NH4V3O8 Rectangular Nanotube: A Novel High-Performance Cathode Material for Lithium Ion Batteries

The morphology and nanosize of cathode materials play a crucial role in the improved electrochemical properties of the electrode material for lithium ion batteries. Herein, we report the synthesis of a novel NH4V3O8 rectangular nanotube via a facile one-pot solvothermal protocol with the use of the mixing solvent containing glycerol, ethanol, and ethylene glycol. The morphology and nanosize evolution of the as-prepared NH4V3O8 materials from the addition of different solvents has been systematically investigated. The electrochemical properties of these materials are closely related to their structure. Compared with other synthesized counterparts with three different morphologies (nanoparticle, ultra-small nanoparticle, and hierarchical microsheet), the resultant NH4V3O8 rectangular nanotube exhibited high reversible capacity with a maximum discharge capacity of 253.8 mAh g−1at 15 mA g−1, and the capacity retention rate is 75% after 50 cycles. This work reveals the relationship between the morphology and electrochemical performance of NH4V3O8 and provides a feasible method for the synthesis of high-performance electrode materials.


Introduction
At present, the application of lithium ion batteries has changed from the traditional electronic field to the new energy storage devices [1]. The performance of the battery requirements continues to increase [2,3]. To improve energy density and power density, many researchers have made contributions [4,5]. Extensive research on the study of lithium ion battery anodes with many studies on the application of nanomaterials such as traditional carbon materials [6][7][8][9], silicon carbon nanocomposite [10][11][12][13], alloy materials [14,15], transition metal oxide [16,17]. To develop a better battery and adapt to the fast-developing anode, the cathode material also needs complementary development. The cathode materials for lithium ion batteries with high energy density, high power density, high safety performance, and long cycle life are the focal point of current energy storage materials [18].
China has abundant vanadium resources and is one of the largest vanadium oxide producers in the world [19]. Vanadium derivatives and oxides have a unique layered crystal structure and good Li-ion storage performance, so they are one of the potentially high-energy and cost-effective cathode materials for lithium ion batteries (LIBs) [20,21]. For example, a versatile oxygen-deficient NaV 6 O 15 and LiV 3 O 8 nanosheets display remarkable capacity properties at various current densities and excellent lithium ion storage stability [22,23]. The 2D nanostructured V 2 O 5 exhibits enhanced lithium storage properties including high reversible capacity, good cycling, and rate performance [24,25]. Recently, the monoclinic systems NH 4 V 3 O 8 are receiving considerable attention as alternative cathode material due to their remarkable features such as larger interlayer spacing, rapid migration of Li + , and the network of N-H···O hydrogen bonds which increase the stability of NH 4 V 3 O 8 structure.
The electrochemical performance of electrode materials for lithium ion batteries and supercapacitors is largely dependent on their nanostructures [26][27][28][29]. Until now, there are several main methods to improve the electrochemical performance of cathode materials. Such as surface coating by carbon material or other high conductivity material, doping, and structural control [30]. The structural control method was demonstrated to affect the materials' physical and electrochemical performance. To improve the electrochemical performance, various nanostructures of NH 4 V 3 O 8 such as nanosheets [31,32], nanoflowers [33], nanorods [34], and nanobelts [35][36][37] have been synthesized. However, through continuous exploration by material researchers, the NH 4 V 3 O 8 with a novel structure can be synthesized.
In this work, the morphology and nanosize evolution on the as-prepared NH 4 V 3 O 8 materials from the addition of different solvents have been systematically investigated. The process is environmentally friendly and economically beneficial. The material morphology is controlled by adjusting the ratio of glycerol, glycol, and ethanol. By adjusting the types of solvents, different morphologies of NH 4 V 3 O 8 were synthesized, including nanoparticle (NP), ultra-small nanoparticle (SNP), rectangular nanotube (RNT), and hierarchical microsheet (HMS). Among these structures, the RNT as the cathode material of lithium ion batteries has the best electrochemical performance. The RNT delivered the discharge capacity of 253.8 mAh g −1 at 15 mA g −1 , and 75.6% capacity retention after 50 cycles. Compared with other samples, the cycle stability of the battery has been greatly improved.

Experimental
All reagents and solvents were analytical grade and purchased from commercial suppliers.

