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• Currently, titanate-based compounds have been considered as hopeful materials for high-performance lithium-ion batteries, then the new category of lithium-rich transition metal oxides Li₂MTiO₄ (M = Fe, Mn, Co, Ni or Cu) have excellent theoretical capacity (about 290 mAh/g), and have been researched as cathode materials for lithium batteries [1-7]. Among these oxides, the redox reaction of Ti⁴⁺ + M²⁺ ↔ Ti³⁺ + M³⁺ is beneficial to the high capacity of Ti⁴⁺/M²⁺ oxidation states in principle, and it realized the two-electron response of each Fe atom [8, 9]. Among these cubic cations disordered rock salt (space group Fm-3m) structure materials, the metal atoms in Li₂FeTiO₄ are disordered in octahedral sites of the cubic closet packing (CCP) array of anions [10]. Li₂FeTiO₄ cathode materials had been attracted attention for the environment friendly, low-cost and high theoretical capacity [11]. However, it is difficult for the Li₂FeTiO₄ to attain its full capacity, because of low electron conductivity and slow diffusion rate of Li⁺ in the olivine structure. To solve these problems, people have made a lot of efforts, one of the most effective methods is fabricating nanostructure [12-15]. Lithium-ionbattery electrodes prepared by nanomaterials or mesoporous nanocomposites have excellent rate capability [16-23]. For example, Yang et al. [24] use nanostructured Li₂FeTiO₄/graphene composites made by sol-gel method with graphene oxide as the template and distributed on the graphene substrate, and the particle size was 20-50 nm. Kuzma et al. [25] reported that novel active materials were made by using transition metal titanates (Li₂MTiO₄) as carbon precursor, which consist of 1020 nm particles embedded in the conductive carbon coating. They also show that it is not the coating but the size of the small particles determines the activity of electrode materials. However, using graphene oxide or carbon precursor to obtain products usually requires multi-steps and special treatment, which was not commercially viable.

  The morphology and size of nanomaterials play an important role in the electrochemical properties [26-28]. The electrode material made by the hydrothermal method with uniform particle size distribution, stable electro-chemical properties, and structure can be controlled, which is easy to realize the nano-crystallization of electrode materials and expected to achieve the industrial production of materials [29-38]. In this paper, Li₂FeTiO₄ composite materials adopting multi-phase modification synthesized by hydrothermal, and the reinforcing agent formed by the growth of the multi-phase modification constitutes the multi-phase composite materials with a super high initial discharge capacity and excellent cycling stability [39]. Li₂FeTiO₄ composite cathode materials have high crystallinity, high specific surface area and uniform particle size, which make it had much superior electrochemical performances than most of the reported Li₄Ti₅O₁₂-based nanocomposites. Well-dispersed Li₂FeTiO₄ composite particles were successfully prepared by the hydrothermal reaction method using tetrabutyl titanate, TiO₂ anatase, hydrogen titanate nanowire (H₂Ti₃O₇NW, HTO-NW) and titanium oxide nanotubes (TiO₂ NB) as the source of titanium. Because different raw materials have a significant influence on the electro-chemical characterization of Li-ion batteries [40], we use different titanium source preparations of Li₂FeTiO₄ in a performance comparison to determine the best reaction materials. The products of Li₂FeTiO₄ have high yield and good industrial application prospects.

