Characterization of a Novel Chloride Li-ion Conductor Li₂LuCl₅

Abstract

Materials with a high Li-ion conductivity and deformability are garnering interest for the facile fabrication of safe all-solid-state batteries with high energy densities. Hence, increasing attention has been focused on Li-containing chloride materials that meet these requirements since they were first reported in 2018 (Asano et al. Advanced Materials 2018, 30 (44), 1803075). In this paper, we report a novel Li-containing chloride of Li₃LuCl₆ with a high Li-ion conductivity (σ_{25 °C} = 3.1 × 10⁻⁴ S cm⁻¹) and sufficient deformability. Furthermore, its defect derivative of Li_3−xLuCl_6−x (x = 1), i.e., Li₂LuCl₅ with a higher Li-ion conductivity (σ_{25 °C} = 5.2 × 10⁻⁴ S cm⁻¹), is synthesized. Scanning electron microscopy confirms the dense packing of both Li₃LuCl₆ and Li₂LuCl₅ as compressed pellets. Hence, Li₂LuCl₅ is presented as a promising solid electrolyte with a high Li-ion conductivity and deformability, which presents a novel opportunity for exploring the composition of Li₂MCl₅ (M: trivalent metal ion) compounds and indicates potential applications as an all-solid-state battery material. Furthermore, as there are no reported cases of high-Li-ion-conductivity chloride materials with the Li₂MCl₅ composition until this work, this study is expected to increase the progress in future studies of LiCl-deficient Li₂MCl₅ compositions with a Li₃MCl₆ composition.

1. Introduction

Li-ion batteries are widely used in small electronic devices. Recently, there have been increasing expectations for large-scale batteries for electric vehicles and stationary energy storage among other applications.¹ However, the leakage and flammability of organic liquid electrolytes in batteries are major concerns for battery safety, which limit their large-scale use in batteries.² The research and development of all-solid-state Li-ion batteries, in which the liquid electrolyte is replaced by a flame-retardant solid electrolyte, have been extensively conducted in recent years.^3–5 Solid electrolytes must have a high Li-ion conductivity that is comparable to that of liquid electrolytes. Recently, solid electrolytes with ionic conductivities that exceed those of liquid electrolytes have been developed.^6,7 However, unlike their liquid equivalents, solid electrolytes, which are composed of ceramic particles, experience intergranular resistance from the nonuniform contact between the particles (disorder in the crystal structure and formation of voids), thereby increasing the overpotential and decreasing the cycling performance.^8–12 Improving particle-to-particle contact to facilitate ion transfer is one of the most important challenges in realizing the facile fabrication of all-solid-state batteries with high energy densities.

Li-ion conductive oxides and sulfide materials have been studied for several decades as candidate solid electrolytes for all-solid-state batteries. In oxide materials, particles are connected via high-temperature sintering. As the sintering process theoretically involves the interdiffusion of constituent elements, undesirable side reactions often occur between the electrode and electrolyte materials.^13,14 In contrast, sulfide materials exhibit high deformability and form strong mechanical contacts without voids upon cold pressing.¹⁵ However, the potential window is narrow, and their electrochemical stability is low.¹⁶ These properties strongly limit the electrode–electrolyte material combination.¹⁷ Therefore, it is necessary to select a material with a wide potential window, high Li-ion conductivity, and high deformability for batteries.

Chloride materials have a wide potential window¹⁸ and are expected to exhibit a high ionic conductivity and deformability in terms of polarizability and Coulombic interactions.^19–22 Since Asano et al. discovered highly Li-ion conductive Li₃YCl₆¹⁹ (∼10⁻³ S cm⁻¹ at room temperature) in 2018, various Li₃InCl₆²⁰ and Li₃ScCl₆²¹ with high ionic conductivities of ∼10⁻³ S cm⁻¹ at room temperature have been reported. Hence, there is ongoing intensive exploration of solid electrolytes with a Li₃MCl₆ composition.^22–24 Recently, the Li-ion conductivity of chloride materials has been suggested to exceed that of sulfide materials.¹⁸ Ishiguro et al. reported an LiTaCl₆⁷ with the Li-ion conductivity of ∼10⁻² S cm⁻¹.

