This work addresses the electrochemical performances of an over-lithiated lithium nickel oxide (Li2NiO2) as a sacrificing positive additive for lithium-ion batteries. Li2NiO2 decomposes along with a crystalline to amorphous phase transition at 3.5V (vs. Li/Li+) in the first charging period, which is far below the charging potential of common positive electrodes (for instance, ca. 4.0V for LiCoO2). The decomposed amorphous phases then deliver a de-lithiation capacity up to >300mAhg−1 in the first charging. The combined feature of easy decomposition and large first de-lithiation capacity demonstrates that Li2NiO2 is a promising positive additive to provide the electric charges/Li+ ions for the charge compensation on negative electrodes. In addition, this over-lithiated lithium nickel oxide delivers a reversible capacity amounting to 70–90mAhg−1 in the continuing cycles, which is an extra capacity to be added to that delivered by main positive electrodes. It is found that the capacity gain (extra capacity) is larger when Li2NiO2 is decomposed at a faster rate due to a smaller charge transfer resistance. This finding suggests that, when Li2NiO2 is used as the sacrificing positive additive, the use of higher current in the first charging is preferred for the capacity gain.