Novel synthesis of LiMnPO₄·Li₃V₂(PO₄)₃/C composite cathode material

Abstract

A carbon-coated LiMnPO₄·Li₃V₂(PO₄)₃ composite cathode material is synthesized from a rod-like MnV₂O₆·4H₂O precursor prepared via aqueous precipitation for the first time, followed by chemical reduction and lithiation with oxalic acid as the reducing agent and glucose as the carbon source. XRD results indicate that orthorhombic LiMnPO₄ and monoclinic Li₃V₂(PO₄)₃ co-exist. SEM results reveal that the thickness of the rod-like MnV₂O₆·4H₂O precursor is about 80 nm and that the LiMnPO₄·Li₃V₂(PO₄)₃/C composite possesses a micro/nano sphere-like morphology. HRTEM results indicate that the sample is a core–shell structure, where the external shell is amorphous carbon and at the core of the sample are LiMnPO₄, Li₃V₂(PO₄)₃ and LiMnPO₄·Li₃V₂(PO₄)₃ unit cells. The initial discharge capacity of the LiMnPO₄·Li₃V₂(PO₄)₃/C composite is 110 mA h g⁻¹, 104.4 mA h g⁻¹, 100.6 mA h g⁻¹ and 80.4 mA h g⁻¹ at the rate of 0.1 C, 1 C, 3 C and 10 C, respectively. The cell shows excellent cycling stability and good rate capability as a cathode for lithium-ion batteries.

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Introduction

Because of the increasing environmental pressure, rechargeable lithium-ion batteries are considered to be excellent candidates because of their high energy density, long cycle life and environmental friendliness. Multiple kinds of materials have been developed for cathodes and anodes by scientists around the world. Among them, metal polyanion materials based on phosphate compounds, such as LiFePO₄,¹ LiMnPO₄,^2,3 LiCoPO₄, Li₃V₂(PO₄)₃,^4–7 have been proposed to play an important role in hybrid electric vehicles, electric vehicles (EVs) and large-scale energy storage due to their high energy density and durable cycle life. Moreover, the strong P–O bond and three-dimensional solid framework in (PO₄)³⁻ anions guarantees both the dynamic and thermal stability needed to fulfill the safety requirements in high power applications mentioned above.

Compared to the commercially successful LiFePO₄, LiMnPO₄ has an equal theoretical capacity (171 mA h g⁻¹) but can provide nearly 20% higher energy density. However, the low electronic conductivity of LiMnPO₄ limits its performance at high currents.⁸ Similar to LiMnPO₄, Li₃V₂(PO₄)₃ has been proposed to be a highly promising cathode material for high-power lithium ion batteries because of its open three-dimensional framework, which allows fast transport of lithium ions.⁹ Many efforts including metal-doping,^10–13 coating with electronically conductive materials, such as carbon, metal, and metal oxide,^14–17 and optimization of particles with suitable preparation procedures have been made to improve the performance of LiMnPO₄ and Li₃V₂(PO₄)₃ cathode materials. Recently, Yang et al.¹⁸ reported that the discharge capacity at 1/20 C in the voltage range of 2.7–4.8 V was 10 mA h g⁻¹ for LiMn_0.95V_0.05PO₄ and 62 mA h g⁻¹ for pure LiMnPO₄. Wang et al.¹⁹ reported the performance of a different ratio of LiMnPO₄ to Li₂V₂(PO₄)₃ in the composite synthesized through spray drying followed by solid-state reaction and demonstrated that the electronic conductivity of the LiMnPO₄ could be enhanced by adding some Li₃V₂(PO₄)₃. Qin et al.²⁰ reported the relationship between the ratios of LiMnPO₄ to Li₃V₂(PO₄)₃ and the electrochemical performances of the composite materials, indicating the best ratio to be 0.6 : 0.4.

In this work, we report a new way to synthesize LiMnPO₄·Li₃V₂(PO₄)₃/C composite cathode material. We used NH₄VO₃ and Mn(CH₃COO)·4H₂O as starting materials to synthesize a rod-like MnV₂O₆·4H₂O precursor via an aqueous co-precipitation route for the first time; LiMnPO₄·Li₃V₂(PO₄)₃/C was prepared from the MnV₂O₆·4H₂O precursor by chemical reduction, and the properties were innovationally investigated.

