Introduction

Recently, with the development of society, unlimited usage of energy source results in energy crisis. The development of lithium-ion batteries (LIBs) with high-specific energy, discharge specific capacity, and excellent cycling stability has become a research focus currently [1,2,3,4,5,6]. And there are some important issues that need to be resolved before LIBs could be widely used. Since battery performances are mainly determined by cathode materials, much work has been done to develop cathode materials for the lithium ion batteries with excellent performance in new era [1, 2, 5,6,7,8,9]. Monoclinic lithium vanadium phosphate (Li3V2(PO4)3, LVP) is a good cathode candidate material for LIBs owing to its stable structure, high theoretical specific capacity, and good ion diffusion coefficient [10,11,12]. Li3V2(PO4)3 three-dimensional network structure is composed of VO6 octahedron and PO4 tetrahedron by sharing the apex oxygen atoms, and this structure enlarge the space of the insert and extraction of the Li ions [11, 13]. With charge voltage rang of 3.0~4.3 V, its discharge specific capacity can be reached 133 mAhg−1, and during the voltage range 3.0~4.8 V, its capacity can be reached 197 mAhg-1 [14,15,16]; at the same time, the three Li ions can be reversibly embedded and escaped from the lattice.

However, some disadvantage exists for Li3V2(PO4)3 with relatively small intrinsic electronic conductivity (10−7 Scm−1) and poor cycle stability [17, 18], which enormously restrict its large-scale applications. Therefore, in recent years, giant efforts have been done to optimize the electrochemical properties of Li3V2(PO4)3. Carbon coating and cation doping are the most efficacious methods [16, 19]. Up to now, multifarious carbon resources and lots of cations have been investigated extensively using various preparation routes to optimize the electrochemical performance of Li3V2(PO4)3. V-site substitutions of Li3V2(PO4)3 by Ni2+, Al3+, Na+, Cr3+ have been extensively investigated [20,21,22,23,24,25] and are considered to be an effective method to improve electrochemical performance. Doping Li3V2(PO4)3 lattice with Cr3+ could stabilize the structure of Li3V2(PO4)3, significantly enhancing the specific capacity and cycle life [25].

Ti–Fe codoped samples Li3V1.9Ti0.05Fe0.05(PO4)3 have much better high-rate discharge capability and long-term cycling performance than those of Li3V2(PO4)3. However, the electrochemical performance of Li3V1.9Ti0.05Mn0.05(PO4)3 is worse than that of Li3V2(PO4)3 [26]. Until now, Li3V2(PO4)3 doped only by Ti4+ with carbothermal reduction method have not been studied with voltage range of 3.0–4.3 V [27, 28]. The carbon coating of Li3V2(PO4)3 by citric acid, glucose, maltose, and alginic acid have been reported [29,30,31,32]. Because of cheapness, stearic acid as both carbon source and surfactant in preparation of Li3V2(PO4)3 has advantage.

In current work, Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.04, 0.06, and 0.08) cathode materials were gotten via stearic acid carbon–thermal reduction method [33]. The crystal phase, microstructures, and electrochemical properties of the composites were characterized by XRD, SEM, and electrochemical performance study. And the results showed that Ti doping can not only improve the first discharge specific capacity of Li3V2(PO4)3/C composites but also improve its cycle performance at different rate.

Experimental

Preparation of composite materials

Ti doping Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.04, 0.06, and 0.08) composite materials were prepared via stearic acid carbon–thermal reduction method, using LiH2PO4, V2O5, TiO2, and stearic acid (CH3(CH2)16COOH, 12.3 wt%) as the raw materials. In which stearic acid acted as the carbon source, chelating agent, and surface active agent [33]. The stoichiometric ratio of Li:V:Ti:P equals to 3:2−x:x:3. The mixtures were ball milled for 8 h at a rate of 300 r/min with absolute ethanol as a medium and then dried at 80 °C for 12 h. After that, precursor was sintered for 10 h with 750 °C and nitrogen atmosphere, Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.04, 0.06, and 0.08) composite powders were obtained.

Characterization of composite materials

The crystalline phases of the Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.04, 0.06, and 0.08) were characterized using X-ray diffraction analyzer (DX-2700) with Cu Kα radiation, and the diffraction angle ranged between 10 and 70° with scan rate of 0.03°/s. The grain morphologies and microstructures of the samples were observed using a scanning electron microscope (FE-SEM, S3400N).

