Influence of Mg on the Li ion mobility in Li4-2xMgxP2S6

Aliovalent substitution of Li in salts by Mg can generate Li vacancies and thus in principle improve the ionic conductivity. In fact the influence of substitution on ionic conductivity is far more complex. Here the impact of Mg substitution on Li ion mobility is studied in the example of Li4P2S6 by a combination of nuclear magnetic resonance experiments on 31P and 7Li at variable temperatures, impedance spectroscopy and X-ray powder diffraction to elucidate the relationship with structural changes and the effect on mobility on different length scales. It is found that substituting Li ions with Mg ions in Li4-2xMgxP2S6 with 0 ≤ x ≤ 0.2 increases the local Li ion mobility up to a certain concentration at which a phase transition induces a different structural realignment of the P2S64- units. The determined activation energies can be assigned to vacancy hopping processes by comparison with nudged elastic band calculations at the density functional level of theory, which shows not only the possibilities but also limitations of substituting Li with Mg.

Improved air-stability and conductivity in the 75Li2S·25P2S5 solid-state electrolyte system: the role of Li7P3S11

Doping modification is regarded as a simple and effective method for increasing the ionic conductivity and air stability of solid state electrolytes. In this work, a series of (100-x)(0.75Li2S·0.25P2S5)·xP2O5 (mol%) (x = 0, 1, 2, 3 and 4) glass-ceramic electrolytes were synthesized by a two-step ball milling technique. Various characterization techniques (including powder X-ray diffraction, Raman and solid-state nuclear magnetic resonance) have proved that the addition of P2O5 can stimulate 75Li2S·25P2S5 system to generate the high ionic conductivity phase Li7P3S11. Through the doping optimization strategy, 98(0.75Li2S·0.25P2S5)·2P2O5 glass-ceramic (2PO) not only had a 3.6 times higher ionic conductivity than the undoped sample but also had higher air stability. Its ionic conductivity remained in the same order of magnitude after 10 minutes in the air. We further investigated the reasons why 2PO has a relatively high air stability using powder X-ray diffraction and scanning electron microscopy in terms of crystal structure degradation and morphology changes. In comparison to the undoped sample, the high ionic conductivity phases (β-Li3PS4 and Li7P3S11) of 2PO were better preserved, and less impurity and unknown peaks were generated over a short period of exposure time. In addition, the morphology of 2PO only changed slightly after 10 minutes of exposure. Despite the fact that the particles aggregated significantly after several days of exposure, 2PO tended to form a protective layer composed of S8, which might allow some particles to be shielded from attack by moisture, slowing down the decay of material properties.

Li-Ion Conductivity of Single-Step Synthesized Glassy-Ceramic Li10GeP2S12 and Post-heated Highly Crystalline Li10GeP2S12

Li₁₀GeP₂S₁₂ is a phosphosulfide solid electrolyte that exhibits exceptionally high Li-ion conductivity, reaching a conductivity above 10^-3 S cm^-1 at room temperature, rivaling that of liquid electrolytes. Herein, a method to produce glassy-ceramic Li₁₀GeP₂S₁₂ via a single-step utilizing high-energy ball milling was developed and systematically studied. During the high energy milling process, the precursors experience three different stages, namely, the 'Vitrification zone' where the precursors undergo homogenization and amorphization, 'Intermediary zone' where Li₃PS₄ and Li₄GeS₄ are formed, and the 'Product stage' where the desired glassy-ceramic Li₁₀GeP₂S₁₂ is formed after 520 min of milling. At room temperature, the as-milled sample achieved a high ionic conductivity of 1.07 × 10^-3 S cm^-1. It was determined via quantitative phase analyses (QPA) of transmission X-ray diffraction results that the as-milled Li₁₀GeP₂S₁₂ possessed a high degree of amorphization (44.4 wt %). To further improve the crystallinity and ionic conductivity of the Li₁₀GeP₂S₁₂, heat treatment of the as-milled sample was carried out. The optimal heat-treated Li₁₀GeP₂S₁₂ is almost fully crystalline and possesses a room temperature ionic conductivity of 3.27 × 10^-3 S cm^-1, an over 200% increase compared to the glassy-ceramic Li₁₀GeP₂S₁₂. These findings help provide previously lacking insights into the controllable preparation of Li₁₀GeP₂S₁₂ material.

Simulated sulfur K-edge X-ray absorption spectroscopy database of lithium thiophosphate solid electrolytes

X-ray absorption spectroscopy (XAS) is a premier technique for materials characterization, providing key information about the local chemical environment of the absorber atom. In this work, we develop a database of sulfur K-edge XAS spectra of crystalline and amorphous lithium thiophosphate materials based on the atomic structures reported in Chem. Mater., 34, 6702 (2022). The XAS database is based on simulations using the excited electron and core-hole pseudopotential approach implemented in the Vienna Ab initio Simulation Package. Our database contains 2681 S K-edge XAS spectra for 66 crystalline and glassy structure models, making it the largest collection of first-principles computational XAS spectra for glass/ceramic lithium thiophosphates to date. This database can be used to correlate S spectral features with distinct S species based on their local coordination and short-range ordering in sulfide-based solid electrolytes. The data is openly distributed via the Materials Cloud, allowing researchers to access it for free and use it for further analysis, such as spectral fingerprinting, matching with experiments, and developing machine learning models.