  The schematic diagram of the preparation procedure of Li₂FeTiO₄ composite cathode materials is presented at Fig. S1 (Supporting information). Commercial LiAC, FeCl₂, tetrabutyl titanate or TiO₂ anatase, hydrogen titanate nanowire (H₂Ti₃O₇NW, HTO-NW) and titanium oxide nanotubes (TiO₂ NB) were mixed in designed molar ratio as 4:1:1 for the hydrothermal method. The mixtures were filled in a Teflon vessel (110 mL). The vessel was placed in a stainless-steel autoclave and heated with 180-220 ℃. After cooling to room temperature, the precipitates were collected by suction filtration and dried in vacuum. In the post-heattreatment, dried products were calcined under nitrogen atmosphere at 700 ℃ for 10 h. The electrolyte was 1 mol/L LIPF₆ synthesized by ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate with volume ratio of 1:1:1, the water content of electrolyte was 10 ppm. The battery was the button battery of CR2025. In addition, commercial LiAC was supplied by the Third Tianjin Chemical Reagent Factory with AR grade purity; FeCl₂ was supplied by Tianjin Damao Chemical Reagent Factory with AR grade purity; tetrabutyl titanate was supplied by Tianjin Guangfu Fine Chemical Research Institute with AR grade purity and TiO₂ anatase was supplied by Beijing Modern Oriental Company with AR grade purity. Hydrogen titanate nanowire (H₂Ti₃O₇NW, HTO-NW) was prepared by mixing solution of TiO₂ anatase and NaOH (NaOH was supplied by Beijing Modern Oriental Company with AR grade purity) in ultrasonic generator for 2 h (ultrasonic power is 0.2–0.5 W/cm²) and then placed at 150 ℃ for 48 h. Titanium oxide nanotubes (TiO₂ NB) was prepared from hydrogen titanate nanowire after heat treatment at 700 ℃.

  The crystal structure of Li₂FeTiO₄ composites were determined bypowder X-ray diffraction (XRD) using Cu-Kα radiation (λ = 0.15406 nm) at scan rate of 0.04°/s (30 kV) with DX-2500 instrument. The morphology of samples were observed and compared using field-emission scanning electron microscopy (FE-SEM, S-4800-II, Hitachi, Japan). The Li₂FeTiO₄ was mixed with acetylene black and polyvinylidene difluoride (PVDF) at the weight ratio of 75:17:8 to prepare electrodes in N-methyl pyrrolidone. The loading mass of active material is 1.96 mg. A LAND CT 2001A system was utilized for battery charge-discharge tests between 1.5 V and 4.8 V vs. Li, and the battery was cycled galvanostatically at room temperature. The specific surface area was determined and the particle size was analyzed under 175 ℃ constant temperature of liquid nitrogen using SSA-4300 instrument. The specific surface area of the Li₂FeTiO₄ was calculated from N₂ adsorption/desorption data by Brunauer-Emmett-Teller (BET) method and the pore size distribution can be obtained by Barrett-Joyner-Halenda (BJH) method. Electro-chemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were obtained by Solartron 1260 + 1287 electrochemical impedance analyzer.

  Fig. 1 shows the XRD pattern of Li₂FeTiO₄ composite cathode materials under hydrothermal conditions. The main reflectance peaks are attributed to the Li₂FeTiO₄ phase, that means the samples have irregular rock-salt-type structure. Three intense diffraction peaks corresponding to the (111), (200) and (222) planes are consistent with reported data [25], demonstrate that synthetic materials are made of Li₂FeTiO₄. Impure phase TiO₂, Fe₂O₃ and FeTiO₄ were detected among the Li₂FeTiO₄ composites with different titanium sources. The observation of such impurities has also been reported by other authors, and it is a common problem with the hydrothermal reaction method [24, 25]. These impurities are used for multi-phase modification of Li₂FeTiO₄ composite materials, and particle dispersion of multi-phase materials make up for uneven distribution caused by differences in the morphology of the nanotubes. The unmarked small peaks in Fig. 1 are the impurity peaks, which have little influence on the material properties. Besides, Li₂FeTiO₄ materials with complete phase structure cannot be synthesized from tetrabutyl titanate, and this sample will not be discussed.

  Figure 1

  

  Figure 1.  XRD of Li₂FeTiO₄composite cathode materials synthesized from (a) tetrabutyl titanate, (b) TiO₂ anatase, (c) HTO-NW and (d) TiO₂ NB.

