Fig. 5(a) shows the charge–discharge test for the LTAPO solid
electrolyte cell. The first charge and discharge capacities of the LTAPO cell reach 18 mAh/g and 6
mAh/g, respectively. The irreversible capacity is as high as 12 mAh/g during the first cycle
although it decreases with further cycling, according to decreasing capacity. The difference
between the average charge and discharge potentials (ΔV) is 1.2 V. The decrease in discharge
capacity and increase in ΔV with cycle mean an increase in cell resistance. This is because of the
increased interface resistances of the electrode/electrolyte, which are created by the volume
change of LiMn2O4 with the extraction/insertion of the lith-
ium ions during the electrochemical reaction.