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Enhanced electrochemical performance of Li3V2(PO4)3 structurally converted from LiVOPO4 by graphite nanofiber addition
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 5403–5413 www.elsevier.com/locate/ceramint
Enhanced electrochemical performance of Li3V2(PO4)3 structurally converted from LiVOPO4 by graphite nanofiber addition Van Hiep Nguyen, Wan Lin Wang, Hal-Bon Gun Department of Electrical Engineering, Chonnam National University, Gwangju 500-757, South Korea Received 3 June 2014; received in revised form 11 November 2014; accepted 18 December 2014 Available online 29 December 2014
Abstract In this work, the structural conversion of LiVOPO4 to Li3V2(PO4)3 due to the addition of graphene nanofiber (GNF) was investigated, and the resulting materials were found to exhibit enhanced capacity and cyclability. First, LiVOPO4 was synthesized using a solid-state method followed by annealing at 900 1C for 12 h under nitrogen atmosphere. Then, the conversion from the triclinic LiVOPO4 structure to the monoclinic Li3V2(PO4)3 structure due to the GNF addition was observed. No impurity peak was observed in the X-ray diffraction patterns of LiVOPO4 or Li3V2(PO4)3, and the structural conversion caused no defects to form in the resulting Li3V2(PO4)3 crystallite. Field emission-scanning electron microscope studies clearly demonstrate that larger corroded-structure-like particles formed which were mixed with GNF. This provided both a large active area and fast transport of lithium ions, which afforded enough active sites for simultaneous intercalation of many lithium ions, leading to improved electrochemical properties of the material. Compared with LiVOPO4, the Li3V2(PO4)3–GNF showed better properties, such as an improved lithium ion diffusion coefficient, improved cyclability, and smaller impedance. Furthermore, the optimized Li3V2(PO4)3–GNF (7%) battery showed the best discharge capacity of 181 mA h g 1 at 0.1 C and lithium ion diffusion coefficient of 6.01 10 9 cm2 s 1. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Li3V2(PO4)3–graphite nanofiber; Solid-state method; Structural conversion; Positive electrode
1. Introduction In recent years, cathode materials based on lithium transition metal oxides have attracted much attention because they are advantageous for applications such as powering electronic devices like cameras, laptops, electrical devices, or hybrid electric vehicles. At present, several materials are under development to overcome certain limitations of these materials related to electron and lithium ion transfer within the lattice, structural stability, low activation energy, low electronic conductivity, and low redox potential versus the Li/Li þ potential. Together, these limitations lead to lowpotential batteries that are not beneficial in terms of energy consumption. Recently, lithium-ion phosphate cathodes with active materials such as LiFePO4 [1–3], LiMnPO4 [4], LiCoPO4 [5], LiNiPO4 [6], and Li3V2(PO4)3 [7] have been extensively utilized n
Corresponding author. Tel.: þ82 62 530 1746; fax: þ 82 62 530 0077. E-mail address: [email protected] (H.-B. Gu).
http://dx.doi.org/10.1016/j.ceramint.2014.12.105 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
for lithium ion batteries. Among them, tremendous attention has been focused on LiFePO4 as an alternative cathode material. In comparison with LiFePO4, LiVOPO4 has the advantages of a higher potential (4.0 V versus Li/Li þ ) with the theoretical capacity of 166 mA h g 1 for charging/discharging and of a triclinic phase [8]. However, the lithium ion diffusion rate in LiVOPO4 decrease with increasing current density because of the separation of VO6 octahedra by PO4 tetrahedra in the monoclinic structure, which leads to a decrease in capacity. Much effort has been devoted to improving the electronic properties of these materials, such as (1) coating them with electronically conductive materials [9], (2) doping them with various elements [10], or (3) modulating the particle properties [11]. With these approaches, LiVOPO4 can be transformed into a better material, namely, Li3V2(PO4)3. Indeed, the phosphate polyanion Li3V2(PO4)3 has been considered a promising material because of its better thermal stability, better ion mobility, higher theoretical capacity (197 mA h g 1), higher operating voltage, and high electronic conductivity of 2.4 10 7 S cm 1 [7].
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Fig. 1. Schematic illustration of LiVOPO4 and Li3V2(PO4)3–GNF preparation.
