Sol–gel-assisted, fast and low-temperature synthesis of La-doped Li₃V₂(PO₄)₃/C cathode materials for lithium-ion batteries

Abstract

A series of La-doped Li₃V₂(PO₄)₃/C cathode materials for Li-ion batteries are synthesized by a sol–gel-assisted, low-temperature sintering process. La(NO₃)₃ acts not only as the La source, but also, together with the intermediate product LiNO₃, promotes combustion, the ultrahigh exothermic energy that is advantageous for the nucleation process. The subsequent sintering process at 600 °C for 4 h is sufficient to produce highly crystalline La-doped Li₃V₂(PO₄)₃/C composites. The as-prepared cathode materials display smaller particle size, lower electron-transfer resistance and faster Li ion migration, which is ascribed to enhanced Li-ion transfer because of the La doping. The resulting Li₃V_1.96La_0.04(PO₄)₃/C cathode has a stable specific capacity of 160 mA h g⁻¹ at low charge–discharge rates over 100 cycles, and retained a stable capacity of up to 116 mA h g⁻¹ at a rate of 5 C, which is 40% higher than the undoped pristine cathode.

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1. Introduction

As an electrochemical energy storage device, Li-ion batteries (LIBs) have been widely used in wireless telephones, laptop computers, digital cameras, and other electronic devices since they were first introduced in the 1990s.^1,2 Recently, the application of LIB technology in electrical energy storage systems for smart grids that are powered by conventional energy sources such as coal, as well as intermittent renewable energy sources such as solar and wind, has attracted considerable attention.

In the present LIB market, any Li-ion cathode needs to satisfy the demands of high energy density, fast charging capability, long shelf-life and safety. To address these requirements, great efforts have been made to develop phosphate-based cathodes because the strongly covalent (PO₄)³⁻ units provide greater structural stability than commercial LiCoO₂ hosts even under deep charging conditions and elevated temperatures.^3–6

Among the known phosphates, monoclinic Li₃V₂(PO₄)₃ (LVP) is a promising LiB cathode because of its higher operating voltage (∼4.8 V) and its ability to deliver a capacity of 197 mA h g⁻¹ as 3 Li-ions are inserted/extracted in the operating voltage range of 3.0–4.8 V.^7–14 However, monoclinic LVP suffers from low electrical conductivity (∼2.4 × 10⁻⁸ S cm⁻¹); hence, achieving its full theoretical capacity and a high rate performance is challenging. The majority of the strategies to overcome these obstacles are focused on doping with metal ions,^15–27 coating with carbon^29–31 or scaling down the LVP particle size.^32–36 Up to now, lots of elements have been extensively investigated, such as alkali metals (Na),²⁸ alkaline-earth metals (Mg),^10,37 transition metals (Ti, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, Zn),^15–27 main group element metals (Al, Sn, Ge),^22,24 and halogens (Cl, F).³⁸ Relative to them, there are few works involving the study of the substitution behavior of lanthanide element (Ce, Nd, and Sm)^23,39 in the vanadium site in LVP systems. Yao et al.⁹ prepared the Ce-doped Li₃V_2−xCe_x(PO₄)₃/C composites by a sol–gel method and the Li₃V_2−xCe_x(PO₄)₃/C (x = 0.05) exhibited the best rate and cycle performance. Dang et al.¹⁹ also synthesis the Ce-doped Li₃V_2−xCe_x(PO₄)₃/C composites by a microwave assisted sol–gel process, and electrochemical performance measurements exhibited that Li₃V_1.98Ce_0.02(PO₄)₃/C exhibited a good cycling performance, with a retention rate of discharge capacity of 94% at 0.2 C after 100 cycles. In addition, Tm-doped Li₃V_2−xTm_x(PO₄)₃/C samples prepared by solid-state reaction could deliver discharge capacity of 181 mA h g⁻¹ at 0.1 C and sustain 95% of capacity retention after 20 cycles.⁴⁰ Although many kinds of doped Li₃V₂(PO₄)₃ have been studied, the electrochemical performance of the composite electrode has not yet reached a satisfactory level.

