Combined XRD, EXAFS, and Mössbauer Studies of the Reduction by Lithium of α-Fe[sub 2]O[sub 3] with Various Particle Sizes

Abstract

The electrochemical reduction of hematite with various particle sizes by metallic lithium has been studied by means of X-ray diffraction ͑XRD͒ Mo ¨ssbauer and extended X-ray absorption fine structure ͑EXAFS͒ spectroscopy. Previous in situ XRD analysis coupled with electrochemical data showed that lithium can be inserted in the nanosized sample up to 1 Li per Fe 2 O 3 whereas bulk material undergoes an irreversible Li-driven transformation from an hexagonal anionic packing to a close cubic packed framework as soon as 0.03 Li is inserted in the corundum structure. The present data show that only 0.6 Li per formula unit are actually inserted in the structure of small particles. The remaining lithium ͑0.4͒ is engaged in irreversible reduction of surface groups, or capacitive behavior. Beyond the solid solution domains, both samples are multiphase, and consist of Li 2 Fe 2 O 3 , Fe 0 clusters ͑10-15 Å͒ and inserted ␣-Fe 2 O 3 , which proportions are used to calculate the mean iron oxidation state in the electrode as the reaction proceeds. From these data, we found that electrolyte decomposition can occur at very different steps of the reduction depending on the texture of the active materials. In addition, during the reduction process, we evidenced a reaction of disproportionation (3Fe 2ϩ → 2Fe 3ϩ ϩ Fe 0 ), an intense electrochemical grinding of the hematite particles and the formation of extremely fine metallic surface clusters. For the first time, the EXAFS/X-ray absorption near-edge structure signature of the divalent intermediate Li 2 Fe 2 O 3 phase is obtained.

Thermal Characteristics of Conversion-Type FeOF Cathode in Li-ion Batteries

“…Owing to the introduction of more covalent M-O bonds into the highly ionic fluoride structure, iron oxyfluoride is expected to have better electrochemical performance than iron fluorides due to the improved electronic conductivity. Moreover, the theoretical capacity is also increased from 712 mAh g −1 for FeF 3 to 885 mAh g −1 for FeOF. Iron oxyfluorides were traditionally synthesized through a solid-state reaction of Fe 2 O 3 , with FeF 3 in a high-pressure inert atmosphere at 950-1000 • C [25][26][27].…”

“…The most important advantage of conversion-type materials over insertion compounds is that, theoretically, all of the metal redox potentials of the active material can be utilized during cycling, which indicates a much higher capacity and energy density. Various conversion-type compounds, such as metal oxides [2][3][4], fluorides [5][6][7][8][9][10][11], sulfides [12][13][14][15], and nitrides [16][17][18][19], have been investigated as active materials for Li-ion batteries. Among them, only metal fluorides could be studied as cathode materials due to their relatively high operating potential, which is induced by highly ionic metal-ligand bonds [20,21].…”

“…From the standpoints of cost and environmental impact, iron fluoride is one of the most promising candidates among the conversion-type cathodes, and has been investigated by many researchers [5][6][7][8]20]. Moreover, FeF 3 was reported to show better thermal stability than lithium transition-metal oxide cathodes in associated electrolytes due to the absence of oxygen atoms [22][23][24]. However, metal fluorides have poor electronic conductivity due to their ionic character, with large energy gaps.…”

“…In larger-scale batteries, it becomes more difficult to keep the thermal balance between heat generation and heat dissipation in the cell, because the heat generation is directly proportional to the cell volume (~r 3 ), while the heat dissipation is proportional to the cell surface area (~r 2 ). Moreover, battery accident risk is proportional to the square of the capacity, because the battery accident risk can be expressed by the product of the accident probability and the accident damage, and each is proportional to the capacity.…”

Hydrothermal synthesis and characterization of MnCo2O4 in the low-temperature hydrothermal process: Their magnetism and electrochemical properties

“…However, to match the growing demand for LIBs, higher discharge capacity is needed. Meanwhile, renewed interest in transition-metal oxides as potential anode materials for use in LIBs starts to grow [8,9]. Compared with graphite (372 mA·h/g), transition-metal cobaltites can deliver a high specific capacity.…”