For 30 years, Li₃MCl₆ compounds (M = Sc, In, Ho, Er, Y, Yb, etc.) have been investigated to exhibit extremely low Li-ion conductivities (Li₃InCl₆: σ_{25 °C} ≈ 10⁻⁵ S cm⁻¹,²⁵ Li₃YCl₆: σ_{110 °C} ≈ 6.0 × 10⁻⁷ S cm⁻¹, Li₃YbCl₆: σ_{110 °C} ≈ 2.5 × 10⁻⁷ S cm^{−1 26}). Further, the recent diversification of the synthesis methods prompted the reexamination of chloride materials, whereby phases with high room-temperature Li-ion conductivities of 10⁻⁴–10⁻³ S cm⁻¹ were noted (Li₃InCl₆:²⁰ σ_{25 °C} = 1.5 × 10⁻³ S cm⁻¹, Li₃ErCl₆:^27,28 σ_{25 °C} = 3.3 × 10⁻⁴ S cm⁻¹, Li₃YCl₆:¹⁹ σ_{25 °C} = 5.1 × 10⁻⁴ S cm⁻¹). Introducing Li vacancies by replacing M with a high-valence cation improves the Li-ion conductivity of Li₃MCl₆ (M = Y, Er) (Li_2.5Y_0.5Zr_0.5Cl₆: σ_{25 °C} = 1.4 × 10⁻³ S cm⁻¹, Li_2.63Er_0.63Zr_0.33Cl₆: σ_{25 °C} = 1.1 × 10⁻³ S cm⁻¹).²⁹

This study focused on trivalent lanthanide ion Lu³⁺ with an ionic radius (86 pm) that is similar to those of Yb³⁺ (r = 87 pm), Er³⁺ (r = 89 pm), and Y³⁺ (r = 90 pm).³⁰ Li₃MCl₆ (M³⁺ = Y³⁺, Er³⁺, and Yb³⁺) exhibits a high Li-ion conductivity (∼10⁻⁴ S cm⁻¹) in the trigonal phase (space group $P\bar{3}m1$).^19,27,28,31 However, the conventional solid-state synthesis of the orthorhombic phase (space group Pnma), instead of the trigonal phase (space group $P\bar{3}m1$), has only been reported in the Li₃LuCl₆ system.²⁶ In particular, the ionic conductivity of Li₃YbCl₆ in the orthorhombic phase is considerably lower than that in the trigonal phase.^26,31 Therefore, we expected Li₃LuCl₆, a Lu³⁺ system with an ionic radius similar to that of Yb³⁺, to demonstrate a similar tendency that expect the possibility of obtaining a trigonal phase with a high ionic conductivity using another synthesis method.

In this study, Li₃LuCl₆ was synthesized using a mechanochemical method. The ionic conductivities of the compressed samples were evaluated using the alternating current (AC) impedance technique. Furthermore, Li₂LuCl₅, which is a defect derivative of Li₃LuCl₆, was targeted to improve the ionic conductivity of this system.

2. Experimental

Li₃LuCl₆ and Li₂LuCl₅ were synthesized using mechanochemical methods. LiCl (99.9 %, Kishida Chemical Co., Ltd.) and LuCl₃ (99.99 %, Sigma-Aldrich Co., LLC) were mixed stoichiometrically in a 45 mL ZrO₂ pot, and 30 g ZrO₂ balls with a diameter of 5 mm each were placed in the pot. The mixture was synthesized using a planetary ball mill apparatus (P-7 classic-line, Fritsch Japan K.K.) with a rotation speed of 500 rpm for a milling time of 15 min and intermittence of 10 min, which was repeated 40 times. The crystal structures of the obtained samples were characterized by X-ray diffraction (XRD; MiniFlex 600, Rigaku Corp. CuK_α line). To enhance the crystallinity, a heat treatment was performed after the milling process by holding the sample at 250 °C for 1 h under an Ar atmosphere in a glove box. The ionic conductivity was measured using the AC impedance method. Pellets (diameter = 10 mm, thickness ≈ 0.50 mm) were prepared by sandwiching the powder (weight ≈ 80 mg) between Au-coated stainless steel plates and compressing them at 382 MPa. An electrochemical analyzer (VSP, BioLogic) was used to apply an AC voltage of 100 mV at 25 °C at the measurement frequency range of 10⁰–10⁶ Hz. For the direct current (DC) polarization measurements, the same apparatus was used, with the pellets sandwiched between Au-coated stainless steel plates and held at an applied voltage of 5 mV at 25 °C, which was held for 25 min. Linear sweep voltammetry (LSV) was measured using the same machine; the cell used for LSV testing is shown below. The cathode compound was a mortar mixture of Li₂LuCl₅ and a conductivity aid (KB, Lion Corporation) at a weight ratio of 85 : 15. Next, the Li₂ZrCl₆³² electrolyte was pressed at 96 MPa, and about 5 mg of the prepared cathode alloy was placed on top of the electrolyte pellet and pressed at 382 MPa. Finally, the cell was fabricated by layering Li-In alloy as the anode (counter electrode). Au-coated stainless steel with a diameter of 10 mm was used as the current collector. Cross-sectional images of the samples were obtained by scanning electron microscopy (SEM; JSM-6360LV, JEOL) to evaluate the deformability of the sample. The fracture surface was obtained by physically cutting the pellets. All procedures were performed in a dry Ar-filled atmosphere.

