Synergic effect of CaI₂ and LiI on ionic conductivity of solution-based synthesized Li₇P₃S₁₁ solid electrolyte

Abstract

Li₇P₃S₁₁ doped with CaX₂ (X = Cl, Br, I) and LiI solid electrolytes were successfully prepared by liquid-phase synthesis using acetonitrile as the reaction medium. Their structure was investigated using XRD, Raman spectroscopy and SEM-EDS. The data obtained from complex impedance spectroscopy was analyzed to study the ionic conductivity and relaxation dynamics in the prepared samples. The XRD results suggested that a part of CaX₂ and LiI incorporated into the structure of Li₇P₃S₁₁, while the remaining part existed at the grain boundary of the Li₇P₃S₁₁ particle. The Raman peak positions of PS₄³⁻ and P₂S₇⁴⁻ ions in samples 90Li₇P₃S₁₁–5CaI₂ and 90Li₇P₃S₁₁–5CaI₂–5LiI had shifted as compared to the Li₇P₃S₁₁ sample, showing that CaI₂ addition affected the vibration of PS₄³⁻ and P₂S₇⁴⁻ ions. EDS results indicated that CaI₂ and LiI were well dispersed in the prepared powder sample. The ionic conductivity at 25 °C of sample 90Li₇P₃S₁₁–5CaI₂–5LiI reached a very high value of 3.1 mS cm⁻¹ due to the improvement of Li-ion movement at the grain boundary and structural improvement upon CaI₂ and LiI doping. This study encouraged the application of Li₇P₃S₁₁ in all-solid-state Li-ion batteries.

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1. Introduction

Sulfide-based solid electrolytes (SE) have high potential for application in all-solid-state batteries because of their relatively high ionic conductivity and suitable mechanical properties compared with oxide-based and polymer-based SEs.^1,2 Li₇P₃S₁₁, Li_9.54Si_1.74P_1.44S_11.7Cl_0.3, Li_5.35Ca_0.1PS_4.5Cl_1.55 and Li_10.1P_2.95Sb_0.05S₁₂I exhibited excellent ionic conductivities of about 17, 25, 10.2 and 5.9 mS cm⁻¹ at 25 °C, respectively.^3–6 Among them, Li₇P₃S₁₁ has been thoroughly studied since it was invented.⁷ Li₇P₃S₁₁ is usually prepared by solid-state reaction and achieves a highest ionic conductivity of about 17 mS cm⁻¹ at 25 °C.⁸ Researchers have studied the effects of many different substances such as Li₃PO₄, Li₃BO₃, Li₂ZrO₃ and P₂O₅ on the ionic conductivity of Li₇P₃S₁₁.^9–12 Murakami et al. studied ^6/7Li and ³¹P solid-state NMR to investigate the origin of high ionic conductivity of Li₇P₃S₁₁ and found that the significant motion fluctuation of the P₂S₇ tetrahedral unit above 310 K facilitated the Li-ion movement, resulting in high ionic conductivity.¹³ Seino et al. found that Li₇P₃S₁₁ glass ceramic with the crystallinity ranging from 50% to 80% reached its highest ionic conductivity.¹⁴

