Lithium Copper(I) Orthophosphates Li3-xCuxPO4: Synthesis, Crystal Structures, and Electrochemical Properties

Along the quasi-binary section Li3PO4 - (Cu3PO4)-P-I three different phases (Li3-xCuxPO4)-P-I each with extended homogeneity range occur under equilibrium conditions (650 <= theta ; 700 degrees C). According to single-crystal X-ray structure analyses Phase 1 (0 < x <= 0.7) adopts the HT- or beta-Li3PO4 structure type [(Li2.6Cu0.4PO4)-P-I, Pnma (no. 62), Z = 4, a = 10.4612(2) angstrom, b = 6.1690(3) angstrom, c = 4.9854(2) angstrom, R-1 = 0.023, wR(2) = 0.062, Goof = 1.12] and Phase 2 (0.9 <= x <= 1.8) is isotypic to LT- or alpha-Li3PO4 [(Li2.05Cu0.95PO4)-P-I, Pnm2(1) (no. 31), Z = 2, a = 6.2113(8) angstrom, b = 5.2597(7) angstrom, c = 4.9904(5) angstrom, R-1 = 0.040, wR(2) = 0.108, Goof = 0.98]. A preliminary structure model for the copper-rich Phase 3 (2.1 <= x <= 2.8) [(Li0.6Cu2.4PO4)-P-I, P (3) over bar (no. 147), a = 6.223(1) angstrom, c = 5.3629(5) angstrom] could be refined to R-1 = 0.07. Sharp P-31-MAS-NMR resonances observed in the spectra of (Li2.6Cu0.4PO4)-P-I (delta(iso) = 10.4 ppm), (Li2.05Cu0.95PO4)-P-I (delta(iso) = 12.4 ppm), and (Li0.84Cu2.16PO4)-P-I (delta(iso) = 10.9 ppm) provide evidence for the absence of paramagnetic Cu2+ ions. Pure copper(I) orthophosphate Cu-3(I)(PO4) exists as a homogeneous melt (theta >= 800 degrees C) and can be obtained as thermodynamically metastable solid by quenching. It is isotypic to Phase 3 [a = 6.284(3) angstrom, c = 5.408(5) angstrom]. Electrochemical delithiation of (Li2.05Cu0.95PO4)-P-I (C/10, C/30) indicates two partially reversible oxidation processes between 3.75 V and 4.80 V (vs. Li-0/Li+).