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  Figs. 2a–c show the morphological features of Li₂FeTiO₄ composite cathode materials (SEM), and Figs. 2d–f are the corresponding pore size distribution profile, which shows the grain size. When the grain size is smaller, the specific surface area of the Li₂FeTiO₄ is more significant, which can efficiently increase the active sites of charge-discharge reaction between Li₂FeTiO₄ and electrolyte. The products synthesized by TiO₂ anatase have the largest grain size, followed by HTO-NW and TiO₂ NB. The synthesis grain size of HTO-NW and TiO₂ NB is relatively small, but the grain distribution of HTO-NW is not uniform, the condensation phenomenon is serious, and products still retain titanate nanowire morphology. The larger grain size, the more easily affect electrochemical properties of the electrode materials [11]. The materials synthesized from TiO₂ NB have homogeneous and loose particles growth density. It indicates that materials of TiO₂ NB can obtain the target material of small particles.

  Figure 2

  

  Figure 2.  SEM particle size distribution and nitrogen adsorption/desorption isotherms (inset: pore size distribution profile) of Li₂FeTiO₄ synthesized from (a, d, g) TiO₂ anatase, (b, e, h) HTO-NW and (c, f, i) TiO₂ NB.

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  The specific surface area determined by the BET method is shown in Figs. 2g–i. The results show that specific surface area of Li₂FeTiO₄ samples prepared by TiO₂ anatase, HTO-NW and TiO₂ NB are 7.699 m²/g, 16.054 m²/g and 12.938 m²/g, respectively. The surface area curves of three Li₂FeTiO₄ samples made by different materials are similar. The nitrogen adsorption-desorption isotherms of all Li₂FeTiO₄ samples can be categorized as typical type IV isotherm, and the H3 hysteres is loop exists under relative pressure of 0-1, indicates that existence of mesoporous structure. The adsorption amount is very low at the low-pressure area, meaning that the interaction between adsorbent and adsorbate is weak. With the increase of pressure, the adsorption amount increased sharply, which indicates that the pores are filled. The large surface area of Li₂FeTiO₄ samples prepared by HTO-NW and TiO₂ NB provide more active sites for Li⁺ insertion/extraction, and promotes effective contact area between electrode and electrolyte, improves reaction kinetics. Inset of Figs. 2g–i show the corresponding BJH pore size distribution plots of Li₂FeTiO₄ and they are consistent with the SEM images. The pore size profile of Li₂FeTiO₄ synthesized from HTO-NW and TiO₂ NB are uniform, and the number of grains with small grain size is much larger than the products synthesized from TiO₂ anatase. A large surface area can provide good liquid absorption capacity, which can effectively increase the area of electro-chemical reaction.

  Fig. 3 shows the C/10 rate against lithium for three samples, and the dotted red line is the theoretical capacity (295 mAh/g) of Li₂FeTiO₄. The discharge cutoff potential is 4.5 and 1.0 V, and the charge-discharge current is 10 mA/g. The efficiency experiment shows that the first discharge capacity of Li₂FeTiO₄ prepared byTiO₂ NB and HTO-NWis 367.8 mAh/g and 246.7 mAh/g, are higher than it made by TiO₂ anatase (86.9 mAh/g). The samples using one-dimensional TiO₂ NB as the raw material can get a high initial discharge capacity compared with theoretical capacity. The initial Coulombic efficiency of TiO₂ NB as the raw material was 82.57%. For comparison, the electrochemical performance of Li₂FeTiO₄ prepared by TiO₂ NB in our work and the similar materials have been reported in the literature are summarized in Fig. S2 (Supporting information). The Li₂FeTiO₄ electrode in this work achieves an excellent specific capacity [41]. It can be seen from the Fig. 3, the structural stability of all electrode samples have slight change and the sample capacity have a certain attenuation. It is because that, Li⁺ is regularly embedded and extracted at the process of charge-discharge, resulting in expansion and contraction accompanied by stress variations. In addition, as shown in Figs. S3 and S4 (Supporting information), the Li₂FeTiO₄ cathode materials from raw materials of TiO₂ NB have good stability and rate properties.

