Introduction

In recent years, renewable and green energy such as solar, wind, and tidal power has greatly attracted the interest of scientific researchers, while the rechargeable lithium-ion batteries (LIBs) are one of the most widely used energy storage devices used in portable electronic devices and electric vehicles [1,2,3,4,5,6]. Among them, LiFePO4 cathode material is considered prospective materials and attracted great interest because it is low cost, environmentally friendly, and energy dense [7,8,9,10]. However, the low electronic conductivity, as well as the slow lithium-ion diffusion rate, is the two major drawbacks of LiFePO4, which restricts its performance at a high rate [11,12,13]. Consequently, various strategies have been attempted to overcome these problems and improve the electrochemical properties of LiFePO4. Among all methods, choosing the proper composite materials and introducing into LiFePO4 can effectively enhance the ionic conductivity and electronic conductivity of the active materials in the electrodes [14,15,16]. Medvedeva et al. synthesized LiFePO4, LiMn2O4, and LiNi0.82Co0.18O2 by ultrasonic treatment and analyzed the electrochemical performance of composites [17]. Liu et al. present a mesostructured LiFePO4/reduced graphene oxide composite material which exhibits a high capacity with 161 mAh g−1 after 200 cycles at 0.2 C, accompanying with a Coulombic efficiency of about 100% [18]. Junhui Jeong et al. enhanced the cycling performance of LIBs by rational designing oxide/carbon composites, the rate capacity was improved which is mainly due to the enhancement of lithium-ion transport through the nanoperforations [19]. Yang et al. prepared a 3D spray-dried micro/mesoporous LiFePO4/porous graphene oxide/C composite material by a three-step process: hydrothermal process, carbon coating step, and spray dry process. The SP-LFP/PGO/C composite exhibits the performance of the discharge capacity is 160, 152, 151 mAh g−1 at 0.1 C, 0.2 C, and 0.5 C rate [20,21,22].

In this paper, we have successfully synthesized composites with high ionic conductivity through the addition of H2Ti3O7 and TiO2 into LiFePO4 and methodically studied the effect of composite materials on the cycling capability and rate capability. Furthermore, their crystal structure and morphology of the as-prepared composite materials have been evaluated by the X-ray diffraction and the scanning electron microscope.

Experimental

Preparation of H2Ti3O7 and TiO2

H2Ti3O7 and TiO2 were fabricated by the ultrasonic chemical hydrothermal approach. First, the TiO2 and NaOH with a certain concentration reacted 2 h in the ultrasonic generator. Subsequently, the mixture was placed in a Teflon autoclave and hydrothermal reaction for 48 h at 120~180 °C. After the reaction, the reaction kettle and the filter cake were washed by deionized water and anhydrous alcohol several times until the pH value reached 7. Finally, the H2Ti3O7 was obtained by drying at 80 °C under vacuum. At the same time, a part of the H2Ti3O7 was calcinated to obtain TiO2 in the electric stove.

Synthesis of LiFePO4/H2Ti3O7 and LiFePO4/TiO2 composites

In the experiment, the procedures for different contents of LiFePO4/H2Ti3O7 and LiFePO4/TiO2 composite materials were synthesized via the convenient sol-gel method, and high purity N2 was used as protecting gas. Firstly, adding LiNO3 and FeCl2·4H2O (in a 1 : 1 M ratio) to a solution of ethanol. Then, the above solution and a certain amount of nanomaterials were sequentially dissolved in NH4H2PO4 and citric acid. After ultrasonic processing for 2 h and vacuum treatments for 12 h, the dried gel was then obtained by heat treating and magnetic stirring at 80 °C. Next, the dry gel was ground into powder and pretreatment of 6~10 h under the protection of N2 at 450 °C. The powder was ball-milled for 4 h, dried in air, and then calcined under N2 atmosphere at 750 °C for 6~10 h to gain composite materials of LiFePO4/H2Ti3O7 and LiFePO4/TiO2. The preparation process of the LiFePO4/H2Ti3O7 and LiFePO4/TiO2 composites is shown in Fig. 1.

