Surfactant-free and controllable synthesis of hierarchically lithiated MoO 3 microspheres

Compared with traditional lead-acid batteries, lithium-ion batteries are highly favoured by academic community and practical industry by virtue of their light weight, high voltage and energy density, relatively long cycle life, and low self-discharge rate [1]. In recent years, new energy vehicles have been realized a leaping development, and lithium-ion batteries as the novel system of electrochemical energy storage have been experienced a new upsurge of scientific research and industrial application [2]. As one of the vital components of lithium ion batteries, cathode materials have played a crucial role on the performance of battery [3]. Molybdenum trioxides (MoO3) has been used as functional materials in a wide range of applications such as synthetic sensitive components, display devices, and catalysts, because of its excellent properties of electrochromism, photochromism, photocatalytic degradation, and gas sensing [4]. Moreover, MoO3 has also become a hot topic for researchers as potential electrode materials. There are three common phases of molybdenum trioxide: thermodynamically stable orthogonal phase, thermodynamically metastable hexagonal phase and monoclinic phase [5]. Moreover, tetrahedral and octahedral holes have been observed in their lattice structures, which formed the unique 2D layered structure, exhibiting alluring lithium ion insertion properties. On account of the open layered structure and the property of generating oxygen vacancies, the molybdenum trioxide crystal was characterized by high energy density (930 Wh/kg) and high theoretical electrochemical capacity (372 mAh/g) as the ideal cathode candidates for lithium ion batteries [6]. Various kinds of MoO3 based materials have been prepared for different applications. Chu et al. obtained the bundles of α-MoO3 nanowires by using (NH4)6Mo7O24⋅4H2O as the Mo sources and aniline as the assembly-forming agent [7]. With the help of aniline, the α-MoO3 nanowires can be attached together to form the hierarchical structures. The cycling stability of the α-MoO3 can be enhanced through the assembly of α-MoO3 nanowires. Qin et al. prepared the special arrays of α-MoO3 nanoplates through the solid-state reaction among (NH4)6Mo7O24⋅4H2O, oxalic acid and polyethyleneglycol (PEG-400) [8]. The large specific surface area and the exposure of the active crystal face in the single crystal of

α-MoO 3 structures resulted in the high sensitivity for the detection of xylene. Zhang et al. modified MoO 3 nanobelts with the NiMoO 4 layers to form core/shell structures [9]. The abundance of active sites and the high porosity of the NiMoO 4 coating can act as the lithium reservoirs to improve the electrochemical performance of MoO 3 nanobelts. However, the hierarchical structures composed of primary MoO 3 plates have been rarely reported by using the surfactant-free method [10]. In this work, we have synthesized hierarchically lithiated MoO 3 structures as the cathode material for lithium ion battery, without the preparation and removal of organic templates. It was a simple synthetic method with low cost, which was relatively energy efficient and suitable for mass production.

Synthetic procedures
First, the ethanol with the volumes of 0, 5, 10, 20, and 40 mL were weighed separately and mixed with 10 mL of distilled water.

Materials characterization
The crystal structure of the sample was characterized by an X-ray powder diffractometer under Cu Kα radiation ranging from 5 • to 70 • at a scan rate 60 • /min. The surface morphology and particle size were evaluated using a field-emitting scanning electron microscope (SEM, FEI Quanta 250). The mass ratio of Li:Mo in samples was measured via inductively coupled plasma optical emission spectroscopy (ICP-OES) (AGILENT ICPOES 730).

Electrochemical test
The electrochemical properties were investigated by 2032 cointype cells assembled in an Ar-filled glovebox. The working electrodes were prepared by mixing 80 wt% active material, 10 wt% super P, and 10 wt% polyvinylidenefluoride (PVDF) in an appropriate amount of N-methyl-2-pyrrolidine with the assistance of ultrasound, which was then pasted on aluminium foil current collector and dried at 120 • C in a vacuum oven overnight. at 25 • C. The electrochemical impedance spectroscopy (EIS) was conducted on VersaSTAT 3F electrochemical workstation with a voltage amplitude of 5 mV in the frequency range from 10 mHz to 100 kHz. The cyclic-voltammetry (CV) was performed at scan rates of 0.2 mA/s in the potential window of 1.5-4.2 V vs. Li + /Li., and it was also carried out on VersaSTAT 3F electrochemical workstation.