  Figure 3

  

  Figure 3.  Discharge capacity cycling performance of Li₂FeTiO₄ samples electrodes at C/10 rate. Li₂FeTiO₄ samples electrodes synthesized from (a) TiO₂ anatase, (b) HTO-NW and (c) TiO₂ NB.

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  The representative discharge/charge curves of Li₂FeTiO₄ samples for the 1^st, 2^nd, 5^th, 10^th and 20^th cycles at a current density of 0.1 A/g are shown in Fig. 4. The initial discharge capacities of Li₂FeTiO₄ samples prepared by TiO₂ anatase, HTO-NW and TiO₂ NB are 99.2, 211.0 and 313.5 mAh/g, respectively. The properties of samples prepared from HTO-NW and TiO₂ NB materials are far better than TiO₂ anatase. The reason for the different shapes of charge and discharge curves of the three materials is that the grain sizes of different titanium sources materials are different, which affects the electro-chemical property of Li₂FeTiO₄ samples. In first discharge process, the Li₂FeTiO₄ samples prepared by TiO₂ NB electrode have a good reversible discharge capacity of 313.5 mAh/ g, higher than single electron Fe³⁺/Fe²⁺ reaction [31]. The Li₂FeTiO₄ samples show a size-dependent excess capacity (beyond the theoretical value of 295 mAh/g) in samples synthesized from TiO₂ NB with mean particle sizes of 67.6 nm. The discharge capacity of Li₂FeTiO₄ samples prepared by TiO₂ NB in the second cycle is about 333.2 mAh/g, and the discharge capacity decreases to 324.4 mAh/g after 10 cycles, it demonstrates that the stability of Li₂FeTiO₄ samples. It is worth noting that, after 20 cycles, the capacity returns to 341 mAh/g, which may be due to a pseudo reaction on the surface of the active substance. The high discharge capacity and stability of the Li₂FeTiO₄ samples prepared by TiO₂ NB suggest that Li₂FeTiO₄ composite materials can be used as the excellent cathode material.

  Figure 4

  

  Figure 4.  Charge/discharge curves of Li₂FeTiO₄ samples electrodes synthesized from (a) TiO₂ anatase, (b) HTO-NW and (c) TiO₂ NB. (d) Cyclic voltammetry profiles of sample electrodes from TiO₂ NB.

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  Fig. 4d shows the cyclic voltammogram (CV) curves of Li₂FeTiO₄ synthesized from TiO₂ NB (scanning rate is 0.1 mV/s), which have a visible redox peak. During the Li⁺ insertion/extraction process, a distinct cathode peak appeared at 1.68 V in the initial cycle, which was caused by the formation of Li₂FeTiO₄ and the transition from Fe³⁺ to Fe²⁺. Furthermore, anode peak was found at 2.06 V during the process of gradual oxidation. The curves of the second and third cycles coincide well, indicates that the electro-chemical reaction of Li₂FeTiO₄ has good reversibility, which results that Li₂FeTiO₄ electrode has good stability.

  The important electro-chemical performance parameters of Li₂FeTiO₄ composite electrode materials can be obtained by electro-chemical impedance spectroscopy (EIS). The Nyquist plots were shown in Fig. S5 (Supporting information). The operating conditions were the ac voltage amplitude of 10 mV and the frequency of 10^-1~10⁵ Hz. There was no significant difference in EIS spectrum of Li₂FeTiO₄ composite materials prepared from different materials. According to calculation, the lithium ion diffusion coefficient of Li₂FeTiO₄ composite cathode materials prepared by TiO₂ NB is 1.62×10^-12 cm²/s. In Fig. S5, the figure is a straight line in the low-frequency range and semicircle shape in the high-frequency range. The impedance at high frequency corresponds to the resistance of electrolyte (R₁), and the intercept at semicircle corresponds to the charge transfer resistance (R₂). The double-layer capacitance (CPE) is a constant phase element, which indicates the charge accumulation on both sides of electrode and electrolyte interface [42]. Because the concave semicircle can be observed in Fig. S5, the CPE can be used to replace the pure capacitance. Besides, the sloped line in the low-frequency region was related to the diffusion of Li⁺ in the electrode, and it can be attributed to the Warburg impedance (W₁). Inset picture of Fig. S5 represents the best fitting equivalent circuit, which can be expressed as follows:

