High-voltage cathode materials, granting high-energy density lithium batteries, face the challenge of developing functional electrolytes enabling their stable operation at voltages higher than 4 V (vs. Li⁺/Li). Non-volatile and poorly flammable ionic liquid electrolytes (ILEs) are successfully applied to overcome this challenge, even with low-Co or Co-free positive electrode materials. With the ionic liquid-based electrolytes, the cathodes exhibit remarkable electrochemical performance, achieving initial specific capacity above 200 mAh g^-1 and outstanding capacity retention (up to 88% over 1000 cycles). More importantly, these electrolytes enable average Coulombic efficiency as high as 99.94%. The excellent compatibility of the ILEs with the high-voltage positive electrode materials enables the realization of cells achieving rather high specific energies.