Table 2: Test parameters of the experiment.

Voltage (V) Electrode spacing (cm) Potential gradient (V/cm) Concentration of CaCl2 (mol/L) Total amount of solution (mL) Charging time (h)
T 1 20 20 1 2 80 20
T 2 20 20 1 2 80 30
T 3 20 20 1 2 80 40