Hui Chen‡
a,
Zong-Kai Wang‡c,
Guo-Dong Li*b,
Fei-Fan Guob,
Mei-Hong Fanb,
Xue-Yan Wud and
Xi-Chuan Cao*a
aSchool of Materials Science and Engineering, China University of Mining and Technology, Xuzhou 221000, China. E-mail: xichuancao@cumt.edu.cn
bState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China. E-mail: lgd@jlu.edu.cn
cSchool of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China
dSchool of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China
First published on 27th March 2015
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.
Layered mixed metal oxides (LiCo1/3Ni1/3Mn1/3O2, LiNi0.8Co0.15Al0.05O2, 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%.
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 (E2g vibration of graphite) of carbon, respectively. The ratio of ID/IG 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.
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.
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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. |
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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 LixV2(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. 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 Rct (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 = Lion2/DLi (Γ represents lithium-ion diffusion time for intercalation in the host material, Lion represents the diffusion distance, DLi 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.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra01992c |
‡ These authors contributed equally. |
This journal is © The Royal Society of Chemistry 2015 |