The structural conversion from LiVOPO4 to Li3V2(PO4)3 can be induced using many methods, such as the hydrothermal method [12–14], the solid-state method [15,16], the sol–gel method [17–19], or spray pyrolysis [20,21]. Despite certain disadvantages such as a long period of calcination, irregular morphology, uncontrollable particle growth, and agglomeration, the solid-state method is still the most commonly used because of its simple synthetic procedure and because it is suitable for mass production [15]. In the solid-state method, solid precursors are ground or ball-milled together and then annealed in a furnace. During annealing, carbon can be directly added or can form by pyrolysis of an organic precursor and can then act as a reduction material to reduce V5þ into V3þ . In this work, graphite nanofiber (GNF) was used as a reduction material. Compared with other carbon sources, GNF has considerably higher surface areas, more inexpensive, a superior performance and lightweight material capable of absorbing and retaining hydrogen at room temperature. Moreover, as in my knowledge, until now there still no report about GNF's properties as carbon source enhancing the electrochemical properties of Li3V2(PO4)3 by transforming the structure from LiVOPO4 with GNF. Therefore, the objective of the present investigation is to examine this method as a way of enhancing the electrochemical properties of Li3V2(PO4)3 by transforming the structure from LiVOPO4 with GNF as a reduction material. Furthermore, the physical, electrical, and electrochemical characteristics of LiVOPO4 and Li3V2(PO4)3–GNF batteries were investigated. 2. Experimental section LiVOPO4 powders were synthesized using a solid-state method. The lithium carbonate (Li2CO3, Aldrich Co., 499%), vanadium oxide (V2O5, Aldrich Co., 498%), and ammonium dihydrogen phosphate (NH4H2PO4, Aldrich Co., 498%) precursor materials were used in stoichiometric amounts. First, the mixtures were ballmilled by a planetary mono-mill (Pulverisette 6, Fritsch, Germany) with N-methyl-2-pyrrolidone (NMP) solvent at 300 rpm for 12 h. After that, the obtained LiVOPO4 slurry was dried at 90 1C for 12 h in air. In order to obtain the Li3V2(PO4)3–GNF composite, the
LiVOPO4 slurry described above was separately mixed with GNF (segment type, length 30 μm, diameter 300 nm, and Carbon Nano-Material Technology Co.; 3%, 5%, 7%, and 10%) and ballmilled for 10 h. The obtained powders were pelletized and then heat-treated at 900 1C for 12 h in a tube furnace (J-FCA, Jisico, Korea) under nitrogen atmosphere. The synthesis process described above is illustrated in Fig. 1. The crystalline phases of LiVOPO4 and the Li3V2(PO4)3–GNF composites were identified by X-ray diffraction (XRD, Dmax/ 1200, Rigaku, Japan) with scanning steps of 0.021 in the range 10–801. The morphologies of the LiVOPO4 and Li3V2(PO4)3– GNF composites were observed with a field emission-scanning electron microscope (FE-SEM, Hitachi S-4700, Japan), which had an accelerating voltage of 15 kV. The stoichiometric molar composition of LiVOPO4 and Li3V2(PO4)3–GNF was analyzed by inductively coupled plasma mass spectrometer (ICP/MS, Nexion 300X, Perkin-Elmer, Canada). The surface areas of samples were measured by Brunauer, Emmett and Teller (BET, Micrometrics Instruments ASAP 2020, USA). The composite electrodes were made from mixtures of LiVOPO4 and Li3V2(PO4)3–GNF composites with carbon black (SP-270) as a conductive material and polyvinylidene fluoride (PVdF) as a binder in a weight percent ratio of 70:25:5. After ball-milling, the slurry was coated onto aluminum foil and dried at 90 1C for 1 h. The electrodes were roll-pressed (0.6 m min 1 and 20 μm) and dried again at 110 1C for 24 h under vacuum. The CR2032-type coin cells were assembled in an argon-filled glove box using lithium as the anode and 1 M LiPF6/EC-DMC (1:1) (Soulbrain, Korea) as the electrolyte. The separator was a Celgard#2500 membrane. The weights of the LiVOPO4 and Li3V2(PO4)3–GNF samples were 1.5–2 mg cm 2. Cyclic voltammetry (CV) was carried out on automatic charge/ discharge equipment (WBCS3000, WonATech, Korea) with a scanning rate of 0.1 mV s 1. The apparent lithium ion diffusion coefficients of LiVOPO4 and Li3V2(PO4)3–GNF were obtained by varying the scanning rate from 0.01 to 0.2 mV s 1. Electrochemical impedance spectroscopy (EIS) was performed using an impedance measurement system (IM6, Germany) with an AC
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voltage of 20 mV in an amplitude over the frequency range from 10 mHz to 2 MHz. The cycling performance was evaluated in the voltage range between 3.0 and 4.8 V at room temperature. The charge–discharge experiment was carried out with WBCS3000 system, the voltage range between 3.0 and 4.8 V at 0.1 C (16.6 mA g 1 for LiVOPO4 and 19.7 mA g 1 for Li3V2(PO4)3– GNF). 3. Results and discussion The XRD patterns of the LiVOPO4 and Li3V2(PO4)3–GNF (7%) samples are depicted in Fig. 2(a). The XRD profile of LiVOPO4, which is identified as that of a triclinic system with a P1 space group (JCPDS card #20537), indicates unit cell
