The application of transition metal fluorides as energy dense cathode materials for lithium ion batteries has been hindered by inadequate understanding of their electrochemical capabilities/limitations. Here, we present an ideal system for mechanistic study through the colloidal synthesis of single crystalline, monodisperse iron(II) fluoride nanorods. Near theoretical capacity (570 mA h g −1 ) and extraordinary cycling stability (>90% capacity retention after 50 cycles at C/20) is achieved solely through the use of an ionic liquid electrolyte (1 m LiFSI/Pyr 1,3 FSI), which forms a stable solid electrolyte interphase and prevents the fusing of particles. This stability extends over 200 cycles at much higher rates (C/2) and temperatures (50 • C). High-resolution analytical transmission electron microscopy reveals intricate morphological features, lattice orientation relationships, and oxidation state changes that comprehensively describe the conversion mechanism. Phase evolution, diffusion kinetics and cell failure are critically influenced by surface specific reactions. The reversibility of the conversion reaction is governed by topotactic cation diffusion through an invariant lattice of fluoride anions and the nucleation of metallic particles on semi-coherent interfaces. This new understanding is used to showcase the inherently high discharge rate capability of FeF 2 .

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In this study, we conduct a comprehensive investigation of the effect of grain, grain boundary and interfacial resistance on the total Li-ion conductivity in Li2OHCl1-xBrx antiperovskite solid electrolytes. We highlight...

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Red phosphorus (RP) is a promising candidate as an anode for sodium-ion batteries because of its low potential and high specific capacity. It has two main disadvantages. First, it experiences 490% volumetric expansion during sodiation, which leads to particle pulverization and substantial reduction of the cycle life. Second, it has an extremely low electronic conductivity of 10–14 S cm–1. Both issues can be addressed by ball milling RP with a carbon matrix to form a composite of electronically conductive carbon and small RP particles, less susceptible to pulverization. Through this procedure, however, the resulting particle-size distribution of the RP particles is difficult to determine because of the presence of the carbon particles. Here, we quantify the relationship between the RP particle-size distribution and its cycle life for the first time by separating the ball-milling process into two steps. The RP is first wet-milled to reduce the particle size, and then the particle-size distribution is measured via dynamic light scattering. This is followed by a dry-milling step to produce RP–graphite composites. We found that wet milling breaks apart the largest RP particles in the range of 2–10 μm, decreases the Dv90 from 1.85 to 1.26 μm, and significantly increases the cycle life of the RP. Photoelectron spectroscopy and transmission electron microscopy confirm the successful formation of a carbon coating, with longer milling times leading to more uniform carbon coatings. The RP with a Dv90 of 0.79 μm mixed with graphite for 48 h delivered 1354 mA h g–1 with high coulombic efficiency (>99%) and cyclability (88% capacity retention after 100 cycles). These results are an important step in the development of cyclable, high-capacity anodes for sodium-ion batteries.