Solvation of lithium ions in mixed organic electrolyte solutions for secondary lithium batteries was investigated by electrospray ionization mass spectroscopy. The electrolyte solutions were mixed binary solutions of diethyl carbonate ͑DEC͒, dimethyl carbonate ͑DMC͒, ethylene carbonate ͑EC͒, ␥-butyrolactone ͑GBL͒, and propylene carbonate ͑PC͒ containing LiClO 4 . Lithium ions solvated mainly to two solvent molecules. The order of the inclination of the solvent molecules solvating to lithium ions is PC Ͼ EC Լ GBL ӷ DEC Ͼ DMC.

Solvation of lithium ions in organic electrolyte solutions, ethylene carbonate ͑EC͒, propylene carbonate ͑PC͒, and ␥-butyrolactone ͑GBL͒ containing LiClO 4 , for lithium batteries was studied by electrospray ionization-mass spectroscopy ͑ESI-MS͒. The mass spectrograms showed that a lithium ion solvated with two or three solvent molecules. In the course of the research and development of advanced batteries, the selection of the electrolyte solution has been an important factor. The solvent molecules react with the negative and positive electrodes and lithium salts in the electrolyte solutions. The solvent molecules solvated to lithium ions would react preferentially with the electrodes and the chemical species near the electrodes. Therefore, the solvation of lithium ion would concern the film formation on the interface between the electrode and the electrolyte. However, the solvation of lithium ions in the electrolyte solutions of lithium batteries has been reported in a few papers. [1][2][3][4][5] In these previous papers, the main procedure was Raman spectroscopy and solvation numbers of the solvated molecules were discussed. On the other hand, electrospray ionization-mass spectroscopy ͑ESI-MS͒ was introduced on the analysis of large biomolecules in 1989. 6 It was then applied on analytical studies of alkali metal cations complexed with 18-crown-6, 7 transition-metal ion complex with solvent, 8 and alkali metal cations-solvent binding. 9 In this work, ESI-MS was used as a first attempt for the evaluation of the solvation number of solvated molecules to lithium ion in organic electrolyte solutions, and reasonable results were obtained. ExperimentalSample solutions were prepared containing LiClO 4 ͑reagent grade͒ and EC, PC, or GBL ͑Lithium battery grade, Ube Industries͒ in methanol ͑MeOH͒ ͑reagent grade͒ to decrease the viscosity of the solutions. The diluted solutions were prepared by mixing MeOH containing LiClO 4 ͑1 mM LiClO 4 /MeOH͒ and EC, PC, or GBL and their volumetric ratio was 9:1.A schematic of ESI-MS instrument is shown in Fig. 1. The instrument was a mass spectrometer ͑JEOL-D300͒ furnished with an electrospray ionization interface. Samples dissolved in methanol were introduced at a rate of 0.15 mL/h to an injector with a nozzle conductor connecting to a high voltage dc source. High dc voltage was applied to the nozzle and the samples contained in methanol were positively charged. The lithium ions solvated with EC, PC, or GBL molecules were introduced together with methanol, through a path of which temperature was controlled by heated nitrogen gas, to a skimmer, and methanol vapor was striped there. The positively charged sample was then injected into a mass analyzer capable of analyzing samples with mass numbers over 50. Results and DiscussionESI-MS spectra of 1 mM LiClO 4 /methanol solution without the aprotic solvent are shown in Fig. 2 Figure 3 shows ESI-MS spectra of EC and 1 mM LiClO 4 /MeOH ͑1:9 vol͒ solution. In this figure, the spectra of ͓Li ͑EC͒ 2 ͔ ϩ (m/z ϭ 183.1) and ͓Li ͑EC͒ 3 ͔ ϩ (m/z ϭ...

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