Metal fluoride and oxides can store multiple lithium-ions through conversion chemistry to enable high energy-density lithium-ion batteries. However, their practical applications have been hindered by an unusually large voltage hysteresis between charge and discharge voltage-profiles and the consequent low energy efficiency (< 80%). The physical origins of such hysteresis are rarely studied and poorly understood. Here we employ in situ X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), density-functional-theory (DFT) calculations, and galvanostatic intermittent titration technique (GITT) to first correlate the voltage profile of iron fluoride ($FeF_3$), a representative conversion electrode material, with evolution and spatial distribution of intermediate phases in the electrode. The results reveal that, contrary to conventional belief, the phase evolution in the electrode is symmetrical during discharge and charge. However, the spatial evolution of the electrochemically active phases, which is controlled by reaction kinetics, is different. We further propose that the voltage hysteresis in the $FeF_3$ electrode is kinetic in nature. It is the result of Ohmic voltage drop, reaction overpotential, and different spatial distributions of electrochemically-active phases (i.e. compositional inhomogeneity). Therefore, the large hysteresis can be expected to be mitigated by rational design and optimization of material microstructure and electrode architecture to improve the energy efficiency of lithium-ion batteries based on conversion chemistry.
Effect of disorder and doping on electronic structure and diffusion properties of Li$_{3}$V$_{2}$O$_{5}$
具有过量锂的处于其$ω$相的V $_{2}$ O $_{5}$（Li $_{3}$ V $_{2}$ O $_{5}$）由于其更安全的电压（相对于Li $^{+}$ /Li（s）为0.6 V）和高锂传输速率，是用于在低温和快速充电条件下的锂离子电池的石墨阳极的潜在替代物。如在大多数有序材料中所观察到的，在操作中阳离子无序可以在电荷补偿机制、阴离子活性、锂扩散和操作电压方面产生显著变化。在这项工作中，我们报告的结构畸变，电子结构和迁移势垒的变化伴随着无序使用第一性原理计算。由于锂原子在无序状态下的分离，我们观察到更大的扭曲，出现金属行为和潜在的阴离子活性从非键合氧状态附近的费米能级。氧化还原容量可以通过掺杂3d金属来调节，这可以调节参与的阳离子状态，并且通过氟取代来稳定或抑制阴离子状态。此外，抑制阴离子活性被发现，以减少结构的扭曲，减轻电压衰减和滞后的关键。在无序存在下的扩散势垒计算表明激活的剩余的3D路径的锂跳跃是不可用的有序配置，解释其快速充电能力在实验中观察到的。
Observing Nucleation and Crystallization of Rocksalt LiF from Molten State through Molecular Dynamics Simulations with Refined Machine-Learned Force Field
Interfacial Effects on Solid Electrolyte Interphase in Lithium-ion Batteries
界面处钝化层的存在是实现现代锂离子（Li离子）电池的主要因素。它们的特性决定了电池的循环寿命、性能和安全性。一种特殊情况是固体电解质中间相（SEI），一种由于电解质的不稳定性和随后在阳极表面的分解而形成的异质多组分膜。SEI充当阻碍电解质进一步分解的钝化层，这对库仑效率是有害的。在这项工作中，我们使用第一性原理模拟研究氟化锂（LiF）和碳酸锂（Li $_2$ CO $_3$）之间的界面的动力学和电子特性，这两种常见的SEI组件存在于具有有机液体电解质的锂离子电池中。我们发现，这些组件之间的一致的接口，限制在他们每个人的应变低于3%。我们发现，界面引起的弗伦克尔缺陷的形成能量的大幅度增加，产生Li空位的LiF和Li晶体中的Li $_2$ CO $_3$负责运输。另一方面，Li间隙跳跃势垒从体Li $_2$ CO $_3$中的$0.3$ eV降低到所考虑的界面结构中的$0.10$或$0.22$ eV，表明界面的有利作用。在异质SEI中控制这两种效应对于在SEI中保持快速离子传输是至关重要的。我们进一步进行了Car-Parrinello分子动力学模拟，以探讨在我们的界面结构，这揭示了增强的界面附近的锂离子扩散的锂离子传导。了解多相SEI的界面特性是实现下一代电池的重要前沿。
