LiFePO 4 powders were synthesized under various conditions and the performance of the cathodes was evaluated using coin cells. The samples were characterized by X-ray diffraction, scanning electron microscope observations, Brunauer, Emmett, and Teller surface area measurements, particle-size distribution measurements, and Mo ¨ssbauer spectroscopy. Ab initio calculation was used to confirm the experimental redox potentials and Mo ¨ssbauer parameters. The choice of a moderate sintering temperature (500°C Ͻ T Ͻ 600°C) and a homogeneous precursor enabled nearly perfect utilization of Ͼ95% of the 170 mAh/g theoretical capacity at room temperature. There are two main obstacles to achieving optimum charge/discharge performance of LiFePO 4 : ͑i͒ undesirable particle growth at T Ͼ 600°C and ͑ii͒ the presence of a noncrystalline residual Fe 3ϩ phase at T Ͻ 500°C.

The development of a stable, functional electrolyte is urgently required for fast-charging and high-voltage lithium-ion batteries as well as next-generation advanced batteries (e.g., Li−O 2 systems). Acetonitrile (AN) solutions are one of the most promising electrolytes with remarkably high chemical and oxidative stability as well as high ionic conductivity, but its low stability against reduction is a critical problem that hinders its extensive applications. Herein, we report enhanced reductive stability of a superconcentrated AN solution (>4 mol dm −3 ). Applying it to a battery electrolyte, we demonstrate, for the first time, reversible lithium intercalation into a graphite electrode in a reduction-vulnerable AN solvent. Moreover, the reaction kinetics is much faster than in a currently used commercial electrolyte. First-principle calculations combined with spectroscopic analyses reveal that the peculiar reductive stability arises from modified frontier orbital characters unique to such superconcentrated solutions, in which all solvents and anions coordinate to Li + cations to form a fluid polymeric network of anions and Li + cations. ■ INTRODUCTIONWith growing public concern about environmental and energy issues, considerable effort has been devoted to the development of cutting-edge electrochemical energy-storage technologies such as high-voltage and fast-charging lithium-ion batteries as well as next-generation lithium−oxygen batteries. 1−4 A key material in such advanced batteries is a stable, functional electrolyte that allows for reversible and rapid positive/negative electrode reactions without suffering from severe oxidative/ reductive decompositions. In particular, an oxidation-tolerant electrolyte is primarily required to meet the recent remarkable progress and diversification of positive-electrode materials for high-voltage advanced batteries.Acetonitrile (AN) is one of the most oxidation-tolerant organic solvents. In addition, due to its high dielectric constant, 5 AN can easily dissolve electrolyte salts to exhibit considerably high ionic conductivity. Because of these attractive features, AN solutions are a promising electrolyte for various electrochemical devices. 6−9 Particularly, applying them to lithium-ion batteries will eliminate the conventional voltage limitation (∼4.2 V) based on the electrochemical window of currently used carbonate-based electrolytes to open the possibility of high-voltage operation with a 5 V-class positive electrode. 4,10 Furthermore, the excellent ionic transport property will possibly realize fast-charging lithium-ion batteries, which are urgently required for automobile applications.Despite these remarkable advantages, AN has not found extensive application in batteries. This is primarily due to its crucially poor reductive stability. AN spontaneously reacts with lithium metal (i.e., a strong reducing agent), and thus, a lithium metal electrode does not work reversibly in AN electrolytes. 11 For the same reason, there is no report on reversible lithium intercalati...

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