Fig. 1
figure 1

Schematic illustration for the synthesis process of the composites

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Characterization

The crystal structures of the as-synthesized materials were characterized via X-ray diffraction (DX-2500) equipped with Cu Kα radiation (λ = 0.15418 nm) from 2θ = 10~80°, which the working voltage of 30 kV and the tube current of 25 mA. The surface morphologies were identified by scanning electron microscopy (SEM, SSX-550). The Brunauer-Emmett-Teller (BET) multiple points method with a specific surface area analyzer (SSA-4300) were used to measure the pore-size distribution and specific surface area. The electrochemical properties were conducted through the button cell (CR-2032) and all cells were assembled in a glove-box under the argon atmosphere. The working electrode was mixed by the 80% active materials, 10% acetylene black, and 10% polyvinylidene fluoride (PVDF). N-methyl-2-pyrrolidone (NMP) was used as a dispersant to mix them together and form a viscous slurry, followed by coating the slurry on Al foils and dried at 80 °C for 6 h to volatilize the NMP [23]. To remove excess moisture from the electrode, the working electrode should be vacuum dried in an oven at 120 °C for 8 h before assembling the battery [24]. The cathode and the separators were lithium tablets and Celgard 2400, respectively. One molar LiPF6 solution was dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 1 : 1, which was used as the electrolyte. The charge and discharge test was conducted by the Land CT2001A battery system between 2.5 and 4.2 V under room temperature.

Results and discussion

Figure 2a exhibits the XRD patterns of the LiFePO4. The sample has characteristic diffraction peaks (200), (101), (111), (211), and (311), which corresponds to the olivine structure LiFePO4 standard card JCPDS 83-2092. In addition, no obvious impurity phases are observed, which indicates a pure phase LiFePO4 has achieved. Furthermore, the diffraction peak of the calcined LiFePO4 is sharp, and the crystallinity is excellent. SEM image of as-prepared LiFePO4 is shown in Fig. 1b. It is obviously seen that slight agglomeration, which is caused by the carbon. And the result is in good agreement with the XRD results [25].

Fig. 2
figure 2

a XRD patterns of the LiFePO4. b SEM image of the LiFePO4

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The surface area and the pore-size distribution of LiFePO4, H2Ti3O7, and TiO2 are presented in Fig. 3 and its inset, respectively. The H2Ti3O7 and TiO2 samples exhibit type III isotherm curves at the relative pressure of 0.2–1.0, suggesting the existence of the mesoporous structure. And with increasing pressure, the adsorption increases slowly. Figure 2a exhibits that the BET specific surface area of H2Ti3O7 (316.009 m2/g) is much higher than that of LiFePO4 (53.758 m2/g). Moreover, the pore-size distribution of H2Ti3O7 has distributed around 0.23 and 25.49 nm with an average pore diameter of 6.82 nm. The large specific surface area will be favorable for the electrolyte to pass through and provide more active sites for Li-ion insertion and extraction, thus accelerating the ionic and electronic diffusion [26, 27]. Figure 3b presents that the specific surface area of TiO2 is 284.4 m2/g. And the pore-size distribution curve displays that the pore-size is distributed between 0.6 and 23.48 nm.

Fig. 3
figure 3

N2 adsorption and desorption isotherms of (a) H2Ti3O7 and LiFePO4 and (b) TiO2 and LiFePO4; the insert was pore-size distribution curve by the Barrette-Joyner-Halenda formula

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Figure 4 a and b display the XRD patterns of composites with different adding amounts of H2Ti3O7 and TiO2. The major diffraction peaks of (200), (101), (111), (211), and (311) exist in the six composites, and the diffraction peaks position and intensities also correspond well to orthorhombic crystal system LiFePO4 with olivine structure. The diffraction peaks of H2Ti3O7 and TiO2 are not observed in the above XRD diffraction pattern, mainly because the addition of H2Ti3O7 nanotubes and TiO2 nanotubes is small and the crystallinity is poor, and the diffraction peaks of A and B are “obscured” by the strong diffraction peaks of lithium iron phosphate. Furthermore, no impurity peaks are detected in the samples, which means the addition of H2Ti3O7 and TiO2 does not significantly change the crystal structure of LiFePO4.

Fig. 4
figure 4

XRD diffraction patterns of different adding amount: (a) H2Ti3O7, (b) TiO2

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The SEM images of the comprised materials with 1%, 3%, and 5% H2Ti3O7 are demonstrated in Fig. 5a–c. It is observed that the particle size distribution is varied with H2Ti3O7 adding content. In addition, with the increase of adding amounts, particle agglomeration is decreased. For comparison, the SEM photographs of different addition of TiO2 are displayed in Fig. 5d–f. It is found that the size of particles decreases and the size distribution is relatively uniform with TiO2 addition contents increasing. Although both of them showed no significant difference in the morphology, the porous structure of LiFePO4/H2Ti3O7 composite, which can shorten the lithium-ion diffusion distance, thus provide better performance.