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parameters of a¼ 6.748(0) Å, b ¼ 7.206(0) Å, c¼ 7.922(0) Å, and V ¼ 343.16 Å3. After GNF was added, the LiVOPO4 transformed completely to Li3V2(PO4)3. The XRD pattern of the transformed material is typical for a monoclinic system with a P21/n space group (JCPDS card #69345) with unit cell parameters of a¼ 8.604(0) Å, b ¼ 8.591(0) Å, c¼ 14.723(0) Å, and V ¼ 889.54 Å3. Moreover, no (002) diffraction peak of GNF located around 24.51 was observed [22]. This also proves that GNF has no effect on the Li3V2(PO4)3 crystallites other than acting as a reduction material. The ICP analysis also confirmed the molar stoichiometric composition of the prepared LiVOPO4 and Li3V2(PO4)3–GNF (7%). In the results, the molar ratios of Li:V:P in the LiVOPO4 and Li3V2(PO4)3– GNF (7%) are 0.97:0.98:1 and 2.98:1.97:3. In general, several factors can contribute to the broadening of X-ray diffraction peaks. One factor that directly affects the Li3V2(PO4)3 crystallite is local microstrain [23,24]. The crystallite size L and local microstrain (e2) can be determined by combining Scherrer's equation and Bragg's law for diffraction using following equation: B2 cos 2 θ ¼ 16e2 sin 2 θ þ ðK 2 λ2 Þ=L2
ð1Þ
where B is the full-width at half-maximum, θ is the diffraction angle, and K is a dimensionless shape factor (K=0.9). The plot of the relationship between B2cos2 θ and sin2 θ is reported in Fig. 2(b) and was well fit by straight lines. The values of the microstrain (e2) and crystallite size (L) are provided in Table 1. From a glimpse at the table, we can recognize that the crystallite size increases as the structure conversion from LiVOPO4 to Li3V2(PO4)3, which leads to the increase in the grain size of Li3V2(PO4)3. This suggestion will be confirmed in the morphological image below (Fig. 4). On the other hand, the microstrains of LiVOPO4 and Li3V2(PO4)3– GNF (7%) are the same, which means that the structural conversion does not cause defects in the resulting Li3V2(PO4)3 crystallite. Fig. 3(a) shows the fitting line for the powder X-ray diffraction data of Li3V2(PO4)3–GNF with a space group of P21/n. The good fitting between the observed and calculated patterns of Li3V2 (PO4)3–GNF can be recognized from the figure. As can also be seen from Fig. 3(b), Li3V2(PO4)3 consists of a framework of metal octahedra and phosphate tetrahedra linked together by common apical oxygen atoms. This framework forms a three-dimensional network in which all the Li, V, P, and O atoms occupy Wyckoff position 4e with different coordinates. This unique arrangement forms three-dimensional pathways in the structures of the lithiated and delithiated phases, leading to better electrochemical properties than those obtained with just the one-dimensional pathway in LiVOPO4. This suggestion will be confirmed in the following results. Table 1 Local microstrain and crystallite size of LiVOPO4 and Li3V2(PO4)3–GNF.
Fig. 2. (a) XRD profiles of the LiVOPO4 and Li3V2(PO4)3–GNF (7%) samples and (b) the relationship between B2cos2 θ and sin2 θ.
Samples
Microstrain (e)
Crystallite size (L) (nm)
LiVOPO4 Li3V2(PO4)3–GNF
0.0197 0.0205
8.32 11.59
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Fig. 4 shows FE-SEM images of LiVOPO4 and Li3V2(PO4)3– GNF (7%) before and after heat-treatment. In Fig. 4(a), after ballmilling, the gravel-like particles can be observed. When GNF was added and ball-milled for 10 h, the morphology of the particles changed, as shown in Fig. 4(b). In the presence of GNF, the size of particle becomes smaller and the holes between the particles were decreased. Further, after the heat-treatment, the particle sizes of LiVOPO4 particles become smaller (Fig. 4(c)). Whereas, the Li3V2(PO4)3–GNF consists of larger particles with sizes of about 3–5 mm mixed with GNF. Larger particles with eroded-like structures were formed, as can be observed in Fig. 4(d). This is because after the heat-treatment process LiVOPO4 was completely converted to Li3V2(PO4)3. Despite of the particles size increase, the different structures of the Li3V2(PO4)3–GNF compound are