The liquid phase synthesis of Li₇P₃S₁₁ has been recently introduced and takes the advantage of electrode composite preparation.^15,16 Dimethoxy ethane was the first solvent ever used to prepared Li₇P₃S₁₁.¹⁷ Ethyl acetate was recently employed to synthesize Li₇P₃S₁₁ which exhibited high ionic conductivity of 1.05 mS cm⁻¹ at 25 °C.¹⁸ Among of the solvents that has been employed in Li₇P₃S₁₁, acetonitrile (ACN) is the most common one because the prepared SEs exhibited relatively high ionic conductivity at 25 °C, ranging from 0.8 to 1.2 mS cm⁻¹.^15,19,20 The solvent-based synthesis of Li₇P₃S₁₁ is complicated and could be described as dissolution-evaporation process. It was found that Li₂S reacts with P₂S₅ in ACN to form soluble Li₄P₂S₇ and Li₃PS₄ precipitate at a molar ratio of 1 : 1. The residue obtained after solvent removal are Li₄P₂S₇·ACN and Li₃PS₄·ACN. Li₇P₃S₁₁ was formed simultaneously with ACN removal from these phases during heat treatment at high temperature. There are many studies on how to increase the ionic conductivity of Li₇P₃S₁₁ produced by solid-phase reaction methods, such as using doping substances or creating defects in the crystal lattice.^21–23 However, there are still lack of the information about the method to improve the ionic conductivity of Li₇P₃S₁₁ prepared using liquid-phase synthesis. It was reported that CaI₂ and CaS addition enhanced the conductivity of Li₇P₃S₁₁.^16,24 Li₇P₃S₁₁ prepared using a mixture of Li₂S, P₂S₅, and excess elemental sulfur in a mixed solvent of acetonitrile, tetrahydrofuran, and ethanol also exhibited high ionic conductivity of about 1.2 mS cm⁻¹ at 25 °C.²⁵

In this study, the ionic conductivity of Li₇P₃S₁₁ prepared using ACN was enhanced by CaX₂ (X = Cl, Br, I) and LiI doping. The 95Li₇P₃S₁₁–5CaI₂ solid electrolyte exhibited the ionic conductivity of about 1.0 mS cm⁻¹ at 25 °C. The 90Li₇P₃S₁₁–5CaI₂–5LiI solid electrolyte showed high ionic conductivity of about 3.1 mS cm⁻¹ at 25 °C, which was comparable to those prepared by solid-state reaction. The data obtained from AC impedance spectroscopy was interpreted in terms of conductivity isotherms, dielectric constant and dielectric loss. It was found that CaI₂ and LiI doping enhanced the Li-ion movement at grain boundary and P₂S₇⁴⁻ ion motion, thus improved ionic conductivity.

2. Experimental

Chemicals

Li₂S (99.9%, Macklin), P₂S₅ (99%, Macklin), CaCl₂ (99.99%, Macklin), CaBr₂ (99.99%, Aladin), CaI₂ (99.999%, Macklin), LiI (99.999%, Macklin) and super dehydrated acetonitrile (Aldrich) were used as-received without any further treatment.

Liquid-phase synthesis of Li₇P₃S₁₁

1.5 g of Li₂S and P₂S₅ (7 : 3 in molar ratio) was weighted and put into a three-necked flask together with 40 ml of ACN. The mixture was stirred at 300 rpm and 50 °C for 24 h, the solvent was then evaporated at 80 °C under low pressure. The residue was carefully grounded using agate mortar prior to be heat treated at 270 °C for 2 h in Ar atmosphere to obtain Li₇P₃S₁₁ solid electrolyte (hereafter denote as LPS).

Liquid-phase synthesis of 95Li₇P₃S₁₁–5CaX₂ (X = Cl, Br, I)

1.5 g of Li₂S and P₂S₅ (7 : 3 in molar ratio) and appropriate amount of CaX₂ to form 95Li₇P₃S₁₁–5CaX₂ (molar ratio) was put into a three-necked flask together with 40 ml of ACN. The mixture was stirred at 300 rpm and 50 °C for 24 h, the solvent was then evaporated at 80 °C under low pressure. The residue was carefully grounded using agate mortar prior to be heat treated at 270 °C for 2 h in Ar atmosphere to obtain 95Li₇P₃S₁₁–5CaX₂ solid electrolytes (hereafter denote as 5CaX₂).

Liquid-phase synthesis of 90Li₇P₃S₁₁–5CaI₂–5LiI

1.5 g of Li₂S and P₂S₅ (7 : 3 in molar ratio), appropriate amount of CaI₂ and LiI to form 90Li₇P₃S₁₁–5CaI₂–5LiI (molar ratio) was put into a three-necked flask together with 40 ml of ACN. The mixture was stirred at 300 rpm and 50 °C for 24 h, the solvent was then evaporated at 80 °C under low pressure. The residue was carefully grounded using agate mortar prior to be heat treated at 270 °C for 2 h in Ar atmosphere to obtain 90Li₇P₃S₁₁–5CaI₂–5LiI solid electrolyte (hereafter denote as 5CaI₂–5LiI).

