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

Abstract

AbstractAlong the quasi‐binary section Li3PO4 ‐ CuI3PO4 three different phases Li3–xCuIxPO4 each with extended homogeneity range occur under equilibrium conditions (650 ≤ ; ≤ 700 °C). According to single‐crystal X‐ray structure analyses Phase 1 (0 < x ≤ 0.7) adopts the HT‐ or β‐Li3PO4 structure type [Li2.6CuI0.4PO4, Pnma (no. 62), Z = 4, a = 10.4612(2) Å, b = 6.1690(3) Å, c = 4.9854(2) Å, R1 = 0.023, wR2 = 0.062, Goof = 1.12] and Phase 2 (0.9 ≤ x ≤ 1.8) is isotypic to LT‐ or α‐Li3PO4 [Li2.05CuI0.95PO4, Pnm21 (no. 31), Z = 2, a = 6.2113(8) Å, b = 5.2597(7) Å, c = 4.9904(5) Å, R1 = 0.040, wR2 = 0.108, Goof = 0.98]. A preliminary structure model for the copper‐rich Phase 3 (2.1 ≤ x ≤ 2.8) [“Li0.6CuI2.4PO4”, P$\bar{3}$ (no. 147), a = 6.223(1) Å, c = 5.3629(5) Å] could be refined to R1 = 0.07. Sharp 31P‐MAS‐NMR resonances observed in the spectra of Li2.6CuI0.4PO4 (δiso = 10.4 ppm), Li2.05CuI0.95PO4 (δiso = 12.4 ppm), and Li0.84CuI2.16PO4 (δiso = 10.9 ppm) provide evidence for the absence of paramagnetic Cu2+ ions. Pure copper(I) orthophosphate CuI3(PO4) exists as a homogeneous melt ( ≥ 800 °C) and can be obtained as thermodynamically metastable solid by quenching. It is isotypic to Phase 3 [a = 6.284(3) Å, c = 5.408(5) Å]. Electrochemical delithiation of Li2.05CuI0.95PO4 (C/10, C/30) indicates two partially reversible oxidation processes between 3.75 V and 4.80 V (vs. Li0/Li+).