- A cell was charged or discharged at 30 mA g-1 rate for 20 min, followed by a 3 h open circuit step to allow relaxation back to equilibrium (defined as dE/dt < 0.2 mV h-1).
- The procedure was continued until the charge (or discharge) voltage reached 1.8 V (0.2 V).
- The Zn2+ ion diffusion coefficients could be calculated using the following equation first outlined by Weppner and Huggins:
where, I is the current (A);
Vm is the molar volume of the ZVO (cm3 mol-1);
ZA is the charge number;
F is the Faraday’s constant (96485 C mol-1);
S is the electrode/electrolyte contact area (cm2);
dE/dδ is the slope of the coulometric titration curve, found by plotting the steady state
voltages E (V) measured after each titration step δ;
dE/d√t is the slope of the linearized plot of the potential E (V) during the current pulse of duration t (s).
If sufficiently small currents are applied for short time intervals, so that dE/d√t can be considered linear and the coulometric titration curve can be also considered linear over the composition range involved in that step, the above equation can be simplified into:
Here, τ is the duration of the current pulse (s);
nm is the number of moles (mol);
Vm is the molar volume of the electrode (cm3 mol-1);
S is the electrode/electrolyte contact area (cm2);
ΔEs is the steady-state voltage change, due to the current pulse and ΔEt is the voltage change during the constant current pulse, eliminating the iR drop.
