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Lithium metal deposition during overcharge in practical lithium ion cells composed of a lithium metal oxide (LiCoO 2 ) positive electrode coated on Al foil, carbon (synthesized graphite and hard carbon) negative electrodes coated on Cu foil, polypropylene separator, and liquid electrolyte was observed using in situ solid state 7 Li nuclear magnetic resonance (NMR) measurements with an original probe featuring a flattened solenoid coil. Li insertion and extraction in carbon electrodes were monitored during charge and discharge and the intensities of certain peaks were found to be proportional to the cell capacity change. The deposition of metallic Li commenced after the cell voltage exceeded the nominal value and almost saturated after 160% of charge at a low current rate. The measurements showed that the deposition of metallic Li was much easier on graphite compared to hard carbon. The metallic Li deposited on hard carbon was almost completely discharged, whereas that on graphite remained after discharging to 2.5 V.

Use of nonflammable methyl nonafluorobuyl ether (MFE) has been studied to develop an inherently safe electrolyte for lithium secondary batteries. A no flash point (NFP) solution was prepared by mixing a proper amount of MFE with a common electrolyte solvent such as dimethyl carbonate or ethyl methyl carbonate (EMC) and the NFP electrolyte was obtained by dissolving organic lithium salts (e.g., normalLifalse[SO2C2F5]2; LiBETI) in the NFP solution. Cell capacities with NFP electrolyte of 1 mol dm−3 (M) LiBETI-MFE/EMC (80:20 vol %) were limited by the charge-discharge process on the graphite anode. Electrolyte components were investigated in terms of modifying the solid electrolyte interface film to improve the charge-discharge performance and cycle life of NFP electrolyte. Adding cyclic carbonate (e.g., ethylene carbonate, EC) and LiPF6 to the electrolyte reduced the interfacial resistance in a graphite/Li cell. A 18650 cylindrical cell with EC and LiPF6 added to 1 M LiBETI-MFE/EMC (80:20) electrolyte discharged more than 90% of its capacity at a 1 C current rate (vs. the capacity at the 0.1 C) and kept more than 80% of its initial capacity after 560 cycles at the 1 C current rate and room temperature. Effects of these additives on charge-discharge capacities in a graphite/Li cell were also investigated in terms of electrochemical spectroscopy, X-ray photoelectron spectroscopy, solid-state 7normalLi nuclear magnetic resonance, and attenuated total reflection infrared spectroscopy. © 2003 The Electrochemical Society. All rights reserved.

(Abstract)Lithium ion cells comprising actual components of positive electrodes (LiCoO2, LiNixCoyAlz, and LiMn2O4) and negative electrodes (graphite and hard carbon) were assembled for in situ 7 Li nuclear magnetic resonance (NMR) experiments. The 7 Li NMR measurements of the cells revealed a "relaxation effect" after overcharging: a decrease of the signal assigned to Li metal deposited on the negative electrode surface by overcharging.The reduction of the Li metal signal was inversely proportional to the increase of the signal of lithium stored in carbon. Therefore, the effect was ascribed to absorption of deposited

To improve the safety of the electrolyte used for lithium secondary batteries, binary mixed solvent electrolytes containing trifluoropropylene carbonate ͑TFPC͒ as cosolvent have been studied. Chloroethylene carbonate ͑ClEC͒, ethylene carbonate, and propylene carbonate were chosen as the other component of the binary mixed solvent for the electrolytes. The solution properties of these electrolytes were characterized using conductivity and nuclear magnetic resonance ͑NMR͒ spectroscopy. The chemical shift of ClEC and TFPC did not vary with the mixing ratio due to their similar enthalpies of solvation as derived by molecular orbital simulation. The ClEC/TFPC electrolyte showed higher discharge capacities with lower irreversible capacity loss in both a graphite/Li cell and Li 1ϩx Mn 2 O 4 /Li cell than other electrolyte systems. Electrochemical impedance spectroscopy measurements were made for cells composed of each electrolyte. The surface of the graphite anode was analyzed using X-ray photoelectron spectroscopy, infrared spectroscopy, and solid 7 Li-NMR spectroscopy.

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