1 research outputs found
Origin of the Voltage Hysteresis in the CoP Conversion Material for Li-Ion Batteries
The electrochemical activity of the CoP conversion electrode
was
investigated through the combination of computational and experimental
techniques. The carbon-free CoP electrode shows better performances
than the carbon-coated electrode, in sharp contrast with the beneficial
role of carbon coating reported in many insertion materials. A two-step
insertion/conversion process associated with the exchange of 3Li is
predicted for this system from the <i>T</i> = 0 K phase
stability diagram performed on bulk structures within the DFT framework.
The voltage hystereses measured for these two processes through a
seven-day relaxation procedure (GITT) are 1 order of magnitude higher
for the conversion process (Δ<i>V</i><sub>conv</sub><sup>exp</sup> = 0.44 V)
than for the insertion process (Δ<i>V</i><sub>ins</sub><sup>exp</sup> = 0.04 V).
The various elementary reactions susceptible to occur at the surface
of the electrode were investigated by means of surface DFT calculations.
This mechanistic study shows that the insertion mechanism is not significantly
affected by the electrode nanosizing (Δ<i>V</i><sub>ins</sub><sup>th</sup> = 0.04 V),
while the conversion reaction does. Asymmetric responses are expected
upon charge and discharge for this system, due to the growth of different
interfaces. This induces different electrochemical equilibriums and
then different voltages in charge and discharge. The hysteresis voltage
computed for the conversion of LiCoP into Li<sub>3</sub>P + Co<sup>0</sup> is again in very good agreement with experiments (Δ<i>V</i><sub>conv</sub><sup>th</sup> = 0.41 V). Such results are very encouraging and open new routes
to the rationalization of the microscopic mechanisms acting as limiting
reactions in electrode materials for Li-ion batteries