battery like electrodes means, transition metal oxides. These metal oxides are also active as electrocatalyst for oxygen evolution. So when you go towards positive potential during charging, at some points, the electrolyte starts decomposing and oxygen evolution starts. for each oxygen evolution, one electron is released, which balances the electron deficiency at positive positive potential. so, to reach the final potential it takes more time. but during returning back to the initial potential during discharging, no such reaction happens and as a result, the discharge time decreases and consequently the coulombic efficiency also decreases.

Thank you very much for this simple explanation....