Experimental section

Rod-like MnV₂O₆·4H₂O particles were synthesized via an aqueous co-precipitation route for the first time. An equimolar solution of Mn(CH₃COO)₂·4H₂O (99 wt%) and NH₄VO₃ (99 wt%) were mixed under vigorous stirring and maintained at 60 °C for 1 h under ultrasonic dispersion. The pH of the solution was adjusted to 6 by using ammonia water. Then, a brick-red precipitate spontaneously appeared and remained. The MnV₂O₆·4H₂O precursor was washed three times with deionized water and then dried in an oven at 60 °C. Then, stoichiometric ratios of the MnV₂O₆·4H₂O precursor, LiH₂PO₄ (99.9 wt%), oxalic acid, and glucose were mixed by ball milling for 4 h in alcohol, where oxalic acid is the reducer and glucose works as a carbon source. The resulting mixture was dried at 60 °C and subsequently fired in an argon atmosphere. Finally, the LiMnPO₄·Li₃V₂(PO₄)₃/C was obtained. The reaction may occur as follows:

MnV₂O₆·4H₂O + 4LiH₂PO₄ + 4HOOCCOOH·2H₂O → LiMnPO₄ + Li₃V₂(PO₄)₃ + 8CO₂↑ + 12H₂O↑ (1)

The mole ratio Mn/V of the as-prepared MnV₂O₆·4H₂O was determined by chemical titration. Powder X-ray diffraction (Rint-2000, Rigaku) using Cu K radiation was employed to identify the crystalline structure and purity of the synthesized materials. The surface morphologies and particle information of samples were observed by SEM (JEOL, JSM-5600LV) and transmission electron microscope (TEM) (Tecnai G12). Carbon content of the composite was determined by C–S analysis equipment (Eltar, Germany).

Electrochemical characterizations were carried out using CR2025 coin-type cell. Typical positive electrode loadings were in the range of 2–2.5 mg cm⁻², and an electrode diameter of 14 mm was used. For positive electrode fabrication, the prepared powders were mixed with 10% of carbon black and 10% of polyvinylidene fluoride in N-methyl pyrrolidinone until a slurry was obtained. The blended slurries were then pasted onto an aluminum current collector, and the electrode was dried at 120 °C for 4 h in air. The test cell consisted of a positive electrode and a lithium foil as negative electrode separated by a porous polypropylene film, and 1 mol L⁻¹ LiPF₆ in EC, EMC and DMC (1 : 1 : 1 in volume) was used as the electrolyte. In addition, the assembly of the cells was carried out in a dry Ar-filled glove box. The coin cells were charged at 0.1 C and discharged at 0.1 C, 1 C, 5 C and 10 C over the voltage range of 2.5–4.5 V at ambient temperature using a battery testing system (Neware BTS-2000). Finally, cyclic voltammogram was carried out with a CHI660D electrochemical analyzer.

Results

Rod-like MnV₂O₆·4H₂O precursor was innovationally prepared via an aqueous co-precipitation process. The Mn/V mole ratio of MnV₂O₆·xH₂O was determined by chemical titration to be 2.003, confirming that the prepared brick-red precipitate is MnV₂O₆·xH₂O.

Fig. 1 shows the XRD patterns of MnV₂O₆·4H₂O. All the peaks are indexed and consistent with the data of JCPDS#86-0720. The crystal structure is identified to be monoclinic, which is the same as that reported by JOEL.²¹

Fig. 1 XRD pattern of MnV₂O₆·4H₂O precursor.

As illustrated in Fig. 2, the main part of the obtained MnV₂O₆·4H₂O precursor are rod-like with the average width of the particles being around 200 nm and the thickness approaching 100 nm. A small number of rods aggregate and generate rods with the width and thickness approaching 2 μm.

Fig. 2 SEM image of MnV₂O₆·4H₂O precursor.

