Liquid electrolytes with boroxine compounds were prepared, and interfacial reactions at the liquid electrolytes/LiMn 2 O 4 thin-film electrodes were investigated. Boroxine compounds, B 3 O 3 ͑OR͒ 3 , with different kinds of substituents ͓R = CH 3 , CH 2 CH 3 , ͑CH 2 ͒ 2 CH 3 , and CH͑CH 3 ͒ 2 ͔ were dissolved in 1 mol kg −1 LiCF 3 SO 3 /ethylene carbonate + EMC ͑1:1 by volume͒. Both cyclic voltammetry and electrical impedance spectroscopy measurements revealed that interfacial resistance depended on the chain length in the substituent of the boroxine compounds, and the resistance decreased with the increase in the chain length. Moreover, the anodic stability of the electrolytes was improved up to 5.0 V ͑vs Li/Li + ͒ on the film electrodes by the addition of tri-isopropoxy boroxine ͓R = CH͑CH 3 ͒ 2 ͔ with a steric structure ͑branched chain substituent͒. These results indicate that mixing boroxine compounds with both appropriate chain length and steric structure can be an effective way to develop advanced liquid electrolytes for high voltage rechargeable lithium ion batteries. © 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3374638͔ All rights reserved. Rechargeable lithium ion batteries have been expected as power sources for hybrid and electric vehicles, and some of them have been commercialized. However, further improvements in the current battery system are strongly demanded at the social end, and one of the most expected properties is the enhancement of energy density to prolong the cruising distance of electric vehicles. In this prospect, increase in battery operating voltage must be a possible way because there are several kinds of 5 V class positive electrode materials such as LiNi 0.5 Mn 1.5 O 4 , 1 LiCoPO 4 , 2 and LiCoMnO 4 . 3One of the drawbacks to realize 5 V class rechargeable lithium ion batteries is to develop advanced electrolytes with a wide potential window, high ionic conductivity, sufficient safety, low cost, etc. Although new kinds of electrolytes, typically ionic liquids 4 and solid electrolytes, 5 have been proposed, several problems such as cost, power density, and large resistance at the interface still remained to put them in practical use.In addition to the developments of the above electrolytes, various boron-based compounds have been studied as a new class of electrolytes. Zhang and Angell discovered the electrolytes with boronbased cosolvent. They showed that the electrolyte obtains an approximately 5 V stability on platinum electrodes and that the electrolyte can work well in both Li/LiMnO 2 and carbon/LiMnO 2 cells at 2.0-4.4 V.6 Barthel et al. proposed new lithium salts with a cherate-type anion of boron. 7,8 Sasaki et al. prepared their derivatives and applied them to a practical lithium ion battery system. 9,10Although the anodic stability of these lithium salts was ca. 4.2 V vs Li/Li + , Xu and Angell discovered lithium bis͑oxalate͒ borate ͑Li-BOB͒, which obtained superior anodic stability up to 4.5 V vs Li/Li + on platinum electrode. 11 The LiBOB obtained other interactiv...

Liquid electrolytes with boroxine compounds were prepared, and interfacial reactions at the liquid electrolytes/LiMn 2 O 4 thin-film electrodes were investigated. Boroxine compounds, B 3 O 3 ͑OR͒ 3 , with different kinds of substituents ͓R = CH 3 , CH 2 CH 3 , ͑CH 2 ͒ 2 CH 3 , and CH͑CH 3 ͒ 2 ͔ were dissolved in 1 mol kg −1 LiCF 3 SO 3 /ethylene carbonate + EMC ͑1:1 by volume͒. Both cyclic voltammetry and electrical impedance spectroscopy measurements revealed that interfacial resistance depended on the chain length in the substituent of the boroxine compounds, and the resistance decreased with the increase in the chain length. Moreover, the anodic stability of the electrolytes was improved up to 5.0 V ͑vs Li/Li + ͒ on the film electrodes by the addition of tri-isopropoxy boroxine ͓R = CH͑CH 3 ͒ 2 ͔ with a steric structure ͑branched chain substituent͒. These results indicate that mixing boroxine compounds with both appropriate chain length and steric structure can be an effective way to develop advanced liquid electrolytes for high voltage rechargeable lithium ion batteries.