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

• Published in: ChemElectroChem Vol. 5; no. 8; pp. 1219 - 1229
• Main Authors: Bhardwaj, Hem Shruti, Ramireddy, Thrinathreddy, Pradeep, Anagha, Jangid, Manoj K., Srihari, Velaga, Poswal, Himanshu K., Mukhopadhyay, Amartya
• Format: Journal Article
• Language: English
• Published: Weinheim John Wiley & Sons, Inc 01.04.2018
• Subjects: Anodes; Batteries; Charge transfer; Correlation analysis; electrochemical behavior; Electrode materials; Electrolytic cells; in situ synchrotron XRD; Multi wall carbon nanotubes; Na2Ti3O7; Na2Ti3O7-MWCNT composites; sodium-ion batteries; Stability
• ISSN: 2196-0216; 2196-0216
• DOI: 10.1002/celc.201701276

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. The real story: Continuous formation of insulating/passivating (impurity) phase(s)/layer(s) owing to reaction with the electrolyte, causing significant rise in the impedance (upon cell assembly and during electrochemical cycling), and not (in)stability/(ir)reversibility of the fully sodiated phase (i. e. Na4Ti3O7), is the major cause for the rapid capacity fade of Na2Ti3O7‐based electrodes. Such issues can be considerably suppressed (and cyclic stability improved) in the presence of stable conducting network (say, with carbon nanotubes) that maintain inter‐particle connectivity during electrochemical cycling.