We report the electrochemical study of cubic and monoclinic NiP2 polymorphs toward Li, as a candidate for anodic applications for Li-ion batteries. We found that the monoclinic form is the most attractive one performance-wise. Monoclinic NiP2 can reversibly uptake 5 lithium per formula unit, leading to reversible capacities of 1000 mAh/g at an average potential of 0.9 V vs Li+/Li°. From complementary X-ray diffraction (XRD) and HRTEM (high-resolution transmission electron microscopy) measurements, it was shown that, during the first discharge, the cubic phase undergoes a pure conversion process (NiP2 + 6 Li+ + 6e- → Ni° + 2Li3P) as opposed to a sequential insertion−conversion process for monoclinic NiP2. Such a different behavior rooted in subtle structural changes was explained through electronic structure calculations. Once the first discharged is achieved, both phases were shown to react with Li through a classical conversion process. More importantly, we report a novel way to design NiP2 electrodes wi...