Development of conversion coatings on iron via corrosion in LiPF6 solution

Synergistic Interaction of Clusters of Iron Oxide Nanoparticles and Reduced Graphene Oxide for High Supercapacitor Performance

Supercapacitors have been recognized as one of the more promising energy storage devices, with great potential use in portable electronics and hybrid vehicles. In this study, a composite made of clusters of iron oxide (Fe₃O₄-γFe₂O₃) nanoparticles and reduced graphene oxide (rGO) has been developed through a simple one-step solvothermal synthesis method for a high-performance supercapacitor electrode. Electrochemical assessment via cyclic voltammetry, galvanostatic charge-discharge experiments, and electrochemical impedance spectroscopy (EIS) revealed that the Fe₃O₄-γFe₂O₃/rGO nanocomposite showed much higher specific capacitance than either rGO or bare clusters of Fe₃O₄-γFe₂O₃ nanoparticles. In particular, specific capacitance values of 100 F g^-1, 250 F g^-1, and 528 F g^-1 were obtained for the clusters of iron oxide nanoparticles, rGO, and the hybrid nanostructure, respectively. The enhancement of the electrochemical performance of the composite material may be attributed to the synergistic interaction between the layers of graphene oxide and the clusters of iron oxide nanoparticles. The intimate contact between the two phases eliminates the interface, thus enabling facile electron transport, which is key to attaining high specific capacitance and, consequently, enhanced charge-discharge time. Performance evaluation in consecutive cycles has demonstrated that the composite material retains 110% of its initial capacitance after 3000 cycles, making it a promising candidate for supercapacitors.

The Role of Electrolyte Composition in Enabling Li Metal-Iron Fluoride Full-Cell Batteries

FeF₃ conversion cathodes, paired with Li metal, are promising for use in next-generation secondary batteries and offer a remarkable theoretical energy density of 1947 Wh kg^-1 compared to 690 Wh kg^-1 for LiNi_0.5 Mn_1.5 O₄ ; however, many successful studies on FeF₃ cathodes are performed in cells with a large (>90-fold) excess of Li that disguises the effects of tested variables on the anode and decreases the practical energy density of the battery. Herein, it is demonstrated that for full-cell compatibility, the electrolyte must produce both a protective solid-electrolyte interphase and cathode-electrolyte interphase and that an electrolyte composed of 1:1.3:3 (m/m) LiFSI, 1,2-dimethoxyethane, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether fulfills both these requirements. This work demonstrates the importance of verifying electrode level solutions on the full-cell level when developing new battery chemistries and represents the first full cell demonstration of a Li/FeF₃ cell, with both limited Li and high capacity FeF₃ utilization.

Laser-Guided Corrosion Control: A New Approach to Tailor the Degradation of Mg-Alloys

Despite corrosion being commonly seen as a problem to be avoided, applications such as batteries or biodegradable implants do benefit from corrosion-like phenomena. However, current strategies address corrosion control from a global perspective for a whole component, without considering local adaptations to functionality specifications or inhomogeneous environments. Here, a novel concept is presented: the local control and guidance of corrosion through a laser surface treatment. Immersion tests in saline solution of AZ31 magnesium alloy samples show degradation rates reduced up to 15 times with the treatment, owing to a fast passivation after the induced microstructural modifications. By controlling the treatment conditions, the degradation can be restricted to delimited regions and driven towards specific directions. The applicability of the method for the design of tailored degradation biomedical implants is demonstrated and uses for cathodic protection systems and batteries can also be anticipated.

Degradation Mechanism of Conversion-Type Iron Trifluoride: Toward Improvement of Cycle Performance

Conversion-type iron trifluoride (FeF₃) has attracted considerable attention as a positive electrode material for lithium secondary batteries due to its high energy density and low cost. However, the conversion process through which FeF₃ operates leads it to suffer from capacity degradation upon repeated cycling. To improve the cycle performance, in this study we investigated the degradation mechanism of conversion-type FeF₃ electrode material. Bulk analyses of FeF₃ upon cycling reveal incomplete oxidation to Fe³⁺ concomitant with the aggregation of LiF at the charged state. In addition, surface analyses of FeF₃ reveal that a film covered the electrode surface after 10 cycles, which leads to a remarkable increase in resistance. We show that the choice of the electrolyte formulation is crucial in preventing the formation of the film on the electrode surface; thus, FeF₃ shows better performance in an electrolyte comprising LiBF₄ solute in cyclic carbonate solvents than in chain carbonate-containing LiPF₆ as the electrolyte. This study underpins that a careful selection of solvent, rather than solute, is significantly essential to improve the cycle performance of the FeF₃ electrode.