responsible of the better performances. These structures have the advantages of a large active area and fast transport of lithium ions, which can afford enough active sites for simultaneous intercalation of many lithium ions. Furthermore, the presence of GNF enhances the connections between Li3V2(PO4)3 particles, resulting in the enhanced diffusion of lithium ions between the particles and the enhanced electrochemical properties of Li3V2(PO4)3–GNF batteries. The structure of the synthesized Li3V2(PO4)3–GNF favors the penetration of the electrolyte into the cathode material, decreasing the diffusion lengths of lithium ions and electrons and improving the cyclability of the Li3V2(PO4)3 cathode. Further, the Brunauer–Emmett–Teller (BET) analysis in estimating the surface area of the LiVOPO4 and Li3V2(PO4)3–GNF (7%) sample to be 13.59 and 21.40 m2 g 1 respectively (Fig. 5). The CV curves for the LiVOPO4 and Li3V2(PO4)3–GNF batteries obtained at a scanning rate of 0.1 mV s 1 in the voltage range of 3.0–4.8 V are shown in Fig. 6. Reduction and oxidation peaks of LiVOPO4 at around 4.13 and 3.74 V, respectively, are observed in Fig. 6(a). During the charge process, as shown in Fig. 6(b), there are four plateaus around 3.71 (A1), 3.82 (A2), 4.26 (A3), and 4.64 (A4) V, which correspond to a sequence of phase transition processes between the single-phase reaction (solid-state behavior) of LixV2(PO4)3 (x¼ 3.0, 2.5, 2.0, 1.0, and 0). In contrast, in the discharge process, there are three peaks around 3.81 (B1), 3.70 (B2), and 3.48 (B3) V corresponding to a sequence of phase transition processes between the two-phase reaction of LixV2(PO4)3 (x¼ 2, 2.5, and 3) [25]. Compared with LiVOPO4, the cyclic performance of Li3V2(PO4)3–GNF was changed dramatically. This is can be explained in two main reasons: (1) GNF was completely converted LiVOPO4 to Li3V2 (PO4)3 which has better theoretical capacity as well as electrochemical performance, and (2) as mentioned above, Li3V2 (PO4)3–GNF has as structure which allows larger active area and faster lithium ion transport. Moreover, the more GNF is added the better the cyclic voltammograms are. However, when the GNF content increased to more than 7%, the values were dropped. This is because an excessive carbon content will reduce the ratio of the active material and the GNF adding web is intrinsically a physical barrier that hinders the diffusion and transition of lithium ion, both of which leading to capacity loss [26]. The phase transition processes of Li3V2(PO4)3 are given in the equations below, which are consistent with early CV studies on Li3V2(PO4)3. A plausible discharge mechanism can be described as follows [27,28]. In the charge process: Li3 V2 ðPO4 Þ3 – 0:5Li þ –0:5e -Li2:5 V2 ðPO4 Þ3
þÞ V3 þ ; Vð4 ð1=2Þ
ð2Þ Li2:5 V2 ðPO4 Þ3 –0:5Li þ –0:5e -Li2 V2 ðPO4 Þ3
V3 þ ; V4 þ ð3Þ
Li2 V2 ðPO4 Þ3 – Li þ – e -LiV2 ðPO4 Þ3 Fig. 3. (a) Fitting line for the Li3V2(PO4)3–GNF (7%) powder X-ray diffraction data. (b) Crystal structure of Li3V2(PO4)3.
LiV2 ðPO4 Þ3 –Li þ –e -V2 ðPO4 Þ3
V4 þ V4:5 þ
ð4Þ ð5Þ
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GNF
Fig. 4. FE-SEM images of LiVOPO4 and Li3V2(PO4)3–GNF (7%) powders (a) and (b) before heat-treatment, (c) and (d) after heat-treatment.
In the discharge process: V2 ðPO4 Þ3 þ 2Li þ þ 2e -Li2 V2 ðPO4 Þ3
V3 þ ; V4 þ
Li2 V2 ðPO4 Þ3 þ 0:5Li þ þ 0:5e -Li2:5 V2 ðPO4 Þ3
ð6Þ
þÞ V3 þ ; Vð4 ð1=2Þ
ð7Þ Li2:5 V2 ðPO4 Þ3 þ 0:5Li þ þ 0:5e -Li3 V2 ðPO4 Þ3
V3 þ ð8Þ
Fig. 5. BET images for LiVOPO4 and Li3V2(PO4)3–GNF (7%).
To deeply explore the properties of LiVOPO4 and Li3V2(PO4)3– GNF, the apparent lithium ion diffusion coefficient was measured. This diffusion coefficient is affected by the reversibility of the intercalation and de-intercalation reactions, stability of the material, and phase conversions during the reactions. CV tests of LiVOPO4 and Li3V2(PO4)3–GNF have been carried out at various scanning rates from 0.01 to 0.2 mV s 1 (Fig. 7(a–e)). As shown in Fig. 7(a–e), the reduction peaks move to higher values and the oxidation peaks simultaneously move to lower ones with increasing scanning rate. Furthermore, the redox peaks get higher while the redox areas get bigger at high
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processes, resulting in irreversible behavior. In addition, at a high scanning rate, anodic peaks A1 and A2 and cathodic peaks B2 and B3 integrate and disappear gradually, respectively. This phenomenon was also observed in previous studies [28,29]. Therefore, only two oxidation peaks (A3 and A4) and one reduction peak (B1) remain, which can be used to characterize the lithium ion diffusion coefficients in Li3V2(PO4)3. Consequently, the apparent lithium ion diffusion coefficient of LiVOPO4 and Li3V2(PO4)3–GNF was estimated using Eqs. (9 and 10) from all the CV results above and the relationship between the peak current and CV scanning rate [27,29–32]: ip ¼ ð2:69 105 ÞðαnÞ1=2 AΔC Li DLi 1=2 v1=2 =Ep E o = ¼
RT αnF
(
ð9Þ
) 1=2 DLi αnFv 1=2 0:78þ ln þ ln RT k0 ð10Þ
A3 A2 A1
B3
B2
A4
B1
Fig. 6. Cyclic voltammetry profiles of (a) LiVOPO4 and (b) Li3V2(PO4)3– GNF (3%, 5%, 7%, and 10%) obtained at a scanning rate of 0.1 mV s 1.
