Rapid and Tunable Assisted-Microwave Preparation of Glass and Glass-Ceramic Thiophosphate "Li7P3S11" Li-Ion Conductors

Progress and Perspective of Glass-Ceramic Solid-State Electrolytes for Lithium Batteries

The all-solid-state lithium battery (ASSLIB) is one of the key points of future lithium battery technology development. Because solid-state electrolytes (SSEs) have higher safety performance than liquid electrolytes, and they can promote the application of Li-metal anodes to endow batteries with higher energy density. Glass-ceramic SSEs with excellent ionic conductivity and mechanical strength are one of the main focuses of SSE research. In this review paper, we discuss recent advances in the synthesis and characterization of glass-ceramic SSEs. Additionally, some discussions on the interface problems commonly found in glass-ceramic SSEs and their solutions are provided. At the end of this review, some drawbacks of glass-ceramic SSEs are summarized, and future development directions are prospected. We hope that this review paper can help the development of glass-ceramic solid-state electrolytes.

Self-organized hetero-nanodomains actuating super Li+ conduction in glass ceramics

Easy-to-manufacture Li2S-P2S5 glass ceramics are the key to large-scale all-solid-state lithium batteries from an industrial point of view, while their commercialization is greatly hampered by the low room temperature Li+ conductivity, especially due to the lack of solutions. Herein, we propose a nanocrystallization strategy to fabricate super Li+-conductive glass ceramics. Through regulating the nucleation energy, the crystallites within glass ceramics can self-organize into hetero-nanodomains during the solid-state reaction. Cryogenic transmission electron microscope and electron holography directly demonstrate the numerous closely spaced grain boundaries with enriched charge carriers, which actuate superior Li+-conduction as confirmed by variable-temperature solid-state nuclear magnetic resonance. Glass ceramics with a record Li+ conductivity of 13.2 mS cm-1 are prepared. The high Li+ conductivity ensures stable operation of a 220 μm thick LiNi0.6Mn0.2Co0.2O2 composite cathode (8 mAh cm-2), with which the all-solid-state lithium battery reaches a high energy density of 420 Wh kg-1 by cell mass and 834 Wh L-1 by cell volume at room temperature. These findings bring about powerful new degrees of freedom for engineering super ionic conductors.

A new universal force-field for the Li2S-P2S5 system

Lithium thiophosphate electrolyte is a promising material for application in all-solid-state batteries. Ab initio molecular dynamics (AIMD) simulations have been used to investigate the ion conduction mechanisms in single-crystalline and glassy compounds. However, the complexity of real materials (e.g., materials with grain boundaries and multiphase glass-ceramics) causes AIMD simulations to have high computational cost. To overcome this computational limitation, we developed a new interatomic potential for classical molecular dynamics (CMD) simulations of Li solid-state electrolytes. The training datasets were generated from representative sulfide electrolytes (β-Li₃PS₄, γ-Li₃PS₄, Li₄P₂S₆, Li₇P₃S₁₁, and Li₇PS₆ crystals and 70Li₂S-30P₂S₅ glass). Using the functional forms of the Class II and Stillinger-Weber potentials, all parameters were optimized by minimizing the differences in forces on atoms, stresses, and potential energies between the CMD and AIMD results. Subsequent validation showed that the optimized parameters can reproduce the dynamics of Li⁺ as well as the structures of the crystalline and glassy materials. The ionic conductivity of Li₇P₃S₁₁ crystal was approximately five times that of the isostoichiometric 70Li₂S-30P₂S₅ glass, indicating that CMD simulations using the developed force-field accurately reproduced the effective conduction path in Li₇P₃S₁₁ from AIMD. The developed force-field parameters make it possible to simulate complex materials including amorphous-crystalline interfaces and multiphase glass-ceramics in the CMD framework.

Subtle Local Structural Details Influence Ion Transport in Glassy Li+ Thiophosphate Solid Electrolytes

Many of the promising, high-performing solid electrolytes for lithium-ion batteries are amorphous or contain an amorphous component, particularly in the Li thiophosphate Li₂S-P₂S₅ (LPS) compositional series. An explicit study of the local structure in four samples of ostensibly identically prepared 70Li₂S-30P₂S₅ glass reveals substantial variation in the ratio between the two main local structural units in this system: PS₄3^- tetrahedra and P₂S₇^4- corner-sharing tetrahedral pairs. Local structural and compositional probes including Raman spectroscopy, X-ray photoelectron spectroscopy, and X-ray pair distribution function analysis are employed here to arrive at a consistent description of the relative amounts of isolated tetrahedral units, which vary by 13% across the samples measured. This local structure variation translates to differences in the activation energies measured by electrochemical impedance spectroscopy in these samples, such that the higher concentration of isolated tetrahedra corresponds to a lower activation energy. The measured temperature-dependent ionic conductivity data are compared to conductivity results across the literature reported on the same compositions, highlighting the variation in the measured activation energy for nominally identical samples. These findings have implications for the critical need to play close attention to the local structure in solid electrolytes, particularly in systems that are glasses or glass ceramics, or those that comprise any amorphous contribution.

