆PS₅Cl exhibited slightly different behavior to the Ca²⁺ doped sample, where the Y³⁺ doped Li₆PS₅Cl consistently exhibited the highest ionic conductivity with values of 13, 22, 33, and 44 mS cm⁻¹ at 50, 70, 90, and 110 °C, respectively. The ionic conductivity at 50 °C of the 0.98Li₆PS₅Cl–0.02YCl₃ (13 mS cm⁻¹) is higher than Li₃PS₄ (approximately 0.5 mS cm⁻¹ at 50 °C),³⁴ Li₇P₃S₁₁ (approximately 10 mS cm⁻¹ at 50 °C),⁷ Li₆PS₅Cl (approximately 2.6 mS cm⁻¹ at 50 °C),³³ and Li₇P₂S₈I (approximately 2.0 mS cm⁻¹ at 50 °C),³⁵ although it was lower than LGPS of approximately 30 mS cm⁻¹ at 50 °C.⁹ Thus, the 0.98Li₆PS₅Cl–0.02YCl₃ has a significant advantage compared to these the state-of-the-art solid electrolytes in terms of the ionic conductivity.

Fig. 5 Temperature dependence of ionic conductivity of (1 − x)Li₆PS₅Cl–xMX (x = 0.02 or 0.03, MX = BaS, SrS, YCl₃, AlCl₃, ZnCl₂).

The activation energy slightly increased in the Y³⁺ doped Li₆PS₅Cl compared with the pure Li₆PS₅Cl solid electrolyte, which was 0.21 eV. This change exhibited a similar tendency to the Ca²⁺ doped Li₆PS₅Cl, which was also similar to other superionic conductors.

The impedance spectroscopies at room temperature and 50 °C of 0.98Li₆PS₅Cl–0.02YCl₃ and Li₆PS₅Cl were illustrated in Fig. 5 for a direct comparison. The resistance value of the pure Li₆PS₅Cl solid electrolyte was 68 Ω and 34 Ω at room temperature and 50 °C, respectively. Meanwhile, the resistance value of 0.98Li₆PS₅Cl–0.02YCl₃ significantly decreased to 21.7 Ω at 50 °C from 120 Ω at room temperature, although the absolute value depends on the sample thickness. These results also confirmed that the 0.98Li₆PS₅Cl–0.02YCl₃ was more substantially improved than the pure Li₆PS₅Cl solid electrolyte between room temperature and 50 °C.