Lanthanum (La) has a relatively large radius and high affinity for oxygen, and La-doping can enhance the superconductivity and stability of materials.⁴¹ La is the second most abundant rare earth element in the world (32 ppm in Earth's crust) and it is inexpensive (e.g. lanthanum ammonium nitrate, 99.9%, $1.93 per pound, Lindsay Chem., USA), making it attractive for large-scale commercial applications. It is noted that the La-doped Li₃V_2−xLa_x(PO₄)₃ samples have been synthesized by Jiang et al. using lanthanum oxide (La₂O₃) as the La source. The Li₃V_2−xLa_x(PO₄)₃ (x = 0.02) sample showed the highest discharge capacity of 168 mA h g⁻¹ at 0.2 C rate in the potential range of 3.0–4.8 V, with a retention rate of discharge capacity of 93% at 0.2 C after 30 cycles.⁴² On the basis of this, we prepared the La-doped carbon coating Li₃V₂(PO₄)₃ composite materials using a low temperature and fast sol–gel method with spontaneous chemical reactions in the present study. The low reaction temperature and the short reaction time are made possible because of the selection of La(NO₃)₃ and the intermediate product of LiNO₃, both of which promote combustion, the ultrahigh exothermic energy of which accelerates the nucleation process. The effects of La substitution on the electrochemical properties and rate performance of as-obtained Li₃V₂(PO₄)₃/C composites are investigated in detail.

2. Experiment

The Li₃V_2−xLa_x(PO₄)₃/C cathode materials (x = 0, 0.02, 0.04, 0.06) were synthesized by a low-temperature and fast sol–gel route in a molar ratio of Li : V : La : P = 3 : 2 − x : x : 3 using Li₂CO₃, V₂O₅, La(NO₃)₃ and NH₄H₂PO₄ as raw materials as shown in Fig. 1. All chemicals used in this work were analytical grade without any pre-treatment. Firstly, V₂O₅ and citric acid were dissolved in deionized water at 70 °C to form VOC₆H₈ solution. Then, the mixed Li₂CO₃, La(NO₃)₃ and NH₄H₂PO₄ solution was added into the prepared VOC₆H₈ solution with magnetic stirring to form brown LVP hydrophilic homogeneous colloids with anionic surface. In order to chelate the metal ions uniformly by citric acid, the pH value of above solution was adjusted to about 8.0–9.0 using NH₃·H₂O. Subsequently, the colloids solution was heated to ∼80 °C for ∼6 hours under active stirring for the purpose of evaporating the water until the gel was formed. Next, the precursor was obtained after drying at ∼80 °C for ∼24 hours in an oven. Afterwards, the obtained precursor was carefully grounded after cooling to room temperature, and was sintered at 600 °C for ∼4 hours in flowing N₂ to yield the La-doping Li₃V_2−x(PO₄)₃/C composite (x = 0, 0.02, 0.04, 0.06, 0.08).

Fig. 1 The processes of the La doped Li₃V₂(PO₄)₃/C composites.

X-Ray diffraction patterns were obtained using an Philips X'Pert diffractometer with Cu K_α radiation. Thermogravimetric (TG) analysis was performed by using a SDTA851E thermoanalyzer using a heating rate of 10 °C min⁻¹. The morphology was obtained with a field-scanning electron microscope (SEM, JSM-6390LA) and a transmission electron microscope (TEM, JEOL JEM-200CX, 200 kV).

Electrochemical properties were evaluated with model CR2025 coin-type cell. The working electrodes were prepared as follows: a mixture of 82 wt% active materials, 10 wt% carbon black, and 8 wt% polyvinylidiene fluoride (PVDF) binder in N-methylpyrrolidinone was create a slurry. The resulting slurry was coated on an Al current collector with a thickness of ∼75 μm. The cathodes were dried in a vacuum furnace at 65 °C for ∼20 hours, following which they were roll-pressed at a pressure of ∼15 MPa. Circular electrodes (12 mm in diameter) were then punched out. The typical weight of the cathode ranged from ∼7 to 8 mg. Finally, the cathode was dried again in a vacuum oven at ∼100 °C for ∼48 hours prior to assembly. The cells were assembled in a high purity argon atmosphere inside a glove box (Mbraunlab Master130, Germany). Celgard® 2325 was used as the separator and the electrolyte was a solution of 1 M LiPF₆ dissolved in a mixture of ethylene carbonate (EC) : dimethyl carbonate (DMC) (1 : 1 vol%). The cells were cycled at different rates between 3.0 and 4.8 V on a cell testing instrument (LAND CT2001A, China). Electrochemical impedance spectra (EIS) measurements were performed on an electrochemical working station (PARSTAT2273, Princeton Applied Research, U.S.). EIS spectra were obtained over a frequency range between 1 mHz and 100 kHz.

