Strategic Structural Design of a Gel Polymer Electrolyte toward a High Efficiency Lithium-Ion Battery

Electrolytes have played critical roles in electrochemical energy storage. In Li-ion battery, liquid electrolytes have shown their excellent performances over decades, such as high ionic conductivity (similar to 10(-3) S cm(-1)) and good contacts with electrodes. However, the use of liquid electrolytes often brought risks associated with leakage and combustion of organic electrolytes. Hence, polymer electrolytes become potential candidates to replace liquid electrolyte systems. Although solid polymer electrolytes (SPEs) offer better safety and good mechanical properties to take over liquid electrolytes, most of them only deliver low ionic conductivities (similar to 10(-8) S cm(-1)) and poor contact with electrodes, resulting in poor cycle performance and low electrical capacity of the batteries. In addition, gel polymer electrolytes (GPEs) have received increasing research attention due to their relevant characteristics, which extend from liquid electrolytes and solid polymer electrolytes. In this review, state-of-the-art samples of gel polymer electrolytes are elucidated with respect to their structural design and electrochemical properties to determine their application potential in Li-ion batteries (LIBs). First, we present the general requirements of GPEs for LIBs applications, followed by important electrochemical properties of GPEs for LIBs including ionic conductivity, transference number, and ionic transport mechanisms. Furthermore, recent progress of common polymers, namely, polyether, polyvinyl, polynitrile, polycarbonate, and polyacrylate, as polymer host of GPEs has been carefully explained. Finally, the alternative polymers were also discussed to provide new approaches for further developments of GPEs to fulfill the demanded properties for practical applications.