P-Redox Mechanism at the Origin of the High Lithium Storage in NiP2-Based Batteries

The Li reactivity of NiP2 is investigated by means of electrochemical tests, in situ XRD, and T-31 NMR characterizations as well as first principles DFT calculations. A two-step insertion/conversion reaction is shown to transform the NiP2 starting electrode into an intermediate Li2NiP2 single phase and then to convert into the Li3P/Ni degrees nanocomposite. The ternary phase is fully characterized and is shown to be structurally very close to the starting NiP2 regarding the Ni ions environment. This demonstrates that its formation results from a P-redox insertion mechanism associated with a very good reversibility. However, its nucleation upon delithiation from the fully converted Li3P/Ni composite is shown to be kinetically limited (poor structural relationship) which strongly suggests that restricted lithiation is required for best cycleability of the NiP2/Li cell.