Synthesis of nickel doped anatase titanate as high performance anode materials for lithium ion batteries
Graphical abstract
Introduction
For several decades, there has been a steady increase in the demand for high performance and long-lasting rechargeable batteries for a wide range of applications, from portable electronics to hybrid vehicles [1], [2], [3], [4]. As one of the most widely used energy storage devices, Li-ion batteries (LIBs) have high intrinsic energy density and thus are promising candidates for these applications [5], [6]. However, current LIB technologies are still unable to meet the soaring industrial demand that requires a significant increase of power supply capacity. After a careful analysis of the major components of LIBs and recent studies, it is not difficult to conclude that although electrolyte plays an important role in the performance of LIBs, our choices are indeed limited to electrodes for a remarkable increase of the discharge capacity. As the most critical part of batteries, the performance of electrodes (including both anodes and cathodes) depends directly on their chemical compositions, crystal structure, microstructural, etc.; it is reasonable to say that one of the top priorities for the improvement on LIB performance is the exploration of highly functional electrode materials. After decades of research, quite a few electrode materials have been selected for further investigation. Among them is titania (TiO2) as the anode for LIBs. Titania is a promising anode material because of its high chemical stability, non-toxicity, abundance, and low cost [7], [8], [9], [10]. Among different TiO2 polymorphs, anatase TiO2 is considered as the one of most promising candidates for energy storage due to its easy fabrication, fast Li+ insertion–extraction reactions, and high theoretical insertion capacity [11], [12], [13], [14]. However, in most applications the real capacity of anatase TiO2 is lower than the predicted maximum of 335 mAhg−1 [10], [15] due to the phase transition from tetragonal crystal structure to orthorhombic crystal structure and the resulting 1-D diffusion limitation in the Li-ordered Li0.5TiO2 [16]. To solve this problem, it is essential to circumvent the impediment of pure Li0.5TiO2. Fabricating a novel TiO2-based compound by modifying the anatase TiO2 chemical composition provides a feasible way to overcome this limitation. Doping is most widely employed in attempts of fabricating TiO2-based new compound and its effectiveness has been demonstrated. For instance, there are several papers reporting the fabrication of nickel ion doped TiO2, using the methods of hydrothermal [17], [18], sol–gel [19] and hot solution reaction [20], respectively. However, routine doping processes are mostly complicated and time-consuming. Besides doping, some other efforts have also been made to modify the surface chemistry of TiO2 by coating, such as atomic layer deposition [21] and solution-based coating [22]. These methods have been successful in small systems but the scale-up either poses an energy input/output efficiency question or involves technical challenges, especially the challenges associated with homogenous coverage [23]. Therefore, a bulk synthesis and simple uniform doping process is still necessary for further exploration and application of anatase TiO2.
Herein, we adopt a two-step strategy for synthesis of surface-doped TiO2. First, layered protonated titanate structures were synthesized via an aqueous solution based reaction at room temperature. Then, the prepared layered structure was easily doped by nickel ions through an ion-exchange process at room temperature. This Ni–TiO2 sample turned into anatase structure after calcination and exhibited noticeable performance improvement in electrochemical tests when used as anode materials, which indicated the prospects of surface doped TiO2 for applications in LIBs.
Section snippets
Preparation of Ni-doped TiO2 structure
For the synthesis of Ni-doped TiO2 structure, the precursor of layered protonated dititanate was first prepared via an aqueous solution based reaction at room temperature similar to reported in the literature [24]. In a typical synthesis process, 2.9762 mL of titanium (IV) isopropoxide (TIP, 97%, Sigma–Aldrich) was added to 10 mL of anhydrous ethanol (>99.5%, Anhydrous, Sigma–Aldrich), followed by the addition of 40 mL of ammonium hydroxide (ACS, 30%, Alfa Aesar) under stirring. The milky white
Characterization of Ni and Mn-doped TiO2 structures
As shown in Fig. 1a, the low-resolution scanning electron microscope (SEM) image shows that the synthesized TiO2 precursor (TiO2–P) is granular in nature. Furthermore, the transmission electron microscope (TEM) image in Fig. 1b reveals that a single particle is formed by layered nanosheets overlapping each other. Fig. 1c shows an SEM image of the TiO2 after calcination at 400 °C for 3 h (TiO2-400). Compared to the sample before calcination, there is little noticeable morphology change. Fig. 1d
Conclusions
In summary, an effective doping strategy was developed based on the ion-exchange process to obtain Ni-doped TiO2 structures at room temperature. Compared to the undoped anatase TiO2, Ni ion doping improved the electrochemical performance of as-prepared TiO2 notably when used as lithium ion battery electrodes. This enhancement can be ascribed to the effect of doped Ni ions on the TiO2 structure, including the inhibition of crystallinity under calcination, by causing lattice distortion. In
Acknowledgments
YG and NPM acknowledge funding provided by the Department of Energy Nuclear Engineering University Program (DOE-NEUP) program under contract number DE-AC0705-ID14517.
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