The electrochemical performance of the Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.04, 0.06, and 0.08) composite materials were tested after the samples were assembled into 2016 coin cells. The cathodes were prepared with Li3V2−xTix(PO4)3/C, PVDF, and acetylene black at the weight ratio of 8:1:1, and N-Methylpyrrolidone(NMP) as solvent. After the NMP evaporated, electrodes were punched into a disc with an active area of 1.54 cm2 and an active loading of ~ 3 mg cm−2. In the 2016 coin cells, lithium metal was used as anode material, and the polypropylene (Celgard2400) was used as the separator membrane. The electrolyte was 1 M LiPF6 with EC+DEC+DMC(volume ratio of 1:1:1) as solvent. And all the cells were assembled in Mikrouna glove box. Constant current charge/discharge measurements were performed with voltage window 3~4.3 V at charge–discharge rates of 0.2 C, 0.5 C, 1.0 C, 2.0 C, and 5.0 C on a Land CT2001 testing system (Wuhan, China) [33]. After 60th charge–discharge cycle, the coin cells were subjected to electrochemical impedance spectroscopy (EIS) with CHI660C electrochemical workstation, and the applied frequency range was in the range of 100 kHZ~1 HZ with 5 mV amplitude.

Results and discussion

Structure and morphology characteristics

X-Ray diffraction profiles of the Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.06, and 0.08) composites are displayed in Fig. 1. As can be seen that the diffraction profiles of the Ti doping composites are very similar to the diffraction profiles of the undoped composite, and no any other extra diffraction reflections can be observed. This shows that atom Ti may enter into the crystal lattice of Li3V2−xTix(PO4)3 rather than forms heterogeneous substance. The peaks of the XRD profile almost have the similar positions with standard monoclinic Li3V2(PO4)3 (P21/n space group, JCPDS #97-009-6962). And which demonstrates the formation of well crystallized Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.06, and 0.08). Residual carbon is not detected on the XRD diffraction profiles of the Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.04, 0.06, and 0.08) composites; its season may be that it has amorphous structure or the carbon film coating on the Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.06, and 0.08) composites is too thin.

Fig. 1
figure 1

XRD profiles of Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.06, and 0.08)

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SEM surface morphologies of the pristine composites and Ti doping Li3V2(PO4)3/C composites are given in Fig. 2a–d. From the figures, we can obtain that the pristine Li3V2(PO4)3/C composite is consists of irregular grain sheets of different sizes. After Ti doping of the composites, the Li3V2−xTix(PO4)3/C(x ~ 0.02, 0.04, 0.06, and 0.08) composites have smaller grain size with more nanorods. Also, some holes can be observed from them. The generate of the pore structure can attribute to the combined action of Ti doping and stearic acid, the decomposition of stearic acid is conducive to the generation of the holes. This indicates that Ti doping can reduce the grain size and help for the generation of the holes. The smaller grain size is, the shorter ion diffusion path will become. And thus, Ti doping composites have smaller relative internal resistance, lower self-polarization, and larger surface area, which improves the safety and rate performance of the samples. Nanorods with 60 nm width in Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.06, and 0.08) composites also plays a big role in increasing the ion diffusion rate and conductivity. Figure 2e shows the TEM characterization of Li3V2−xTix(PO4)3/C with x = 0.06, carbon layer can be seen at the surface of the LVP, which is beneficial for the electron conductivity improving.

Fig. 2
figure 2

SEM of the Li3V2−xTix(PO4)3/C with ax = 0, bx = 0.02, cx = 0.06, dx = 0.08; HRTEM of e Li3V2−xTix(PO4)3/C with x = 0.06

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First charge–discharge performance

To determine the electrochemical properties of the Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.06, and 0.08) composites powder, the charge–discharge curves were measured with a certain current density. Figure 3 presents the first charge/discharge characteristics of all the composites at 0.2 C and 3.0~4.3 V voltage. The charge profile of the pristine composite has three platforms with voltages of approximate 3.60, 3.68, and 4.08 V, respectively. Similarly, there are three discharge platforms during the discharge process at about 3.55, 3.63, and 4.03 V, respectively. Each platform of the charge/discharge curves corresponds to a charge and discharge reaction of lithium ions. There are two charging platforms (3.60 and 3.68 V) for the first lithium ion to extract during charging because of the existence of Li2.5V2(PO4)3 ordered phase. After that, the second Li-ion extract, which corresponds a single charging platform at about 4.08 V [34]. The three potential platforms correspond to the redox reaction of V3+/V4+ pairs.

Fig. 3
figure 3

The first charge–discharge characteristics of Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.06, 0.08) electrodes with 0.2 C

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Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.06, and 0.08) samples with different x have the same electrochemical reaction mechanism because they have the similar charging and discharging platform. The voltage platform of the charge and discharge curve varies with the amount of Ti doping. During charging, the voltage of the platforms gradually reduced with x increased, and the sample with x of 0.06 has the lowest voltage platform. During discharging, the voltage of the platforms gradually increased with x increased, and the sample with x of 0.06 also has the highest voltage platform. These phenomena show that Li3V2−xTix(PO4)3/C (x = 0.06) powder has the weakest polarization and so it has the highest discharge platform voltage. In addition, it is also apparent that the first discharge performances of the Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.06, and 0.08) samples are dependent on the Ti doping amounts. The discharging capacities are approximately 112.4 mAhg−1, 115.5 mAhg−1, 121.9 mAhg−1 and 120.8 mAhg−1, respectively. In general, Ti doping samples have higher discharging capacity than undoped sample, the Li3V2−xTix(PO4)3/C sample with x ~ 0.06 have the highest discharge platform and discharge voltage, and also the highest discharging capacity. The reason is that Ti doping has been verified to be effective to improve the intrinsic electron transfer conductivity by decreasing the band gap [35] and smaller grain size of Li3V2−xTix(PO4)3/C make the ion diffusion path shorter.