Fig. 3 shows the typical Rietveld refinement XRD patterns of LMP·LVP/C synthesized at 700 °C for 10 h. The refined lattice parameters and phase content are listed in Table 1. The observed and calculated patterns match and the reliability factor (R_w) is acceptable. In addition, the weight ratio of LiMnPO₄ (marked as LMP) to Li₃V₂(PO₄)₃ (marked as LVP) is 27.75 : 72.25, which is consistent with the molar ratio of 1 : 1, indicating the purity of the rod-like MnV₂O₆·4H₂O precursor. It can be seen that the cell volume of LMP in the LMP·LVP/C composites decreases compared with the pure orthorhombic LMP (space group Pnma, 25834-ICSD), which may be attributed to the V doping on the Mn sites because the ionic radius of V³⁺ (0.74 Å) is smaller than that of Mn²⁺ (0.80 Å). However, the cell volume of LVP in the LMP·LVP/C composites increases compared with the monoclinic LVP (space group P21/n, 96962-ICSD), indicating that Mn is also doped into the LVP host lattice. The abovementioned results predict that the Mn²⁺ doping LVP and V³⁺ doping LMP co-exist in the two-component composites for the utilization of MnV₂O₆·4H₂O. As we know, this two-way behavior improved their electronic conductivity and electrochemical performance.^18,22

Fig. 3 Rietveld refinement result of LiMnPO₄·Li₃V₂(PO₄)₃/C composite.

Table 1 Refined cell lattice parameters of Li₃V₂(PO₄)₃ and LiMnPO₄ in LiMnPO₄·Li₃V₂(PO₄)₃/C and standard data of LiMnPO₄ (25834-ICSD) and Li₃V₂(PO₄)₃ (96962-ICSD)

Sample	a (nm)	b (nm)	c (nm)	Volume (nm^3)	Weight ratio (%)	Rw (%)
LMP in LMP·LVP	0.60958	1.04348	0.47418	0.30162	27.75	10.34
LMP	0.610	1.046	0.4744	0.30269	—	—
LVP in LMP·LVP	0.86068	0.85922	1.20362	0.89010	72.25	10.34
LVP	0.86056	0.85917	1.20370	0.88998	—	—

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Based on the C–S analysis result, the carbon from the glucose coating on the surface of the LMP·LVP/C composite and existing in the form of a carbon web between the particles add up to about 1.7 wt%. Oxalic acid acted as the reductant and ultimately decomposed into CO₂ and H₂O and ultimately, was over-contained in the composite. The SEM image of LMP·LVP/C synthesized at 700 °C for 10 h is shown in Fig. 4a. The particles in all the samples have sphere-like morphology. In order to investigate the distribution and the carbon coating information in the LMP·LVP/C composites, TEM was employed. Fig. 4b and c are the TEM images of the LMP·LVP/C synthesized at 700 °C for 10 h. The primary particle size is about 200–400 nm, and a conductive carbon network among the particles can also be observed in Fig. 4b and c. HRTEM of region I was used to further study the microstructure of the as-prepared composites. Two regions of different framework are presented in Fig. 4d. The lattice spacing of 0.23 nm of the right region corresponds to (012) of LMP phase and the lattice spacing of 0.26 nm of the left region corresponds to (113) of LVP phase. Fig. 4d indicates that the sample is a core–shell structure, where the external shell is amorphous carbon and at the core of the sample are LiMnPO₄ and Li₃V₂(PO₄)₃ unit cells. The thickness of the carbon shell is about 4.8 nm.

Fig. 4 (a) SEM image of LiMnPO₄·Li₃V₂(PO₄)₃/C synthesized at 700 °C for 10 h. (b) and (c) TEM images of LiMnPO₄·Li₃V₂(PO₄)₃/C synthesized at 700 °C for 10 h. (d) HRTEM image of LiMnPO₄·Li₃V₂(PO₄)₃/C synthesized at 700 °C for 10 h.

Discussion

Fig. 5a shows the initial charge–discharge curves of Li/LiMnPO₄·Li₃V₂(PO₄)₃/C cells at the rates of 0.05 C, 0.1 C, 3 C and 10 C. The excellent electrochemical performance of the composites may be due to the homogeneous distribution and the co-carbon source. With the proper addition of glucose and oxalic acid, the composites involve a conductive network among the particles to improve the electronic conductivity of the material. Thus, the structure of Li/LiMnPO₄·Li₃V₂(PO₄)₃/C was advantageous to the rate capability and cyclic performances of the composite materials.

