The equations show the standard electrode potentials of the half-cells used in a sodium-ion battery:
Na++e−⇌NaE⊖=−2.71 V \text{Na}^+ + \text{e}^- \rightleftharpoons \text{Na} \quad E^{\ominus} = -2.71\text{ V} Na++e−⇌NaE⊖=−2.71 V CoO2+Na++e−⇌NaCoO2E⊖=+0.68 V \text{CoO}_2 + \text{Na}^+ + \text{e}^- \rightleftharpoons \text{NaCoO}_2 \quad E^{\ominus} = +0.68\text{ V} CoO2+Na++e−⇌NaCoO2E⊖=+0.68 VWhich statement is correct for this cell during discharge?
The standard cell potential is 2.03 V2.03\text{ V}2.03 V.
The overall cell reaction is Na+CoO2→NaCoO2\text{Na} + \text{CoO}_2 \rightarrow \text{NaCoO}_2Na+CoO2→NaCoO2.
The reaction at the positive electrode is NaCoO2→CoO2+Na++e−\text{NaCoO}_2 \rightarrow \text{CoO}_2 + \text{Na}^+ + \text{e}^-NaCoO2→CoO2+Na++e−.
The oxidation state of cobalt changes from +3+3+3 to +4+4+4.