On the basis of the similarity between spinel and rocksalt structures, it is shown that some spinel oxides (e.g., MgCo2O4, etc) can be cathode materials for Mg rechargeable batteries around 150 °C. The Mg insertion into spinel lattices occurs via “intercalation and push‐out” process to form a rocksalt phase in the spinel mother phase. For example, by utilizing the valence change from Co(III) to Co(II) in MgCo2O4, Mg insertion occurs at a considerably high potential of about 2.9 V vs. Mg2+/Mg, and similarly it occurs around 2.3 V vs. Mg2+/Mg with the valence change from Mn(III) to Mn(II) in MgMn2O4, being comparable to the ab initio calculation. The feasibility of Mg insertion would depend on the phase stability of the counterpart rocksalt XO of MgO in Mg2X2O4 or MgX3O4 (X = Co, Fe, Mn, and Cr). In addition, the normal spinel MgMn2O4 and MgCr2O4 can be demagnesiated to some extent owing to the robust host structure of Mg1−xX2O4, where the Mg extraction/insertion potentials for MgMn2O4 and MgCr2O4 are both about 3.4 V vs. Mg2+/Mg. Especially, the former “intercalation and push‐out” process would provide a safe and stable design of cathode materials for polyvalent cations.

The influence of Mn-ion dissolved in electrolyte solution on the electrochemical properties of graphite negative-electrodes was investigated using edge plane highly oriented pyrolytic graphite (HOPG) by cyclic voltammetry, electrochemical impedance spectroscopy, and in situ atomic force microscopy (AFM). Redox currents due to intercalation and de-intercalation reactions of Li-ion at an edge plane HOPG electrode significantly decreased with an increase in the cycle number. Both the surface film and interfacial Li-ion transfer resistances remarkably increased in the presence of Mn-ion, and particularly at potentials below 0.6 V, indicating that some irreversible reactions should occur. X-ray photoelectron spectra indicate that Mn metal was deposited on the edge plane HOPG, and then oxidized into divalent (or higher) Mn under open-circuit conditions. These results suggest that the deposited Mn metal should be oxidized to decompose the electrolyte itself and/or the original surface film reductively. Electrochemical AFM observation showed that very fine particles smaller than 0.1 μm were formed on edge plane HOPG in the initial potential cycle in electrolyte containing Mn-ion, and then larger particles were observed after further potential cycles. The effects of film-forming additives on the deposition of Mn on the edge plane HOPG electrode were also investigated.Various kinds of lithium 3d-transition metal oxides have been used as active materials in positive electrodes in commercially available lithium-ion batteries. The working potentials depend largely on reduction reactions of the 3d-transition metals within oxides, which involve the intercalation (insertion) of Li-ion from the electrolyte and the injection of electrons from the external circuit. Hence, lithium 3d-transition metal compounds containing Mn, Fe, Co and Ni have been widely explored to develop active materials with high working potentials and large discharge capacities. As a result, a variety of promising active materials have been created, but some of them are known to dissolve in electrolyte solution, particularly at elevated temperatures. 1,2 The dissolution of active materials results in a decrease in the reversible capacities of positive electrodes. In addition, dissolved 3d-transition metal-ion, and particularly Mn-ion, is known to cause a deterioration in the performance of graphite negative-electrodes. 3,4 The Mn-ion should deposit on a negative electrode because the working potentials, e.g., ca. 0.25 V vs. Li/Li + for graphite, are usually very low compared to the deposition potentials. 5,6 These technical issues must be solved because Mn-based active materials including spinel LiMn 2 O 4 are promising active materials for the positive electrode in lithium-ion batteries for large-sized and/or high-power applications. Many studies have been conducted to suppress the dissolution of LiMn 2 O 4 positive-electrodes, and several effective methods have been reported, such as surface coating of LiMn 2 O 4 , 7,8 lithium excess Li 1+x Mn 2 O 4 9 and el...

No abstract

Various kinds of fluoroalkyl ethers were investigated as diluents to reduce the high viscosity of highly concentrated LiBF4/PC electrolyte solutions. 1,1,2,2–tetrafluoroethyl 2,2,3,3–tetrafluoropropyl ether (HFE) was the most suitable diluent because of a high solubility of LiBF4. 2.50 mol kg−1 LiBF4/PC+HFE (2:1 by volume) was a reasonable compromise to attain a low viscosity (51.7 mPa s) and a low PC/Li molar ratio (2.39). Raman spectroscopy revealed that the fraction of free PC molecules in 2.50 mol kg−1 LiBF4/PC+HFE (2:1) was much smaller than 2.50 mol kg−1 LiBF4/PC (PC/Li molar ratio = 4.00), and that the interactions of HFE with Li+ cations and BF4− anions were very weak. LiNi0.5Mn1.5O4 positive electrodes showed high charge/discharge performance with low irreversible capacities in 2.50 mol kg−1 LiBF4/PC+HFE (2:1), which showed that the highly concentrated LiBF4/PC system can be diluted with HFE without losing the high stability against oxidation.

No abstract