Ionics DOI 10.1007/s11581-017-2053-6 ORIGINAL PAPER Low-temperature one-step solid-phase synthesis of carbon-encapsulated TiO2 nanocrystals as anode materials for lithium-ion batteries Boyang Liu1 & Yingfeng Shao 2 & Xin Xiang1 & Jiayuan Ren1 & Wenge Li 3 Received: 14 September 2016 /Revised: 27 February 2017 /Accepted: 27 February 2017 # Springer-Verlag Berlin Heidelberg 2017 Abstract A simple and highly efficient method is developed for after oxidative dehydrogenation coating on the TiO2 in situ one-step preparation of carbon co-encapsulated anatase nanocrystals, resulting in the formation of core-shell structure. and rutile TiO2 nanocrystals (TiO2@C) with core-shell structure The TiO2 nanocrystals prepared by titanocene dichloride have for lithium-ion battery anode. The synthesis is depending on the an equiaxed morphology with a small diameter of 10–55 nm and solid-phase reaction of titanocene dichloride with ammonium the median size is 30.3 nm. Hundreds of TiO2 nanocrystals are persulfate in an autoclave at 200 °C for 30 min. The other three encapsulated together by the worm-like carbon shell, which is titanocene complexes including bis(cyclopentadienyl)dicarbonyl amorphous and about 20–30 nm in thickness. The content of titanium, cyclopentadienyltitanium trichloride, and TiO2 nanocrystals in the nanocomposite is about 31.1 wt.%. cyclopentadienyl(cycloheptatrienyl)titanium are used instead to This TiO2@C anode shows stable cyclability and retains a good comprehensively investigate the formation mechanism and to reversible capacity of 400 mAh g−1 after 100 cycles at a current improve the microstructure of the product. The huge heat gener- density of about 100 mA g−1, owing to the enhanced conductiv- ated during the explosive reaction cleaves the cyclopentadiene ity and protection of carbon shell. ligands into small carbon fragments, which form carbon shell Keywords Solid-phase synthesis . Core-shell structure . TiO2 Electronic supplementary material The online version of this article nanocrystals . Lithium-ion battery (doi:10.1007/s11581-017-2053-6) contains supplementary material, which is available to authorized users. Introduction * Boyang Liu [email protected] Nowadays, lithium-ion batteries (LIBs) have been generally Yingfeng Shao accepted as power sources for consumer electronics and [email protected] electric/hybrid electric-vehicles because of their merits in Xin Xiang terms of high energy density, long cycle life, tiny memory [email protected] effect, low toxicity, and cost [1–3]. The battery performance Jiayuan Ren is depending on the intrinsic characteristics of electrode mate- [email protected] rial, and commercial carbonaceous anodes show an increased Wenge Li resistance at high rate, which is inappropriate for advanced [email protected] high-power LIBs in future [4]. Therefore, a lot of efforts have been made for developing various candidates with different 1 College of Ocean Science and Engineering, Shanghai Maritime nanostructures and physicochemical properties to improve the University, Shanghai 201306, China battery performance. Titanium dioxide (TiO2) is a widely used 2 State Key Laboratory of Nonlinear Mechanics, Institute of material in industrial applications because of its abundance, Mechanics, Beijing 100190, China relatively low price, nontoxicity, and excellent stability [5]. 3 Merchant Marine College, Shanghai Maritime University, Most TiO2 polymorphs (including rutile, anatase, and brook- Shanghai 201306, China ite) and their hybrids are regarded as attractive options for Ionics lithium storage owing to their superior inherent safety and rate capability and cycling performance of the LIBs. chemical compatibility with the electrolyte [6]. Compared Consequently, it is still an ongoing issue to explore simple, with carbon anodes, TiO2 exhibits a comparable theoretical efficient, high yield, and environmentally friendly preparation capacity of 335 mAh g−1 according to the accommodation techniques on the basis of new formation mechanism for over- of one Li per TiO2 [7]. It also has a higher voltage in the range coming the shortcomings of current methods. of 1.5–1.8 V relative to lithium depending on different poly- In the present work, a novel solid-phase reaction between morphs, thus preventing the formation of lithium dendrites titanium metallocene and ammonium persulfate ((NH4)2S2O8, and side reactions between the electrode and electrolyte [8]. abbreviated as APS) has been developed for in situ one-step Furthermore, its volume expansion is only about 3.7% within preparation of carbon-encapsulated TiO2 nanocrystals the crystalline lattice upon lithium insertion/extraction, ren- (TiO2@C) with core-shell structure at 200 °C. TiO2@C with dering the incredible stability under long and