Structural characterization

The structure of the prepared samples was characterized with X-ray diffraction (XRD; X8, Bruker), Raman spectroscopy (Horiba LabRam HR spectrometer, 532 nm) and SEM (S4800, Hitachi) and EDS (ULTIM MAX, Oxford Instrument).

The samples were prepared in an Ar-filled glove box. The prepared sample was loaded into an air-tide sample holder for characterization.

AC electrochemical impedance spectroscopy

The electrical conductivity measurement was performed on a pellet prepared by uniaxially cold pressing approximately 100 mg of the powder at a pressure of 510 MPa as reported previously.²⁶ The alternating-current impedance spectroscopy measurement was carried out using a potentiostat (PGSTAT302N, Autolab, Herisau, Switzerland) from 10 MHz to 10 Hz. Since 5CaI₂–5LiI exhibited high ionic conductivity at room temperature, a pellet with thickness of about 4.2 mm was prepared to get accurate data.

3. Results and discussion

Fig. 1a showed the XRD patterns of LPS, 5CaX₂ and 5CaI₂–5LiI solid electrolytes. The patterns of all samples could be assigned to Li₇P₃S₁₁ crystal phase.^7,27 No peaks of CaX₂ and LiI was detected. The position of the most intense peak in the doped samples was almost similar to that of LPS indicating that only a small amount of CaX₂ incorporated to the crystal structure of Li₇P₃S₁₁. It was also reported that the full doping amount of CaI₂ to Li₇P₃S₁₁ was about 3 mol%.¹⁶ Ujiie et al. reported that LiX (X = F, Cl, Br, I) dissolved into LPS crystal structure at less than 10% molar ratio.²⁷ Hikima et al. found that CaS and CaI₂ mainly remained at the grain boundary of Li₇P₃S₁₁.²⁴ Thus, the patterns in Fig. 1a suggested that a part of CaX₂ and LiI incorporated into the structure of LPS, while the remained part existed at the grain boundary of the LPS particle. Fig. 1b showed the Raman spectra of LPS, 5CaI₂ and 5CaI₂–5LiI. Fig. 1c and d showed the deconvolution spectra of LPS and 5CaI₂, respectively. The spectrum of LPS had two peaks at 421 and 411 cm⁻¹, which corresponded to the vibration of PS₄³⁻ and P₂S₇⁴⁻.²⁸ The spectrum of 5CaI₂ also revealed two peaks of PS₄³⁻ and P₂S₇⁴⁻ located at 421 and 403 cm⁻¹. The spectrum of 5CaI₂–5LiI showed two peaks located at 420 and 400 cm⁻¹, which could be assigned to the local structure unit of PS₄³⁻ and P₂S₇⁴⁻ in Li₇P₃S₁₁, respectively. The peak positions of PS₄³⁻ and P₂S₇⁴⁻ ions in samples 5CaI₂ and 5CaI₂–5LiI had shifted as compared to the LPS sample showing that CaI₂ addition affected the vibration of PS₄³⁻ and P₂S₇⁴⁻ ions. The results from Raman spectra demonstrated that a portion of CaI₂ incorporated into the crystal structure of LPS. SEM-EDS results for 5CaI₂–5LiI is shown in Fig. 1e. The SEs are in the form of particles with a size of several tens of micrometers. EDS results indicated that CaI₂ and LiI were well dispersed in the prepared powder sample. It can be reasonably concluded that Ca²⁺ and I⁻ were doped in the Li₇P₃S₁₁ structure based on the experimental results: the disappearance of the peak corresponding to CaI₂ and LiI in the XRD results, the peak shift in the Raman spectrum, Ca and I are also dispersed in the particles as shown in the EDS results.

