Spray-drying synthesis of Li₄Ti₅O₁₂ microspheres in pilot scale using TiO₂ nanosheets as starting materials and their application in high-rate lithium ion battery

Highlights

• TiO₂ with sheet-like structure has been prepared as the starting material.

• LTO microspheres with pure phase have been prepared by spray-drying.

• The reactivity of LTO precursor has been enhanced by grinding treatment.

• Excellent rate performance along with superior cycling life is achieved.

Abstract

Spray-drying technique has been widely used for synthesis of energy storage materials due to its low cost and easy scale up. However, in mass production, this method usually suffers from the incomplete solid-state reaction owing to the aggregation or poor reactivity of precursors caused by their large particle size and unfavorable morphology. In this study, spinel Li₄Ti₅O₁₂ (LTO) has been synthesized by using TiO₂ nanosheets as precursor through spray-drying for large-scale production. The TiO₂ nanosheets are prepared via a facile and scalable wet grinding method. The high aspect ratio TiO₂ nanosheets can efficiently reduce the diffusion length of Li element during the solid-state reaction leading to higher reactivity. It has been found that the temperature required for the formation of LTO phase can be significantly reduced by using the two-dimensional (2D) TiO₂ nanosheets as starting materials. As a result, through a pilot-scale spray drying process, the LTO reacted from the TiO₂ nanosheets shows a pure spinel structure due to the better morphology of TiO₂ nanosheets. In contrast, using the unprocessed TiO₂ as precursor, the resulting LTO still reveals other impure phases leading to a poor electrochemical performance. The pure LTO shows a higher discharge capacity of ∼160.8 mAh/g at 0.1 C, with an excellent rate performance and superior cycling life in comparison with the LTO containing impurity phases.

Introduction

In past decades, the world is facing a serious environmental crisis due to the fossil fuel consumption and global warming. With the awareness of environmental protection, the technological developments for pure electric vehicles (EVs)/hybrid electric vehicles (HEVs) with low emissions have become increasingly important [[1], [2], [3]]. Lithium-ion batteries (LIBs) generally have high energy and power densities which are adapted for applications in HEVs/EVs. Unfortunately, the conventional graphite anodes cannot fulfill the demands of large-scale energy-storage devices due to its low Li-intercalation potential voltage (almost 0.1 V vs Li⁺/Li) and a poor kinetics of Li-ion diffusion rate. In recent years, LTO with a spinel structure has drawn much attention as an appealing anode material. Compared with the traditional graphite-based anode materials, LTO can deliver a flat charge/discharge plateau at a relatively high potential voltage of 1.55 V (vs Li⁺/Li) [[4], [5], [6]]. Moreover, LTO exhibits excellent cycling life and thermal stability due to its nearly zero volume change during the Li-ion insertion/extraction process and no formation of solid-electrolyte interphase layer [[7], [8], [9]]. These properties enable spinel LTO to meet the requirements for application in EVs.

LTO is convectional prepared by sol–gel [10,11], hydrothermal [[12], [13], [14]] and co-precipitation approaches [15,16]. These methods are usually attempted to homogeneously mix Ti and Li in atomic or molecular level and then calcine or age the precursor in elevated temperature to synthesize the nanocrystalline LTO. The resultant LTO nanoparticles can show excellent electrochemical properties due to the shorten Li-ion diffusion distance resulted from their nanostructure. However, the preparation procedure of these methods is time-consuming and difficult to scale up. The nano-sized LTO prepared from these liquid methods also exhibits relatively low volumetric energy density and difficulties in coating process for electrode film [17]. Furthermore, the nano-sized LTO generally adsorb much moisture due to its large specific surface area resulting in the serious gas swelling and poor cell lifetime for the application of LIBs [18,19].

The spray-drying technique has been widely applied in the field of chemical engineering for preparation of functional materials or composites in submicro- and micro-scale because of its simple apparatus, easy operation and mass production. In addition, the foreign elements can be easily added in the synthesis process through the spray drying in a single step within a short time [[20], [21], [22]]. The spray drying also can enhance the contact between the stating materials and facilitate the solid-state reaction during the post-thermal annealing. However, it is hard to obtain a single phase LTO without other impure phases such as TiO₂ and Li₂TiO₃ by spray drying due to the poor dispersion, insufficient mixing and too large particle size of the starting precursors [[23], [24], [25], [26], [27]]. The unreacted TiO₂ or secondary phase of Li₂TiO₃ can act as a resistance component leading to lower charge capacity and rate performance [28]. Moreover, the existence of unreacted TiO₂ means that there is also unreacted Li₂CO₃ within the LTO. It has been reported that the Li₂CO₃ can be chemically decomposed to LiF as a solid product, CO₂ and POF₃ as gaseous species during the charge/discharge process [29,30]. These gaseous species would cause serious damage for the cycle life of LIBs. To overcome the above problems, some effort has been made to synthesize pure LTO by controlling the Li and Ti ratios in the range of 0.800–0.900 or changing the parameters of thermal annealing through the spray-drying method [31]. It was found that pure LTO phase can be obtained with the starting Li/Ti ratio higher than 0.86. However, the higher Li/Ti ratio in the starting precursor indicates some unreacted Li-containing materials within the final product. This would increase the irreversible capacity in the first charge-discharge cycle and deliver some gaseous species [29,30]. Moreover, the requirement of more rigorous annealing conditions such as higher temperature or longer time also raise the production cost.

In this study, we have tried to prepare 2D TiO₂ with sheet-like structure through a simple high-energy grinding process. The TiO₂ particles experience miniaturization and deformation during the grinding process leading to the morphological change from roughly spherical to sheet-like structure. The 2D-TiO₂ reveals much higher surface area compared with the bare TiO₂, which can provide much interface between TiO₂ and Li₂CO₃. Besides, the thin thickness of the 2D-TiO₂ also can reduce the diffusion length of Li element during the post-thermal annealing. The dispersability and chemical composition of the slurry also can be enhanced and mixed much well after the grinding treatment. All the effects are favorable for the solid-state reaction between TiO₂ and Li₂CO₃ leading to better reactivity. Owing to the grinding treatment, we can get the starting materials of spherical LTO precursor, in which the 2D-TiO₂ and Li₂CO₃ are homogeneously mixed by spray-drying in large-scale production. After that, we can obtain the well-crystallized and highly pure LTO without prolonging heating time or elevating the calcination temperature.