Electrochemical Reactivity and Design of NiP₂ Negative Electrodes for Secondary Li-Ion Batteries

Abstract

2We report the electrochemical study of cubic and monoclinic NiP 2 polymorphs towards Li, as candidate for anodic applications for Li-ion batteries. We found that the monoclinic form is the most attractive one performance-wise. Monoclinic NiP 2 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 XRD and HREM measurements it was shown that during the first discharge the cubic phase undergoes a pure conversion process (NiP 2 + 6 Li + + 6e - Ni° + 2Li 3 P) as opposed to a sequential insertionconversion process for monoclinic NiP 2 . Such a different behaviour 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 NiP 2 electrodes with enhanced capacity retention and rate capabilities. It consists in growing the monoclinic NiP 2 phase, through a vapour phase transport process, on a commercial Nifoam commonly used in Ni-based alkaline batteries. These new self-supported electrodes, based on chemically made interfaces, offer new opportunities to fully exploit the capacity gains provided by conversion reactions.

Metal phosphides as potential thermoelectric materials

“…6(b)). This is consistent with the Ni-P (2.29 Å) and P-P (2.19 Å) bond lengths, which are both smaller than the sum of the covalent radii (P: 1.10 Å; Ni: 1.23 Å), 97 indicating, together with the small charge transfer between Ni and P, a weak interaction and low-frequency modes. These modes could lower both the frequency of the acoustic phonons and k min .…”

Beyond Intercalation‐Based Li‐Ion Batteries: The State of the Art and Challenges of Electrode Materials Reacting Through Conversion Reactions

“…Both the cubic and the monoclinic polymorphs convert to Li 3 P and Ni, with a intermediate step of insertion for the latter, [ 235 ] upon discharge to 0 V, producing a total of 1000-1300 mAh g − 1 . [ 234 ] Fair reversibility is observed upon the fi rst charge, and the capacity in powder electrodes decays noticeably within the fi rst 10 cycles, even at low rates. In an attempt to solve this issue, monoclinic NiP 2 was grown on Ni foam, which also acts as a current collector, through a vapour-phase procedure.…”

“…In an attempt to solve this issue, monoclinic NiP 2 was grown on Ni foam, which also acts as a current collector, through a vapour-phase procedure. [ 234 ] The strategy is rather successful; not only was the retention ameliorated, but these integrated electrodes showed remarkably enhanced rate capabilities.…”

Key challenges in future Li-battery research

“…. ), the reversible uptake of two (CoO) (Poizot et al 2000) to six Li + (NiP 2 ) (Gillot et al 2005) per 3d transition metal at potentials that depend on the iconicity of the M-X bond, with the highest redox potential for the highest electronegative anion (e.g. F−).…”