Li-S batteries have been considered promising candidates for the next-generation energy storage devices because of their extremely high energy densities and low cost. However, Li dendrite formation/dissolution and shuttle of high-order polysulfides prevent their practical applications. Herein, we demonstrate a highly concentrated electrolyte, 12 M lithium bis(fluorosulfonyl)imide (LiFSI) salt in DME solvent (12 M LiFSI/DME), that can effectively suppress both the Li dendritic growth on the anode and the polysulfide shuttle reactions on the cathode side. The highly concentrated electrolyte along with the robust solid electrolyte interphase (SEI) formed therein play the key role in achieving high coulombic efficiencies for both Li stripping/plating (> 99.2%) and S cathode (> 99.7%). Based on the in-depth understanding of the interactions between electrodes and highly concentrated electrolyte, we designed a novel dilute electrolyte (1 M LiFSI/HFE + DME), which achieves similar electrochemical performances in Li-S batteries as the concentrated electrolytes. These Li-S batteries with the highest CE for Li anode and sulfur cathode maintains a high reversible capacity of 786 mA h/g at 0.1 A/g after 300 cycles, or 644 mA h/g at 300th cycle even at 1 A/g without any detectable shuttle reactions.