vehicles and the grid.[ 2 ] Future large-scale LIB designs could benefi t from metalcation-based electrode materials capable of multiple-electron transfers (METs) per metal cation, [ 3 ] resulting in higher energy density compared to commonly employed intercalation-type electrodes. [ 4 ] Many electrode materials were reported to undergo MET reactions, such as metal oxides, fl uorides, nitrides and sulfi des, [ 4 a,f ] layered dichalcogenides, layered oxides, vanadyl phosphate, [ 4 g,h] and also mixed-anion and mixed-cation compounds. [ 5 a,c] Many of these electrode materials share a common feature in their structures, i.e., the closepacked anion framework with tetra hedral and octahedral sites being occupied by cations. The MET reactions in these materials very often involve both intercalation and conversion processes, leading to multiple phase transformations that can profoundly affect the rate capability and cycling stability. [ 6 ] Thus, a detailed mechanistic investigation is needed for better understanding of the complex MET reactions and associated structural changes. Metal oxides of the general formula, M 3 O 4 , such as Fe 3 O 4 , [ 4 b ,d, 7 ] Co 3 O 4 , [ 8 ] and Mn 3 O 4 , [ 9 ] have been considered as possible MET compounds; although they share the same formulation and have similar metal oxidation states, Fe 3 O 4 is an inverse spinel, Co 3 O 4 is a normal spinel, and Mn 3 O 4 is a tetragonally distorted spinel. For a spinel structure, M 2+ and M 3+ cations are located in tetrahedral (8a) and octahedral (16d) sites of a cubicclose-packed (ccp) O-anion array, respectively, while for an inverse spinel, the tetrahedral site is occupied by one of the M 3+ cations while the other M 3+ cation and the M 2+ cation occupy octahedral sites of a ccp O-anion array. These metal oxides are currently under active investigation for potential use as lithium insertion electrodes, [ 10 ] as well as conversion electrodes capable of delivering theoretically high capacities through full reduction of the transition metals. [ 1 , 6 a] One of the inverse spinel oxides, Fe 3 O 4 , has been intensely studied for battery applications, due to its low cost, natural abundance, and low toxicity. [ 6 b] A recent review highlights the signifi cance of the particle size and morphology of Fe 3 O 4 , as well as the role of the heterostructure encompassing the active material. [ 11 ] The mesoscale electrode environment of nanocrystalline Fe 3 O 4 has also been recently evaluated, indicating the signifi cant infl uence of agglomeration on functional capacity. [ 12 ] Metal oxides, such as Fe 3 O 4 , hold promise for future battery applications due to their abundance, low cost, and opportunity for high lithium storage capacity. In order to better understand the mechanisms of multiple-electron transfer reactions leading to high capacity in
Here, we synthesized a series of α-MnO2 samples with differing K+ content but similar physical properties allowing direct study of the role of tunnel K+ on the electrochemistry of α-MnO2 cathodes.
The iron oxide magnetite, FeO, is a promising conversion type lithium ion battery anode material due to its high natural abundance, low cost and high theoretical capacity. While the close packing of ions in the inverse spinel structure of FeO enables high energy density, it also limits the kinetics of lithium ion diffusion in the material. Nanosizing of FeO to reduce the diffusion path length is an effective strategy for overcoming this issue and results in improved rate capability. However, the impact of nanosizing on the multiple structural transformations that occur during the electrochemical (de)lithiation reaction in FeO is poorly understood. In this study, the influence of crystallite size on the lithiation-conversion mechanisms in FeO is investigated using complementary X-ray techniques along with transmission electron microscopy (TEM) and continuum level simulations on electrodes of two different FeO crystallite sizes. In situ X-ray diffraction (XRD) measurements were utilized to track the changes to the crystalline phases during (de)lithiation. X-ray absorption spectroscopy (XAS) measurements at multiple points during the (de)lithiation processes provided local electronic and atomic structural information. Tracking the crystalline and nanocrystalline phases during the first (de)lithiation provides experimental evidence that (1) the lithiation mechanism is non-uniform and dependent on crystallite size, where increased Li diffusion length in larger crystals results in conversion to Fe metal while insertion of Li into spinel-FeO is still occurring, and (2) the disorder and size of the Fe metal domains formed when either material is fully lithiated impacts the homogeneity of the FeO phase formed during the subsequent delithiation.
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