3. Results and discussion

The thermogravimetric (TG) curves for the dry gel precursors of Li₃V_2−xLa_x(PO₄)₃/C (x = 0, 0.02, 0.04, 0.06) are shown in Fig. 2. The data were acquired in the temperature range from 25 °C to 800 °C at a heating rate of ∼10 °C min⁻¹ under N₂ flow. Similar curves were obtained for all dry gel precursors. Approximately ∼40.6% weight loss is observed for the dry gel precursor of Li₃V_1.96La_0.04(PO₄)₃/C in the temperature sweep to ∼600 °C, above which the weight loss is insignificant. In the TG curves, there are three main weight loss stages: below 200 °C, 200–450 °C and 450–600 °C. The 13.9% weight just below ∼200 °C is attributed to loss of crystalline water, absorbed water, ammonia and hydrogen fluoride. From 200 to 450 °C, the weight loss is approximately 18.5 wt%, which is owing to the pyrolysis of citric acid. From 450 to 600 °C, the weight loss is approximately ∼8.2%, which can be attributed to the pyrolysis of the remaining organic compounds. As previously mentioned, there was no significant weight loss observed in the TG curve above 600 °C, which indicates that the stable composites of La-doped Li₃V₂(PO₄)₃/C composites are formed and that the chemical reactions had reached completion.

Fig. 2 TG curve for the dry gel precursor of Li₃V_2−xLa_x(PO₄)₃/C (x = 0, 0.02, 0.04, 0.06) under N₂ flow at a heating rate of 10 °C min⁻¹.

On the base of the TG analysis, the La-doped Li₃V₂(PO₄)₃/C composites were prepared by a sol–gel method at ∼600 °C for ∼4 h. The low reaction or sintering temperature and the short reaction time are made possible by the selection of La(NO₃)₃ and the intermediate product of LiNO₃. Because of these molecules are combustion promoters, a large amount of energy will be released during the reaction, which is advantageous for the nucleation process.

The XRD experiments were conducted to investigate the structural changes caused by incorporation of different concentration of La ions in the Li₃V_2−xLa_x(PO₄)₃/C (x = 0, 0.02, 0.04, 0.06) samples. As shown in Fig. 3, the XRD patterns of the Li₃V_2−xLa_x(PO₄)₃/C samples are similar to that of the undoped sample. No impurity phases are detectable at the resolution of the X-ray diffractometer. This confirms that La ion are doped into the crystalline structure of Li₃V₂(PO₄)₃. The typical monoclinic structure can be indexed with the space group P21/n (ICSD #96962, #01-072-7074) by all fundamental peaks,⁴³ which suggests that La was successfully incorporated into the Li₃V₂(PO₄)₃/C crystal lattice, and the basic crystal structure of Li₃V₂(PO₄)₃/C is unchanged. Accordingly, the unit cell lattice parameters of all the samples have been refined, and are shown in Table 1. An increase in the unit cell volume is caused by the replacement of V by La in the monoclinic structure, which is consistent with the fact that the radius of V³⁺ in the VO₆ octahedra is smaller than La³⁺. The cell parameters of the Li₃V₂(PO₄)₃/C sample also agree well with those reported in the literature.⁴⁴ The previous studies differ from the present study only in terms of the different carbon sources (e.g. humic acid, glucose) and synthesis methods. The results indicate that in the crystal lattice, La ions are positioned in the V³⁺ sites, and the crystallinity of Li₃V₂(PO₄)₃ is retained even after La doping. This high crystallinity of Li₃V₂(PO₄)₃ is a critical factor that can significantly improve the cycling stability of the Li₃V₂(PO₄)₃ cathode.⁴⁵

Fig. 3 XRD patterns of the Li₃V_2−xLa_x(PO₄)₃/C (x = 0, 0.02, 0.04, 0.06) samples as a function of La ion doping.