scanning rates. This is because the peak area divided by the scanning rate yields the capacity of the electrode, which should be a constant [27]. It is noted that the oxidation peaks are slightly higher than the reduction peaks, indicating a slight kinetic difference between the lithium insertion and extraction processes, especially at a scanning rate of 0.2 mV s 1. This is because of disparities in the insertion and extraction capacities the electrode for lithium ions at high scanning rates, which means that the forward and reverse reaction rates have a wide disparity. In particular, the ionic transport across particle boundaries is not the same for the forward and reverse
where Ip is the peak current, n is the number of electrons per lithium ion (n=1), ν is the scanning rate for the CV, ΔCLi is the concentration of lithium ions (3.7 10 3 mol cm 3) [27], A is the electrode's surface area which contacts between electrode material and electrolyte [27,30], /Ep Eo/ is calculated as half of the difference between redox peak potentials, α is the transfer coefficient, and k0 is the standard rate constant. The relationships between /Ep Eo/ and ln ν, between Ip and ν1/2 are illustrated in Fig. 8(a–d). The transfer coefficient (α) is obtained from Fig. 8(a and c) and Eq. (10). The values of DLi for LiVOPO4 and Li3V2(PO4)3–GNF can be obtained from Fig. 8(b and d) with knowledge of the obtained α and Eq. (9). The peak current (Ip) exhibits a linear relation with the square root of the scanning rate (ν1/2), as shown in Fig. 8(b and d). It is found that LiVOPO4 displays an apparent diffusion coefficient lower than that of Li3V2(PO4)3 in the extraction process, which confirms that the extraction of lithium ions from Li3V2(PO4)3–GNF is better than that from LiVOPO4. In particular, when 7% GNF is added to Li3V2(PO4)3, the apparent diffusion coefficient during extraction is 6.01 10 9 cm2 s 1, whereas 8.32 10 11 cm2 s 1 was obtained for LiVOPO4. Detailed results are shown in Table 2. The higher DLi for Li3V2(PO4)3–GNF is due to the charge-transfer interface reaction, which is an important factor in lithium ion diffusion and thus leads to different rate performance. For the case of LiVOPO4 where the most places on a particle surface contact directly with the electrolyte, the reaction can only occur on the selected spots in which the electronic conductive path is built, the effective reaction area is limited. However, after adding carbon sources-GNF, by the appearance of GNF and large active area, lithium ions can move from this particle to the others more easily, thus enhancing the connection availability between particles, leading to the change in the transfer coefficient and the apparent diffusion of Li3V2(PO4)3–GNF, especially with Li3V2(PO4)3–GNF (7%). Further, after adding 10% GNF, as mentioned above an excessive carbon content will reduce the ratio of the active material and the GNF adding web is intrinsically a physical barrier that hinders the diffusion and transition of lithium ion, leading to the decrease in the value of 10% GNF added Li3V2(PO4)3.
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Fig. 7. Cyclic voltammetry profiles of (a) LiVOPO4, (b)–(e) Li3V2(PO4)3–GNF (3%, 5%, 7%, 10%) obtained at various scanning rates.
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Fig. 8. Relationships (a) and (c) between /Ep Eo/ and ln ν, (b) and (d) between Ip and ν1/2 for LiVOPO4 and Li3V2(PO4)3–GNF (3%, 5%, 7%, and 10%).
Table 2 Transfer coefficient and apparent diffusion coefficient obtained by CV. Samples LiVOPO4 3% 5% 7% 10%
Peaks
α
DLi (cm2 s 1)
A3 A4 A3 A4 A3 A4 A3 A4
0.35 0.59 0.57 0.56 0.71 0.25 0.30 0.23 0.22
8.32 10 11 2.91 10 10 4.31 10 11 7.07 10 10 7.95 10 11 6.01 10 9 1.20 10 9 4.06 10 9 8.17 10 10
EIS results for the LiVOPO4 and Li3V2(PO4)3–GNF batteries are shown in Fig. 9. The semicircles from the high to medium frequency are mainly related to a complex reaction process at the electrolyte/ cathode interface. The impedance spectra can be interpreted on the basis of an equivalent circuit in which Zw is Warburg impedance, Rct is charge-transfer resistance, Cd is capacitance of a double layer, and Rs is ohmic resistance. Furthermore, the exchange current density is
calculated using following equation: RT ð11Þ nFRct The diameter of the high-frequency combined semicircle of the batteries decreases when GNF is added and is especially small for Li3V2(PO4)3–GNF (7%). The impedance parameters of the batteries prepared from LiVOPO4 and Li3V2(PO4)3–GNF are shown in Table 3. It is found that the charge transfer resistance (Rct) of the Li3V2(PO4)3–GNF battery is significantly smaller than that of the LiVOPO4 battery, and the exchange current density is increased. This phenomena can be explained that after adding GNF, Li3V2(PO4)3–GNF has larger active area which reduces resistance between the active material and the surface film [28], as well as allows solid state diffusion of lithium ion easily, leading to a decrease in the charge transfer resistance. Adding GNF (7%) significantly improved the performance of the Li-battery, leading to the lowest resistance of 40 Ω and the largest exchange current density of 2.14 10 4 mA cm 2. However, when the GNF content increases to 10%,
io ¼
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Fig. 9. EIS results for LiVOPO4 and Li3V2(PO4)3–GNF (3%, 5%, 7%, and 10%).