Synthesis
The NH 4 V 3 O 8 nanopowders were prepared via a facile one-pot solvothermal protocol. In a typical experiment, 0.7 g ammonium metavanadate (NH 4 VO 3 ) (purchased from Dezhou Fukai Chemical Co., Ltd) powder was added to a mixed organic solvent containing glycerol (Sinopharm Chemical Reagent Co., Ltd), ethanol (Fuyu Fine Chemical Co., Ltd) and glycol (Sinopharm Chemical Reagent Co., Ltd) either in a volume ratio of 1:2:1. The total volume of the solvent is 80 mL. Keep the filling ratio is 80%. Heat the solution to 30 ℃ and being stirred for 1 h. After that a proper amount of hydrochloric acid (1 mol L −1 ) (Tianjin Bodi Chemical Co., Ltd) was further added dropwise into the above solution under continuous stirring. Finally, the resulting mixing solution was transferred into a 100 mL Teflon-lined stainless autoclave. The autoclave was sealed and heated at 160 ℃ for 12 h. After the solvothermal treatment, the autoclave was cooled down to room temperature and the precipitates were collected by vacuum filtration, and then washed with deionized water and ethanol three times. The final sample was obtained after drying at 80 ℃ overnight. Then, the NH 4 V 3 O 8 sample NP was obtained. For comparison, the NH 4 V 3 O 8 SNP was prepared by the above method without adding glycerol solvent, maintain the volume ratio ethanol: glycol = 2:1. NH 4 V 3 O 8 RNT was prepared through the ratio of ethanol to glycol is 1:1. Preparation of NH 4 V 3 O 8 HMS, the solvent, was only ethanol.

Structure Characterization
Powder X-Ray diffraction (XRD) patterns were recorded on a Rigaku D/max 2200PC V diffractometer operating with Cu Kα radiation (λ = 1.5406 Å) to characterize the crystal structure of as-prepared products. The morphology was investigated by field emission scanning electron microscopy (FESEM, S-4800, Hitachi, Japan).

Electrochemical Characterization
To prepare the working electrode, the active material was mixed with sodium carboxymethyl cellulose (CMC) binder and super P together at a weight ratio of 8:1:1 in N-methyl pyrrolidone and pasted on aluminum foil. Then, it was dried in a vacuum oven at 80 °C for 24 h and punched into small disks with a diameter of 15.8 mm. Electrochemical measurements were performed using CR2032 half cells, which were assembled in an argon-filled glove box (Mbraun, Germany, O 2 , and H 2 O contents < 0.5 ppm). Pure Li foil was used as the counter electrode and microporous polypropylene film as the separator. The electrolyte was a solution containing 1 M LiPF 6 dissolved in a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) (1:1:1 in volume). The galvanostatic charge/discharge tests were conducted using a Neware battery testing system with a voltage range of 1.5 to 4.0 V (vs. Li/Li + ). The other electrochemical characterizations (cyclic voltammetry and electrochemical impedance spectroscopy) were conducted using a CHI 660E electrochemistry workstation.