The effect of solvent on reactivity of the Li2S-P2S5 system in liquid-phase synthesis of Li7P3S11 solid electrolyte

Synthesis technology for sulfide-based solid electrolytes based on liquid-phase processing has attracted significant interest in relation to achieving the optimal design for all-solid-state batteries. Herein, guidelines to solvent selection for the liquid-phase synthesis of superionic conductor Li₇P₃S₁₁ are described through systematic examination. 70Li₂S-30P₂S₅ system, a source of Li₇P₃S₁₁, is treated via a wet chemical reaction using eight organic solvents with different physical and chemical properties (i.e., dielectric constant, molecule structure, and boiling point). We reveal that the solvent's polarity, characterized by the dielectric constant, plays an important role in the formation of crystalline Li₇P₃S₁₁ via wet chemical reaction. In addition, acetonitrile (ACN) solvent with a high dielectric constant was found to lead to high-purity crystalline Li₇P₃S₁₁ and intrinsically high ionic conductivity. Further, solvents with a high boiling point and ring structures that cause steric hindrance were found to be unfavorable for the wet chemical synthesis of Li₇P₃S₁₁ solid electrolyte. Overall, we demonstrate that ACN solvent is the most suitable for the liquid-phase synthesis of a crystalline Li₇P₃S₁₁ solid electrolyte with high purity based on its dielectric constant, molecular structure, and boiling point.

Modified Li7P3S11 Glass-Ceramic Electrolyte and Its Characterization

Li₇P₃S₁₁ glass ceramics have high conductivities competitive with liquid electrolytes, making them good candidates as solid-state electrolytes for all-solid-state lithium-ion batteries. However, the metastable nature and performance of Li₇P₃S₁₁ glass ceramics remain mysterious. Herein, modified Li₇P₃S₁₁ glass ceramics with compositions of 70Li₂S-30P₂S₅ were prepared via two-step mechanical milling and thermal annealing. Li₇P₃S₁₁ glass ceramics synthesized using the conventional method (mechanical milling and thermal annealing) were again ball-milled to obtain amorphous 70Li₂S-30P₂S₅ with a peculiar glass structure. Further thermal annealing was carried out to crystallize the glass. The obtained crystalline phase was analogous to the original Li₇P₃S₁₁ phase, but the conductivity was enhanced by a factor of 1.7. Based on ³¹P solid-state nuclear magnetic resonance (NMR) spectroscopy, the Li₇P₃S₁₁ phase contained an additional PS₄^3- unit. A rational deconvolution procedure for the ³¹P solid-state NMR spectra based on crystalline Li₇P₃S₁₁ was developed and applied to the samples. The analysis can resolve the additional crystalline PS₄^3- unit in the Li₇P₃S₁₁ structure. Based on two-dimensional double-quantum ³¹P NMR spectroscopy, the additional PS₄^3- unit is located adjacent to the P₂S₇^4- unit, suggesting that P₂S₇^4- is divided into two PS₄^3- units in the Li₇P₃S₁₁ phase. The flip motion of Li⁺ was also investigated based on the ⁷Li spin-lattice relaxation time. The independent activation energy of spin-lattice relaxation with respect to temperature in the Li₇P₃S₁₁ phase was attributed to a conduction path between the two PS₄^3- units. The findings provide a synthetic route that can be used to develop metastable solid-state electrolytes.

On the underestimated influence of synthetic conditions in solid ionic conductors

The development of high-performance inorganic solid electrolytes is central to achieving high-energy- density solid-state batteries. Whereas these solid-state materials are often prepared via classic solid-state syntheses, recent efforts in the community have shown that mechanochemical reactions, solution syntheses, microwave syntheses, and various post-synthetic heat treatment routines can drastically affect the structure and microstructure, and with it, the transport properties of the materials. On the one hand, these are important considerations for the upscaling of a materials processing route for industrial applications and industrial production. On the other hand, it shows that the influence of the different syntheses on the materials' properties is neither well understood fundamentally nor broadly internalized well. Here we aim to review the recent efforts on understanding the influence of the synthetic procedure on the synthesis - (micro)structure - transport correlations in superionic conductors. Our aim is to provide the field of solid-state research a direction for future efforts to better understand current materials properties based on synthetic routes, rather than having an overly simplistic idea of any given composition having an intrinsic conductivity. We hope this review will shed light on the underestimated influence of synthesis on the transport properties of solid electrolytes toward the design of syntheses of future solid electrolytes and help guide industrial efforts of known materials.

Li5VF4(SO4)2: A Prototype High-Voltage Li-Ion Cathode

A Li-rich polyanionic compound based on V³⁺ with a previously unknown structure, Li₅VF₄(SO₄)₂, has been developed as a high-voltage cathode material for Li-ion batteries. The solvothermal preparation of this material, crystal structure solution, and initial electrochemical characterization are presented. An analysis based on density functional theory electronic structure calculations suggests that a high voltage close to 5 V is required to extract two Li ions and to reach the oxidation state of V⁵⁺. However, the use of conventional carbonate-based electrolytes, which exhibit increasing degradation above a potential of 4.3 V, does not permit the full capacity of this compound to be achieved at this time.

Sulfide and Oxide Inorganic Solid Electrolytes for All-Solid-State Li Batteries: A Review

Energy storage materials are finding increasing applications in our daily lives, for devices such as mobile phones and electric vehicles. Current commercial batteries use flammable liquid electrolytes, which are unsafe, toxic, and environmentally unfriendly with low chemical stability. Recently, solid electrolytes have been extensively studied as alternative electrolytes to address these shortcomings. Herein, we report the early history, synthesis and characterization, mechanical properties, and Li+ ion transport mechanisms of inorganic sulfide and oxide electrolytes. Furthermore, we highlight the importance of the fabrication technology and experimental conditions, such as the effects of pressure and operating parameters, on the electrochemical performance of all-solid-state Li batteries. In particular, we emphasize promising electrolyte systems based on sulfides and argyrodites, such as LiPS5Cl and β-Li3PS4, oxide electrolytes, bare and doped Li7La3Zr2O12 garnet, NASICON-type structures, and perovskite electrolyte materials. Moreover, we discuss the present and future challenges that all-solid-state batteries face for large-scale industrial applications.