3. Results and Discussion

The mechanochemically synthesized Li₃LuCl₆ has a white color (Fig. S1a). The XRD patterns of the synthesized sample exhibit broad peaks that differed from those of the raw materials (LiCl, LuCl₃) (Fig. 1a). Although these peaks are not consistent with the previously reported pattern of Li₃MCl₆, the overall pattern is similar to that of LiCl, which has a highly symmetric space group $Fm\bar{3}m$ and a small number of peaks; these results confirm the highly symmetric structure like LiCl of the obtained compound. The Nyquist plot of the AC impedance measurement for the compressed pellet (Fig. 1b) follows a semicircle trend with an ion-blocking-derived spike. Therefore, the ionic conductivity of the compact, including the bulk and grain boundaries, was calculated from resistance R_total shown in the figure. The calculated conductivity is 3.1 × 10⁻⁴ S cm⁻¹, which is more than five orders of magnitude higher than the electronic conductivity (1.8 × 10⁻⁹ S cm⁻¹) measured by DC polarization (Fig. S2a). This relatively high conductivity is considered to be originated from the ion (transport number of 5.8 × 10⁻⁶ for the electronic conduction). A possible reason for the high ionic conductivity of Li₃LuCl₆ is the high symmetry of its crystal structure since highly symmetrical structures often have low diffusion activation energies.³³

Figure 1.

(a) XRD patterns of the synthesized Li₃LuCl₆ and raw materials LiCl and LuCl₃. (b) Nyquist plot of the compressed pellet of Li₃LuCl₆ at 25 °C. The arrows correspond to the resistance derived from both the bulk and grain boundary, R_total.

The mechanically milled sample was heat treated at 250 °C for 1 h to increase the degree of crystallinity. The XRD patterns after heat treatment are shown in Fig. S3. Although a small amount of LiCl precipitated, the XRD pattern of Li₃LuCl₆ after heat treatment was similar to that of trigonal Li₃ErCl₆ (space group $P\bar{3}m1$, ICSD No. 50151), which is different from the pattern²⁶ of the stable phase of orthorhombic Li₃LuCl₆ (space group Pnma). This suggests that the mechanochemical method induced the trigonal phase (space group $P\bar{3}m1$), which is a metastable and highly ionic conductive phase. The small amount of LiCl precipitation, also seen in other Li₃YbCl₆,³¹ is thought to be due to the ∼2 % weight loss of TG shown in Fig. S4. Since the main XRD pattern is Li₃LuCl₆, volatilization corresponding to the ∼2 % weight loss occurred only from the surface during heat treatment, which produced a small amount of LiCl. Through the AC impedance measurement of Li₃LuCl₆ after heat treatment, conductivity of σ_{25 °C} = 2.4 × 10⁻⁴ S cm⁻¹ (Fig. S5a) was determined, which is slightly lower than that of the mechanochemically synthesized sample. Therefore, the higher ionic conductivity of the mechanochemically synthesized sample can be ascribed to the precipitation of the crystalline phase with a higher symmetry like $Fm\bar{3}m$ than that of the structure after heat treatment. The DC polarization measurements noted the electronic conductivity of σ_{25 °C,e} = 9.3 × 10⁻¹⁰ S cm⁻¹ (Fig. S2b), indicating a high ionic-conductivity transport number.

The Li-ion conductivity of chloride Li₃MCl₆ (M = Y, In, Yb, Er) can be improved by the introduction of Li vacancies by replacing M with a high-valence cation.²⁹ Hence, we aimed to improve the Li-ion conductivity by introducing Li vacancies with replacing 3Li⁺ with Lu³⁺, resulting in Li_3−xLuCl_6−x (x = 1; Li₂LuCl₅).

