Morphology/Properties Correlations in TiO2 Nanostructured Material As Negative Electrodes in Sodium Ion Batteries
Authors: Ruffo Riccardo , Longoni Gianluca , Mari Claudio Maria , Pena Cabrera Rosita Lisetta , Darienzo Massimiliano 
Published: February 2020

Lithium ion batteries play a predominant role in the market of power sources for cordless devices with a production higher than 100 million cells/month and about 1500 ton/month of electrode materials. These figures may increase in the future  due to the implementation of such batteries in automotive application which implies the increasing of  lithium raw material (Li2CO3) consumption. Nowadays, the availability of Li2CO3 is restricted to few countries and lithium may become a strategic material in the near future with the booming of its cost and the raising of geopolitical issues. For this reasons Na-ion batteries (SIBs) are getting increasing attention thanks to the higher availability of raw materials and the possibility to drive down the energetic demand connected to raw materials extraction and processing. However, the development of a sodium ion based battery technology requires the discovery and the investigation of new electrode materials with reversible Na+intercalation reaction.

Many efforts have already been made in the design and characterization of both anode and cathode materials for SiBs so far. For what concern cathode side, promising results have been achieved in relation to the stability and charge retention of electroactive intercalation materials. Anode materials, conversely, still represent a challenging topic needy to be investigated. Many solutions have been proposed to overcome the intrinsic limits of negative electrode materials, namely the low practical specific charge and the fast degradation of electrode characteristics and several classes of materials have been taken into account. Transposition of graphitic anodes already employed in LIBs to sodium environment has been considered at first glance: nevertheless, since intercalation of the larger sodium ion into graphite sheets structure induces exfoliation, nano-structured hard-carbons have been investigated instead. Sodium alloying with 14th and 15th group elements (Sb, Sn, Ge and In) has been also exploited. Transition metal oxides (MOxwith M=Fe, Co, Cu, Ti), with electrochemical active transition metal ions, have drawn the attention as possible low cost and easy-to-manufacture SIB anode materials. Their reactivity, electrochemical performances, and effects of morphology and structural properties on lithium storage, have been widely analyzed. On the contrary, same type of studies related to reactivity with Na, remain poorly explored. In particular, titanium based materials are emerging as interesting materials due to soft chemistry routes widely employed to prepare different structures and to control the morphology.

Aim of the present contribution is the description of the morphological, structural, and electrochemical properties of four anatase samples showing different particle size and shape. The samples were prepared by hydrothermal reaction in presence of different capping agents in order to control the particle growth and assembling. Sample surface investigation was also performed in order to develop an optimized protocol to further functionalize the surface and obtain Carbon/TiO2 core/shell particles (see inset of Figure 1). Electrodes obtain by such materials show a reversible electrochemical behavior in the potential range 2.0-0.1 V  with a pseudo-plateau at 0.8 V vs. Na+/Na. The specific capacity is about 150 mAh/g at 50 mA/g and decreases increasing the current density. In particular, 130 (cycles 31/60) and 100 (cycles 61/100) mAh/g are obtained at 100, and 500 mA/g, respectively (Figure 1). The good results are also confirmed by the capacity retention since no capacity degradation was observed after 250 cycles.

Figure 1