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...

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.

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To suppress the degradation of graphite negative-electrodes that is caused by the dissolution of Mn-containing positive-electrode materials, the effects of additives that should strongly interact with Mn ion in electrolyte solution were examined. The edge plane of highly oriented pyrolytic graphite (HOPG) was used as a model graphite electrode, and its electrochemical properties were investigated by cyclic voltammetry and electrochemical impedance spectroscopy in Mn-ion-containing electrolyte solution. 100 ppm Mn ion dissolved in the electrolyte solution caused a significant decrease in redox currents due to the intercalation and de-intercalation reactions of Li+ at an edge plane HOPG electrode, and a remarkable increase in interfacial resistance consisting of surface film resistance and charge transfer resistance due to Li+ transfer at an interface between the electrode and electrolyte solution. This performance deterioration could be effectively suppressed by the addition of a certain type of cyclic ether to the electrolyte solution. The influence of the additive concentration on interfacial resistance was also investigated.

Surface films formed on edge plane pyrolytic graphite electrodes were characterized by attenuated total reflection (ATR)-Fourier transform infrared spectroscopy (FTIR). FTIR-ATR spectra revealed that the surface films consisted mainly of LiOCO 2 R (R = alkyl or lithium alkyl carbonate), Li 2 CO 3 and (CH 2 CH 2 O) n on both the edge and basal planes, while the formation reactions at the edge plane proceeded more rapidly than those at the basal plane. The major constituents of the surface films formed at 0.1 and 0.8 V were LiOCO 2 R and Li 2 CO 3 , respectively. We discuss this difference based on the results in the literature. In addition, surface films derived from film-forming additives, such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC), were investigated by ATR-FTIR and X-ray photoelectron spectroscopy (XPS), and the formation mechanisms are discussed by considering the reactivity of intermediate radical compounds.