Enhanced electrochemical performance of Li₃V₂(PO₄)₃ microspheres assembled with nanoparticles embedded in a carbon matrix

Abstract

We report on uniform Li₃V₂(PO₄)₃ microspheres with a size distribution of 1–3 μm assembled with nanoparticles embedded in a carbon matrix. LVP particles have a size of about 50–100 nm and carbon accounts for about 6% in total mass for the one with the best electrochemical performance. An initial capacity of 121 mA h g⁻¹ or 101 mA h g⁻¹ was achieved when cycled at 1 C or 10 C at room temperature with an admissible capacity fading of 8% after 100 cycles. The high rate capability and cycling stability may attribute to the unique microsize, electron conductive continuous carbon matrix and stable 3D skeleton of Li₃V₂(PO₄)₃.

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Introduction

In order to abate the increasing pressure caused by the energy crisis and global environmental issues, great attention has been drawn to lithium ion batteries (LIBs) for their high energy density both in weight and volume, durable cycle life and environmental benignity.^1–3 As to anode materials, traditional graphite which has a theoretical capacity of 372 mA h g⁻¹ has been widely industrialized already. A large variety of new anode materials has developed rapidly.^4–6 However, it is the cathode material which is improving slowly and hinders the commercialization of LIB.⁷ LiCoO₂ with a capacity of 120–150 mA h g⁻¹ is one of the popular cathode materials in current commercial LIBs, however, it faces a series of problems in that the phase transformation from hexagonal into monoclinic accompanies the lithium ion extraction process, as well as the change of lattice constant and hence the poor stability. What is worse, the scarcity and safety concern further inhibits its wide application.^8–10 Therefore, it is rather urgent to develop high performance, low-cost and environmentally friendly cathode materials for energy conversion and storage systems.

Layered mixed metal oxides (LiCo_1/3Ni_1/3Mn_1/3O₂, LiNi_0.8Co_0.15Al_0.05O₂, etc.), spinel oxides (LiMn₂O₄, etc.) and the transition metal phosphates (LiMnPO₄, LiFePO₄, etc.) have also attracted research interest.^11–15 Among them, monoclinic Li₃V₂(PO₄)₃ (LVP) material which exhibits high theoretical specific capacity, large operating voltage and high safe performance makes it promising candidate for polyanion-type cathode materials¹⁶ since LF Nazar et al. first reported it as cathode material of LIB in 2002.¹⁷ LVP owns stable three dimensional framework consisting of slightly distorted VO₆ octahedron and PO₄ tetrahedron and high theoretical capacity of 197 mA h g⁻¹ when cycled at 3.0–4.8 V and three lithium ions are all extracted, the large polyanion structure is inherently stable and facilitates lithium ion migration.¹⁶

However, similar to LiFePO₄, LVP have not been widely used for the sake of intrinsically poor electronic conductivity (2.4 × 10⁻⁷ S cm⁻¹ at room temperature). To overcome this obstacle, tremendous efforts have been devoted to reasonable synthesis routes of LVP over the past decades.^18–20 In this regard, coating with an electronically conductive layer and doping with metal cations at a lithium site are universal tactics to improve the electronic conductivity of polyanion-type cathode materials.^21–23 Carbon coating is usually more preferred for its simplicity and cost-efficiency.^24–29 By adding organic compounds as carbon source, and subsequently calcining at high temperature in inert atmosphere, carbon coating can be achieved. In this process, on one hand, V⁵⁺ can be reduced to V³⁺, on the other hand, residual carbon can be deposited on the surface of the material after organic compounds decomposition, and the as-obtained core–shell structure can enhance the electron conductivity evidently.^30–32 What is more, the core–shell LVP/C materials have high capacity and improved cyclability.

Besides, nano-sized materials benefit much from its nano-size effect because the diffusion distance of Li⁺ decreases dramatically and enhanced reaction interface favors the electrode kinetics process.^33–35 While the highly nano-sized particles have high specific surface area and high surface energy, and hence tend to agglomerate, which consequently leads to quick capacity fading.³⁶ It is also the case with core–shell LVP/C nanocomposites in previous reports.^18,28,29,37 Herein, we report a uniformly dispersed LVP/C spheres with hierarchical structure. LVP particles with a size of about 50–100 nm are embedded in carbon matrix, and the LVP/C further assembled into stable microspheres. Compared with traditional core–shell structured materials, this unique hierarchical architecture can provide the following two important features simultaneously: (1) efficient and stable electron conduction enabled by the continuous carbon matrix, (2) uniformly dispersed microspheres are easily to be soaked in electrolyte, ensuring fluid ion transport. The as-synthesized material exhibited reasonable discharge capacity. When cycled at 1 C and 10 C in 3–4.3 V after 100 cycles, specific capacity retained more than 92%.

