Thermodynamic
Aspects of Copper Oxides Used as Electrodes for Lithium Ion Batteries
The
overall driving force across an electrochemical cell is determined by the
change in the standard Gibbs free energy from the cell reaction as defined by
the Nernst equation. Evaluated thermodynamic descriptions of multi component
systems containing the Gibbs free energy functions of all phases can be used to
calculate equilibrium cell potentials and plateau capacities resulting from
phase transformations during charge/discharge.
The
Li-Cu-Fe-O quaternary system is of interest for lithium ion batteries because 3
d transition metal oxides in this system can be used as anode materials based
on the conversion mechanism. However, since there is no available description
for the Li-Cu-O ternary sub-system the aim of this work is to create a
consistent thermodynamic dataset for this system valid at room temperature. With this dataset, titration curves were
calculated.
To
generate additional experimental data to describe the temperature dependence of
the ternary compounds in the Li-Cu-O system, differential thermal analysis and
calorimetry were performed. The oxide samples were prepared using solid-state
reaction method from copper oxides and lithium carbonate. The heat capacities
for these compounds were measured in the temperature range from -40 °C to 200
°C and the phase stabilities under air and argon were determined. Additionaly the
standard enthalpies of formation of the oxides were measured using high
temperature drop solution calorimetry at 700 °C.
To test
the battery performance, CuO was used as active material and coin cells of the
type CR2032 were assembled. Electrochemical measurements with different C-rates
as well as open circuit voltage measurements were conducted.
The
experimental results will be discussed in view of the thermodynamic theory.