The Phase Diagram CuO/Cr2O3/P2O5
Structure, Thermal and Electrochemical Behavior of Cu5Cr2O2(PO4)4
Erik Fink, Iren Weimann, Jörg Feller; Hochschule für Technik und Wirtschaft (HTW) Dresden
Zdirad Zak; Masaryk University, Brno/Czech Republic
17. Vortragstagung der Fachgruppe Festkörperchemie und Materialforschung
15.-17. September 2014 in Dresden
Intention
• Investigation of the quasi-ternary system CuO/Cr2O3/P2O5 concerning
existence of phases and phase equlibria
• Electrochemical characterization of copper(II) chromium(III) phosphates
concerning open circuit voltage, discharge capacity and discharge
behavior
The structure is formed by layers of units containing one [CuO6] octahedron
(dark green, fig. 3) and four [CuO5] trigonal pyramids (light green). This
layers are connected via corners by two edge-linked [CrO6] octahedra
(blue) and by [PO4] tetrahedra (red).
Experimental
• Preparation of copper(II) chromium(III) phosphates from CuO, Cr2O3
and (NH4)2HPO4 in porcelain crucibles by gradually heating in the
region from 160 to 950 °C for 96 h according to the equat ion:
x CuO + y Cr2O3 + 2 (NH4)2HPO4 → CuxCr2yP2O5+x+3y + 4 NH3 + 3 H2O
• Characterization of the reaction products by XRPD, EDX und DTA/TG
• Construction of primary batteries for electrochemical characterization as
follows: anode Li foil, cathode 70% active material, 25 % graphite, 5 %
PTFE binder, DMC elektrolyte with LiClO4
Results of the Phase Preparation
• Conformation of known copper phosphates and chromium phoshates
• Identification of a new oxide phosphate Cu5Cr2O2(PO4)4
Fig. 3 Cu5Cr2O2(PO4)4 Structure Representation
Thermal Analysis of Cu5Cr2O2(PO4)4
The compound shows the typical decomposition behavior (fig. 4) of copper
oxide phosphates, which decomposes incongruently into more phosphaterich compounds besides copper(I) oxide and oxygen.
2 Cu5Cr2O2(PO4)4 → Cu3Cr4(PO4)6 + Cu2P2O7 + 2.5 Cu2O + 2.5 O2
Fig. 4
Thermal Decomposition of
Cu5Cr2O2(PO4)4
• Determination of the phase diagram CuO/Cr2O3/P2O5 at 950 °C
Phase Diagram at 950 °C
A: Cu0.5CrP2O7
Results of Electrochemical Investigations
B: Cu3Cr4(PO4)6
• The properties like the number of exchangeable electrons, the calculated
capacity by Faraday‘s law, and the measured open circuit voltage of the
new quarternary phase Cu5Cr2O2(PO4)4 in comparison with the used
standard material Cu4O(PO4)2 are summarized in table 2.
Tab. 2 Electrochemical Properties
compound
Exchangeable
electrons
Molar mass
[g/mol]
Theoretical capacity
[mAh/g]
Open circuit
voltage [V]
Cu4O(PO4)2
8
460,13
466,00
2,2
Cu5Cr2O2(PO4)4
16
833,60
514,44
2,1
C: Cu5Cr2O2(PO4)4
Fig. 1 Phase Diagram CuO/Cr2O3/P2O5 at 950 °C
Crystal Structure of Cu5Cr2O2(PO4)4
Single crystals of the new compound were
obtained by a melting procedure starting at
1100 °C and cooling down to 900 °C with
1K/min.
The
crystallograhic
data
for
Cu5Cr2O2(PO4)4
(ICSD
426626)
are
summarized in table 1.
Calculated powder pattern
Measured powder pattern
mm
Tab. 1 Lattice Constants
of Cu5Cr2O2(PO4)4
C 2/m (No. 12)
a = 12.9195(18) Å
b = 6.3530(8) Å
c = 8.2823(12) Å
β = 111.971(4) °
Z
2
Volume
630.42(15) Å3
Density (cal.) 4.392 g/cm3
• The open circuit voltage of the phase Cu5Cr2O2(PO4)4 is similar to the
voltage of Cu4O(PO4)2 (fig. 5).
• The short plateau in the discharge curve of Cu5Cr2O2(PO4)4 indicates the
partial discharge of Cu(II) and not the formation of a mixed potential.
• The effective capacity of Cu5Cr2O2(PO4)4 corresponds to 40 % of the
theoretical value.
Space group
Cell
In figure 2, the comparison
of the measured and
calculated powder pattern
is
represented.
The
additional reflexes in the
measured
pattern
are
caused by impurities of
Cu2P2O7.
Fig. 2 Comparison of Measured and Calculated Powder Pattern
Cu5Cr2O2(PO4)4
Fig. 5
Discharge Curves at 500 µA
Reference
E. Fink: Untersuchungen zur Darstellung von Kupferchromphosphaten sowie Kupferchromvanadaten und deren Charakterisierung, Master thesis,
HTW Dresden (2012)