Boosting the Power of Na0.44MnO2: Unlocking Its Potential for Aqueous Sodium-Ion Storage through Nanostructuring and Hybridization

We report an effective processing route, combining nanostructure formation and hybridization, to improve the rate performance of the tunnel-structure sodium manganese oxide Na0.44MnO2 (NMO) as a cathode material for aqueous sodium ion storage. We use hydrothermal synthesis to prepare an NMO/CNF (Carbon NanoFiber) hybrid, consisting of uniform oxide nanowires with an average width of 70 nm and length in the range of several tenths of µm. The highly dispersed CNFs impart high conductivity to the NMO/CNF electrode, allowing high-rate performance at a C-rate of up to 20 C, with a delivered capacity of more than half the theoretical value in a 1 M Na2SO4 electrolyte. Moreover, the NMO/CNF hybrid shows good electrochemical stability under several hundred cycles at a high C-rate. However, the NMO nanowire electrodes reveal a lower-than-expected capacity, probably as a result of the tendency of nanowires to form bundles, which prevents direct contact with conductive fibers and induce the under-utilization of active material. With this study, we demonstrate a strong improvement of the otherwise inherently low-rate performance of NMO through oxide nanostructuring and hybridization with carbon fibers, paving the way for further research on NMO-based materials for aqueous sodium ion storage

Boosting the Power of Na_0.44MnO₂: Unlocking Its Potential for Aqueous Sodium-Ion Storage through Nanostructuring and Hybridization

We report an effective processing route, combining nanostructure formation and hybridization, to improve the rate performance of the tunnel-structure sodium manganese oxide Na0.44MnO2 (NMO) as a cathode material for aqueous sodium ion storage. We use hydrothermal synthesis to prepare an NMO/CNF (Carbon NanoFiber) hybrid, consisting of uniform oxide nanowires with an average width of 70 nm and length in the range of several tenths of µm. The highly dispersed CNFs impart high conductivity to the NMO/CNF electrode, allowing high-rate performance at a C-rate of up to 20 C, with a delivered capacity of more than half the theoretical value in a 1 M Na2SO4 electrolyte. Moreover, the NMO/CNF hybrid shows good electrochemical stability under several hundred cycles at a high C-rate. However, the NMO nanowire electrodes reveal a lower-than-expected capacity, probably as a result of the tendency of nanowires to form bundles, which prevents direct contact with conductive fibers and induce the under-utilization of active material. With this study, we demonstrate a strong improvement of the otherwise inherently low-rate performance of NMO through oxide nanostructuring and hybridization with carbon fibers, paving the way for further research on NMO-based materials for aqueous sodium ion storage

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