Phase transformations driven by compositional change require mass flux across a phase boundary. In some anisotropic solids, however, the phase boundary moves along a non-conductive crystallographic direction. One such material is Li_XFePO₄, an electrode for lithium-ion batteries. With poor bulk ionic transport along the direction of phase separation, it is unclear how lithium migrates during phase transformations. Here, we show that lithium migrates along the solid/liquid interface without leaving the particle, whereby charge carriers do not cross the double layer. X-ray diffraction and microscopy experiments as well as ab initio molecular dynamics simulations show that organic solvent and water molecules promote this surface ion diffusion, effectively rendering Li_XFePO₄ a three-dimensional lithium-ion conductor. Phase-field simulations capture the effects of surface diffusion on phase transformation. Lowering surface diffusivity is crucial towards supressing phase separation. This work establishes fluid-enhanced surface diffusion as a key dial for tuning phase transformation in anisotropic solids.

由成分变化驱动的相变需要跨相界的质量通量。但是，在某些各向异性固体中，相边界沿非导电晶体学方向移动。一种这样的材料是Li _X FePO ₄，一种用于锂离子电池的电极。由于沿相分离方向的整体离子传输不良，因此尚不清楚锂在相变过程中如何迁移。在这里，我们表明锂沿着固/液界面迁移而没有离开颗粒，从而电荷载流子不会越过双层。X射线衍射和显微镜实验以及从头算分子动力学模拟表明，有机溶剂和水分子可促进该表面离子扩散，从而有效地使Li _XFePO ₄是三维锂离子导体。相场模拟捕获表面扩散对相变的影响。降低表面扩散率对于抑制相分离至关重要。这项工作确立了增强流体的表面扩散作为调整各向异性固体中相变的关键指标。