Table 1 Lattice parameters of Li₃V_2−xLa_x(PO₄)₃/C cells (x = 0, 0.02, 04, 0.06) samples and comparative data for Li₃V_2−xLa_x(PO₄)₃ phase were taken from X. Zhou et al.⁴⁴

Samples	a (nm)	b (nm)	c (nm)	β (°)	V (nm^3)
Li3V2(PO4)3/C	0.8602 (4)	0.8594 (2)	1.2042 (5)	89.89	0.8896
Li3V1.98La0.02(PO4)3/C	0.8611 (3)	0.8596 (5)	1.2061 (4)	90.32	0.8924
Li3V1.96La0.04(PO4)3/C	0.8620 (3)	0.8601 (4)	1.2076 (3)	90.46	0.8953
Li3V1.94La0.06(PO4)3/C	0.8624 (5)	0.8597 (2)	1.2095 (4)	90.41	0.8967
Li3V2(PO4)3 (ref. 44)	0.8602	0.8585	1.2020	90.53	0.8878

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The carbon content of the Li₃V_2−xLa_x(PO₄)₃/C (x = 0, 0.02, 0.04, 0.06) samples was determined from elemental analysis to be ∼8.14%, ∼9.37%, ∼8.29%, and ∼8.18%, respectively (not shown here). Such a high carbon content is critical for keeping an effective electrical contact between particles.³¹ However, no corresponding peaks were detected by the XRD spectra (Fig. 3), which could be associated with the fact that the residual carbon is amorphous in the Li₃V_2−xLa_x(PO₄)₃/C composites.⁴⁶ Consequently, the presence of the amorphous carbon does not affect the Li₃V_2−xLa_x(PO₄)₃/C crystalline structure, and improves the mechanical stability and the electronic conductivity of the Li₃V₂(PO₄)₃ materials.

Scanning electron microscopy (SEM) images of the La-doped and undoped Li₃V₂(PO₄)₃/C samples are shown in Fig. 4. All the powders show flake-like surface profiles. The Li₃V₂(PO₄)₃/C particles are less than 1 μm in diameter and slightly agglomerated as displayed in Fig. 4A. Thus, the Li₃V_1.96La_0.04(PO₄)₃/C composite has the most uniform morphology and small particle size (i.e. highest specific surface area) of the three doped samples. Such a microstructure will serve to shorten the Li ion diffusion distance from the cathode bulk phase to the electrolyte and facilitate the electrolyte penetration, thereby ensuring better wettability. The high-resolution transmission electron microscopy (HRTEM) image of the composites are shown in Fig. 5 It can be seen that there is an amorphous carbon layer (∼5–8 nm thick) on the Li₃V_1.96La_0.04(PO₄)₃ surface. Such an amorphous carbon network can facilitate better inter-particles electronic contact, and, as a results, reduce the inter-particle contact resistance and improve the electrochemical properties of the composites.

Fig. 4 SEM images of Li₃V_2−xLa_x(PO₄)₃/C samples with different La contents: (a) x = 0, (b) x = 0.02; (c) x = 0.04; (d) x = 0.06.

Fig. 5 TEM images of Li₃V_1.96La_0.04(PO₄)₃/C.

To determine the electrochemical properties of the composites, the charge–discharge characteristic were measured at constant current density. The initial charge–discharge curves of pristine and La-doped Li₃V_2−xLa_x(PO₄)₃/C (x = 0.02, 0.04, 0.06) composites at a charge–discharge rate (C-rate) of ∼0.2 C between 3.0 V and 4.8 V are shown in Fig. 6. All composites display four voltage plateaus during the charging process at approximately ∼3.60, ∼3.70, ∼4.10 and ∼4.60 V, and during the discharging process, there are three voltage plateaus at approximately ∼3.56, ∼3.65, and ∼4.1 V. These results indicate a sequence of phase transition processes taking place in the Li_xV₂(PO₄)₃ (x = 3.0, 2.5, 2.0, 1.0), respectively.¹⁹ It is also apparent that the initial discharge capacities of the Li₃V_2−xLa_x(PO₄)₃/C samples are dependent on the La-doping amounts. The charge capacities of Li₃V_2−xLa_x(PO₄)₃/C composites (x = 0, 0.02, 0.04, 0.06) are approximately ∼175, ∼180, ∼179, and ∼181 mA h g⁻¹, respectively; the discharging capacities are approximately ∼161, ∼168, ∼171 and ∼164 mA h g⁻¹, respectively. The coulombic efficiencies of these composites are approximately ∼91%, ∼93%, ∼95%, and ∼92%, respectively. The high coulombic efficiencies of these composites are indicative of reversible phase transformations in the monoclinic Li₃V_2−xLa_x(PO₄)₃/C composites, which correspond to lithiation and delithiation steps, and accordingly lead to reversibility during continuous cycling.⁴⁷ It is noted that the discharge capacity is improved considerably when the La ion doping level is x = 0.04. The high specific discharge capacity of Li₃V_1.96La_0.04(PO₄)₃/C could arise because of a decrease of grain size and favorable homogeneity (as shown in Fig. 4). Fig. 7 shows the cycling performance of Li₃V_2−xLa_x(PO₄)₃/C composites (x = 0, 0.02, 0.04, 0.06) in the voltage range of 3.0–4.8 V at a rate of 0.2 C rate. The discharge capacity retention is ∼84%, ∼91%, ∼94%, and ∼89% after 100 cycles for the Li₃V_2−xLa_x(PO₄)₃/C (x = 0, 0.02, 0.04, 0.06) composites, respectively. These results demonstrate that La doping considerably improves the cycling stability of Li₃V₂(PO₄)₃, with the optimum doping level being x = 0.04. The enhanced cycling stability will be explained by the structure of Li₃V₂(PO₄)₃/C being stabilized by La-doping. To summarize, Li₃V_1.96La_0.04(PO₄)₃/C demonstrates the most promising electrochemical performance of all the composites as tested, exhibiting both high initial capacity and capacity retention. More details about the effect of the various lanthanide ion doping on the electrochemical performance of Li₃V₂(PO₄)₃/C cathodes are listed in Table 2. It can be found that the La doping system in this work have promising performance among these lanthanide ion-doped systems.

