Modification of the lithium metal surface by nonionic polyether surfactants II.: Investigations with microelectrode voltammetry and in situ quartz crystal microbalance

Li metal has a high specific capacity, and films formed on the surface of Li metal must thus be stabilized in some way. We used a nonionic surfactant resembling poly(ethylene glycol) dimethyl ether (PEGDME) to achieve this stabilization. The success of this stabilization as a function of the molecular weight of the polyether (Mw 90 to 2000) was investigated with microelectrode voltammetry (MEV) and the in situ quartz crystal microbalance method, In I mol dm(-3) LiClO4/PC, the activation energy (Delta G*) for charge transfer of the Li/Li+ couple (as obtained from MEV measurements) increased by II to 60 kJ mol(-1) as the molecular weight (g mol-L) was increased from 90 to 400; and remained almost unchanged as it was increased from 400 to 2000. The variation of Delta C* indicates that Li ions were preferentially coordinated with ethylene oxide (EO) chains and were taken into the helix structures of the EO chains when PEGDME (which possesses repeated EO units in the range of 180 to 2000 Mw) was added to electrolytes. As for the stoichiometry, the surface film formed in the presence of PEGDME with Mw greater than or equal to 180 causes no accumulation during the Li deposition and dissolution cycles. Such a stable film including a few EO units eventually functioned as a uniform path for Li ions in both deposition and dissolution processes. The extent of inactivation of the deposited Li for various molecular weights of the PEGDME was consistent with the change of solvation state and surface chemistry. Namely, the inactivation for the PEGDME-added (with Mw greater than or equal to 180) systems was diminished markedly (by ca. 40 to 65%) as compared with surfactant-free and monomer (Mw 90)-added systems. (C) 2000 The Electrochemical Society. S0013-4651(99)05-037-5. All rights reserved.