The mechanochemically synthesized Li₂LuCl₅ has a white color, similar to that of Li₃LuCl₆ (Fig. S1b). The XRD patterns of the synthesized sample showed a broad peak different from the raw material peaks (LiCl and LuCl₃) (Fig. 2a). The XRD peak of the strongest line in Li₃LuCl₆ (∼33°) only shifts to a lower angle in Li₂LuCl₅. Therefore, the Li₂LuCl₅ is considered to have a similar structure of the trigonal phase (space group $P\bar{3}m1$) in Li₃LuCl₆ after heat treatment. The trigonal phase shows lower activation energy than the orthorhombic phase in the system Li₃MCl₆ compounds.^22,34 Similar to Li₃LuCl₆, the AC impedance measurements noted a high conductivity of σ_{25 °C} = 5.2 × 10⁻⁴ S cm⁻¹ (Fig. S3c). The results of the DC polarization measurements showed that the electronic conductivity is σ_{25 °C,e} = 3.6 × 10⁻⁸ S cm⁻¹ (Fig. S2c), which corresponds to the transport number of 6.9 × 10⁻⁵ for the electronic conduction. These results indicate that Li₂LuCl₅ exhibits a higher ion conductivity than Li₃LuCl₆ by decreasing the amount of LiCl with Li₃LuCl₆.

Figure 2.

(a) XRD patterns of the mechanochemically synthesized Li₂LuCl₅, annealed Li₂LuCl₅, and Li₃ErCl₆ (ICSD No. 50151). (b) Arrhenius plot of the synthesized Li₂LuCl₅ measured every 10 °C in the temperature range of 30–100 °C.

The XRD patterns after heat treatment are shown in Fig. 2a. The result shows that LuCl₃ is precipitated after heat treatment owing to phase separation, indicating that Li₂LuCl₅ is synthesized in a metastable state by a mechanochemical method. Through the AC impedance measurements of Li₂LuCl₅ after heat treatment, a conductivity of σ_{25 °C} = 1.6 × 10⁻⁴ S cm⁻¹ (Fig. S5d) was determined, which is lower than that of Li₂LuCl₅ without heat treatment, similar to the case of Li₃LuCl₆. Through DC polarization measurements, a low electronic conductivity of σ_{25 °C,e} = 2.4 × 10⁻⁸ S cm⁻¹ (Fig. S2d) was determined, which indicates the high ionic transport number. The Arrhenius plots for Li₂LuCl₅ without heat treatment shows a linear trend and low activation energy of 0.33 eV (Fig. 2b). For Li₃LuCl₆ after heat treatment, which has a trigonal phase similar to Li₂LuCl₅ before heat treatment, the Arrhenius plot shows a linear trend and the activation energy is as low as 0.30 eV (Fig. S6). Among the reported Li-Cl-containing ternary compounds, Li₂LuCl₅ has a relatively high room-temperature ion conductivity before heat treatment (Table 1). Li₃LuCl₆ and Li₂LuCl₅ exhibit similar activation energies, but Li₂LuCl₅ exhibits higher ionic conductivity than Li₃LuCl₆ due to increased carrier concentration by introducing Li vacancies. Furthermore, LSV measurements on Li₂LuCl₅ before heat treatment, which had the highest ionic conductivity, suggest an oxidation limit ∼4.0 V (vs. Li/Li⁺) (Fig. S7).

Table 1. Room-temperature Li-ion conductivities of the Li–Cl-containing ternary compounds.

Material (Li–X–Cl)	σRT/S cm−1	Reference
LiTaCl6	1.0 × 10−2	7
Li3ScCl6	3.0 × 10−3	21
Li3InCl6	1.5 × 10−3	20
Li3TiCl6	1.0 × 10−3	35
Li2LuCl5	5.2 × 10−4	This study
Li3YCl6	5.1 × 10−4	19
Li2ZrCl6	5.1 × 10−4	32
Li3ErCl6	3.3 × 10−4	27, 28
Li3YbCl6	1.0 × 10−4	31

Finally, the SEM images (Fig. 3, Fig. S8) of Li₃LuCl₆ and Li₂LuCl₅ obtained before heat treatment synthesized by a mechanochemical method demonstrate the occurrence of necking between the particles. This indicates the high deformability of the chloride materials. The high deformability of Li₃LuCl₆ and Li₂LuCl₅ is useful for the Li-ion conduction and formation of a strong solid–solid interface for all-solid-state batteries.

Figure 3.

SEM cross-section images (×5,000) of the (a) Li₃LuCl₆ and (b) Li₂LuCl₅ pellets synthesized by the mechanochemical method.