Fig. 1 Structural characterization of the prepared samples. (a) XRD patterns of Li₇P₃S₁₁ (LPS), 95Li₇P₃S₁₁–5CaX₂ (5CaX₂) and 90Li₇P₃S₁₁–5CaI₂–5LiI (5CaI₂–5LiI) solid electrolytes; (b) Raman spectra of LPS, 5CaI₂ and 5CaI₂–5LiI; (c and d) deconvolution of the main Raman peaks of LPS and 5CaI₂, respectively; (e) SEM-EDS results of 5CaI₂–5LiI.

The temperature dependence of the ionic conductivity of LPS, 5CaX₂ and 5CaI₂–5LiI solid electrolytes were illustrated in Fig. 2a. The electrochemical impedance spectra of 5CaI₂–5LiI, which were employed to calculate ionic conductivity, were illustrated in Fig. 2b. Those of LPS and 5CaI₂ were shown in Fig. S1 (ESI).† The ionic conductivity at 25 °C of LPS, 5CaCl₂, 5CaCl₂, 5CaCl₂ and 5CaI₂–5LiI were 0.15, 0.19, 0.25, 1.0 and 3.1 mS, respectively. At 70 °C, the ionic conductivity of all the samples increased and reached the value of 1.05, 2.0, 2.2, 10.1 and 13 mS for LPS, 5CaCl₂, 5CaCl₂, 5CaCl₂ and 5CaI₂–5LiI, respectively. It could be seen that log₁₀(σ) satisfied an almost linear dependence on inversed temperature and therefore followed Arrhenius equation σ = σ₀ exp(−E_a,DC/(k_BT)). The activation energy E_a,DC was then calculated and shown in Table 1. The activation energy E_a,DC of LPS, 5CaCl₂, 5CaCl₂, 5CaCl₂ and 5CaI₂–5LiI were 36, 37, 36, 30 and 25 kJ mol⁻¹, respectively. The results showed that CaI₂ and LiI addition greatly improved the ionic conductivity and activation energy of LPS.

Fig. 2 (a) Temperature dependence of Li₇P₃S₁₁, 5CaX₂ (X = Cl, Br, I) and 5CaI₂–5LiI solid electrolytes; (b) electrochemical impedance spectra of the prepared 5CaI₂–5LiI solid electrolyte.

Table 1 DC activation energy E_a,DC, activation energy E_a,m of ion migration at grain boundary, and characteristic time τ_0,m of ion migration at grain boundary

	LPS	5CaCl2	5CaBr2	5CaI2	5CaI2–5LiI
Ea,DC/kJ mol^−1	36	37	36	30	25
Ea,m/kJ mol^−1	37	38	37	31	19
τ0,m/s	5.0 × 10^−10	3.4 × 10^−10	4.4 × 10^−10	6.3 × 10^−9	4.1 × 10^−7

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The study of dielectric properties provides information on electrical energy decay in materials with alternating electric fields. The real part of the dielectric constant, ε′, reflects the amount of energy stored in the form of polarization when an electric field is applied.²⁹ The real part of the permittivity, ε′, was calculated using the following equation:

where C₀ = ε₀(A/d) is the free space capacitance of the cell, ε₀ is the permittivity of the free space (8.854 × 10⁻¹² F m⁻¹), and A and d are the surface area and thickness, respectively, of the sample pellet.

Fig. 3 showed the frequency dependence of the real part of dielectric constant, ε′, of (a) LPS, (b) 5CaI₂, (c) 5CaI₂–5LiI measured at different temperature and (d) frequency dependence of ε′ of LPS, 5CaCl₂, 5CaBr₂, 5CaI₂, 5CaI₂–5LiI measured at room temperature. The observed upturn in all plots at lower frequencies could be attributed to the electrode–electrolyte interface polarization. This is because the accumulation of charged ions near the electrode leads to the formation of space charge layer, which in turn block the electric field and enhance electrical polarization. As frequency increased, the dielectric constant decreased. This phenomenon is a typical property of ionic conducting materials.³⁰ An increase in ε′ with an increase in temperature was observed at low and intermediate frequency region in all the samples suggesting that charge carrier movement at grain boundary was thermally activated (Fig. 3 and S2†). The plots of LPS, 5CaCl₂ and 5CaBr₂ at room temperature continuously decreased in intermediate and high frequency region; however, the plots of 5CaI₂ and 5CaI₂–5LiI at room temperature exhibited a maxima at 10⁵–10⁶ Hz (Fig. 3d). The maxima were observed at 10⁵–10⁶ Hz in all the plots at 50 °C or above (Fig. 3 and S1†). Those observation suggested that CaI₂ and LiI addition enhanced dielectric properties of Li₇P₃S₁₁. This observation was in agreement with Raman spectra (Fig. 1b) and ionic conductivity at room temperature of the prepared samples (Fig. 2).

