In highly concentrated LiBF4/dimethyl carbonate (DMC) electrolyte, Ni-rich LiNi0.8Co0.1Mn0.1O2 (NCM811) positive-electrode exhibited a good cycle performance and the crack formation in NCM811 particles was remarkably suppressed. However, the high concentration causes some problems, such as a high viscosity, a low ionic conductivity and a high cost. To solve these problems, we chose 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) to dilute the concentrated LiBF4/ and LiPF6/DMC systems. The cycle performance of NCM811 electrode remarkably deteriorated in 0.96 mol kg(-1) LiBF4/DMC+HFE (1:2 by volume). Raman spectra indicated that the fraction of freeDMCincreased in the diluted electrolyte, which led to instability against oxidation. However, in the LiPF6/DMC system, the fraction of free DMC had rarely increased after diluted with HFE. The diluted electrolyte showed a high stability against oxidation comparable to the nearly saturated electrolyte, and the NCM811 electrode showed an excellent cycle performance. (C) 2019 The Electrochemical Society.