Table 3 Impedance parameters of the cells prepared from LiVOPO4 and Li3V2(PO4)3– GNF (3%, 5%, 7%, and 10%). Samples
Rct (Ω)
i0 (mA cm 2)
LiVOPO4 3% 5% 7% 10%
421 252 206 40 165
6.10 10 5 3.39 10 5 4.16 10 5 2.14 10 4 5.19 10 5
the charge transfer resistance dropped. That is because the higher GNF carbon content is over thick for easily penetration and intercalation of lithium ions. Actually, only appropriate amount of GNF residue would effectively enhance the electrochemical performance as well as cyclic property. This is in accordance with the CV results. Fig. 10 shows the charge–discharge curves and the cycling performance of the LiVOPO4 and Li3V2(PO4)3–GNF batteries charged–discharged at 0.1 C between 3.0 and 4.8 V. The discharge capacities of LiVOPO4 at the 1st and 16th cycles are 113 and 89 mA h g 1, respectively. Thus, only 79% of the initial capacity was retained after 16 cycles. This demonstrates poor stability of the structure. The Li3V2(PO4)3–GNF (7%) retained 94% of the initial capacity even after 16 cycles, since the capacity was reduced from 177 to 164 mA h g 1. Further, the coulombic efficiencies of LiVOPO4 and Li3V2(PO4)3–GNF (7%) are 93% and 98%, respectively. In here, the coulombic efficiencies do not reach to 100% after 10 cycles due to the dendrite phenomenon formed on electrode surfaces. The dendrites may grow large enough to penetrate the separating barrier and touch the cathode. In the charging– discharging process, after some cycles solid electrolyte interfaces (SEI) layer will form once Li metal contact liquid electrolyte. Lithium ions diffuse through SEI layer and deposit on Li surface difficulty. Formation and stability of SEI layer are the main factors
Fig. 10. (a) Charge–discharge performance at first cycle and (b) cycling performance of LiVOPO4 and Li3V2(PO4)3–GNF (3%, 5%, 7%, and 10%) at 0.1 C.
affecting the coulombic efficiency of Li deposition/stripping processes. Taking into account the XRD, SEM, CV, and apparent lithium ion diffusion results, we can conclude that the intercalation/deintercalation of lithium ions into the active material in the Li3V2(PO4)3–GNF (7%) electrode takes place more easily during the charging/discharging process. Furthermore, Li3V2(PO4)3–GNF (7%) delivers the highest discharge capacity of 181 mA h g 1 at the second cycle. Thus, the electrochemical performance was improved by the structure conversion from LiVOPO4 to Li3V2(PO4)3– GNF due to the addition of GNF. Fig. 11 delivers the LiVOPO4 and Li3V2(PO4)3–GNF batteries charged–discharged at different current densities of 0.1 C, 0.2 C, 0.5 C, 1 C and 5 C. The discharge capacity of each sample declines with an increase in current density. When the same current density is applied, Li3V2(PO4)3–GNF (7%) exhibits larger
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of the batteries. Further, by the presence of GNF, lithium ions can move from this particle to the others more easily, thus enhancing the connection availability between particles, leading to the change in the charge transfer resistance and the apparent lithium ion diffusion of Li3V2(PO4)3–GNF. Because of the structural conversion, the Li3V2(PO4)3–GNF (7%) batteries showed the best properties, with apparent lithium ion diffusion coefficient of 6.01 10 9 cm2 s 1, an impedance of 40 Ω, an exchange current density of 2.14 10 4 mA cm 2, and a discharge capacity of 181 mA h g 1 at the 2nd cycle. Acknowledgments This research was received support from Brain Korea 21 Plus. References
Fig. 11. C-rate performance of LiVOPO4 and Li3V2(PO4)3–GNF .
