Highly concentrated electrolytes, although promising, are of high cost and show high viscosity and unsatisfactory wettability toward electrodes and separators, making them unfavorable for practical applications. A more rational electrolyte design is thus needed. Here, we investigate moderately concentrated electrolytes and find that the lithium bis(fluorosulfonyl)imide (LiFSI) concentration effects on the capacity, rate capability, and cycling stability of Si anodes in an ethylene carbonate (EC)/diethyl carbonate (DEC) mixed electrolyte are opposite to those in a fluoroethylene carbonate (FEC) electrolyte. The reasons for these results are systematically examined using Raman spectroscopy, transmission electron microscopy, electrochemical impedance spectroscopy, and the galvanostatic intermittent titration technique. A detailed X-ray photoelectron spectroscopy analysis is performed to study the solid electrolyte interphase chemistry. Al corrosion that occurs with the EC/DEC-based electrolyte can be effectively suppressed with the FEC-based electrolyte if an adequate LiFSI concentration is used. In the proposed 2 mLiFSI/FEC electrolyte, the Si anode has reversible capacities of 2630 and 855 mA h g-1 at 0.2 and 5 A g-1, respectively, and ∼75% capacity retention after 200 cycles (remarkably higher than that obtained with the EC/DEC-based electrolyte). This electrolyte also shows great compatibility with the high-energy-density LiNi0.8Co0.1Mn0.1O2 (NMC-811) cathode, allowing a stable charge-discharge of a Si//NMC-811 full cell.