稀有金属(英文版) 2015,34(08),586-589
收稿日期：12 July 2013
基金：financially supported by the National Natural Science Foundation of China (Nos. 51164007 and 51204114);
Synthesis and electrochemical properties of Ti-doped Li3V2(PO4)_3/C cathode materials
Sheng-Kui Zhong You Wang Ling Wu Jie-Qun Liu
School of Iron and Steel, Soochow University
Abstract：
Ti-doped Li3V2(PO4)3/C cathode materials were synthesized by a solid-state method. The properties of the samples were characterized by X-ray diffraction(XRD), scanning electronic microscopic(SEM), and electrochemical tests. XRD results indicate that Ti-doping and carbon coating do not alter the structure of Li3V2(PO4)3.SEM images show that Ti-doping could reduce the particle size of Li3V2(PO4)3. Electrochemical tests reveal that Li3V1.95Ti0.05(PO4)3/C possesses the best electrochemical performances. It delivers the first discharge specific capacities of 124.4, 123.5, 120.9, and 114.9 m Ahág-1at0.1C, 1.0C, 2.0C, and 3.0C rates, respectively, and shows excellent cycling performance. The improved electrochemical performances are attributed to the Ti-doping,which increases the electronic and ionic conductivities of the cathode material.
Keyword：
Lithium-ion battery; Cathode material; Li3V2(PO4)3; Ti-doping;
Author： Ling Wu,e-mail: lwu@suda.edu.cn;
Received： 12 July 2013
1 Introduction
Lithium transition metal oxide Li Co O2is the first commercialized cathode material for lithium-ion batteries. However, cobalt is rare and toxic, which is not favorable for the further development of Li Co O2. Since Li Fe PO4was firstly reported as a cathode material for lithium-ion batteries in 1997 [1], a series of lithium transition metal phosphates such as Li MPO4(M = Fe, Mn, Co, and Ni) [2– 4], Li VPO4F [5], and Li3V2(PO4)3[6–9] have been widely studied due to their excellent thermal stability and electrochemical properties.
Li3V2(PO4)3involves three Li crystal locations with a theoretical discharge specific capacity of 197 m Ah?g-1, and the average working voltage is about 3.9 V, which is much higher than that of Li Fe PO4(3.4 V). It seems that Li3V2(PO4)3is one of the most promising cathode material candidates on the basis of the above advantages. However, the drawback of separated [VO6] leads to the poor electronic and ion conductivities of Li3V2(PO4)3, which results in the unacceptable fade of capacity and makes it impossible to realize commercialization for Li3V2(PO4)3. Modifications were made to overcome the disadvantages of the material with the purpose of improving the electrochemical performances of Li3V2(PO4)3cathode material.
Various methods such as carbon coating and metal cations doping were used to improve the electrochemical performances of Li3V2(PO4)3. For example, Fu et al. [7] prepared Li3V2(PO4)3/C cathode material by a sol–gel method using glucose as carbon source, the electrochemical tests reveal that better rate capability and higher discharge capacity are obtained by carbon coating. Zhong et al. [10] prepared Y-doped Li3V2(PO4)3by a carbothermal reduction process, the results indicate that metal cations doping do not change the monoclinic structure of Li3V2(PO4)3, but can reduce the charge transfer resistance and enhance the electrode reaction reversibility. In addition, Ti4?, Mn2?, Al3?, Nb5?, Mg2?, Fe3?etc. [11–17], were also used as doping cations.
Titanate coupling agent TC-Wt is an important chemical raw material which can be used as dispersant, surfactant, coupling agent, etc. It was first reported as titanium and carbon sources for preparation of Ti-doped Li Fe PO4/C by Gong et al. [18]. However, there are still few reports about the modifications of Li3V2(PO4)3by titanate coupling agent (TC-Wt). In this paper, Ti-doped Li3V2(PO4)3/C samples were prepared by a solid-state method using the titanate coupling agent (TC-Wt) as titanium and carbon sources. The effect of Ti-doping on the electrochemical performances of Li3V2(PO4)3was studied.