capacity than LiVOPO4. The discharge capacity of Li3V2(PO4)3– GNF (7%) varies in the range 177–97 mA h g 1 at the rate of 0.1–5 C (113–36 mA h g 1 for LiVOPO4). The improvement may be associated with the decreased charge transfer resistance, increased conductivity and enhanced electrochemical kinetic induced by GNF adding. Li3V2(PO4)3–GNF (7%), with lesser resistance to charge-transfer which will increase the lithium ion diffusion between particles, obviously has the higher stability at the high rate performance. The superior performance and the good stability should be attributed mainly to the suitable content of carbon, which formed not only a homogeneous conductive coating on the surface of each particles but also a uniform web connected the whole active material, and a strong adhesion to maintain the composite film stable during longtime cycling processes. The film derived from higher GNF content (10%) displayed good cyclic stability but frustrating storage properties. The film exhibited only 168 and 85 mA h g 1 at discharge rates of 0.1 C and 5 C, respectively. Since 10% GNF adding was over thick and extensively inhibited the diffusion of lithium ions, resulting in lower intercalation ability, especially at higher discharge current density. 4. Conclusions In summary, LiVOPO4 and Li3V2(PO4)3–GNF were synthesized successfully using the solid-state method. The structural conversion of LiVOPO4 to Li3V2(PO4)3 occurred easily with the help of the GNF reducing agent. No impurity peak was observed in the XRD patterns of LiVOPO4 and Li3V2(PO4)3, and the structural conversion does not cause defects in the resulting Li3V2(PO4)3 crystallite. Larger particles with corroded-like structures formed and were mixed with GNF particles, which provided the advantages of a large active area and fast transport of lithium ions, leading to an improvement in the electrochemical properties
[1] S.Y. Chung, J.T. Bloking, Y.M. Chiang, Electronically conductive phospho-olivines as lithium storage electrodes, Nat. Mater. 1 (2002) 123–128. [2] Y.G. Wang, Y.R. Wang, E.J. Hosono, K.X. Wang, H.S. Zhou, The design of a LiFePO4/carbon nanocomposite with a core–shell structure and its synthesis by an in situ polymerization restriction method, Angew. Chem. Int. Ed. 47 (2008) 7461–7465. [3] V.H. Nguyen, E.M. Jin, H.B. Gu, Synthesis and electrochemical properties of LiFePO4–graphite nanofiber composites as cathode materials for lithium ion batteries, J. Power Sources 244 (2013) 586–591. [4] M. Pivko, M. Bele, E. Tchernychova, N.Z. Logar, R. Dominko, M. Gaberscek, Synthesis of nanometric LiMnPO4 via a two-step technique, Chem. Mater. 24 (2012) 1041–1047. [5] N.N. Bramnik, K. Nikolowski, C. Baehtz, K.G. Bramnik, H. Ehrenberg, Phase transitions occurring upon lithium insertion extraction of LiCoPO4, Chem. Mater. 19 (2007) 908–915. [6] J. Wolfenstine, J. Allen, Ni3 þ /Ni2 þ redox potential in LiNiPO4, J. Power Sources 142 (2005) 389–390. [7] M.Y. Saidi, J. Barker, H. Huang, J.L. Swoyer, G. Adamson, Electrochemical properties of lithium vanadium phosphate as a cathode material for lithium-ion batteries, Electrochem. Solid-State Lett. 5 (2002) A149–A151. [8] B.M. Azmi, T. Ishihara, H. Nishiguchi, Y. Takita, Cathodic performance of VOPO4 with various crystal phases for Li ion rechargeable battery, Electrochim. Acta 48 (2002) 165–170. [9] P. Fu, Y. Zhao, Y. Dong, X. An, G. Shen, Low temperature solid-state synthesis routine and mechanism for Li3V2(PO4)3 using LiF as lithium precursor, Electrachim. Acta 52 (2006) 1003–1008. [10] Y.H. Chen, Y.M. Zhao, X.N. An, J.M. Liu, Y.Z. Dong, L. Chen, Preparation and electrochemical performance studies on Cr-doped Li3V2(PO4)3 as cathode materials for lithium-ion batteries, Electrochim. Acta 54 (2009) 5844–5850. [11] H. Huang, S.C Yin, T. Kerr, N. Taylor, L.F. Nazar, Nanostructured composites: a high capacity, fast rate Li3V2(PO4)3/carbon cathode for rechargeable lithium batteries, Adv. Mater. 14 (2002) 1525–2528. [12] M.R. Gao, Y.F. Xu, J. Jiang, S.H. Yu, Nanostructured metal chalcogenides: synthesis, modification, and applications in energy conversion and storage devices, Chem. Soc. Rev. 42 (2013) 2986–3017. [13] J.J. Chen, M.J. Vacchio, S.J. Wang, N. Chernova, P.Y. Zavalij, M.S. Whittingham, The hydrothermal synthesis and characterization of olivines and related compounds for electrochemical applications, Solid State Ion. 178 (2008) 1676–1693. [14] H. Yu, X.H. Rui, H.T. Tan, J. Chen, X. Huang, C. Xu, W.L. Liu, D.Y.W. Yu, H.H. Hng, H.E. Hoster, Q.Y. Yan, Cu doped V2O5 flowers as cathode material for high-performance lithium ion batteries, Nanoscale 5 (2013) 4937–4943.
