We study the formation of the Li I lambda 6708 Angstroms resonance doublet in spectra of X-ray novae during the stage of outburst decay. We discuss the general conditions on accretion disc parameters which allow the detection of any spectral line absorption component in spectra of self-irradiated accretion discs. We obtain a simple analytical formula to estimate the equivalent width of the lithium line in the spectra of these discs and, adopting a simple model of self-irradiation, we have conducted more precise numerical calculations of these equivalent widths in the spectra of accretion discs around Schwarzschild black holes. We find that the equivalent width of the Li I doublet can reach ~ 20-100 m{ Angstroms} during outburst decay if the luminosity becomes less than 0.01 Eddington luminosity and if the lithium abundance in the discs is about cosmic (log N(Li) ~ 3). If some lithium production takes place in these discs higher abundances would be possible and consequently stronger lines could be formed. The larger the outer disc radius and the smaller the disc inclination angle to line of sight, the greater the equivalent width of the line.