Charging processes of Na4Mn9O18 electrode in aqueous electrolyte

Recent trends in electrochemical energy storage –the renewed interest in aqueous electrolytes, the development of nanostructured and/or hybridized materials, the advent of unconventional systems– call for detailed analyses of charging processes. We address this issue in studying a sodium manganese oxide (Na4Mn9O18, NMO) electrode in aqueous environment. Charge storage is examined by cyclic voltammetry (CV) in a wide range of sweep rate (ν) and by equivalent circuit modelling of the electrode impedance response. Voltammetry shows that, with increasing ν, the insertion process evolves from a quasi-equilibrium behavior (ν ≤ 0.1 mV s–1) towards a diffusion controlled regime overlapping with capacitive charging (ν ≥ 0.2 mV s–1), and culminates at even higher rate (ν > 2 mV s–1) in mixed mass transport ohmic control. Impedance analysis permits to discriminate the varying character of charge storage, revealing the low frequency dominance of faradaic insertion and the rising contribution of pseudocapacitive and double layer charging at higher frequency. We show that the frequency decomposition of charging mechanisms obtained by this analysis can be reconciled with the CV analysis. For further clarification of the above analysis in particular, and as a relevant aspect of the NMO behavior in general, we evaluate the chemical diffusion coefficient of Na-ion as a function of potential.