Scientists realize a new Li-B-S system solid electrolyte

On May 25, 2023, the team of Professors Yi Cui, William C. Chueh and Evan J. Reed of Stanford University published an article entitled “Experimental Discovery of a Fast and Stable Lithium Thioborate Solid Electrolyte, Li6+2x” in the journal ACS Energy Letters[B10S18]Sx (x ≈ 1)”, reporting the latest research results of a new single-crystalline phase Li-B-S solid electrolyte.

The study found Li6+2x[B10S18]The Sx (x ≈ 1) sample exhibits a high ionic conductivity of 1.3×10−4 S cm−1 and an electrochemical stability window of 1.3-2.5 V, and its symmetrical lithium battery can withstand current densities in excess of 1 mA cm-2 and exhibit high cycling stability for more than 140 hours at 0.3 mA cm-2. This work provides guidelines for the synthesis of solid electrolytes in LBS systems, and promotes the further development and wide application of solid electrolytes in sulfide systems.

Cui Yi, William C. Chueh (Que Zongyang), Evan J. Reed as co-corresponding authors; Ma Gingko, Wan Jiayu, and Xu Xin are co-first authors.

While traditional lithium-metal batteries (LMBs) based on organic electrolytes have safety risks such as volatility and flammability and explosion, all-solid-state batteries (ASSBs) have attracted tremendous attention in academia and industry due to potential safety improvements and more ideal energy density and operating temperature ranges. Solid electrolytes (SSEs) are an important part of ASSBs, but the rational design of new electrolytes is a major scientific challenge. A high-performance solid electrolyte must exhibit both rapid lithium-ion conduction, a wide electrochemical stability window, and mechanical resistance to lithium penetration. Among various solid electrolyte systems, sulfide solid electrolytes (such as lithium thiophosphate (LPS)) have high ionic conductivity, certain ductility and low mass density, which are candidates for potential applications. However, traditional sulfide solid electrolytes tend to have a narrow electrochemical stability window, limiting the operating voltage of the full battery.

Recently, through density functional theory (DFT) calculations and giant potential phase analysis, it has been predicted that the four lithium thioborate phases have ultra-high single crystal ion conductivity, wide electrochemical stability window, low cost, and low mass density, which are comparable to the best known oxide ceramic electrolyte materials. However, to date, the pure phase synthesis of these materials is difficult and has rarely been studied experimentally. Known lithium thioborate (Li-B-S) materials include Li5B7S13, Li3BS3, Li9B19S3, Li2B2S5 and Li10B10S20. The Li10B10S20 phase was first synthesized and reported as Li6+2x in 1990[B10S18]Sx(x ≈ 2)。

The researchers synthesized single-phase crystal Li6+2x by solid-phase reaction[B10S18]Sx (x ≈ 1), hereinafter referred to as LBS, and the electrochemical performance of LBS was comprehensively studied. LBS shows an ionic conductivity of 1.3 × 10–4 S cm–1 at room temperature and has an electrochemical stabilization window of 1.3-2.5 V for metallic Li, which is much larger than the stabilization window of electrolytes in most sulfide systems. The assembled symmetrical Li-Li battery can withstand a current density of 1 mA cm–2 and cycle at 0.3 mA cm–2 for more than 140 hours, and the results show that LBS can effectively inhibit the growth of lithium dendrites.

Material synthesis and structural characterization

In Figure 1, the researchers synthesized a new LBS solid electrolyte by designing a solid-phase reaction high-temperature sintering experiment. Compared with the traditional synthesis of LBS system SSE experiment, this experiment can effectively shorten the synthesis time and improve the synthesis efficiency. Based on the refinement analysis of synchrotron radiation XRD data, the synthetic product was determined to be Li6+2x[B10S18]Sx (x ≈ 1) structure. This structure has highly disordered non-frame sulfur and lithium atoms with unrestricted coordinates and occupancy, contributing to the high migration of lithium within the material.

Figure 1: (a) Li6+2x[B10S18]The crystal structure of Sx (x ≈ 1 or 2). (b) Schematic diagram of LBS powder synthesis. (c) Synchrotron XRD patterns and their Rietveld refinement results. (d) Photograph of LBS powder sample. (e, f) Photograph of LBS tablet specimen.

Material morphology and structural characterization

In Figure 2, the researchers observed the surface topography of tablet samples and powder samples through SEM and EDS, and the results showed that B and S were evenly distributed in the material. Notably, LBS materials are sensitive to electron beams, and significant beam damage can be observed after 1 and 2 min of focusing on the same particle. The researchers determined the spacing between different crystal planes of the material through low-dose, high-resolution frozen TEM observation, and the results were consistent with XRD.

Fig. 2: (a) SEM diagram of the surface of an LBS tableted specimen. (b, c) Corresponding LBS sample pieces EDX plot: distribution of (b) B and distribution of (c) S. (d) SEM image of LBS particle samples. (e, f) corresponding LBS particle EDX plot: distribution of (e) B and distribution of (f) S. (g) High-resolution cryo-transmission electron microscopy (cryo-TEM) plot of LBS particles. (h-j) Selective electron diffraction image (SADP) of LBS particles.

