Both Li and F belong to period 2.
Across a period, the screening effect is the same while the effective nuclear charge increases.
As a result, the outer electron is held more tightly and therefore, the ionization enthalpy increases across a period.
Hence, F will have higher ionization enthalpy than Li.
Thus, ionization enthalpy of Li is 520 kJ mol-1 while that of F is 1681 kJ mol-1.