Understanding the Cyclic (In)stability and the Effects of Presence of a Stable Conducting Network on the Electrochemical Performances of Na2Ti3O7

Despite being a promising anode material for the Na-ion battery system, Na-titanate (viz., Na2Ti3O7) lacks in terms of cyclic stability; the cause(s) for which are under debate. Against this backdrop, through electrochemical measurements and insitu synchrotron X-ray diffraction studies, the present work develops insights into the aspects concerning electrochemical reversibility of the fully sodiated phase (i.e., Na4Ti3O7), possible occurrence of irreversible reactions in Na-ion cells, influences of the same towards cyclic instability, and a strategy towards alleviating this problem. The insitu studies rule out (in)stability/(ir)reversibility of Na4Ti3O7 as being a major cause for the capacity fade; rather they indicate the formation of 'impurity' phase(s) due to reaction with the electrolyte. Incorporation of multi-walled carbon nanotubes (MWCNTs; uniformly 'wrapping' the rod-shaped Na2Ti3O7 particles) significantly improved the cyclic stability (ca. 78% reversible capacity retention after 50 cycles, as compared to ca. 6% without MWCNTs) and rate capability (with nearly flat potential plateaus at 5C). The same suppressed the increase in charge-transfer resistance upon cycling by an order of magnitude and also changed the sodiation reaction from being primarily surface to diffusion controlled. Correlation of the results/analysis indicate that, in the absence of a stable conducting network, loss in electrical connectivity owing to the formation of insulating/passivating (surface) phase(s) is the major cause for capacity fade of Na2Ti3O7