Improvement of ionic conductivity

The synchrotron XRD pattern of the 0.98Li₆PS₅Cl–0.02YCl₃ doped sample was evaluated, where the wavelength 1.541 Å was converted from 0.6199 Å in the raw data (Fig. 6). No peak shifts were observed. The Li₂S starting material was observed in all doped samples and the peak intensities of Li₂S were slightly different in the pure Li₆PS₅Cl, 0.98Li₆PS₅Cl–0.02YCl₃, and 0.9Li₆PS₅Cl–0.1YCl₃ samples. The 0.9Li₆PS₅Cl–0.1YCl₃ sample exhibited diffraction peaks at 2θ ≈ 33°and 47°, which confirmed the presence of YCl₃ based on the reference synchrotron XRD patterns of YCl₃ (ICSD #15684). The slight variations in the amount of Li₂S starting material between the pure Li₆PS₅Cl and 0.98Li₆PS₅Cl–0.02YCl₃ samples had no effect on the ionic conductivity. The XRD pattern provides an indication of the average structure of the sample, thus slight changes in the local structure were not detected by XRD. Therefore, TEM was conducted to inspect the local structural changes at a nano-scale. The TEM image, HCDF image, and selected area electron diffraction patterns of the 0.98Li₆PS₅Cl–0.02YCl₃ sample were evaluated (Fig. 7). The HCDF images revealed precipitated nanocrystals with a particle size ranging from ca. 10 to 30 nm randomly distributed throughout the sample. Considering the earlier SEM observations (Fig. 3) and previous reports on TEM observation at an atomic level, each multivalent cation doped Li₆PS₅Cl particle was expected to include several nanocrystals with domain boundaries.³⁶ The TEM observation of the 0.9Li₆PS₅Cl–0.1YCl₃ sample revealed that the size and distribution of the nanocrystals were similar to the 0.98Li₆PS₅Cl–0.02YCl₃ sample (Fig. S3†). The electron diffraction pattern of the 0.98Li₆PS₅Cl–0.02YCl₃ sample was compared with the XRD intensity of Li₆PS₅Cl (ICSD #259205) and all peaks in the pattern were attributed to Li₆PS₅Cl (Fig. 7d). No peak shifts were observed in pure Li₆PS₅Cl (ICSD #259205), further indicating that the nanocrystals observed in the HCDF images were likely Li₆PS₅Cl. The TEM observations indicated that a solid solution was not formed.²⁰ These findings indicated that the doping element remained at the grain boundary between the particles and/or the domain boundary among the nanocrystals, leading to reduced grain boundary resistance. Besides, the temperature dependence of ionic conductivity (Fig. 2a and 5) was changed above 50 °C at all doped samples compared to the undoped sample, which was different from the previous report.²⁰ This phenomenon might be affected by doping element remained at the grain boundary. There are several previous reports on the effects of grain boundary. H. Yamada et al. reported that the grain boundary resistance of a solid electrolyte (Li_1.3Al_0.3Ti_1.7(PO₄)₃) was suppressed by coating a poorly conducting solid electrolyte (Li₂SiO₃).³⁷ H. Xu et al. also reported on Li₃OCl solid electrolytes in which the reduction of the grain boundary significantly influenced total ionic conductivity through an amorphous route.³⁸ However, there was no direct evidence of the doping element at the grain boundary, nor of a change in grain boundary resistance, thus further analysis will be performed.

Fig. 6 Synchrotron XRD patterns of (1 − x)Li₆PS₅Cl–xYCl₃ (x = 0, 0.02, 0.10) standardized by highest intensity of Li₆PS₅Cl at 2θ ≈ 30° with a wavelength of 1.541 Å converted from 0.6199 Å at raw data.

Fig. 7 (a) TEM image, (b) the HCDF image, (c) selected area electron diffraction patterns and (d) the corresponding intensity profiles of 0.98Li₆PS₅Cl–0.02YCl₃ with the XRD patterns of Li₆PS₅Cl (ICSD #259205).

Conclusion

This study explored the modification of grain boundaries for improved overall ionic conductivity of Li₆PS₅Cl solid electrolytes. Multivalent cation doped Li₆PS₅Cl was directly synthesized via mechanical milling synthesis with no heat treatment. The addition of a multivalent cation drastically improved ionic conductivity at higher temperatures (above 50 °C), where the 0.98Li₆PS₅Cl–0.02YCl₃ sample exhibited an ionic conductivity five times higher (13 mS cm⁻¹) than Li₆PS₅Cl (2.6 mS cm⁻¹). However, the difference in ionic conductivity at room temperature was slight. The XRD pattern of the 0.98Li₆PS₅Cl–0.02YCl₃ sample exhibited no shifts, even when using synchrotron XRD. Nanocrystals with a particle size ranging from ca. 10 to 30 nm were observed using TEM and neither peak shifts nor new peaks were observed in the electron diffraction patterns of these nanocrystals. These synchrotron XRD measurements and TEM observations revealed that the doping element remained at the grain boundary to lower the grain boundary resistance. These results are expected to offer insight for the development of other lithium-ion conductors for use in all-solid-state batteries, especially when the potential of bulk conductivity of solid electrolytes is to be completely investigated.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was financial supported by the SOLiD-EV project (JPNP18003) of the New Energy and Industrial Technology Development Organization (NEDO). We also thank Ms. Makiko Oshida and Mr Keisuke Shinoda for their help for FE-SEM and EDS measurements at the National Institute for Materials Science (NIMS) Battery Research Platform. The synchrotron radiation experiments were performed at the BL02B2 of SPring-8 facility by Prof. Toshiyuki Matsunaga and Prof. Kentaro Yamamoto at Kyoto University with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2019B1500).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra02545c

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