Disentangling Phase and Morphological Evolution During the Formation of the Lithium Superionic Conductor Li10 GeP2 S12

The structural and morphological changes of the Lithium superionic conductor Li₁₀ GeP₂ S₁₂ , prepared via a widely used ball milling-heating method over a comprehensive heat treatment range (50 - 700 °C), are investigated. Based on the phase composition, the formation process can be distinctly separated into four zones: Educt, Intermediary, Formation, and Decomposition zone. It is found that instead of Li₄ GeS₄ -Li₃ PS₄ binary crystallization process, diversified intermediate phases, including GeS₂ in different space groups, multiphasic lithium phosphosulfides (Li_x P_y S_z ), and cubic Li₇ Ge₃ PS₁₂ phase, are involved additionally during the formation and decomposition of Li₁₀ GeP₂ S₁₂ . Furthermore, the phase composition at temperatures around the transition temperatures of different formation zones shows a significant deviation. At 600 °C, Li₁₀ GeP₂ S₁₂ is fully crystalline, while the sample decomposed to complex phases at 650 °C with 30 wt.% impurities, including 20 wt.% amorphous phases. These findings over such a wide temperature range are first reported and may help provide previously lacking insights into the formation and crystallinity control of Li₁₀ GeP₂ S₁₂ .

Aluminium ion doping mechanism of lithium thiophosphate based solid electrolytes revealed with solid-state NMR

We investigate the impact of Al incorporation on the structure and dynamics of Al-doped lithium thiophosphates (Li_3-3xAl_xPS₄) based on β-Li₃PS₄. ²⁷Al and ⁶Li magic-angle spinning NMR spectra confirm that Al³⁺ ions occupy octahedral sites in the structure. Quantitative analyses of ²⁷Al NMR spectra show that the maximum Al incorporation is x = 0.06 in Li_3-3xAl_xPS₄. The ionic conductivity of β-Li₃PS₄ is enhanced by over a factor 3 due to Al incorporation. Further increase of the Al doping level leads to the formation of a more complicated material consisting of multiple crystalline and distorted phases as indicated by ³¹P NMR spectra and powder X-ray diffraction. Consequently, novel Li ion diffusion pathways develop leading to a very high ionic conductivity at room temperature. NMR relaxometry shows that the activation barrier for long-range Li ion diffusion in β-Li₃PS₄ hardly changes upon Al incorporation, but the onset temperature for motional narrowing comes down significantly due to Al doping. The activation barrier in the subsequently formed multiphase material decreases significantly, however, indicating a different more efficient Li ion conduction pathway.

Artificial Intelligence-Aided Mapping of the Structure-Composition-Conductivity Relationships of Glass-Ceramic Lithium Thiophosphate Electrolytes

Lithium thiophosphates (LPSs) with the composition (Li₂S) _x (P₂S₅)_1-x are among the most promising prospective electrolyte materials for solid-state batteries (SSBs), owing to their superionic conductivity at room temperature (>10^-3 S cm^-1), soft mechanical properties, and low grain boundary resistance. Several glass-ceramic (gc) LPSs with different compositions and good Li conductivity have been previously reported, but the relationship among composition, atomic structure, stability, and Li conductivity remains unclear due to the challenges in characterizing noncrystalline phases in experiments or simulations. Here, we mapped the LPS phase diagram by combining first-principles and artificial intelligence (AI) methods, integrating density functional theory, artificial neural network potentials, genetic-algorithm sampling, and ab initio molecular dynamics simulations. By means of an unsupervised structure-similarity analysis, the glassy/ceramic phases were correlated with the local structural motifs in the known LPS crystal structures, showing that the energetically most favorable Li environment varies with the composition. Based on the discovered trends in the LPS phase diagram, we propose a candidate solid-state electrolyte composition, (Li₂S) _x (P₂S₅)_1-x (x ∼ 0.725), that exhibits high ionic conductivity (>10^-2 S cm^-1) in our simulations, thereby demonstrating a general design strategy for amorphous or glassy/ceramic solid electrolytes with enhanced conductivity and stability.

Dimensional reduction upon calcium incorporation in Cs0.3(Ca0.3Ln0.7)PS4 and Cs0.5(Ca0.5Ln0.5)PS4

A series of Ca-containing lanthanide thiophosphates has been obtained and their structural evolution from 3D for LnPS₄ and Cs_0.3(Ln_0.7Ca_0.3)PS₄ to 2D in Cs_0.5(Ln_0.5Ca_0.5)PS₄ was shown as a function of Ca content.

Local-structure effects on 31P NMR chemical shift tensors in solid state

The effect of the local structure on the ³¹P NMR chemical shift tensor (CST) has been studied experimentally and simulated theoretically using the density functional theory gauge-independent-atomic-orbital approach. It has been shown that the dominating impact comes from a small number of noncovalent interactions between the phosphorus-containing group under question and the atoms of adjacent molecules. These interactions can be unambiguously identified using the Bader analysis of the electronic density. A robust and computationally effective approach designed to attribute a given experimental ³¹P CST to a certain local morphology has been elaborated. This approach can be useful in studies of surfaces, complex molecular systems, and amorphous materials.