4. Conclusion

In this study, Li₃LuCl₆ and Li₂LuCl₅ were synthesized using a mechanochemical method, and their ion conductivities and deformabilities were evaluated. Li₂LuCl₅ has a metastable phase with a ion conductivity of 5.2 × 10⁻⁴ S cm⁻¹ at 25 °C, which is relatively high among Li-Cl containing ternary compounds and higher than that of the Li₃LuCl₆ (3.1 × 10⁻⁴ S cm⁻¹). The SEM images also showed a high deformability. Therefore, Li₂LuCl₅ can be utilized as a solid electrolyte for all-solid-state batteries because of its high Li-ion conductivity and deformability. Furthermore, as there are no reported cases of high-ion-conductivity chloride materials with the Li₂MCl₅ composition until this work, this study is expected to increase the progress in future studies of LiCl-deficient Li₂MCl₅ compositions with a Li₃MCl₆ composition.

Symbols

σ: Ion conductivity

R_total: Resistance derived from both the bulk and grain boundary.

T: Temperature

Z′: Real components in a Nyquist plot

Z′′: Imaginary component in a Nyquist plot

θ: Plane angle

Abbreviations

XRD: X-ray diffraction

AC impedance: Alternating current impedance

DC polarization: Direct current polarization

SEM: Scanning electron microscopy

Acknowledgments

This work was partially supported by Grants-in-Aid for Scientific Research (Grant Nos. 19H05815, 19K15657, 20H02436, 21H01625, 21J14422, and 21K14715) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan; the Program for Promoting Research on the Supercomputer Fugaku (Fugaku Battery & Fuel Cell Project) of MEXT (Grant No. JPMXP1020200301) and a CREST grant from the Japan Science and Technology Agency, Japan (Grant No. JPMJCR21O6). Data Creation and Utilization Type Material Research and Development Project (grant number JPMXP1122712807) from MEXT, and Fujikura Foundation (23-010).

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.24296884.

• 1) Materials with a high Li-ion conductivity and deformability are garnering interest for the facile fabrication of safe all-solid-state batteries with high energy densities. Hence, increasing attention has been focused on Li-containing chloride materials that meet these requirements since they were first reported in 2018 (Asano et al. Advanced Materials 2018, 30 (44), 1803075). In this paper, we report a novel Li-containing chloride of Li₃LuCl₆ with a high Li-ion conductivity (σ_{25 °C} = 3.1 × 10⁻⁴ S cm⁻¹) and sufficient deformability. Furthermore, its defect derivative of Li_3−xLuCl_6−x (x = 1), i.e., Li₂LuCl₅ with a higher Li-ion conductivity (σ_{25 °C} = 5.2 × 10⁻⁴ S cm⁻¹), is synthesized. Scanning electron microscopy confirms the dense packing of both Li₃LuCl₆ and Li₂LuCl₅ as compressed pellets. Hence, Li₂LuCl₅ is presented as a promising solid electrolyte with a high Li-ion conductivity and deformability, which presents a novel opportunity for exploring the composition of Li₂MCl₅ (M: trivalent metal ion) compounds and indicates potential applications as an all-solid-state battery material. Furthermore, as there are no reported cases of high-Li-ion-conductivity chloride materials with the Li₂MCl₅ composition until this work, this study is expected to increase the progress in future studies of LiCl-deficient Li₂MCl₅ compositions with a Li₃MCl₆ composition.

Author Contributions

The manuscript was written with contributions from all the authors. All the authors approved the final version of the manuscript.

CRediT Authorship Contribution Statement

Shin Aizu: Data curation (Lead), Formal analysis (Equal), Validation (Equal), Visualization (Lead), Writing – original draft (Lead)

Naoto Tanibata: Conceptualization (Lead), Formal analysis (Lead), Funding acquisition (Equal), Investigation (Lead), Methodology (Lead), Project administration (Equal), Resources (Equal), Supervision (Equal), Writing – review & editing (Equal)

Hayami Takeda: Formal analysis (Equal), Writing – review & editing (Equal)

Masanobu Nakayama: Funding acquisition (Lead), Project administration (Equal), Supervision (Equal), Writing – review & editing (Lead)

Conflict of Interest

There is no conflict of interest to declare.

Funding

Ministry of Education, Culture, Sports, Science and Technology: 19H05815, 20H02436, 21H01625, 21J14422, and 21K14715

Ministry of Education, Culture, Sports, Science and Technology: 19K15657

Ministry of Education, Culture, Sports, Science and Technology: JPMXP1020200301

Japan Science and Technology Agency: JPMJCR21O6

Ministry of Education, Culture, Sports, Science and Technology: JPMXP1122712807

Fujikura Foundation: 23-010

Footnotes

S. Aizu: ECSJ Student Member

N. Tanibata and M. Nakayama: ECSJ Active Members

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