Li2S-Li3N固态电解质的高温高压合成及电化学性能表征

固态锂离子电池凭借其结构紧凑、安全性能高以及能量密度大等特点， 在电池技术领域备受瞩目。其中，研发出性能卓越的固态电解质显得尤为 重要。在诸多固态电解质材料中，硫化锂的电化学窗口相对较宽，然而， 其在室温下的锂离子电导率却不尽如人意。相较之下，氮化锂在室温下展 现出了理想的锂离子电导率，但是它的电化学稳定性问题仍然突出。值得 注意的是，两者都表现出了对锂金属的良好的电化学稳定性。所以如果将 其应用于全固态电池中，使锂金属负极成为可能，则可以提高电池的整体 能量密度。 本研究希望结合硫化锂和氮化锂两者的优点，采用球磨-高温高压的方 法将氮化锂掺杂进硫化锂，合成电化学性能优异的Li2S1-1.5xNx（0 ≤ x < 2/3） 系列固态电解质。数据表明，在一定温压范围内，温度越高，压力越大， 保温时间越长，越有利于氮化锂的掺杂。通过调整相关参数，本研究确定 了样品合成的最佳工艺条件。我们成功合成了 Li2S1-1.5xNx 系列固态电解质， 并对其进行了深入的表征分析。在离子电导率方面，样品的锂离子电导率 与氮化锂掺杂量呈正相关，其中x = 0.6时，样品的离子电导率最高（25 ℃， 4.53×10-4 S cm-1），活化能也达到最小值（0.28 eV）。在晶体结构解析方 面，Li2S0.25N0.5 呈现立方相结构（Fm3 ̅m），晶格常数为 5.3126 Å，远低于 纯相硫化锂。这表明晶格常数的减小，缩短了锂离子间距离，有助于离子 的高效输运。此外，氮占据了硫的位置的同时还产生了阴离子空位，锂占 据 8c 位点的同时还部分占据 32f 位点，这进一步提升了电解质的离子传输 性能。同时结合同步热分析数据可知，该电解质在较宽的温度范围内能保 持良好的热稳定性，使其能够适应不同工作环境下的温度变化。在电池性 能测试方面，基于 Li2S0.25N0.5 组装的锂对称电池和锂铟对称电池均可稳定 循环数千圈以上，说明该固态电解质对锂金属及锂铟合金负极具有良好的 兼容性。此外，以 LiCoO2 和 TiS2 为正极的全固态电池也都展现出了优异 的循环性能和较高的容量保持率，进一步验证了该系列固态电解质在固态 锂电池中的应用潜力，为新型固态电解质的研发提供了新思路。

Solid-state lithium-ion batteries have attracted much attention in the field of battery technology due to their compact structure, high safety and high energy density. As of the current issues related to solid-state batteries, the development of solid electrolytes with excellent performance is particularly important. Among the many solid-state electrolyte materials, Li2S has a relatively wide electrochemical window (~2.3 V), however, its lithium-ion conductivity at room temperature is low. In contrast, Li3N exhibits ideal lithium-ion conductivity at room temperature, but its electrochemical window is narrow (~0.44 V). Notably, both exhibit good electrochemical stability to lithium metal, a property that, if applied to solid-state lithium metal batteries, could lead to the improvement of the overall energy density of the battery. In this study, we hope to harvest the advantages of Li2S and Li3N through the use of ball milling technique and high-temperature high-pressure (HPHT) method to dope Li3N into Li2S and synthesize Li2S1-1.5xNx series (0 ≤ x < 2/3) solid electrolytes. The results show that the increase of temperature, pressure and holding duration is more conducive to the doping of Li3N in Li2S. By adjusting the relevant parameters, the optimal conditions for sample synthesis were determined. We successfully synthesized and characterized the Li2S1 1.5xNx series solid electrolytes. The lithium ionic conductivity of the sample is positively correlated with the doping amount of Li3N. The ionic conductivity of Li2S0.1N0.6 is the highest (25 ℃, 4.53×10-4 S cm-1), while the corresponding Ea is also the lowest (0.28 eV). In terms of crystal structure analysis, Li2S0.25N0.5 possesses a cubic phase structure (Fm3 ̅m) with a lattice constant of 5.3126 Å, which is much lower than that of pure phase Li2S. The reduction of the lattice constant shortens the distance between lithium ions within the crystalline structure, which is conducive to the efficient transport of lithium ions through hopping mechanism. In addition, nitrogen occupies the position of sulfur and in the meantime also creates anion vacancies, and lithium occupies the 8c site while also partially occupying the 32f site, which further improves the ionic transport performance of the electrolyte. With the combined TG-DSC analysis, it can be seen that the electrolytes as synthesized have good thermal stability in a wide temperature range, so that it can adapt to temperature changes in different working environments. In terms of battery performance testing, the lithium symmetrical battery and lithium-indium symmetrical battery assembled based on Li2S0.25N0.5 can be stably cycled for more than thousands of cycles, indicating that the solid-state electrolyte has good compatibility with lithium metal and lithium indium alloy anodes. In addition, all-solid-state batteries with LiCoO2 and TiS2 as the cathodes also show excellent cycling performance and high capacity retention, which further verify the application potential of this series of solid-state electrolytes in solid-state lithium batteries and provide new ideas for the research and development of new solid-state electrolytes.

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