Fig. 6 Initial charge–discharge profiles of Li₃V_2−xLa_x(PO₄)₃/C (x = 0, 0.02, 0.04, 0.06) at the 0.2 C rate in the voltage of 3.0–4.8 V.

Fig. 7 Cycling performance of Li₃V_2−xLa_x(PO₄)₃/C (x = 0, 0.02, 0.04, 0.06) at 0.2 C discharge rate between 3.0 V and 4.8 V.

Table 2 Comparative electrochemical performance data for the various lanthanide ion doped Li₃V₂(PO₄)₃/C cathodes in 3.0–4.8 V

Sample	Method	Electrochemical performance	Ref.
Li3V1.95Ce0.05(PO4)/C	Sol–gel reaction	In 3.0–4.8 V, 120 mA h g^−1 with 96.8% capacity retention after 100 cycles at 0.2 C	9
Li3V1.98Ce0.02(PO4)/C	Microwave assisted sol–gel reaction	In 3.0–4.8 V, 171.6 mA h g^−1 with 94.0% capacity retention after 100 cycles at 0.2 C	19
Li3V1.97Ce0.03(PO4)/C	Colloidal crystal template method	In 3.0–4.8 V, 169 mA h g^−1 with 87.5% capacity retention after 100 cycles at 1 C	39
Li3V1.85Nd0.15(PO4)/C	Sol–gel reaction	In 3.0–4.8 V, showing an initial capacity of 162 mA h g^−1 with a capacity retention of about 86.6% at 0.5 C after 50 cycles	23
Li3V1.97Tm0.03(PO4)3/C	Solid-state reaction	In 3.0–4.8 V, showing an initial capacity of 181.2 mA h g^−1, with a capacity retention of 92.4% after 30 cycles.	40
Li3V1.98La0.02(PO4)/C	Microwave-assisted carbothermal reduction	In 3.0–4.8 V, 168.8 mA h g^−1 with 93% capacity retention after 30 cycles at 0.2 C	42
Li3V1.96La0.04(PO4)/C	Sol–gel assisted low temperature reaction	In 3.0–4.8 V, 171 mA h g^−1 with 94% capacity retention after 100 cycles at 0.2 C	This work

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Fig. 8 shows the contrast in the rate performance between the undoped pristine and La-doped Li₃V_1.96La_0.04(PO₄)₃/C composites. It can be seen that the undoped pristine Li₃V₂(PO₄)₃/C material can delivers a specific capacity of ∼153 mA h g⁻¹ at a rate of ∼1 C. When the C-rate is increased from ∼1 C to ∼2 C, the specific capacity retains ∼86%. For the La-doped Li₃V_1.96La_0.04(PO₄)₃/C composite, a specific capacity of ∼162 mA h g⁻¹ is obtained at a rate of ∼1 C, which drops to ∼93% (∼148 mA h g⁻¹) when the rate is increased ∼2 C. When the rate is increased from ∼2 C to ∼5 C, the difference in the specific capacity between the pristine and La doped composites becomes pronounced. For the La-doped Li₃V_1.96La_0.04(PO₄)₃/C composites, the specific capacity is ∼116 mA h g⁻¹ (∼70%) at 5 C, whereas that of the undoped pristine sample is only ∼82.0 mA h g⁻¹ (∼50%). It is also noted that the specific capacity of the Li₃V_1.96La_0.04(PO₄)₃/C composites recovers to ∼159 mA h g⁻¹ (∼94%) when the C-rate is returned back to 1 C from 5 C, whereas that of the undoped pristine Li₃V₂(PO₄)₃/C recovers to only ∼138 mA h g⁻¹ (∼83%). The high-rate performance of Li₃V_1.96La_0.04(PO₄)₃/C composites is attributed to faster Li ion migration, more uniform particle distribution via the sol–gel method and the low-temperature sintering process with spontaneous chemical reactions. The above results demonstrate that at the higher rates (5 C), the electrochemical dynamics for the Li₃V_1.96La_0.04(PO₄)₃/C composites are influenced by the Li ion diffusion in the bulk material rather than only by the electrochemical reaction rate.

