The propensity of metals to form irregular and nonplanar electrodeposits at liquid-solid interfaces has emerged as a fundamental barrier to high-energy, rechargeable batteries that use metal anodes. We report an epitaxial mechanism to regulate nucleation, growth, and reversibility of metal anodes. The crystallographic, surface texturing, and electrochemical criteria for reversible epitaxial electrodeposition of metals are defined and their effectiveness demonstrated by using zinc (Zn), a safe, low-cost, and energy-dense battery anode material. Graphene, with a low lattice mismatch for Zn, is shown to be effective in driving deposition of Zn with a locked crystallographic orientation relation. The resultant epitaxial Zn anodes achieve exceptional reversibility over thousands of cycles at moderate and high rates. Reversible electrochemical epitaxy of metals provides a general pathway toward energy-dense batteries with high reversibility.

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Rechargeable magnesium-ion batteries are a promising candidate technology to address future electrical energy storage needs of large scale mobile and stationary devices, due to the high environmental abundance of magnesium metal and divalent character of magnesium ion. With the recent increase in reports discussing cathode materials for magnesium-ion batteries, it is instructive to assess recent research in order to provide inspiration for future research. This review is a summary of the different chemistries and structures of the materials developed for magnesium ion cathodes. The particular strategies which may lead to future research initiatives are amplified.

The propensity of metal anodes of contemporary interest (e.g., Li, Al, Na, and Zn) to form non-planar, dendritic morphologies during battery charging is a fundamental barrier to achievement of full reversibility. We experimentally investigate the origins of dendritic electrodeposition of Zn, Cu, and Li in a three-electrode electrochemical cell bounded at one end by a rotating disc electrode. We find that the classical picture of ion depletion–induced growth of dendrites is valid in dilute electrolytes but is essentially irrelevant in the concentrated (≥1 M) electrolytes typically used in rechargeable batteries. Using Zn as an example, we find that ion depletion at the mass transport limit may be overcome by spontaneous reorientation of Zn crystallites from orientations parallel to the electrode surface to dominantly homeotropic orientations, which appear to facilitate contact with cations outside the depletion layer. This chemotaxis-like process causes obvious texturing and increases the porosity of metal electrodeposits.

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

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A multi-scale mathematical model, which accounts for mass transport on the crystal and agglomerate length-scales, is used to investigate the electrochemical performance of lithium-magnetite electrochemical cells. Experimental discharge and voltage recovery data are compared to three sets of simulations, which incorporate crystal-only, agglomerate-only, or multi-scale transport effects. Mass transport diffusion coefficients are determined by fitting the simulated voltage recovery times to experimental data. In addition, a further extension of the multi-scale model is proposed which accounts for the impact of agglomerate size distributions on electrochemical performance. The results of the study indicate that, depending on the crystal size, the low utilization of the active material is caused by transport limitations on the agglomerate and/or crystal length-scales. For electrodes composed of small crystals (6 and 8 nm diameters), it is concluded that the transport limitations in the agglomerate are primarily responsible for the long voltage recovery times and low utilization of the active mass. In the electrodes composed of large crystals (32 nm diameter), the slow voltage recovery is attributed to transport limitations on both the agglomerate and crystal length-scales. Large increases in the use of distributed and intermittent energy sources (i.e., wind and solar) have increased the need for cost effective, reliable, and efficient energy storage technologies.1 To address these needs, significant research efforts have focused on the development of next generation materials for secondary batteries, which can provide inexpensive and long lasting energy storage solutions.2-4 In particular, considerable work has focused on the advancement of magnetite (Fe 3 O 4 ) as an electrode in lithium-ion batteries due to its high theoretical capacity (926 mAh g −1 ), low cost and safety (non-toxic). 5-14Despite these advantages, one of the major challenges limiting the advancement of magnetite electrodes is a considerable difference between the maximum, theoretical capacity and the observed, experimental capacity of the active material. This difference increases the anticipated cost of magnetite batteries because it requires the electrodes to be overdesigned with excess amounts of active material. The difference between the theoretical and experimental capacity is related to the close-packed inverse spinel structure of Fe 3 O 4 , which restricts the transport of lithium in the material. To address this issue, several authors have synthesized Fe 3 O 4 nano-crystallites in attempts to minimize the path length for ion transport.9-14 The smaller path length increases the utilization of the active material by making it possible for ions to penetrate to the center of the crystals, especially at high rates of discharge. Electrodes fabricated with nano-crystalline magnetite have shown significant improvement in capacity; however, the theoretical capacity has still proven difficult to obtain.11 Further improvements in capacity may requir...

Operando, spatiotemporal resolved synchrotron X-ray fluorescence mapping measurements on a custom aqueous Zn/α-MnO2 cell provided direct, quantitative evidence of a Mn dissolution-deposition faradaic mechanism that governs the electrochemistry.