Department of Chemistry, University of Wisconsin-Madison , Madison, Wisconsin 53705, United States.
Department of Materials Science and Engineering, University of Wisconsin-Madison , Madison, Wisconsin 53706, United States.
J Am Chem Soc. 2016 Mar 2;138(8):2838-48. doi: 10.1021/jacs.6b00061. Epub 2016 Feb 19.
Metal fluorides 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, transmission electron microscopy, density functional theory calculations, and galvanostatic intermittent titration technique to first correlate the voltage profile of iron fluoride (FeF3), 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 FeF3 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.
金属氟化物和氧化物可以通过转化化学存储多个锂离子，从而实现高能密度锂离子电池。然而，它们的实际应用受到充电和放电电压曲线之间异常大的电压滞后以及由此产生的低能量效率（<80%）的阻碍。这种滞后的物理起源很少被研究，也理解得很差。在这里，我们采用原位 X 射线吸收光谱、透射电子显微镜、密度泛函理论计算和恒电流间歇滴定技术，首先将代表性转化电极材料氟化铁（FeF3）的电压曲线与电极中中间相的演变和空间分布相关联。结果表明，与传统观念相反，电极中的相演变在放电和充电过程中是对称的。然而，受反应动力学控制的电化学活性相的空间演变是不同的。我们进一步提出，FeF3 电极中的电压滞后具有动力学性质。它是欧姆电压降、反应过电位和电化学活性相（即组成不均匀性）不同空间分布的结果。因此，通过合理设计和优化材料微结构和电极结构，可以减轻大滞后，从而提高基于转化化学的锂离子电池的能量效率。