high-rate cycling homogeneous morphology exhibits high specific capacity and [9]. However, both of the lithium-ion diffusivity and electron- rate capability, as well as excellent cycling stability. ic conductivity of TiO2 are relatively low, which can adversely affect the rate capability of LIBs [10]. To address the negative issues, TiO2 in various nanocrystalline forms are widely used Experimental to reduce the diffusion path length for lithium-ions and in- crease high contact area between electrolyte and electrode, Materials and preparation thereby improving both storage capacity and rate capability [11–13]. Another common strategy involves incorporation of All the chemicals were received as analytical reagent grade TiO2 with carbonaceous materials, which can enhance the and used without further purification. Four titanium electronic conductivity and suppress the aggregation of TiO2 metallocenes, including bis(cyclopentadienyl)titanium nanocrystals, thus increasing the anode stability during cy- dichloride ((C5H5)2TiCl2, abbreviated as Cp2TiCl2), cling [14–16]. Consequently, TiO2@carbon nanocomposite bis(cyclopentadienyl)dicarbonyl titanium (Cp2Ti(CO)2), is considered as a promising high-performance anode material cyclopentadienyltitanium trichloride (CpTiCl3), and in LIBs, and several synthesis methods have been developed. cyclopentadienyl(cycloheptatrienyl)titanium (CpTi(C7H7)), Conventionally, TiO2 nanomaterials, such as nanoparticles, were used to react with APS in an autoclave at 200 °C with nanorods, and nanotube arrays, can be obtained by flame an identical process, respectively. Taking sample #1 for exam- spray pyrolysis, sol-gel method, electrochemical anodization, ple, 4 mmol of Cp2Ti2Cl2 and 8 mmol of APS were weighed and hydrothermal process [17–20]. Otherwise, carbon nano- and manually milled by an agate mortar. Then, the homoge- structures are mainly fabricated by self-recombination of car- neously mixed reactants were sealed in an autoclave with a bon clusters through physical evaporation of graphite, chem- 50-ml PTFE liner and held at 200 °C in an electric heat oven ical pyrolysis of organic precursors, and hydrothermal carbon- for 30 min. After the reaction, the as-prepared powder was ization of carbohydrate sources in different temperature ranges thoroughly rinsed with deionized water and ethanol in se- [21–23]. Therefore, TiO2 and carbon are generally prepared quence to remove the soluble by-products, and TiO2@C was individually and then hybridized because of their different eventually obtained after being dried in a vacuum oven at formation mechanisms and incompatible starting materials. 100 °C. Other samples were also prepared by different precur- For example, Oh et al. first synthesized TiO2 spheres using sors which can affect the morphology and microstructure of TiCl4 as the precursor by hydrothermal method at 120 °C for the products (Table 1). 24 h, which were then mixed with sucrose in a solution. Carbon coating was further obtained by annealing of the ho- Characterization mogeneous mixture at 400 °C in Ar atmosphere [24]. Wang et al. prepared TiO2 hollow nanospheres using monodisperse Phase structures of the samples were characterized by X-ray silica spheres as templates, and then, a resorcinol- diffraction (XRD) using a PANalytical X’Pert PRO diffrac- formaldehyde layer was coated on these spheres, which sub- tometer with Cu Kα radiation. Raman spectrum of the samples sequently turned to carbon shell under Ar atmosphere at was recorded by a Bruker Senterra micro Raman spectrometer 700 °C [25]. It is also a common strategy to grow TiO2 with an excitation wavelength of 633 nm at 2 mW. The surface nanomaterials on the high-performance carbon nanotubes morphology, microstructure, and composition of the samples and graphenes by hydrothermal method [26–28]. However, were analyzed by field-emission scanning electron microsco- it is evident that multi-step, long reaction time and high tem- py (SEM, JEOL JSM 7500F) and transmission electron mi- perature are usually required for preparation of TiO2@carbon croscopy (TEM, JEOL JEM 2010) equipped with an X-ray nanocomposites. Additionally, it is difficult to ensure control- energy-dispersive spectroscopy system (EDS, EDAX). The lable and homogeneous distribution of the nanosized TiO2 and reaction behaviors of the reactants and the final product of carbon in scalable synthesis, which may lead to the unsatisfied sample #1 in ambient atmosphere were determined by Ionics Table 1 Synthesis parameters of No. Reactants Vol. of Temp. Time Major carbon and TiO2 nanocomposites autoclave (°C) (min) products Titanium mmol Ammonium mmol (ml) metallocenes persulfate #1 (C5H5)2TiCl2 4(NH4)2S2O8 8 50 200 30 TiO2@C #2 (C5H5)2Ti(CO)2 4 8 Carbon, S8 and TiO2 #3