Fig. 5
figure 5

SEM images of different adding amount: (a–c) H2Ti3O7 and (b–f) TiO2

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Figure 6 a shows the rate capacities of composites degrees with different adding amounts of H2Ti3O7 from 0.2 to 2.0 C. Obviously, as the current density increases, all the samples present a decrease in discharge capacities systematically. Compared with the LiFePO4 electrode, the LiFePO4/H2Ti3O7 samples exhibit a better rate of the property. In all the samples, the 1% content LiFePO4/H2Ti3O7 composite electrode exhibits the highest discharge capacity at 0.5 C. In addition, the 1% content LiFePO4/H2Ti3O7 composite electrode shows relatively moderate capacity fading, which is compared with the other samples. The maximum discharge capacity of 1% content LiFePO4/H2Ti3O7 at 0.5 C is 161.1 mAh g−1, while that of LiFePO4 is only 75.1 mAh g−1.

Fig. 6
figure 6

a, b Rate capability of LiFePO4 different amounts LiFePO4/H2Ti3O7 LiFePO4/TiO2 composite at different rate capability. c Discharge profiles of 1% LiFePO4/H2Ti3O7 and LiFePO4/TiO2 at various rates from 0.1 to 10 C. d Initial charge-discharge curves of 1% LiFePO4/H2Ti3O7 and LiFePO4/TiO2 composites

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For comparison, Fig. 6b exhibits the discharge capacities of composites degrees with different adding amounts of TiO2 at different rate capability. It is clearly seen that the 1% TiO2 sample exhibits more excellent rate capabilities than LiFePO4. For samples at 0.2, 0.5, 1.0, and 2.0 C, the discharge capacities are 137.7, 134.9, 123.5, and 118.9 mAh g−1, respectively. Obviously, the 1% content LiFePO4/H2Ti3O7 composite material exhibits the best rate capability among all the synthesized samples.

The initial discharge curves of 1% H2Ti3O7 and TiO2 from 0.2 to 2.0 C are illustrated in Fig. 6c. As observed from the curves, the LiFePO4/H2Ti3O7 electrode shows a superior discharge capacity to the LiFePO4/TiO2 electrode at all discharge rates. Moreover, the first discharge capacity of 1% H2Ti3O7 is 151.9 mAh g−1 at 0.5 C, while the 1% TiO2 is 129.9 mAh g−1. In other words, 1% of H2Ti3O7 added provides better pathways for rapid ion diffusion. Besides, it is noted that with the increasing current rate, the discharge plateau shows a drop trend and the discharge capacity decreases for all samples.

Figure 6 d show the initial charge-discharge curves of 1% content LiFePO4/H2Ti3O7 and LiFePO4/TiO2 composite material at 0.5 C. Obviously, typically reversible voltages of ∼3.4 V were displayed for LiFePO4/H2Ti3O7 and LiFePO4/TiO2, respectively, corresponding to the Fe3+/Fe2+ redox couple. And the initial discharge capacities are 151.9 mAh g−1 and 129.9 mAh g−1 for two composites, which indicates that the electrochemical performance of LiFePO4 is improved effectively after H2Ti3O7 and TiO2 were introduced. Additionally, the gap between charge and discharge plateaus for two samples is narrower, which demonstrating that the samples have lower overall resistance and can dramatically reduce the polarization.

Conclusions

In summary, the LiFePO4/H2Ti3O7 and LiFePO4/TiO2 samples were successfully synthesized via a simple sol-gel method. The introduction of H2Ti3O7 and TiO2 can reduce particle size and improve the uniformity of size distribution in a certain range. The H2Ti3O7 and TiO2 had a higher specific surface area than the LiFePO4. The electrochemical performance of LiFePO4 has been significantly improved after composited. Among all the samples, the 1% H2Ti3O7 exhibited the best electrochemical properties with the maximum discharge capacity of 161.1 mAh g−1 and capacity retention is 105.83% at 0.5 C. Therefore, the appropriate introduction of H2Ti3O7 is an efficient way to enhance the cycle stability and rate performance of LiFePO4.