This SBIR Phase I project will develop a multifunctional electrolyte for high energy density abuse-tolerant lithium ion batteries with 5 V cathodes such as LiCoPO4. The multifunctional electrolyte comprises of a non-flammable ionic liquid as solvent and a porous ionomer membrane which serve as both separator and lithium cation source. No additional lithium salt such as LiPF6 or LiTFSi is required. The ionic liquid and the ionomer are electrochemically stable at 5 V vs Li/Li+ for many charge-discharge cycles, therefore will enable a lithium ion battery with high voltage and high energy density. The ionomer is a high temperature polymer with anions chemically bonded to the polymer chain. With a super-CO2 facilitated processing technology, porous membrane can be formed with the ionomer with interconnected pores (~50% porosity and pore size of 0.3 ~0.6um). In addition, the porous ionomer membrane has high mechanical modulus and strength and high thermal stability >150 degree Celsius. The high energy density abuse-tolerant lithium ion battery will be enabled by combining: 1. high voltage 5 V cathode; 2. abuse-tolerant cathode that fail mildly; 3. high voltage electrolyte with cycling stability; 4. Superior ionic conductivity ~ 1 mS/cm at -20 degree C; 5. non-flammable electrolyte with ionic liquid; and 6. mechanically and thermally stable porous ionomer membrane. The energy density can be further improved by using lithium metal anode.