Supercapacitive properties of MnO2 and underlying kinetics by distribution of relaxation time method

Charging and discharging processes in supercapacitors and secondary batteries are critically dependent on elementary rate determining sub-processes (ERDSPs) with different relaxation time (τ) at different frequencies. In this work, distribution of relaxation time (DRT) method is employed to investigate the ERDSPs and relevant kinetics of electrodeposited MnO2 electrodes. Two ERDSPs are identified: ion diffusion at low frequencies (100-0.1Hz), and the mechanism of ERDSP at high frequencies (105-102 Hz) is found to be charge transfer across the MnO2/electrolyte interface. The rate capability is found to reduce with the increasing electrode thickness (l), which is attributed to the increasing relaxation time of diffusion (τd), proportional to l2. Oxygen vacancies (VO••) can lead to an increased specific capacitance. Meanwhile, VO•• with positive effective charge may also lead to repulsion to cations, and thus increase the activation energy (Ea) of charge transfer (Ea-ct) and diffusion (Ea-diff) processes. Enhancing cycling stability with reducing Mn3+ and VO•• concentration may result from reducing Mn element loss due to disproportionation reaction of Mn3+, and alleviating polarization and increasing reversibility due to reducing Ea-ct and Ea-diff.