Improved Compatibility of α-NaMnO₂ Cathodes at the Interface with Ionic Liquid Electrolytes

The behaviour and compatibility of monoclinic sodium manganite, α-NaMnO₂, cathodes at the interface with electrolytes based on the 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIFSI) and N-trimethyl-N-butylammonium bis(fluorosulfonyl)imide (N₁₁₁₄FSI) ionic liquids is presented and discussed. The Na⁺ insertion process was analysed through cyclic voltammetry tests combined with impedance spectroscopy measurements and the cell performance was tested by charge-discharge cycles. XPS and FIB-SEM measurements allowed analysis of the surface composition and the morphology of post-mortem cathodes. Overall, the α-NaMnO₂ cathode showed high reversibility in N₁₁₁₄FSI-based electrolyte, delivering 60 % of the initial capacity after 1200 cycles in conjunction with a Coulombic efficiency above 99 %. To our knowledge, these very promising results are the best result obtained till now for monolithic α-NaMnO₂ cathodes, are ascribable to the formation of a stable passive layer onto the electrode surface, as confirmed by spectroscopic analysis.