Stability of the species Li₂, Li₂^– and Li₂⁺ increases in the order of :

Bond order = Li₂ > Li⁺₂ = Li

stability order  ${\mathrm{Li}}_2>{\mathrm{Li}}_2^{+}>{\mathrm{Li}}_2^{-}$

when two molecular species have same bond order then the species with maximum number of electrons in antibonding molecular orbitals is less stable.

 Number of ABMO electrons in ${\mathrm{Li}}_2^{-}$= 3

Number of ABMO electrons in  ${\mathrm{Li}}_2^{+}$= 2