Li migration channel simulation

In Figure 3, the researchers performed density functional theory molecular dynamics (DFT-MD) simulations to identify the major ion conduction pathways. During the 40 ps simulation, the mean square displacement (MSD) of Li exceeded 100 Å2, comparable to the calculations of other previously reported Li-B-S phases over a considerable amount of simulation time. To explore the fastest conduction position of Li, the research team constructed a heat map showing the trajectories most often occupied by Li (Figures 3c, d). These heat maps show that Li tends to be along[B10S186–]The external conduction of structures does not pass through the gaps between these structures. Calculating the MSD of Li along the lattice vectors of a, b, and c and the MSD of the ab, bc, and ac planes shows that Li diffuses in all directions, but is not completely isotropic; Diffusion in the C direction and in the BC and AC planes is favorable, while diffusion along the A and B directions is low, about one-third to one-half (Figure 3B). To directly compare one- and two-dimensional MSDs, the research team normalized the MSD values in Figure 3b by dimension d, where d = 1 represents MSD for a, b, and c, and d = 2 represents MSD for ab, bc, and ac. To visualize the conduction pathway, the three-dimensional Li probability density is folded into two planes for visualization: the plane perpendicular to the c lattice vector (Figure 3c) and the bc plane (Figure 3d). These heat maps are shown in[B10S186–]The apparent absence of Li strength in the gaps between structures indicates that Li diffuses outside the structure but does not cross the gap between adjacent structures.

Figure 3: (a) In the DFT-MD simulation, comparing the mean-squared displacement (MSD) of 40-50 ps between Li10B10S19 and other Li-B-S phases shows that this phase has fast ion conduction properties. (b) Li MSD along the direction (a, b, c) and plane (ab, bc, ac) defined by the lattice vector indicates that Li diffusion is more favorable in the c direction and the associated position (ac, bc). (c) Li probability density heat map as observed in the c direction. (d) Li probability density heat map projected into the bc plane. These heat maps indicate that Li diffusion occurs[B10S186-]The exterior of the structure, rather than through an open channel.

Electrochemical characterization and analysis

High ionic conductivity and wide electrochemical stability window are the most important properties pursued by SSE materials. In Figure 4, impedance spectroscopy shows that the ionic conductivity of SSE is 1.3 × 10–4 S cm–1. Based on the LI/LBS/LBS-C cell CV data, the electrochemical stabilization window of LBS is 1.3-2.5 V.

Figure 4: (a) Schematic diagram of an In/LBS/In cell for measuring ionic conductivity. (b) Impedance spectrum of LBS. (c) Schematic diagram of a Li/LBS/LBS-graphite cell for CV measurements. (d) CV test curve.

Lithium symmetric battery test analysis

To demonstrate the performance of LBS symmetrical cell configurations at room temperature, the researchers assembled Li/LBS/Li symmetrical cells. The results show that LBS can withstand a current density of more than 1 mA cm–2 and has a charge-discharge capacity of 1 mAh cm–2. In the long cycle test, the symmetrical Li-Li cell was able to cycle stably for more than 140 h at 0.3 mA cm–2 (0.3 mAh cm–2 charge-discharge capacity). Therefore, LBS has a good ability to inhibit the growth of lithium dendrites and can play a role in high-power lithium metal batteries. In addition, no decrease in ion conductivity was observed in symmetrical cells after more than two weeks, which also indicates that LBS solid electrolyte has good stability and is suitable for long-term storage and use.

Figure 5: (a) Schematic of a Li/LBS/Li symmetrical cell. (b) CCD to lithium test curve. (c) Room temperature cycling test of Li/LBS/Li batteries at 0.3 mA cm-2. (c) Impedance spectra of symmetrical cells after standing for different days. (e) Ionic conductivity of LBS after standing for different times.

Summary and outlook

In this paper, the researchers successfully synthesized a new lithium thioborate solid electrolyte (Li6+2x) by solid-phase reaction[B10S18]Sx,x ≈1)(LBS), and its electrochemical performance was studied through experiments and simulations. LBS has the characteristics of crystal single phase, high purity, good uniformity, low density, good processability and high synthesis efficiency. LBS exhibits high ionic conductivity and a wide electrochemical stability window (1.3-2.5 V). In addition, cyclic testing of symmetrical Li/LBS/Li cells can withstand current densities of more than 1 mA cm–2 at room temperature. The symmetrical Li/LBS/Li cells also exhibited good cycling stability for more than 140 hours at a current density of 0.3 mA cm–2 and a charge-discharge capacity of 0.3 mAh cm–2Sex. In addition, LBS solid electrolytes have stable ionic conductivity and are suitable for long-term storage and use. This study is the first comprehensive report on the electrochemical performance of lithium thioborate. Therefore, the research group provides an efficient technique to synthesize pure phase LBS with low mass density, fast ion conduction, wide electrochemical stability window, and good cycle stability. In addition, the research group provides guidelines for the synthesis of lithium thioborate-derived solid electrolytes, which promotes the further development and wide application of chalcogenide solid electrolytes, and the doping technology used to improve the conductivity of LBS ions is also a promising research direction. (Source: Science Network)

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