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Na2Ti3O7 (NTO) is a potential anode material with low discharge voltage for room‐temperature sodium‐ion batteries. In this work, NTO nanotubes were prepared by a hydrothermal method. XRD, SEM, TEM, and HRTEM studies are performed to investigate the composition, morphology, and structure. As anodes for sodium‐ion batteries, NTO nanotubes show a reversible capacity of 126.2 mAh g−1 at 100 mA g−1. The discharge capacity can still reach 109 mAh g−1 even for 2000 cycles. Solid‐state sodium‐ion batteries containing NTO‐nanotube anodes and an Na3Zr2Si2PO12 solid electrolyte display a comparable performance to batteries with a liquid electrolyte. Moreover, the prepared solid‐state sodium battery is also demonstrated to store mechanical energy harvested by triboelectric nanogenerators (TENGs). The discharge capacity reaches 121 mAh g−1 at 5 mA g−1. [ABSTRACT FROM AUTHOR]

Abstract: 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 in situ 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 in situ 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. [ABSTRACT FROM AUTHOR]