2 Experimental
Li3V2-xTix(PO4)3/C (x = 0, 0.03, 0.05, and 0.07) cathode materials were prepared by a solid-state method using Li2CO3(A.R.), V2O5(A.R.), NH4H2PO4(A.R.), and titanate coupling agent TC-Wt as starting materials. Titanate coupling agent TC-Wt played the roles of dispersant, surfactant, titanium, and carbon sources. Stoichiometric amount of Li2CO3, V2O5, NH4H2PO4, and titanate coupling agent TC-Wt was dispersed in ethanol and then thoroughly ground for 1 h. The mixtures were dried at 80 °C for 24 h in vacuum, and then transferred into a tube furnace. The samples were calcined at 800 °C for 12 h under argon atmosphere to obtain the Li3V2-xTix(PO4)3/C samples.
The phase analysis of the samples was conducted via X-ray diffraction (XRD, XPert-Pro) using Cu Ka radiation scanning in the range of 108 to 808 (2h). The morphology of the samples was observed by scanning electron microscope (SEM, GSM-6380LV).
The electrochemical characterizations were performed using a CR2025 coin-type cell. The positive electrodes were fabricated from 10 wt% acetylene black, 10 wt% polyvinylidene fluoride (PVDF) binder, and 80 wt% active material. The blended slurries were pasted onto an aluminum current collector, dried at 120 °C for 4 h in vacuum. Then, the cells were assembled in an argon-filled glove box with Li3V2-xTix(PO4)3/C as the cathode, Li metal as the anode and 1 M Li PF6solution in a mixture of ethylene carbonate and dimethyl carbonate with a volumetric ratio of 1:1 as the electrolyte. The cells were charged and discharged in the voltage range of 3.0–4.2 V at room temperature. Electrochemical impedance spectroscopy (EIS) tests were performed with a CHI660D electrochemical work station. EIS experiments were carried out in the frequency range of 0.01–100.00 k Hz.
3 Results and discussion
Figure 1 shows the XRD patterns of Li3V2-xTix(PO4)3/C samples. It can be seen that all the samples are well consistent with monoclinic Li3V2(PO4)3(space group P21/n). No impurity phases are indexed, indicating that Ti ions are doped into the crystal lattice of Li3V2(PO4)3, and Ti-doping does not change the crystal structure of Li3V2(PO4)3. Carbon is not observed for all the samples, which reveals that the residue carbon is amorphous.
Fig.1 XRD patterns of Li3V2-xTix(PO4)3/C samples
Fig.2 SEM images of Li3V2-xTix(PO4)3/C samples: a x = 0, b x = 0.03, c x = 0.05, and d x = 0.07
Figure 2 shows the SEM images of Li3V2-xTix(PO4)3/C. It is found that all the samples are composed of slightly agglomerated primary particles. And the smaller and more homogeneous grains can be observed for the Ti-doped samples, which illustrate that the particle size of Li3V2(PO4)3can be reduced by the Ti-doping. It is well known that small particles will have an advantage in the rate performance of a battery because of the short diffusion paths of Li-ions. Therefore, it can be deduced that the electrochemical properties of Li3V2(PO4)3/C could be enhanced by the Ti-doping.
