Lithium copper(I) orthophosphates Li_3-xCu_xPO₄: synthesis, crystal structures, and electrochemical properties

Along the quasi-binary section Li₃PO₄ - Cu^I₃PO₄ three different phases Li_3-xCu^I_xPO₄ each with extended homogeneity range occur under equilibrium conditions (650 ≤ v ≤ 700 C). According to single-crystal X-ray structure analyses Phase 1 (0 < x ≤ 0.7) adopts the HT- or β-Li₃PO₄ structure type [Li_2.6Cu^I_0.4PO₄, Pnma (no. 62), Z = 4, a = 10.4612(2) Aa, b = 6.1690(3) Aa, c = 4.9854(2) Aa, R₁ = 0.023, wR₂ = 0.062, Goof = 1.12] and Phase 2 (0.9 ≤ x ≤ 1.8) is isotypic to LT- or α-Li₃PO₄ [Li_2.05Cu^I_0.95PO₄, Pnm2₁ (no. 31), Z = 2, a = 6.2113(8) Aa, b = 5.2597(7) Aa, c = 4.9904(5) Aa, R₁ = 0.040, wR₂ = 0.108, Goof = 0.98]. A preliminary structure model for the copper-rich Phase 3 (2.1 ≤ x ≤ 2.8) [''Li_0.6Cu^I_2.4PO₄'', P anti 3 (no. 147), a = 6.223(1) Aa, c = 5.3629(5) Aa] could be refined to R₁ = 0.07. Sharp ³¹P-MAS-NMR resonances observed in the spectra of Li_2.6Cu^I_0.4PO₄ (δ_iso = 10.4 ppm), Li_2.05Cu^I_0.95PO₄ (δ_iso = 12.4 ppm), and Li_0.84Cu^I_2.16PO₄ (δ_iso = 10.9 ppm) provide evidence for the absence of paramagnetic Cu²⁺ ions. Pure copper(I) orthophosphate Cu^I₃(PO₄) exists as a homogeneous melt (v ≥ 800 C) and can be obtained as thermodynamically metastable solid by quenching. It is isotypic to Phase 3 [a = 6.284(3) Aa, c = 5.408(5) Aa]. Electrochemical delithiation of Li_2.05Cu^I_0.95PO₄ (C/10, C/30) indicates two partially reversible oxidation processes between 3.75 V and 4.80 V (vs. Li⁰/Li⁺). (Copyright copyright 2014 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)