Self‐Assembled Ultrathin Anatase TiO2 Nanosheets with Reactive (001) Facets for Highly Enhanced Reversible Li Storage

Abstract

Nanostructued TiO2 polymorphs have attracted attention as promising materials for photocatalysis, solar cells, photochromic devices, gas sensors, surface coatings and energy-storage applications, particularly for Li-ion batteries (LIBs), which are well-known electrochemical energy-storage systems that have dominated in many miniature electronic appliances. Moreover, various polymorphs of TiO2, such as anatase, rutile and monoclinic phases, have been widely investigated as prospective anode materials for LIBs because of their good rate capability and the absence of lithium platting issues during highcurrent operation, and because solid electrolyte interfaces (SEIs) are unlikely to be formed, compared to graphitic anodes. However, their poor reversibility (0.5 moles of Li), higher operating potential and lower theoretical capacity (335 mAhg ) compared to graphitic anodes (372 mAhg) is a drawback of titanate anodes. Tuning the structural (orientation of the crystals) and morphological aspects of anatase phase is reported to be a noteworthy approach to realize its theoretical capacity with high reversibility. Apart from the synthesis technique, there are several attempts, such as carbon coating, composite with carbonaceous materials, metal doping and integrated structure with other transition metal oxides, that are reported to increase the reversibility beyond 0.5 moles of Li per formula unit. 11–13] However, the aforementioned approaches provide only a marginal improvement in the reversibility. Another approach to enhance the reversibility without significantly affecting the volumetric capacity is the utilization of high-surface-energy facets. In the case of the anatase phase, the (001) facets exhibit a high reactivity because of their higher surface energy (0.90 Jm ) compared to the (100) facets (0.44 Jm ) and the thermodynamically more stable (101) facets (0.53 Jm ). This clearly shows that Li diffusion is more favored along the (001) direction rather than along the (101) direction, and the Li diffusion coefficients are approximately found to be (2.0 0.8) 10 13 and (7 2) 10 14 cms , respectively, as reported by Hengerer et al. Therefore, research is devoted to engineering the utilization of high-energy (001) facets for prospective LIB anodes with high reversibility and morphological features. However, few reports are available on the utilization of such high-energy facets for LIB anodes; for example, Sun et al. reported an initial discharge capacity of ~200 mAhg 1 for (001) facets at a current density of 167 mAg 1 with severe capacity fading. Chen et al. reported the assembly of hierarchical microspheres consisting of (001) facets and delivered a first discharge capacity of ~204 mAhg 1 at a current density of 170 mAg 1 with capacity fading. Hence, there is a continuing effort anticipated for highly exposed (001) facets to enhance the Li reversibility during electrochemical cycling, since the observed values from the literature are more or less the same as that of conventional nanostructured anatase phase (~0.5 moles of Li). In this line, we made an attempt to employ highly exposed (001) facets of anatase phase TiO2 nanosheets by a conventional hydrothermal approach and subsequently assembled them together as a stack to increase the Li-ion reversibility. A half-cell assembly is conducted to evaluate the Li-storage properties of self-assembled and asprepared (non-assembled) TiO2 nanosheets of (001) facets ; these are described in detail.