Figure 3 shows the initial charge–discharge curves of samples at 0.1C rate between the potential range of 3.0–4.2 V. As shown, the charge curves contain three plateaus around 3.60, 3.68, and 4.08 V, which correspond to the extraction of Li-ions and the phase transition of LiyV2(PO4)3from y = 3.0 to 2.5, 2.0, and 1.0, respectively. The first Li-ion is extracted in two steps (3.60 and 3.68 V) because of the existence of an ordered-Li2.5V2(PO4)3phase at a mixed V3?/V4?state. Then, a single step for extracting the second Li-ion at 4.1 V can be observed, which corresponds to the complete oxidation of V3?to V4?. Three corresponding plateaus around 4.04, 3.65, and 3.56 V during the discharge process are owing to the insertion of two Li-ions that accompanied the phase transition of LiyV2(PO4)3from y = 1.0 to 1.5, 2.0, and 3.0. The initial charge specific capacities of Li3V2-xTix(PO4)3/C with x = 0, 0.03, 0.05, and 0.07 are 108.6, 118.2, 131.7, and 120.0 m Ah?g-1, respectively, and the initial discharge capacities are 101.1, 112.4, 124.4, and 118.1 m Ah?g-1, respectively. The corresponding coulombic efficiencies are 93.1, 95.1, 94.5, and 98.5 %, respectively. It is obvious that the discharge capacity and the first coulomb efficiency are affected by Ti-doping content, and the optimal content is x = 0.05. The improved electrochemical performances of Ti-doped samples should be ascribed to the enhanced electronic and ionic conductivities by Ti-doping.
Fig.3 Initial charge–discharge curves of Li3V2-xTix(PO4)3/C at 0.1C rate
Fig.4 Discharge curves of Li3V1.95Ti0.05(PO4)3/C at various rates
Fig.5 Cycling performance of Li3V2-xTix(PO4)3/C at 1.0C rate
Figure 4 shows the rate performance of Li3V1.95Ti0.05(PO4)3/C. As shown, the discharge specific capacity sightly declines with the increase of current density. The initial discharge specific capacities are 124.4, 123.5, 120.9, and 114.9 m Ah?g-1at 0.1C, 1.0C, 2.0C, and 3.0C, respectively. Nevertheless, the discharge curves exhibit serious polarization, which should be ascribed to the micron-sized particles.
Figure 5 shows the cycling performance of samples at 1.0C rate between the potential range of 3.0–4.2 V. When the Ti content x is 0, 0.03, 0.05, and 0.07, the initial discharge specific capacities at 1.0C rate are 89.2, 110.5, 123.5, and 112.8 m Ah?g-1, respectively. After 30 cycles, the discharge capacities remain 71.8, 97.9, 117.7, and 101.8 m Ah?g-1, and show 80.5, 88.6, 95.3, and 90.2 % of its initial discharge capacities, respectively. The cycling performance of Li3V2(PO4)3is remarkably enhanced by Tidoping, which should be attributed to the enhanced structural stability by Ti-doping. It can be found that the optimal Ti content is 0.05.
The Nyquist plots of Li3V2(PO4)3/C and Li3V1.95Ti0.05(PO4)3/C are shown in Fig. 6. The cells cycled 4 times at 0.1C rate before EIS testing. As shown, both Nyquist plots are comprised of a depressed semicircle in high-frequency region and a straight line in low-frequency region. The intercept at real axis in the very high-frequency region identifies the solution resistance (Rs). The diameter of semicircle along the real axis in mid-frequencies represents the charge transfer resistance (Rct). The inclined line in low frequencies corresponds to the Warburg impedance, which represents the diffusion of lithium ions in the cathode material. Is is found that both the electrodesshow similar Rs. However, the Rctof Li3V1.95Ti0.05(PO4)3/ C is much smaller than that of Li3V2(PO4)3/C, which indicates that the charge transfer speed of the electrochemical reaction is significantly increased by Ti-doping.
Fig.6 Nyquist plots of Li3V2(PO4)3/C and Li3V1.95Ti0.05(PO4)3/C. Inset enlarged view of high-frequency region (discharge state: 2.5 V)
4 Conclusion
Li3V2-xTix(PO4)3/C (x = 0, 0.03, 0.05, and 0.07) cathode materials were successfully synthesized by a solid-state method. It is found that Ti-doping and carbon coating do not change the crystal structure of Li3V2(PO4)3. Ti-doping can reduce the particle size of Li3V2(PO4)3. Electrochemical tests reveal that Ti-doping can significantly increase the first discharge capacity, improve the rate and cycling performances of Li3V2(PO4)3/C.