Experimental

Chemicals and reagents

All the solvents and chemicals were used without further purification and deionized water was used in all experiments.

Synthesis of Li₃V₂(PO₄)₃/C

The LVP/C microspheres were synthesized via a facile method by combining hydrothermal process and solid-state carbothermal reduction reaction. Typically, 0.003 mol ammonium metavanadate (NH₄VO₃) was added into 30 mL distilled water under magnetic stirring at 70 °C until complete dissolution to get homogenous light yellow solution, then 60 mL isopropanol and 30 g glycerol was added into the above solution, followed by stoichiometric amounts of lithium acetate (CH₃COOLi₂·H₂O) and ammonium dihydrogen phosphate (NH₄H₂PO₄), stirring for 1 hour to obtain a red brown solution before transferred to 50 mL Teflon-lined autoclaves, sealed and maintained at 180 °C for 6 hours. After cooling to room temperature naturally, the product was washed with ethanol for three times and dried at 60 °C overnight and black precursor was obtained. The as-obtained precursor was dispersed in glucose–ethanol solution and grinded in a motor. The molar ratio of glucose to vanadium was controlled as 1 : 8. The mixture was dried under magnetic stirring at 70 °C, and heated at 120 °C for another 6 hours. Finally, the completely dried mixture was calcined at 800 °C for 6 hours under the protection of nitrogen.

General characterizations

The structure and phase purity of the synthesized compound were examined with a Rigaku D/Max 2550 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). The morphology and microstructure of the as-synthesized product was then characterized by a JEOL JSM 6700F electron scanning electron microscopy (SEM) and a Philips-FEI Tecnai G2S-Twin microscope equipped with a field emission gun operating at 200 kV (TEM). Elemental analyses of possible species in the LVP/C sample were also conducted using a Perkin-Elmer Optima 3300 DV ICP spectrometer. To determine the amount of carbon in the sample, the thermogravimetric analysis (TGA) was performed using with a NETZSCH STA 449C TG thermal analyser by heating the samples from 35 to 800 °C at a heating rate of 10 °C min⁻¹ in air. The carbon layer was analysed with a Renishaw Raman system model 1000 spectrometer with a 20 mW air-cooled argon ion laser (514.5 nm) as the exciting source.

Electrochemical measurements

The electrochemical performance of the LVP/C material was evaluated at room temperature using CR2016 coin-type half-cell with Li foil as anode. The electrode laminate was prepared by mixing LVP, acetylene black (AB) and polyvinylidene fluoride (PVDF) with a mass ratio of 8 : 1 : 1 into N-methyl-2-pyrrolidone (NMP). After ultrasonic for 5 minutes, the obtained homogenous slurry was then coated onto aluminum foil uniformly and dried in vacuum at 120 °C for 10 hours. With metal Li employed as anode, 1 M LiPF₆ dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethylene methyl carbonate (EMC) with a volume ratio of 1 : 1 : 1 as electrolyte, the cells were assembled in an argon-filled glove box (Mikrouna). The cells were cycled at different charge–discharge rates within the potential range of 3.0–4.3 V on a LAND CT2001A cell testing system. The cyclic voltammetry (CV) spectrum was collected on a CHI660d electrochemical workstation between 3.0 and 4.3 V at the scan rate of 0.2 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) was also recorded on this electrochemical workstation with the frequency ranging from 100 kHz to 10 mHz and an AC signal of 5 mV in amplitude as the perturbation. All of the electrochemical measurements were conducted at room temperature.

Results and discussion

Fig. 1A shows the XRD patterns of the obtained LVP/C composites which can be well indexed to the monoclinic space group P2₁/n with lattice constants a = 8.606 Å, b = 8.592 Å, c = 12.037 Å, and β = 90.61° (JCPDS no. 01-072-7074) indicating the high purity and the sharp peaks reflect high crystallinity of the as-synthesized sample. No diffraction peaks corresponding to carbon or graphite can be observed in the patterns, indicating that the carbon network is not well crystallized here. And the glucose serves as carbon source for the reaction and meanwhile plays a role as a reducing agent in reducing V⁵⁺ to V³⁺ during the heat treatment under Ar. The crystal structure of the monoclinic LVP is shown in Fig. 1B. A range of VO₆ octahedron and PO₄ tetrahedron connected by oxygen vertices constituting the host V₂(PO₄)₃ 3D framework which makes lithium 3D pathways feasible, the lithium ions can reversibly extracted and reinserted from the monoclinic structure of vanadium phosphate.

