Stability of species${\mathrm{Li}}_2 , {\mathrm{Li}}_2^{-}$and ${\mathrm{Li}}_2^{+}$ increases in the order of:

The electronic configuration of' ${\mathrm{Li}}_2,{\mathrm{Li}}_2^{+},{\mathrm{Li}}_2^{-}$ and their bond orders are 

$\underset{\mathrm{Bond} \mathrm{order} =1}{{\mathrm{Li}}_2(\sigma 2\mathrm{s}{)}^2 }\underset{\mathrm{Bond} \mathrm{order} =1/2}{{\mathrm{Li}}_2^{+}(\sigma 2\mathrm{s}{)}^1 }\underset{\mathrm{Bond} \mathrm{order}= 1/2}{{\mathrm{Li}}_2^{-}\left(\sigma 2\mathrm{s}{)}^2\right(\sigma \ast 2\mathrm{s}{)}^1}$

The species ${\mathrm{Li}}_2$ will have larger electronic repulsion as compared to ${\mathrm{Li}}_2^{+}$ and thus will be less stable. Thus, the stability order is ${\mathrm{Li}}_2<{\mathrm{Li}}_2^{+}<{\mathrm{Li}}_2^{-}$