Stability of the species Li2, Li2 and Li+2 increases in the order:

For Li2 = 1s²σ1s²2s² B.O. = 4
Li2 = 1
Li+2(5) = σ1s²σ1s²σ2s¹ B.O. = 3
Li2 = 0.5
Li2(7) = σ1s²σ1s²σ2s²σ2s¹ B.O. = 4
Li2 = 0.5
Li+2 is more stable than Li2 because Li2 has more numbers of antibonding electrons. More the bonding electrons, more is the stability.