Synthesis and characterization of low-temperature lithium-ion conductive phase of LiX (X=Cl, Br)-Li₃PS₄ solid electrolytes

Highlights

• A new class of LiX (X = Cl, Br)-Li₃PS₄ solid electrolytes are synthesized by using conventional solid reaction process.

• 0.25LiCl-0.75LiBr-2Li₃PS₄ solid electrolyte shows a high Li⁺ conductivity of 1.7mScm⁻¹ at 25 °C.

• All-solid-state cells exhibits a discharge capacity of 185 mAhg⁻¹_-NCA at 25 °C.

Abstract

A new class of sulfide electrolytes, (1-x)LiCl-xLiBr-2Li₃PS₄ (x = 0, 0.25, 0.50, 0.75, and 1.0), was synthesized, and characterized their stable temperature range, ionic-conductivity and applicability for all-solid-state lithium metal battery. The optimal heat-treatment temperature of the synthesis process is relatively low, i.e., 200 °C. Ionic conductivity of the electrolyte increases with increasing LiBr ratio, and maximum ionic conductivity at 25 °C is 1.7 mScm⁻¹ with x = 0.75. At higher heat-treatment temperature, above 250 °C, a low-ionic-conduction crystalline phase appears in the electrolyte, and at that time, ionic conductivity decreases to a similar value to that of the amorphous precursor used in the synthesis process. Since the electrolyte does not contain a transition metal, it is applicable to the lithium-metal secondary system. A reversible lithium deposition-stripping reaction was demonstrated by cyclic voltammogram, the results of which indicate that the electrolyte is applicable to a typical 4-V rechargeable lithium-metal secondary system. The Coulombic efficiency of the first charge-discharge cycle is approximately 75%, and reversible capacity of 185 mAhg⁻¹ was verified by a Li/0.25LiCl-0.75LiBr-2Li₃PS₄/NCA pelletized-cell test. It is concluded from these results that the proposed low-temperature-synthesized sulfide electrolytes will reduce energy consumption for material production, and such a low-carbon-footprint process meets the demand of the future sustainable society.

Introduction

Lithium-ion conductive solid electrolytes have been gaining interest because of their high lithium-ion conductivity, incombustibility, and applicability to lithium-metal secondary batteries. Recently, various types of inorganic electrolyte having high lithium-ion conductivity, for example, sulfide-based [[1], [2], [3], [4]] and oxide-based [1,2,5,6] electrolytes, have been studied. To realize a solid-state battery operating at ambient temperature, sulfide solid electrolytes have been attracting attention, especially in regard to application to xEV batteries, because of their low grain-boundary resistance due to unique mechanical properties [7]. It is also known that some sulfide-based electrolytes, e.g., the Li₁₀GeP₂S₁₂ (LGPS) family [8], Li_9.54Si_1.74P_1.44S_11.7Cl_0.3 [9] with a LISICON-like structure, Li₇P₃S₁₁ [10], and Ge-doped Li₆PS₅I [11], support fast ion conduction exceeding 10 mScm⁻¹, and net lithium-ion conduction is now theoretically approaching a similar or higher value than that of a liquid electrolyte. Moreover since less concentration polarization is expected in the case of a cathode composite using such single-ion conductors, a thick electrode with fewer current collectors can be designed that increases energy density of a battery. Since solid electrolytes including a transitional metal element are usually reduced and decomposed near the redox potential of Li⁰/Li⁺, argyrodite-type glass ceramics have been gaining interest as an electrolyte that shows both high ionic conductivity (above 1 mScm⁻¹) and high electrochemical stability [12,13].

Discovered by Deiseroth et al. [14], lithium argyrodite, Li_7-xPS_6-xX_x (0 ≤ x ≤ 1; X = Cl, Br, or I), is considered to be a promising solid electrolyte because of its high ion conductivity and stability in regard to lithium redox potential. Conductivities of lithium argyrodites at room temperature are influenced by the halide ion (X): 4 × 10⁻⁴ mScm⁻¹ for Li₆PS₅I [15], 3 mScm⁻¹ for Li₆PS₅Cl [16], and 7 mScm⁻¹ for Li₆PS₅Br [16]. In addition, the atomic-site disorder generated by excess halogen substitution [17], substitution of germanium by phosphorus [11], and rapid quenching in the synthesis process [18,19] enhances the diffusion of lithium. These sulfide-based crystalline electrolytes do not require very high temperature, above 1000 °C like Li₇La₃Zr₂O₁₂ (LLZO), to synthesize them; however, their synthesis still requires heat-treatment above 500 °C. From the viewpoint of mass production with low carbon footprint, lower-temperature heat treatment is preferable.

Recently, in the case of the halogen-containing Li₃PS₄ system, a low-temperature crystalline phase with high conductivity has been reported [[20], [21], [22], [23], [24], [25], [26], [27], [28]]. For instance, 2Li₃PS₄-LiX (X = Br and I; crystallization temperature T_c = 160 °C) has demonstrated high conductivity, σ_25°C = 4.7 mScm⁻¹ [26]. Similarly, high ionic conductivity in such binary anion systems with different halogen species and molar ratio has been verified [27,28]. Recent reports on a halogen-containing sulfide-based solid electrolyte synthesized at around 200 °C are summarized in Table 1. However, especially in the case of the binary anion system, the number of studies on that phase is still limited. There are uncertainties regarding the crystallographic phase formed under certain material composition and heat-treatment temperature as well as the ionic conductivity of the halogen-containing lithium phosphorus sulfide of that phase. In addition, this high-ionic-conductivity phase is considered meta-stable only in a narrow temperature range, and it changes to a lower-conductivity crystalline high-temperature phase above 230–250 °C [26,28].

To apply the proposed solid electrolytes to lithium-metal secondary batteries, electrochemical stability in regard to the lithium potential is required. Although Li₁₀P₃S₁₂I [25] and Li₇P₂S₈Br_0.5I_0.5 [26] have improved stability in regard to the lithium redox potential, performance of solid state battery with these materials was not reported. In addition, it is known that the existence of iodine generally decreases cathodic decomposition stability [21]. And the electrolyte of lithium batteries must have a wide potential range. An electrolyte based on X = Cl or Br may be suitable if oxidation stability is therefore taken into account in the electrolyte design.

In this study, which aims to secure a feasible electrolyte with low carbon footprint, a new class of sulfide-based crystalline electrolytes with optimized composition based on LiX(X = Cl, Br)-2Li₃PS₄ was synthesized, and their temperature stability and ionic-conductivity was evaluated. The applicability of this class of electrolytes to a 4-V lithium-metal secondary battery was preliminary demonstrated by testing using a pelletized cell.