Features of carbon-based reinforcements influencing the electrochemical behaviour of bi-phase Na-titanate based anode materials for Na-ion batteries

Na-titanate is a class of potentially ‘safe’ anode material for Na-ion batteries, with the combination of Na2Ti3O7 and Na2Ti6O13, as ‘Bi-phase NTO’, often performing better than the single-phase counterparts. Nevertheless, good coverage and intimate contact of the Na-titanate particles with conducting reinforcements is needed for achieving superior electrochemical performance by suppressing deleterious reactions with electrolyte and providing connectivity across particles. Against this backdrop, the present manuscript reports on the effects of dimensionality, functional groups and conductivity of carbon-based reinforcements [viz., pristine and functionalized multi-walled carbon nanotubes (MWCNTs), reduced graphene oxide, carbon black] towards the performances of ‘Bi-phase NTO’/carbon-based electrodes. High aspect ratio, along with the presence of functional groups, of functionalized MWCNTs results in complete coverage/‘wrapping’ of Na-titanate surface; thus, leading to the best improvements of Na-storage capacity and cyclic stability of ‘Bi-phase NTO’, along with negligible increment in impedance over electrochemical sodiation/desodiation cycles. Nevertheless, higher overall electronic conductivity bestowed by the less defected and less functional group containing pristine MWCNT leads to the best rate-capability. However, carbon black, lacking in terms of both aspect ratio and presence of functional groups, and leading to a very poor coverage of the Na-titanate surface, does not improve the electrochemical performances of ‘Bi-phase NTO’.