Understanding Sodium Ion Storage and Diffusivity into CNT-TiO2 Hybrid Electrodes
Authors: Martha Surendra Kumar , Ghosh Sourav , Kiran Kumar V , Biswas Sanjay 
Published: February 2020

Sodium ion batteries (NIBs) have attracted much attention of researchers worldwide due to its competence to be used in stationary applications, where cost plays a major role than the size and weight of the battery. Till date, a large variety of materials have been investigated as anodes for sodium ion battery.1 Graphite, the state-of-art of the lithium ion battery anode, can host a negligible amount of sodium during electrochemical intercalation/deintercalation processes.2 Considerable efforts have been made to develop alternative anodes for NIBs such as tin and antimony based alloys3, metal phosphides/oxides/sulphides4, spinel oxides5, and hard carbons6. Hard carbons show a promising sodium ion storage capacity of about 200-400 mAh g-1, but low operating potential of the hard carbon may rise to severe safety issues of dendrite formation and metal plating. Alloy based anodes have very high theoretical capacity between 600 (Na3Sb) to 2600 mAhg-1
(Na3P). Unfortunately, very poor electrochemical performance has been observed for alloy based anodes because of volume changes during sodiation/desodiation which leads to pulverisation. Consequently, an appropriate anode material for NIB is desired which is free from both metal plating and pulverisation.
Titanium dioxide (TiO2)7, which is extensively used as multifunctional material because of its stability, non-toxicity, robustness and physio-chemical properties with the advantage of its moderate cost have also been investigated as anodes for NIBs. Titanium based compounds are very stable and safe, delivers long cycle life of over 1000s of cycles. The 3-dimensional open structure of anatase TiO2 offers possible pathways for Na ion insertion and deinsertion. TiO2 offer capacities close to 335 mAh g-1 due to redox reaction of Ti4+/Ti3+. But, electrochemical performance in terms of capacity and rate capability of TiO2 is limited due to its poor conductivity. Therefore, most of the research in this material is focussed on to improve sluggish kinetics of Na+ ion in TiO2 to gain high capacity and good C rate performance.
To improve the charge storage and kinetics of TiO2 anodes for NIBs, in this work we present a hybrid material of TiO2 nanoparticles (NPs) is incorporated onto multiwall carbon nanotube (CNTs) (Figure 1a). The cycling stability and C-rate performance of CNT-TiO2 hybrid as anodes for NIBs demonstrate two and half times enhancement in electrochemical performance in the presence of CNT. CNT - TiO2 hybrid shows discharge capacity of about 100 mAh g-1 at current density of 1 A g-1 and cycles for 1000 cycles with little loss in capacity. The overall charge storage properties of pristine TiO2 and CNT-TiO2 hybrid is quantified into pseudo capacitive and diffusion-control Na+ intercalation (Figure 1b). Two times increase in Na+ diffusivity in CNT-TiO2 hybrid is achieved in relation to pristine TiO2. Sodium ion full cell based on CNT-TiO2 hybrid and phosphate based cathode delivers capacity of 184 mAh g-1 and cycles very well. The outstanding electrochemical performance is due to the synergetic effect of TiO2 nanoparticles and conducting CNT network which provides efficient charge storage as well Na+ diffusivity. This work not only develops the fundamental understanding of Na+ storage and kinetics of TiO2 electrode but also its practical realisation in sodium ion full cells.

Acknowledgements

SG acknowledges MHRD, VKK acknowledges UGC-NET, Govt. of India, and SKM acknowledges Research Center Imarat (DRDO), Hyderabad under grant no. RCI/CAAT/8151/CARS-358 for the financial support.

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Figure 1