Fig. 3 Frequency dependence of the real part of dielectric constant, ε′, of (a) LPS, (b) 5CaI₂, (c) 5CaI₂–5LiI measured at different temperature and (d) frequency dependence of ε′ of LPS, 5CaCl₂, 5CaBr₂, 5CaI₂, 5CaI₂–5LiI measured at room temperature.

Fig. 4a–c showed the frequency dependence of the loss factor, tan δ, of LPS, 5CaI₂ and 5CaI₂–5LiI, respectively. The plot at room temperature of LPS showed two peaks at low and high frequency, which could be attributed to ion migration at grain boundary and bulk (Fig. 4a). As temperature increased, two peaks appeared at low frequency region. The one at lower frequency was assigned to electrode polarization. The peak at higher frequency shifted toward high frequency as temperature increased and could be attributed to grain boundary migration. The migration energy E_a,m of the Li⁺ ion moving at grain boundary could be obtained from the temperature dependence of the peak position at low frequency in tan δ. Temperature dependence of the inverse relaxation time (τ_m⁻¹ = F_max) was plotted, and the migration characteristic time τ_0,m was then obtained by numerically fitting the data using the Arrhenius equation: τ_m = τ_0,m exp(−E_a,m/(k_BT)). The obtained E_a,m and τ_0,m were illustrated in Table 1. The migration activation energy at grain boundary E_a,m of the prepared samples were similar to DC activation energy E_a,DC, suggesting that charge carrier moving at grain boundary was the critical process in those samples. The characteristic migration time of LPS, 5CaCl₂ and 5CaBr₂ were 5.0 × 10⁻¹⁰, 3.4 × 10⁻¹⁰ and 4.4 × 10⁻¹⁰ s, respectively. The characteristic migration time of 5CaI₂ and 5CaI₂–5LiI were 6.3 × 10⁻⁹ and 4.1 × 10⁻⁷ s, respectively. Therefore, the charge carrier movement at grain boundary was enhanced by CaI₂ and LiI addition; it changed from short range diffusion in LPS to long range diffusion in 5CaI₂ and 5CaI₂–5LiI. This phenomenon was also consistent with the article by Tu et al., which reported the influence of LiI doping on the impedance at the grain boundary of Li₂S–AlI₃ solid solution.³¹

Fig. 4 Frequency dependence of the loss factor, tan δ, of (a) LPS, (b) 5CaI₂ and (c) 5CaI₂–5LiI.

4. Conclusion

Li₇P₃S₁₁ solid electrolytes doped with CaX₂ (X = Cl, Br, I) and LiI were successfully prepared by liquid-phase synthesis using ACN as reaction medium. Results from XRD, Raman spectroscopy and SEM-EDS proved that CaX₂ and LiI incorporated into the crystal structure of Li₇P₃S₁₁. Results from AC electrochemical impedance spectroscopy showed that CaI₂ and LiI addition enhanced dielectric properties of Li₇P₃S₁₁. In addition, the charge carrier movement at grain boundary changed from short range diffusion in LPS to long range diffusion in 5CaI₂ and 5CaI₂–5LiI. As a result, very high ionic conductivity of 3.1 mS cm⁻¹ at 25 °C was obtained.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We acknowledge Ho Chi Minh City University of Technology (HCMUT), VNU-HCM for supporting this study.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra00442f

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