Fig. 8 Cycling performance of Li₃V₂(PO₄)₃/C and Li₃V_1.96La_0.04(PO₄)₃/C at different discharge rate of 0.2 C, 1 C, 2 C, and 5 C, respectively, between 3.0–4.8 V.

The kinetic properties of Li₃V_1.96La_0.04(PO₄)₃/C can be further analyzed by electrochemical impedance spectroscopy (EIS). Fig. 9 shows the Nyquist plots for the Li₃V₂(PO₄)₃/C and Li₃V_1.96La_0.04(PO₄)₃/C electrodes. The inset equivalent circuit is used to simulate the impedance spectra. The low-frequency semicircle shows the Li-ion diffusion in the bulk of the electrode and the process is related to the Warburg impedance (W₁). The middle frequency is associated with the charge–discharge process, which includes charge transfer resistance (R₁), the particle-to-particle resistance (R₂), the double-layer capacitance (CPE₁) and the capacitance at the electrode–electrolyte interface (CPE₂). The high frequencies are attributed to the interfacial resistance on the electrodes and the intercept corresponds to the electrolyte resistance (R_e). As seen in Fig. 9, the charge-transfer resistance of the Li₃V_1.96La_0.04(PO₄)₃/C composite (∼187 Ω) is significantly lower than that of the Li₃V₂(PO₄)₃/C composite (∼294 Ω), which is likely because of the increase in electron conductivity that results from La-doping. In the impedance spectra, the low impedance values are related to the diffusion of Li ions (low-frequency spike) in the Li₃V_1.96La_0.04(PO₄)₃/C composites. The low impedance is associated with higher surface area, uniform particle size distribution and better electrolyte wettability. This may be related to the use of La(NO₃)₃ and the intermediate product of LiNO₃ in the sintering process. As discussed earlier, these molecules promote combustion, the ultrahigh exothermic energy of which accelerates the nucleation reaction. These factors results in greater accessibility to active sites for the Li ions, faster Li ion diffusion, shorter diffusion distances, and, consequently, improve the rate performance of the Li₃V_1.96La_0.04(PO₄)₃/C electrode.

Fig. 9 Impedance spectra of the Li₃V₂(PO₄)₃/C and Li₃V_1.96La_0.04(PO₄)₃/C composites. Inset: equivalent circuit corresponding to the impedance diagrams.

4. Conclusions

Li₃V₂(PO₄)₃/C and La-doped Li₃V₂(PO₄)₃/C composites were synthesized by a sol–gel-assisted, low-temperature sintering process. La(NO₃)₃ acted not only as the La source, but also, together with the intermediate product of LiNO₃, promoted combustion, which accelerated the nucleation process. We analyzed the influence of La doping on the electrochemical performance and structural characteristics of Li₃V₂(PO₄)₃/C. The results demonstrated that La could be incorporated into the crystal structure of Li₃V₂(PO₄)₃/C and the as-prepared cathode materials had smaller particle sizes, faster Li ion migration kinetics, and lower electron-transfer resistance. The Li₃V_1.96La_0.04(PO₄)₃/C cathode had a stable specific capacity of ∼160 mA h g⁻¹ at low charge–discharge rates over 100 cycles, and the cathode could still attain a stable capacity of up to ∼116 mA h g⁻¹ at a rate of 5 C rates (40% higher than its undoped pristine cathode).

Acknowledgements

Z. Yang and N. Gu acknowledges funding support by the National Natural Science Foundation of China (Grant no. 20963007, 21263016, 21363015) and Science Research Program in Jiangxi Province (no. GJJ13117, 2011BBE50022) for the financial support.

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