Synthesis, structures and properties of the new lithium cobalt(II) phosphate Li4Co(PO4)2

α-Li4Co(PO4)2 has been synthesized and crystallized by solid-state reactions. The new phosphate crystallizes in the monoclinic system (P21/a, Z=4, a=8.117(3) Å, b=10.303(8) Å, c=8.118(8) Å, β=104.36(8) Å) and is isotypic to α-Li4Zn(PO4)2. The structure of α-Li4Co(PO4)2 has been determined from single-crystal X-ray diffraction data {R1=0.040, wR2=0.135, 2278 unique reflections with Fo>4σ(Fo)}. The crystal structure, which might be regarded as a superstructure of the wurtzite structure type, is build of layers of regular CoO4, PO4 and Li1O4 tetrahedra. Lithium atoms Li2, Li3 and Li4 are located between these layers. Thermal investigations by in-situ XRPD, DTA/TG and quenching experiments suggest decomposition followed by formation and phase transformation of Li4Co(PO4)2:α-Li4Co(PO4)2⟹442°Cβ-Li3PO4+LiCoPO4⇌773°Cβ-Li4Co(PO4)2⟹quenchingto25°Cα-Li4Co(PO4)2According to HT-XRPD at ϑ=850°Cβ-Li4Co(PO4)2 (Pnma, Z=2, 10.3341(8) Å, b=6.5829(5) Å, c=5.0428(3) Å) is isostructural to γ-Li3PO4. The powder reflectance spectrum of α-Li4Co(PO4)2 shows the typical absorption bands for the tetrahedral chromophore [CoIIO4].

Graphical abstractThe complex formation and decomposition behavior of Li4Co(PO4)2 with temperature has been elucidated. The crystal structure of its α-phase was determined from single crystal data, HT-XRPD allowed derivation of a structure model for the β-phase. Both modifications belong to the Li3PO4 structure family.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Li4Co(PO4)2 exhibits complex thermal behavior. ► The new phosphate belongs to the Li3PO4 structure family. ► A single-crystal structure analysis is provided for the metastable α-Li4Co(PO4)2. ► From HT-XRPD data a cation distribution model is developed for β-Li4Co(PO4)2. ► No electrochemical delithiation is observed up to 5 V.