The emerging technology of sodium ion batteries has attracted a great deal of attention due to the abundant availability and cost efficiency of the concerned raw materials. The technology still needs improvements, primarily in terms of finding suitable anode material with superior Na-capacity, cyclic stability and also rate capability. This is mainly caused by graphitic carbon not being able to intercalate sodium, unlike in the case of lithium. Layered Na₂Ti₃O₇, because of its low potential against Na/Na⁺and decent theoretical Na-capacity (~200 mAh/g), has emerged as a promising anode material among other Na-based intercalation compounds [1]. However, in addition to its poor rate capability owing to inherently low electronic conductivity in pristine form, its cyclic stability is also far from being satisfactory. Poor structural stability during sodiation/desodiation is one of the reasons for the same.

Against this backdrop, the present work focuses on improving the overall electrochemical behaviour, not only by reducing size of the Na₂Ti₃O₇ particles to nano-sized regime, but also by incorporating conducting reinforcements like multi-walled carbon nanotubes (MWCNTs). However, the challenges associated with synthesizing active materials having nanoscaled dimensions via facile and cost-effective wet-chemical synthesis routes, while at the same time achieving intimate contact with carbon nanotubes usually pose practical hindrances towards improving rate capability and cyclic stability of promising ceramic electrode materials. Developing MWCNTs-incorporated composite electrode slurry via the usual physical mixing route may not allow intimate contact; rather may lead to contamination. Growth of MWCNTs on electrode-active particles via vapour deposition is not a cost-effective and scalable process. These disadvantages have been overcome in our work by developing an innovative, but facile sol-gel based route, which helped in synthesizing Na₂Ti₃O₇ nanoparticles with near-perfect control over stoichiometry and also in intimate contact with MWCNTs. In more precise terms, the sol-gel based route has been tuned for successfully developing Na₂Ti₃O₇ nanoparticles with MWCNTs uniformly wrapped around them allowing intimate contact with the MWCNTs, as was clearly evident from the high-resolution TEM images. The results once again highlight the success of this route based on direct addition of MWCNTs into the ‘matrix’ sol, followed by rapid gelation and subsequent crystallization in presence of MWCNTs (for Na-ion battery electrode), as recently demonstrated also for LiFePO₄-MWCNT electrodes (for Li-ion batteries) [2].

The specific Na-capacities, as recorded during galvanostatic cycling, were considerably higher upon incorporation of the MWCNTs, as compared to those with the active particles of the same size/composition, prepared using the same route, but sans MWCNTs. More importantly, the cyclic stability improved significantly in the presence of the MWCNTs. The differences were more notable at the higher current densities, signifying considerably improved rate capability. In more specific terms, for over a period of 100 discharge/charge cycles, the capacity retention for Na₂Ti₃O₇/MWCNT was superior to that for Na₂Ti₃O₇, sans MWCNTs, by a factor of ~16. Another important observation has been the significant reduction in voltage hysteresis in presence of the MWCNTs. In addition to increasing the overall electrical conductivity, the anchoring/buffering action provided by the MWCNTs against the high volume changes during phase transformation reactions, might possibly be some of the reasons for the higher capacity and improved cyclic stability in the case of MWCNTs-containing Na₂Ti₃O₇ nanoparticles. The work, to be presented, also includes in-situ synchrotron X-Ray diffraction studies for obtaining better insights into some of the fundamental aspects concerning phase transformations during charge/discharge at such ultra-fine dimension, in the presence/absence of MWCNTs (in intimate contact) and also in the presence of impurity phases (as deliberately introduced). The results provide clarity on the sodiation/de-sodiation mechanisms and correlations between the electrochemical performances, phase assemblage and phase evolutions of Na₂Ti₃O₇ and Na₂Ti₃O₇/MWCNT anodes for Na-ion batteries.

Keywords: Na₂Ti₃O₇/MWCNT, Na-ion batteries, Cyclic stability, In-situ synchrotron X-Ray diffraction

References:

1. S. W. Kim, D. H. Seo, X. Ma, G. Ceder, K. Kang, Advanced Energy Materials, 2 (2012) 710-721.
2. M. K. Satam, R. Natarajan, S. Kobi, M. K. Jangid, Y. Krishnan, A. Mukhopadhyay, Scripta Materialia, 124 (2016) 1–5.