Lithium Diffusion Pathways in 3R-Li_<i>x</i>TiS₂: A Combined Neutron Diffraction and Computational Study

Layered lithium transition-metal sulfides have long been discussed as early electrode materials for lithium-ion batteries. However, fundamental knowledge of lithium-ion migration in these solids is still lacking. In this study, we report on the diffusion dynamics in lithium-deficient high-temperature polymorphs of lithium titanium sulfides (3R-LixTiS2; x = 0.7, 0.9) as analyzed using powder neutron diffractometry and density functional theory (DFT) climbing-image nudged-elastic-band (cNEB) calculations. Two classes of probable migration pathways have been identified from the scattering-length density distributions (filtered using the maximum-entropy method [MEM]) and the probability density functions (PDFs, modeled from anharmonic Debye–Waller factors): direct diffusion in the (001) plane as the major mechanism and indirect diffusion through adjacent tetrahedral voids as a minor mechanism. Calculated activation barriers agree well with one-particle potentials (OPPs) derived from measurements for Li0.7TiS2 (0.484[14] and 0.88[4] eV) but deviate for Li0.9TiS2. The discrepancy at low defect concentration is attributed to the failure of the OPP derivation and the different nature of the methods (space-time averaged vs individual-ion perspective). This work elucidates the pathways of lithium-ion diffusion in 3R-LixTiS2 and points out pitfalls in established experimental/computational methods.层状锂过渡金属硫化物（layered lithium transition-metal sulfides）长期以来被视为锂离子电池的早期电极材料。然而，学界对这类固体中锂离子迁移的基础认知仍较为匮乏。本研究针对锂缺陷型硫化钛锂高温多晶型（3R-LixTiS2；x=0.7、0.9）中的扩散动力学展开研究，采用粉末中子衍射法（powder neutron diffractometry）与密度泛函理论（Density Functional Theory, DFT）爬坡式弹性带（climbing-image nudged-elastic-band, cNEB）计算完成分析。研究从散射长度密度分布（经最大熵方法（Maximum-Entropy Method, MEM）滤波处理）以及由非简谐德拜-沃勒因子（anharmonic Debye–Waller factors）建模得到的概率密度函数（Probability Density Functions, PDFs）中，识别出两类潜在的锂离子迁移路径：以(001)晶面内的直接扩散为主要迁移机制，以通过相邻四面体空位的间接扩散为次要迁移机制。计算得到的活化能垒与基于Li0.7TiS2实验数据推导得到的单粒子势（One-Particle Potentials, OPPs）吻合度较高（分别为0.484[14]与0.88[4] eV），但与Li0.9TiS2的对应结果存在偏差。低缺陷浓度下出现的这种偏差，可归因于单粒子势推导方法的局限性，以及两种分析方法本质上的差异——时空平均视角与单离子视角的不同。本研究阐明了3R-LixTiS2中锂离子的扩散路径，并指出了现有主流实验与计算方法中存在的认知误区。