An Investigation of Na1-XLi2x MnyNizOd Compounds for High Performance Sodium-Ion Batteries
Compounds of the type P2-NaxMO2 are intensively investigated due to their suitability for Na-ion intercalation . However, a drawback of these compounds is the de-stabilization during cycling attributed to layer-gliding and phase transformations (often seen as steps and kinks in GCPL curves) . Recently, C.S Johnson et al. has proposed that introduction of Li-ions in compounds of the type P2-NaLix MnyNizOδ and Na1-xLix MnyNizOδ prvents layer-gliding during cycling, stabilizing the P2 phase (for the former compound) and therefore enabling an improved power and rate performance [2-4]. In the pristine compounds, the transition metals are in the Mn (4+) and Ni (2+) states (implying values of δ > 2.0, i.e oxygen rich materials) . The authors confirmed that during cycling, only the Ni (+2) provides redox activity whereas the manganese remains inactive providing stability to the compounds . In the case of P2-NaLix MnyNizOδ, Li was found to reside on both the alkali and the transition metal layer and the lack of layer gliding was attributed to the presence of a Li2MnO3-like phase where Li remained immobile . In the case of Na1-xLix MnyNizOδ , the stability was attributed to the inter-growth of P2/O3 phases.
We are intrigued by the complex nature of these compounds and the associated stabilizing mechanism during Na-ion insertion/de-insertion processes. We are interested on exploring alternative stoichiometries, particularly those that perhaps will allow stability to a higher upper cut-off voltages enabling a higher energy density while maintaining a high power performance. We believe that besides adequate stoichiometry, the performance of these compounds can be improved by adequate electrolyte formulation. In this work, compounds of the type Na1-xLi2xMnyNizOδ were synthesized using a facile sol-gel technique The electrochemical performance of the compounds was investigated in a two-electrode cell using the metal oxide as working electrode, metallic sodium as pseudo-reference and counter electrodes and a 1 M NaClO4/EC/PC/additive as electrolyte. The main electrochemical techniques used were GCPL and EIS.The morphology and crystal structure were investigated by XRD, SEM and TEM.
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