A diisocyanate/sulfone binary electrolyte based on lithium difluoro(oxalate)borate for lithium batteries

Abstract

A new binary electrolyte containing tetramethylene sulfone (TMS) and hexamethylene diisocyanate (HDI) with lithium difluoro(oxalate)borate (LiODFB) as the lithium salt has been prepared and investigated for physicochemical properties. A linear relationship between the frontier molecular orbital energies and the oxidation/reduction potentials is preliminarily confirmed. Compared to the pure TMS electrolyte, a mixture of TMS and HDI exhibits a wider electrochemical stability window, better wettability and an improved low temperature performance. Combined with the mixed electrolyte, LiCoO₂ and LiNi_1/3Mn_1/3Co_1/3O₂ cathode materials show specific capacities of nearly 134.5 mA h g⁻¹ and 168.3 mA h g⁻¹ after 50 cycles, respectively, which is superior to those containing the traditional electrolyte. Furthermore, the composite electrolyte exhibits a good compatibility with the high voltage LiNi_0.5Mn_1.5O₄ cathode material which has a specific capacity close to 120 mA h g⁻¹ after 50 cycles. The enhanced battery performance is mainly due to HDI, which has a high oxidation potential (5.2 V), good wettability, a low melting point and an outstanding ability to form effective solid electrolyte interface layers. In addition, LiODFB makes a contribution to the compatibility of the electrolyte due to its passivation toward aluminum, its high solubility and its ability to support reversible metallic lithium cycling. All of the properties above indicate that the LiODFB/HDI/TMS mixed electrolyte is a promising material and can have applications in the field of lithium batteries.