Studies on Solvation of Lithium Ions in Organic Electrolyte Solutions by Electrospray Ionization-Mass Spectroscopy

Abstract

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...

In situ FTIR spectra at the Cu electrode/propylene carbonate solution interface

“…Thus, both solvated species reach equilibrium in the solution. In the ESI-MS study, it was reported that Li + (PC) 3 was observed [12][13][14]. However, there is no contradiction between the ESI-MS study and the present work.…”

“…On the other hand, it is well known that when lithium ions are solvated using nonaqueous organic solvents [6][7][8][9][10][11][12][13][14], the solvents affect the reduction potential and the subsequent decomposition reactions. When considering rechargeable lithium ion batteries, it is important to understand the behavior of species solvated to lithium ions during the charging and discharging phases.…”

Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries

“…129,[169][170][171] Using a new mass spectrum (MS) technique, a more recent determination of the coordination number (CN) for lithium ions seems to support these computational results: among the various solvated lithium ion species, the peaks corresponding to Li(solv) 2∼3 + are the most abundant no matter whether the solvent is EC, PC, or γ-butyrolactone (γBL). 172,173 This latter experimental observation also revealed the stability of the solvation sheath, which obviously remains intact even during the electrospraying under vacuum and the subsequent ionization process. Therefore, there should be sufficient confidence in the general belief that the composition of solvated lithium ion remains unchanged during its migration in an electrolyte solution.…”

In situ microscope FTIR spectroscopic studies of interfacial reactions of Sn–Co alloy film anode of lithium ion battery

“…When E S is cathodically polarized from 0.7 to 0.02 V, lithium ions desolvate firstly from solvated solvent molecules, and then alloy with Sn-Co anode. As a result, if taking account of solvation/desolvation reactions in the lithiation/delithiation processes [23], the Eq. (2) could be rewritten as:…”