Electrolyte for lithium secondary battery with Ni-rich cathode materials comprises of 1)LiFSI; 2)First solvent is carbonate, ester, ether or their fluorinated compounds. The exampled solvent is dimethoxyethane (DME), 1-Methoxy-2-(trifluoromethoxy)ethane (MTE), dimethyl carbonate (DMC) or ethyl propanoate (EP); 3)Second solvent is fluorinated tetrahydrofuran, C4HmFnO,m+n is 8, the exampled solvent is 3,3,4,4-Tetrafluorotetrahydrofuran (TFT); 4)Additives from VC, EC, FEC, VEC, LiBOB, LiDFOB, LiDFOP, LiNO3, TFPi, ES, 1,3-PS, DTD, MMSDS. The mole ratio among LiFSI : First solvent: Second solvent is x:y:z, where x=1, y= 0.5-10, Z=0.5-10. If z <=0.5, first solvent is fluorinated ether like MTE, and x:y is 1:1 to 1:5.