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...

We evaluated the charge−discharge performance of a Sn 4 P 3 negative electrode in an ionic liquid electrolyte comprised of Nmethyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide (Py13-FSA) and NaFSA. We also conducted cyclic voltammetry and transmission electron microscopy for the Sn 4 P 3 electrode to reveal the reaction mechanism. It was suggested that Na 15 Sn 4 and Na 3 P are formed via phase separation in the first sodiation and that elemental Sn and elemental P formed by following a desodiation reaction with Na ions in the subsequent cycles. The Sn 4 P 3 electrode exhibited a high Coulombic efficiency of 99.1% at the fourth cycle and an excellent cycling performance with a high reversible capacity of 750 mA h g −1 even at the 200th cycle. We demonstrated that there are two important factors to improve the performance: (i) higher volume fraction of Sn than P and (ii) uniform dispersion of Sn nanoparticles in a P matrix. The ionic liquid electrolyte showed good applicability to the Sn 4 P 3 negative electrode due to its superior electrochemical stability.

The effect of phosphorus (P)-doping on the electrochemical performance of Si negative electrodes in lithium-ion batteries was investigated. Field-emission scanning electron microscopy was used to observe changes in surface morphology. Surface crystallinity and the phase transition of Si negative electrodes before and after a charge-discharge cycle were investigated by Raman spectroscopy and X-ray diffraction. Li insertion energy into Si was also calculated based on computational chemistry. The results showed that a low P concentration of 124 ppm has a meaningful influence on the electrochemical properties of a Si negative electrode; the cycle performance is improved by P-doping of Si. P-doping suppresses the changes in the surface morphology of a Si negative electrode and the phase transition during a charge-discharge cycle. Li insertion energy increases with an increase in the P concentration; Li insertion into P-doped Si is energetically unfavorable, which indicates that the crystal lattice of Si shrinks as a result of the replacement of some Si atoms with smaller P atoms, and therefore, it is more difficult to insert Li into P-doped Si. These results reveal that suppression of the phase transition reduces the large change in the volume of Si and prevents a Si negative electrode from disintegrating, which helps to improve the otherwise poor cycle performance of a Si electrode.

Elemental Si has a high theoretical capacity and has attracted attention as an anode material for high energy density lithium-ion batteries. Rapid capacity fading is the main problem with Si-based electrodes; this is mainly because of a massive volume change in Si during lithiation–delithiation. Here, we report that combining an ionic-liquid electrolyte with a charge capacity limit of 1000 mA h g–1 significantly suppresses Si volume expansion, improving the cycle life. Phosphorus-doping of Si also enhances the suppression and increases the Li+ diffusion coefficient. In contrast, the Si layer expands significantly in an organic electrolyte even with the charge capacity limit and even in an ionic-liquid electrolyte without the limit. We demonstrated that the homogeneously distributed Si lithiation–delithiation, phase-transition control from the Si to Li-rich Li–Si alloy phases, formation of a surface film with structural and/or mechanical stability, and faster Li+ diffusion contribute to suppressing Si volume expansion.

Changes in the surface morphology of the edge planes of graphite during a potential sweep were studied using highly oriented pyrolytic graphite (HOPG) in an ethylene carbonate (EC) + diethyl carbonate (DEC)-based electrolyte solution by in situ atomic force microscopy (AFM). The effects of the microscopic structures of graphite, i.e., edge and basal planes, on surface film formation are discussed. The formation of fine particles and precipitates was observed depending on the electrode potential between 1.0 and 0 V. These were considered to be remnants of blisters that could be observed at the basal plane and decomposition products of the electrolyte solution. The surface films were 56 and 66 nm thick after the first and second cycles, respectively. The precipitate layer formed on the edge plane was thinner than that observed on the basal plane after the second cycle. These results enabled us to elucidate the difference in the formation of surface films on the edge and basal planes of HOPG.