Rate and cycle performance

The cycling characteristics of all Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.06, and 0.08) composites at 0.2 C rate are shown in Fig. 4. As can be seen from the figure, that capacity decline of the Ti doping composites is slower than that of the pristine one, and the Ti doping composites show better cycle stability at 0.2 C. For the x ~ 0, 0.02, 0.06, and 0.08 samples, 93.95%, 94.46%, 98.69%, and 94.62% of the first discharge capacity was retained after 100 cycles at 0.2 C, respectively. And among them, Li3V1.94Ti0.06(PO4)3/C has the highest capacity retention of 98.69%, and also the maximum specific capacity of 120.3 mAhg−1.

Fig. 4
figure 4

Cycle performance of Li3V2−xTix(PO4)3/C(x ~ 0, 0.02, 0.06, 0.08) with charge/discharge voltage range 3.0~4.3 V

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To study the rate characteristics of the Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.06, and 0.08) composites, all the samples were cycled at four current densities (0.5 C, 1 C, 2 C, and 5 C) with 3.0~4.3 V, and the cyclic results are presented in Fig. 5. The first discharge capacities of the Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.06, and 0.08) samples at 0.5 C are approximately 108.1 mAhg−1, 120.3 mAhg−1, 123.5 mAhg−1, and 113.5 mAhg−1, respectively. After 15 cycles, discharge capacities of the samples are approximately 108.6 mAhg−1, 120.6 mAhg−1, 121.5 mAhg−1, and 115.2 mAhg−1, respectively, and there’s basically no reduction in capacity.

Fig. 5
figure 5

Cyclic characteristics of Li3V2−xTix(PO4)3/C electrodes at different rates with x ~ 0, 0.02, 0.06, 0.08

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After cycled at 0.5 C for 15 times, the composites were cycled at 1 C, 2 C, and 5 C, respectively. Although the discharge capacity of each composite was different, all composites showed good circulation performance. The composites with x ~ 0.06 and 0.08 have higher discharge capacity than undoped composites at four current densities (0.5 C, 1 C, 2 C, and 5 C), and the composites with x ~ 0.06 have the highest discharge capacity. Therefore, moderate Ti doping can improve cycle and discharge performance with big current rate.

EIS measurements

EIS measurements were performed to analyze the structure of the electrochemical system and the nature of the electrode process with Ti doping over 100 kHz~1 Hz frequency range. Figure 6 shows the EIS profiles of Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.06, and 0.08). Each profile consists of a semicircle in the high frequency zone and a slant in the low frequency zone. The semicircle reflects charge transfer reaction resistance, and the slant line reflects Li-ion diffusion resistance in the solid [36]. The depressed semicircles of Li3V2−xTix(PO4)3/C with x ~ 0.02, 0.06, and 0.08 are much smaller than the sample with x ~ 0, which shows the charge transfer resistance for Ti doping samples is reduced by doping, and it is beneficial for the redox reaction of cathode material and its kinetic process of the reaction. Because Ti doping reduce the grain size, Li3V2−xTix(PO4)3/C with x ~ 0.02 and 0.06 have steeper slant lines and it show Ti doping accelerate the solid state diffusion of Li+, so Li3V2−xTix(PO4)3/C with x ~ 0.02 and 0.06 have bigger diffusion coefficients than the pristine Li3V2(PO4)3. And so the rate and cycle performance will be improved by Ti doping, especially for x ~ 0.06 samples.

Fig. 6
figure 6

EIS profiles of Li3V2−xTix(PO4)3 samples

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Conclusions

In summary, Ti doping Li3V2−xTix(PO4)3/C (x ~ 0, 0.02, 0.06, 0.08) was synthesized by carbon–thermal reduction method. Research showed Ti doping have no effect on the XRD profile of Li3V2(PO4)3/C composites and there have no extra diffraction reflections at the profiles of the Ti doping samples, which shows that Ti successfully enter into the crystal structure of Li3V2(PO4)3. Ti doping can improve the cycling performance, which is due to its structural stability characteristics. The rate performance is improved by Ti doping with x ~ 0.06, which is because its lower charge transfer resistance. And these results can also be attributed to four factors, particle size reduction, more nanorods, more pores in the microstructure of Li3V2−xTix(PO4)3/C(x ~ 0.02, 0.06, 0.08) and the high Li-ion diffusivity of Ti doping Li3V2(PO4)3/C.