| Abstract
| - We report the electrochemical study of cubic and monoclinic NiP2 polymorphs toward Li, as a candidatefor anodic applications for Li-ion batteries. We found that the monoclinic form is the most attractive oneperformance-wise. Monoclinic NiP2 can reversibly uptake 5 lithium per formula unit, leading to reversiblecapacities of 1000 mAh/g at an average potential of 0.9 V vs Li+/Li°. From complementary X-raydiffraction (XRD) and HRTEM (high-resolution transmission electron microscopy) measurements, it wasshown that, during the first discharge, the cubic phase undergoes a pure conversion process (NiP2 + 6Li+ + 6e- → Ni° + 2Li3P) as opposed to a sequential insertion−conversion process for monoclinicNiP2. Such a different behavior rooted in subtle structural changes was explained through electronicstructure calculations. Once the first discharged is achieved, both phases were shown to react with Lithrough a classical conversion process. More importantly, we report a novel way to design NiP2 electrodeswith enhanced capacity retention and rate capabilities. It consists in growing the monoclinic NiP2 phase,through a vapor-phase transport process, on a commercial Ni-foam commonly used in Ni-based alkalinebatteries. These new self-supported electrodes, based on chemically made interfaces, offer new opportunitiesto fully exploit the capacity gains provided by conversion reactions.
- The electrochemical reactivity of two NiP2 polymorphs toward Li shows the monoclinic form to be the most attractive one performance-wise. Upon cycling, both phases react with Li through a conversion process: NiP2 + 6Li+ ↔ Ni° + 2Li3P. A novel way to design NiP2 electrodes enhances capacity retention and rate capabilities.
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