Fig. 1 (A) XRD pattern of LVP composites, (B) crystal structure of monoclinic Li₃V₂(PO₄)₃.

In order to further study the presence of carbon in the product, the Raman spectrum (Fig. 2) was collected in the range of 500–2000 cm⁻¹. The characteristic peaks located at ∼1360 and ∼1600 cm⁻¹ are assigned to the D-band (disorder-induced phonon mode) and G-band (E_2g vibration of graphite) of carbon, respectively. The ratio of I_D/I_G is about 1.1, this indicates that the graphite carbon content is close to 50%, which can promote electron conductivity of the material and thus electrochemical performance.^26–28,39 However, the graphite crystallites in the residual carbon are very little and in a disordered form, hence, it is difficult to observe any graphite crystallites using XRD. In addition, there are two weak Raman bands at 1011 and 1142 cm⁻¹, which can be assigned to the vibrations of Li₃V₂(PO₄)₃.³⁸ To determine the exact carbon content in the composite, TGA were carried out (Fig. S1, ESI†). The composite is composed of about 6 wt% of carbon and 94 wt% of Li₃V₂(PO₄)₃, which is further confirmed by ICP-OES analysis.

Fig. 2 Raman spectrum of the as-prepared Li₃V₂(PO₄)₃/C composite.

The morphology and microstructure of the as-synthesized product were further investigated by SEM and TEM. The Li₃V₂(PO₄)₃/C are uniform microspheres and are 1–3 μm in size (Fig. 3A) similar to its precursor (Fig. S2 and S3, ESI†). The TEM image (Fig. 3A, inset) further confirms that Li₃V₂(PO₄)₃/C is spherical with a smooth surface. The cages and channels in the microspheres are propitious to the mass transfer of electrolyte. The microstructure of the composite can be directly observed from TEM (Fig. 3B and C) that the microsphere is composed of LVP nanoparticles. The LVP nanoparticles with a size distribution of 50–100 nm were well-embedded in the layer of carbon, constructing a stable phase with low static energy. The superior electron conductivity of the carbon filling in the interstitial is vital to the intrinsically poor-conductive LVP and is advantageous to protect the 3D skeleton when Li⁺ insertion/extraction take place. The 3D structure of the microspheres facilitates Li⁺ transportation in all directions at the same time. The HRTEM (Fig. 3D) image taken on an individual nanoparticle displays clear lattice fringes with a d-spacing of 0.428 nm, corresponding to the [121] plane of monoclinic Li₃V₂(PO₄)₃. It can be seen that the outer carbon layer doesn't own evident lattice. And the possible Li⁺ and electron transformation pathways processed in the battery is schematically shown in Fig. 4.

Fig. 3 Typical (A) low-magnification and SEM micrographs, (B) low-magnification TEM micrographs, (C) high-magnification TEM micrographs, (D) HRTEM image of Li₃V₂(PO₄)₃/C composite.

Fig. 4 Schematic illustration of Micro-sized LVP/C spheres with 3D pathways for Li⁺, electron and electrolyte molecules processed in the battery.

To evaluate the performance of the as-synthesized Li₃V₂(PO₄)₃/C composite, electrochemical tests were carried out. The cyclic voltammetry (CV) curves of the first three times were shown in Fig. 5A. It can be clearly seen that there are three cathodic peaks located at 3.65 V, 3.75 V, 4.17 V and their corresponding anodic peaks located at 3.51 V, 3.60 V, 3.96 V, respectively. And the three pairs in association with the V³⁺/V⁴⁺ redox couples during the charge and discharge steps. It is apparent that the curves are remarkably similar except that the first cycle deviate slightly due to the formation of the solid electrolyte interface (SEI),⁴⁰ indicating the high stability of the material. Fig. 5B and S4 (see ESI†) Show the 1st, 10th, 50th, and the 100th charge–discharge curves at 1 C and 10 C over a potential range of 3.0–4.3 V. Obviously, there are three flat plateaus located at around 3.61/3.55 V, 3.69/3.63 V, and 4.09/4.03 V on each branch originating from reversible phase transformation of Li_xV₂(PO₄)₃ (x = 2.5, 2.0, 1.0) occurred during the Li⁺ insertion/extraction. The nearly identical flat plateau after 100 cycles shown in charge–discharge profiles indicates that the LVP/C composite is rather stable.