  The R₂ of Li₂FeTiO₄ electrodes synthesized from titanium sources of TiO₂ NB is markedly lower than Li₂FeTiO₄ electrodes manufactured from HTO-NW and TiO₂ anatase, indicating that Li₂FeTiO₄ electrodes synthesized of TiO₂ NB have high electrical conductivity and fast charge transfer process. As shown in Figs. S5 and S6 (Supporting information), there were small differences between the different samples before and after the cycle. This demonstrates that Li₂FeTiO₄ electrodes synthesized of TiO₂NB have high electrical conductivity and fast charge transfer process.

  The experimental design proposed herein enables the assessment of Li₂FeTiO₄ from raw materials of HTO-N Wand TiO₂NB, which have good electro-chemical performances compared with TiO₂ anatase. The Li₂FeTiO₄ composites material shows a primary particle size of 50-200 nm of high crystallinity staggered. The specific surface area of Li₂FeTiO₄ was measured via nitrogen adsorption/desorption isotherms, demonstrating the potential application of Li₂FeTiO₄ and the synthesis grain size of HTO-NW and TiO₂ NB were relatively small and uniform. As shown in Fig. S7 (Supporting information), the specific capacity of optimized Li₂FeTiO₄ cathode materials from raw materials of TiO₂NBreached367.8 mAh/g, and it showed good circular performance in the discharge process at C/10 rate. Therefore, Li₂FeTiO₄ cathode materials synthesized from HTO-NW and TiO₂ NB had good properties.

  Declaration of competing interest

  No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (Nos. 51874079, 51674068), Natural Science Foundation of Hebei Province (No. E2018501091), The Training Foundation for Scientific Research of Talents Project, Hebei Province (No. A2016005004), The Fundamental Research Funds for the Central Universities (Nos. N172302001, N182312007, N182306001), Hebei Province Key Research and Development Plan Project (No. 19211302D), Qinhuangdao City University Student of Science and Technology Innovation and Entrepreneurship Project (Nos. PZB1810008T-46, PZB1810008T-14).

  Appendix A. Supplementary data

  Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2020.05.036.

  
  ^* Corresponding authors.
  E-mail addresses: tianyanglsh@163.com (S. Luo), wangswork@126.com (Q. Wang).
  
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  Figure 1  XRD of Li₂FeTiO₄composite cathode materials synthesized from (a) tetrabutyl titanate, (b) TiO₂ anatase, (c) HTO-NW and (d) TiO₂ NB.

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  Figure 2  SEM particle size distribution and nitrogen adsorption/desorption isotherms (inset: pore size distribution profile) of Li₂FeTiO₄ synthesized from (a, d, g) TiO₂ anatase, (b, e, h) HTO-NW and (c, f, i) TiO₂ NB.

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  Figure 3  Discharge capacity cycling performance of Li₂FeTiO₄ samples electrodes at C/10 rate. Li₂FeTiO₄ samples electrodes synthesized from (a) TiO₂ anatase, (b) HTO-NW and (c) TiO₂ NB.

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  Figure 4  Charge/discharge curves of Li₂FeTiO₄ samples electrodes synthesized from (a) TiO₂ anatase, (b) HTO-NW and (c) TiO₂ NB. (d) Cyclic voltammetry profiles of sample electrodes from TiO₂ NB.

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