Fig. 5 (a) Rate performance of LiMnPO₄·Li₃V₂(PO₄)₃/C synthesized at 700 °C for 10 h. (b) Cyclic voltammetry profiles of the LiMnPO₄·Li₃V₂(PO₄)₃/C synthesized at 700 °C for 10 h. (c) Cycle performance of LiMnPO₄·Li₃V₂(PO₄)₃/C synthesized at 700 °C for 10 h.

The cyclic voltammetry curves of LMP·LVP/C at a scan rate of 0.1 mV s⁻¹ are presented in Fig. 5b. It shows the redox peaks over the voltage range of 3–4.3 V for the composite corresponding to the plateaus of the charging and discharging curves. Three couples of redox peaks can be seen in the curves. The oxidation peaks of 3.64 and 3.73 V correspond to the extraction of the first Li⁺ in two steps: Li₃V₂^3+/3.5+(PO₄)₃ to Li_2.5V₂^3+/3.5+(PO₄)₃ and Li_2.5V₂^3.5+/4+(PO₄)₃ to Li₂V₂^3.5+/4+(PO₄)₃. The sharp oxidation peaks of 4.14 V corresponds to the extraction of the second Li⁺:Li₂V₂^4+/5+(PO₄)₃ to Li₁V₂^4+/5+(PO₄)₃. The reduction peaks around 3.53 V, 3.61 V and 3.99 V correspond to the insertion of the two Li⁺. No characteristic peaks of LMP can be observed in the curves. This phenomenon is unique and may be attributed to the peaks of LMP being covered by the peaks of LVP.

As seen in Fig. 5c, the initial discharge capacity of LMP·LVP/C at a rate of 0.05 C is about 136.5 mA h g⁻¹ (the theoretical specific capacity (C_T) of LMP·LVP is calculated by the following equation: C_T = (171X₁ + 133X₂) (mA h g⁻¹), where 171 and 133 are the theoretical capacities of LMP and LVP (<4.3 V), respectively; X₁ and X₂ are the weight content of LMP and LVP, respectively. Based on our Rietveld refinement results (Table 1 and Fig. 3), the weight ratio of LVP is 72.25%, so the C_T for LMP·LVP/C synthesized in this paper is obtained as 143 mA h g⁻¹). The real capacity of the synthesized LiMnPO₄ is about 47 mA h g⁻¹, and about 96 mA h g⁻¹ for the Li₃V₂(PO₄)₃.

While the initial discharge capacity of LMP·LVP/C at the rate of 0.1 C, 3 C and 10 C can reach 126.2 mA h g⁻¹, 115.7 mA h g⁻¹ and 80.4 mA h g⁻¹, respectively, the discharge capacity is about 113.8 mA h g⁻¹ and 78.2 mA h g⁻¹ after 30 cycles at the rate of 3 C and 10 C, respectively. The cell retains 98.35% and 97.3% of its initial discharge capacity. The composite material performs well at high rate; however, the discharge plateaus of LMP·LVP/C at different rates are obviously fading.

Conclusions

LiMnPO₄·Li₃V₂(PO₄)₃/C composite cathode material was synthesized from rod-like MnV₂O₆·4H₂O prepared by aqueous precipitation for the first time, following chemical reduction and lithiation with oxalic acid as the reducer and glucose as carbon source. The LiMnPO₄·Li₃V₂(PO₄)₃/C compound synthesized at 700 °C for 10 h showed excellent electrochemical performance at high rate, and its discharge capacity is about 110 mA h g⁻¹, 104.4 mA h g⁻¹, 100.6 mA h g⁻¹ and 80.4 mA h g⁻¹ at the rate of 0.1 C, 1 C, 3 C and 10 C, respectively. The cell retains 97.1%, 97.0%, 92.1% and 97.3% of its initial discharge capacity after 20 cycles.

Acknowledgements

We gratefully acknowledge the financial support for this work provided by the National Natural Science Foundation of China (General Program) under grant number 51272290 and 51402365.

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