Results and Discussion
The phase structures of NH 4 V 3 O 8 samples with different morphologies were analyzed by XRD. Fig. 1 represents the powder XRD pattern of prepared NH 4 V 3 O 8 samples with different morphologies. From the curves, we can deduce that the diffraction peaks of the four samples can be indexed to a monoclinic crystalline NH 4 V 3 O 8 phase (JCPDS Card No. 88-1473, space group P2 1 /m). As seen, no other phases have been detected, showing that the products with high purity. For LiV 3 O 8 , the higher intensity of diffraction intensity (001), the better the degree of crystallinity [31]. Notably, with the decrease in the ethanol and glycerol, the diffraction intensity (001) peak increases, suggesting a bigger crystallite size of NH 4 V 3 O 8 [35]. Unfortunately, the good ordering of crystal is disadvantageous for the diffusion of Li + intercalation and de-intercalation.
The morphology and micro-nano structure of the NH 4 V 3 O 8 were characterized by SEM. Figure 2 shows the SEM images of the as-prepared NH 4 V 3 O 8 . When glycerol, ethanol, and ethylene glycol were used as the solvent, the NH 4 V 3 O 8 showed nanoparticle morphology with an average size of ~ 150 nm (Fig. 2a, b). When the ratio of the solvent to ethanol and ethylene glycol was 2: 1, the NH 4 V 3 O 8 showed ultra-small nanoparticles morphology with an average particle size of ~ 40 nm (Fig. 2c, d). Interestingly, when the ratio of the solvent to ethanol and ethylene glycol was 1: 1, the rectangular nanotube morphology formed (Fig. 2e, f). The average size of the tube is about ~ 140 nm. After removed ethylene glycol from the solvent, there are great morphology changes of the NH 4 V 3 O 8 , it changed from nanoscale order of magnitude to hierarchical microsheet (Fig. 2g, h). From the perspective of the crystal, morphology change is because of the oriented crystal plane.
To understand the superiority of the NH 4 V 3 O 8 rectangular nanotube, the performance of the NH 4 V 3 O 8 NP, SNP, RNT, and HMS as cathodes in lithium ion battery was investigated. Fig. 3 exhibits the cycling performance and rate performance of the as-prepared NH 4 V 3 O 8 . The voltage range is 1.5-4.0 V (vs. Li/Li + ). The cycle performance of the electrodes is shown in Fig. 3a. After 50 cycles, RNT exhibits a discharge capacity of 189.5 mAh g −1 at 15 mA g −1 , while the discharge capacity for NP, SNP, and HMS are only 157.8, 154, and 46.8 mAh g −1 , respectively. It shows that the RNT sample has a significantly improved lithium storage capability. We can get the Coulomb Efficiency (CE) of the sample RNT. The CE of RNT has been maintained at high levels, coincides with better cycling performance. Fig. 3b shows the rate performance of the different NH 4 V 3 O 8 samples at a current density from 15 to 300 mA g −1 .
The rate performance of RNT shows higher capacity retention than the other three samples. After 300 cycles, the discharge capacity of RNT can revert to 131.1 mAh g −1 , while those for NP, SNP, and HMS are only 85.4, 72.1, and 40 mAh g −1 , respectively. The reason for the good electrochemical performance of NH 4 V 3 O 8 is the structure of rectangular nanotube which promotes the contact area between NH 4 V 3 O 8 and electrolyte, enhances the diffusion efficiency. Table 1   litter better than sample RNT, the number of cycles is small. In this paper, after 50 cycles, the discharge capacity retention rate maintains 75%.
The Nyquist plots of the as-prepared samples are compared in Fig. 4a. Firstly charged the battery to 4.0 V, and then, measure electrochemical impedance spectroscope in the frequency range from 0.01 Hz to 100 kHz. NH 4 V 3 O 8 RNT shows a much lower charge transfer resistance is 98.4 Ω, suggesting that it is easy to transfer for Li + and electrons during the charge and discharge process. To better understand the plots, an equivalent circuit model was used to fit the impendence in Fig. 4a. The parameters R e , CPE, R ct , and W correspond to the Ohmic resistance of the electrolyte and electrode in the battery, constant phase element, the charge transfer resistance of corresponding electrochemical reactions, and the Warburg impedance related to Li + diffusion, respectively. The value of R ct of different samples could be simulated from the EIS date by using the equivalent circuit. Fig. 4b is the histogram of comparing each resistance value. It could be seen that the R ct value for RNT, HMS, NS, and NP was 83.2, 106.1, 86.8, and 97.2, respectively. The value of the total resistance of RNT is smaller than the other three samples. The results of electrochemical impedance spectroscope measurements are in good agreement with the electrochemical performance.
The charge/discharge curves of the sample RNT at different cycles shows in Fig. 5a.The discharge capacity of NH 4 V 3 O 8 RNT at the 1st, 2nd, 10th, 25th, and 50th cycle is 269, 250, 233, 215, and 208 mAh g −1 , respectively. There are three discharge voltage plateaus at 2.75, 2.49, and 2.1 V, and the voltage plateaus indicate the multi-step  Li + intercalation/de-intercalation process. Fig. 5b depicts the galvanostatic voltage profile of NH 4 V 3 O 8 RNT at different specific currents. The charge capacity increase and the discharge specific capacity decrease with the variation of current densities from 15 to 300 mA g −1 , because of the sluggish Li + diffusion kinetics at high current densities [41].
To clarify the better electrochemical performance of NH 4 V 3 O 8 RNT, the CV measurement at different sweep rates was conducted. The CV analysis is performed to estimate the kinetic and mass transport during the redox process of cathode materials. Fig. 6a shows the CV curves of the NH 4 V 3 O 8 RNT measured in the voltage range of 1.5-4.0 V at a scan rate of 0.1 mV s −1 . The NH 4 V 3 O 8 RNT shows a peak at 2.48 V and vanishes in the following cycles, corresponding to irreversible lithium insertion into NH 4 V 3 O 8 in the first two cycles. Fig. 6b shows the CV curves of the NH 4 V 3 O 8 RNT measured in the voltage range of 1.5-4.0 V at different scan rates. As the scan rate increase from 0.1 to 0.8 mV s −1 , the shape of the redox peaks is well preserved, implying that small polarization, fast Li + insertion/desertion, and good reversibility of NH 4 V 3 O 8 RNT.

Conclusions
In summary, shape-controlled NH 4 V 3 O 8 was prepared via a facile one-pot solvothermal method. By adjusting the ratio of the solvent, the morphology of the material can be effectively adjusted. The lithium storage performance of as-prepared was through systematic research. The synthesized NH 4 V 3 O 8 RNT exhibited a high discharge capacity of 253.8 mAh g −1 at 15 mA g −1 and good cyclic stability. The excellent electrochemical performance of as-prepared NH 4 V 3 O 8 RNT is benefited from the hollow rectangular nanotube which can provide more reactive active sites. This facile synthetic strategy is expectable to offer an interesting clue for the morphological engineering of other nano-sized electrode materials currently employed in lithium-based batteries.