Chemical and Electrochemical Insertion of Lithium into Two Allotropic Varieties of NbPO₅

The chemical and electrochemical insertions of lithium into α-NbPO 5 ( P 4/ n ) and β-NbPO 5 ( P 2 1 / c ) are compared. Both materials were evaluated as positive electrodes in lithium-type rechargeable batteries through galvanostatic, GITT, and PITT cycling modes within various voltage windows. They present distinct behaviors with regard to lithium insertion. β-Li x NbPO 5 rapidly and reversibly uptakes lithium leading to a sustained capacity of 90−120 mA·h/g (i.e., operating to the Nb 5+ /Nb 4+ couple) for more than 100 cycles at ∼2 V vs Li + /Li. This reversibility was confirmed by in situ X-ray diffraction in the range 0 ≤ x ≤ 0.7 that additionally revealed a series of single- and two-phase insertion mechanisms. On the other hand, α-Li x NbPO 5 displays an insertion plateau at ∼1.7 V vs Li + /Li indicative of a two-phase process that was found to be poorly reversible. Both materials, when discharged at voltages lower than 1 V, present irreversible reactions with complete amorphization.