The concentration polarization, in addition to the activation and ohmic polarizations, limits the fast operation of electrochemical cells such as Li-ion batteries (LIBs). We demonstrate an approach to mitigate the concentration polarization by regulating the effective concentration (i.e., the mean ionic activity) of Li ions. The use of an acrylate-based gel polymer electrolyte (A-GPE) improved the rate capability of LIBs compared with its liquid counterpart. Electrochemical and spectroscopic evidence confirms that the unexpected power performance of the A-GPE is ascribed to the unique solvation structure surrounding the Li ions. The solvation structure suppresses an abnormal increase in the activity of Li ions and thus mitigates the concentration polarization during high-rate discharge. Importantly, this study rejects the common wisdom that the solid or semisolid electrolytes discourage the fast charge/discharge of LIBs and suggests an avenue to simultaneously enhance both the safety and high-power performance of rechargeable batteries.