RESULTS AND DISCUSSION
To study the phase and crystallinity of the lithiated MoO 3 samples prepared in the presence of different amounts of ethanol, the XRD patterns were obtained as shown in Figure 1. the intensity of (040) peak was decreased when more ethanol was added into the reaction system (such as Samples LMO-3 and LMO-4), indicating the negative influence of ethanol on the lithiation of MoO 3 . In addition, the ICP results (Table 1) also indicated the decrease of lithium content in the samples with the increase of the adding amount of ethanol during the synthetic process. The crystal morphology and size distribution of the obtained lithiated MoO 3 spheres were carefully studied by using their SEM images, as shown in Figures 2 and 3. The panoramic images ( Figure 2) and their corresponding enlarged images (Figure 3) indicated that all the prepared products were composed by the lithiated MoO 3 micro-spheres of 2-6 μm. The existence of primary particles with the sizes of several hundred nanometres on the spherical surfaces indicated the hierarchical structures of lithiated MoO 3 . The primary particles were little flakes with the thickness of several ten nanometres for Sample LMO-1, LMO-2 and LMO-3, while the primary particles of LMO-4 were composed of the larger rectangular prisms with the thickness of several hundred nanometres (Figure 4). During the synthesis process, the reaction between molybdenum and H 2 O 2 resulted in the formation of the molybdenum oxides and hydrates in the solution. The interactions among molybdenum species including Vander Waals force and hydrogen bonding induced the aggregation and assembly of the nano-sized crystals [12]. Meanwhile, the continuous growth of molybdenum-based nano-crystals was also carried out in the presence of lithium ions. As a result, the hierarchical structures assembled by the lithiated molybdenum oxide particles can be formed through the hydrothermal reaction. The microsphere shape may be the most stable for the hierarchical structures due to the fact that the interfacial free energy of spherical structure is minimal [13].
At least 100 spheres of each sample were measured for calculating the size distribution of micro-spheres and primary particles, and the results were displayed in Figure 5(a,b), respectively. Based on the statistical analysis, the average sizes of the lithiated MoO 3 spheres and the primary particles on the spheres were further calculated to study the influence of ethanol   The cycling performance of four lithiated MoO 3 samples was studied by the galvanostatic charge/discharge method, showing the initial capacities of nearly 300 mAh/g at 30 mA/g with the voltage range of 1.5-4.5 V (Figure 6(a)). The capacity losses of LMO-1, LMO-2, LMO-3 and LMO-4 were 40.8%, 38.7%, 26.7% and 33.3% in the first ten cycles, respectively. It was mainly because of the gradual collapse of the layered MoO 3 structure [5] and the irreversible insertion of lithium ions into the MoO 3 crystals during the electrochemical cycles [14]. With the cycling was carried out from the 11st to the 50th cycles, the discharging capacities were decreased more slowly than those in the first ten cycles. At the 50th cycles, the discharging capabilities of LMO-1, LMO-2, LMO-3 and LMO-4 were 75.6, 99.1, 155.9 and 111.3 mAh/g, respectively, indicating the capacity retention of 24.3%, 31.4%, 52.5% and 38.2%, respectively. The electrochemical reversibility of the lithiated MoO 3 samples was not very high, which may be due to the structural instability of MoO 3 [7] and inevitable dissolution of Mo in the organic electrolyte [15]. The rate capabilities of the lithiated MoO 3 samples were displayed in Figure 6(b) at different current densities of 30-500 mA/g. The increase of current  density resulted in the reduction of discharging capability, due to the enhanced polarization at higher rates [16]. LMO-3 showed the best rate performance, delivering the average capacities of 258.0, 184.0, 131.4, 84.2 and 41.8 mAh/g at 30, 50, 100, 200 and 500 mA/g.
In order to understand the electrochemical kinetics of the lithiated MoO 3 samples, their electrochemical impedance spectroscopy (EIS) tests were carried out in the frequency domain ranging from 100 kHz to 10 mHz. The typical Nyquist plots and their equivalent circuit model were given in Figure 6(c). The symbols of R s , R ct , Z w and C d were used to represent the ohmic resistance of the electrode, the charge transfer resistance, the Warburg impedance and the double layer capacitance, respectively. The values of R s , R ct1 and R ct2 were simulated by using the Zview software [17], as listed in Table 1 (Table 1). Therefore, the R ct1 +R ct2 values of the lithiated MoO 3 samples were first decreased and then increased. The Nyquist plots of all the samples exhibited similar shapes, including a depressed semicircle in the high-frequency region and a sloped line in the low-frequency region. The highfrequency semicircle can be assigned to the charge transfer resistance (R ct ), and the low-frequency line can be attributed to the Warburg diffusion process [18]. The values of lithium ion diffusion coefficient (D Li+ ) for the lithiated MoO 3 samples can be obtained by applying the following Equations (1) and (2) [19]: (1) Herein, R is the gas constant (8.314 J/K/mol), T is the absolute temperature (298.15 K), A is the surface area of the electrode (1.13 cm 2 ), n is the electron number per molecule during Li ion insertion, F is the Faraday constant (96,500 C/mol), C is the concentration of Li ions, and σ is the Warburg impedance coefficient associated with Z re (the real part of cell impedance) and ω (the angular frequency in the low frequency region, ω = 2πf). The plot of Z re against ω −1/2 in the low-frequency region was shown in Figure 6(d). The relationship between Z re and ω −1/2 can be fitted as the straight lines with the slop of σ [20]. Therefore, the calculated values of D Li+ were 4.3 × 10 −15 , 8.2 × 10 −14 , 1.8 × 10 −13 and 8.7 × 10 −14 cm 2 /s. The cyclic voltammograms (CV) was used at room temperature within the range from 1.5 to 4.2 V (vs Li/Li + ) for studying the insertion and extraction process of lithium ions during the electrochemical cycling. The typical CV results of the Sample LMO-3 were shown in Figure 7, displaying the curves of the initial five consecutive cycles. The area of the CV curves represented the insertion amount of lithium ions [21]. The area percentages of the second, third, fourth and fifth cycles were 97.3%, 96.8%, 96.6% and 96.1% compared with the first cycle of sample, indicating the continuous decrease of capacity during

CONCLUSION
In summary, the hierarchically lithiated MoO 3 microspheres can be fabricated in the presence of the lithium salts without the use of any surfactant. The lithiated MoO 3 microspheres with the diameter of 2.88-3.44 μm were composed of nano-sized flakes or rectangular prisms and the shapes of their primary particles can be well controlled by adjusting the feeding amount of ethanol during their synthetic process. The discharging capacity of the lithiated MoO 3 microspheres can be nearly 300 mAh/g at 30 mA/g and the capacity retention was 52.5% after 50 cycles. The electrochemical performances were enhanced first, and then were reduced with the decrease of lithium content in the lithiated MoO 3 samples. In addition, the method in this work can be utilized for the fabrication of hierarchical structures of other oxide materials.