Fig. 5 (A) CV curves of the first three times at a sweep rate of 0.2 mV s⁻¹ in the potential range of 3.0 to 4.3 V vs. Li/Li⁺, (B) charge–discharge profiles in the potential region of 3.0–4.3 V, (C) the cycling performance at rates of 1 C and 10 C for 100 cycles, (D) Galvanostatic cycling behavior at various rates from 1 C to 20 C of Li₃V₂(PO₄)₃/C composite.

Fig. 5C shows the cycling performances at 1 C and 10 C for 100 times. It delivered a capacity of 121 mA h g⁻¹ for the first cycle when performed at 1 C, with a capacity retention of 92% after 100 cycles. And when operated at 10 C, the initial discharge capacity was 101 mA h g⁻¹ and the capacity retention was also 92% after 100 cycles. The superior cycling performance of the as synthesized LVP/C at 10 C is attractive.

To further understand the high rate performance of the composite, the batteries were charged and discharged at different rates ranging from 1 C to 20 C in the potential range of 3.0–4.3 V (Fig. 5D and S5, ESI†). The discharge capacity at 1 C, 3 C, 5 C, 10 C, 20 C rate is 117, 112, 103, 95, and 81 mA h g⁻¹, respectively. When the current rate returned to 1 C, the discharge capacities returned to their initial states, indicating good electrochemical reversibility of the materials.

In order to study the effect of carbon, LVP/C with different glucose to vanadium ratio (1 : 4, 1 : 8 and 1 : 12, denoted as S4, S8 and S12) and with quantitative graphite as a substitute of glucose (denoted as Sg) were also synthesized and tested under the same condition (see ESI Fig. S6†). When cycled at 1 C over a potential range of 3.0–4.3 V, S4, S8 and S12 deliver capacity of 79 mA h g⁻¹, 121 mA h g⁻¹ and 100 mA h g⁻¹, respectively which is superior to Sg, that may be caused by structure deterioration. Fig. S7† shows the EIS of S4, S8 and S12 measured at the same fully discharge state of 4.1 V after 50 cycles at 1 C, all the three profiles consist of a semicircle in the high frequency region and a straight line in the low-frequency region. The semicircle is ascribed to the lithium ion migration through the interface between the surface layer of the particles and the electrolyte. S8 shows the smallest R_ct (charge-transfer resistance) of the three, indicating that the excellent electrical conductivity of the material which benefits from well-dispersed microspheres and suitable amount of carbon layer. And the result is in consistent with cycling tests (see ESI Fig. S6†). Perhaps, we can infer that the different electrochemical behavior of them may be attributed to the fact that a low amount of glucose might form a thin but hardly a full coating layer on the surface of the LVP, while an excessively thick carbon coating layer would act as a barrier for Li⁺ diffusion.

The outstanding electrochemical performance of the material most probably due to the uniformly dispersed microspheres and unique hierarchical structure with nano-sized monoclinic LVP embedded in partially graphite carbon with 3D channels for Li⁺ transportation. The structure keeps intact in the lithium insertion/extraction cycling process and the partially graphite carbon layer improves the electron conductivity of the intrinsically poor LVP. Nano materials usually have many advantageous merits such as enhanced kinetics and activity. However, it faces disadvantages such as low thermodynamic stability and increased side reactions at the same time. In this paper, the as-synthesized nanosized LVP particle with a layer of carbon agglomerate into micro sized particles. According to Γ = L_ion²/D_Li (Γ represents lithium-ion diffusion time for intercalation in the host material, L_ion represents the diffusion distance, D_Li represents lithium diffusion coefficient) reported in previous work,²⁸ the smaller the particle size, the shorter the lithium transport distance, and hence the higher charge–discharge rates can be achieved.

Conclusions

In summary, with glucose as carbon source, we have successfully synthesized LVP/C composite as efficient high rate cathode material for lithium ion battery via a facile hydrothermal post-carbonization method. When cycled at 1 C and 10 C, it delivered an initial capacity of 121 mA h g⁻¹ and 101 mA h g⁻¹ with a capacity retention of 92% after 100 cycles. It exhibited impressive rate capability and cycling stability resulting from the continuous carbon network and stable 3D structure. The moderate carbon layer surrounding nanoparticles suppresses the structure degradation and the side reactions during cycling of the composite, resulting in excellent cycling performance. In this sense, the Li₃V₂(PO₄)₃/C may be promising candidates for lithium ion batteries and other energy storage devices.

Acknowledgements

This work was supported by the NSFC (21371070), Jilin province science and technology development projects (20140101041JC, 20130204001GX) and the Fundamental Research Funds for the Central Universities (no. 2010LK303).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra01992c

‡ These authors contributed equally.

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