V.H. Nguyen et al. / Ceramics International 41 (2015) 5403–5413 [15] Y. Zhang, Q.Y. Huo, P.P. Du, L.Z. Wang, A.Q. Zhang, Y.H. Song, Y. Lv, G.Y. Li, Advances in new cathode material LiFePO4 for lithiumion batteries, Synth. Metals 162 (2012) 1315–1326. [16] P. Fu, Y.M. Zhao, Y.Z. Dong, X.N. An, G.P. Shen, Synthesis of Li3V2(PO4)3 with high performance by optimized solid-state synthesis routine, J. Power Sources 162 (2006) 651–657. [17] D. Choi, P.N. Kumta, Surfactant based sol–gel approach to nanostructured LiFePO4 for high rate Li-ion batteries, J. Power Sources 163 (2007) 1064–1069. [18] H. Liu, Y.P. Wu, E. Rahm, R. Holze, H.Q. Wu, Cathode materials for lithium ion batteries prepared by sol–gel methods, J. Solid State Electrochem. 8 (2004) 450–466. [19] W. Yuan, J. Yan, Z.Y. Tang, O. Sha, J.M. Wang, W.F. Mao, L. Ma, Synthesis of Li3V2(PO4)3 cathode material via a fast sol–gel method based on spontaneous chemical reactions, J. Power Sources 201 (2012) 301–306. [20] Y.N. Ko, J.H. Kim, S.H. Choi, Y.C. Kang, Electrochemical properties of spherically shaped dense V2O5 cathode powders prepared directly by spray pyrolysis, J. Power Sources 211 (2012) 84–91. [21] Y.N. Ko, H.Y. Koo, J.H. Kim, J.H. Yi, Y.C. Kang, J.H. Lee, Characteristics of Li3V2(PO4)3/C powders prepared by ultrasonic spray pyrolysis, J. Power Sources 196 (2011) 6682–6687. [22] C. Xu, X. Wang, J.W. Zhu, X.J. Yang, L. Lu, Deposition of Co3O4 nanoparticles onto exfoliated graphite oxide sheets, J. Mater. Chem. 18 (2008) 5625–5629. [23] M. Kope¢, A. Yamada, G. Kobayashi, S. Nishimura, R. Kanno, A. Mauger, F. Gendron, C.M. Julien, Structural and magnetic properties of Lix(MnyFe1 y)PO4 electrode materials for Li-ion batteries, J. Power Sources 189 (2009) 1154–1163.
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[24] N. Amdouni, F. Gendron, A. Mauger, H. Zarrouk, C.M. Julien, LiMn2 yCoyO4 (0ry r1) intercalation compounds synthesized from wet-chemical route, Mater. Sci. Eng. B 129 (2006) 64–75. [25] Y. Yang, H.S. Fang, J. Zheng, L. Li, G.S. Li, G.F. Yan, Towards the understanding of poor electrochemical activity of triclinic LiVOPO4: experimental characterization and theoretical investigations, Solid State Sci. 10 (2008) 1292–1298. [26] N. Zhou, E. Uchaker, Y.Y. Liu, S.Q. Liu, Y.N. Liu, G.Z. Cao, Effect of carbon content on electrochemical performance of LiFePO4/C thin film cathodes, Int. J. Electrochem. Sci. 7 (2012) 12633–12645. [27] X.H. Rui, N. Ding, J. Liu, C. Li, C.H. Chen, Analysis of the chemical diffusion coefficient of lithium ions in Li3V2(PO4)3 cathode material, Electrochim. Acta 55 (2010) 2384–2390. [28] X.H. Rui, Q.Y. Yan, M. Skyllas-Kazacos, T.M. Lim, Li3V2(PO4)3 cathode materials for lithium-ion batteries: a review, J. Power Sources 258 (2014) 19–38. [29] S. Franger, C. Bourbon, F. Le Cras, Optimized lithium iron phosphate for high-rate electrochemical applications, J. Electrochem. Soc. 151 (2004) A1024–A1027. [30] L. Cheng, X.L. Li, H.J. Liu, H.M. Xiong, P.W. Zhang, Y.Y Xia, Carboncoated Li4Ti5O12 as a high rate electrode material for Li-ion intercalation, J. Electrochem. Soc. 154 (2007) A692–A697. [31] V.H. Nguyen, W.L. Wang, E.M. Jin, H.B. Gu, Impacts of different polymer binders on electrochemical properties of LiFePO4 cathode, Appl. Surf. Sci. 282 (2013) 444–449. [32] K. Wang, R. Cai, T. Yuan, X. Yu, R. Ran, Z.P. Shao, Process investigation, electrochemical characterization and optimization of LiFePO4/C composite from mechanical activation using sucrose as carbon source, Electrochim. Acta 54 (2009) 2861–2868.
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 5403–5413 www.elsevier.com/locate/ceramint
Enhanced electrochemical performance of Li3V2(PO4)3 structurally converted from LiVOPO4 by graphite nanofiber addition Van Hiep Nguyen, Wan Lin Wang, Hal-Bon Gun Department of Electrical Engineering, Chonnam National University, Gwangju 500-757, South Korea Received 3 June 2014; received in revised form 11 November 2014; accepted 18 December 2014 Available online 29 December 2014
Abstract In this work, the structural conversion of LiVOPO4 to Li3V2(PO4)3 due to the addition of graphene nanofiber (GNF) was investigated, and the resulting materials were found to exhibit enhanced capacity and cyclability. First, LiVOPO4 was synthesized using a solid-state method followed by annealing at 900 1C for 12 h under nitrogen atmosphere. Then, the conversion from the triclinic LiVOPO4 structure to the monoclinic Li3V2(PO4)3 structure due to the GNF addition was observed. No impurity peak was observed in the X-ray diffraction patterns of LiVOPO4 or Li3V2(PO4)3, and the structural conversion caused no defects to form in the resulting Li3V2(PO4)3 crystallite. Field emission-scanning electron microscope studies clearly demonstrate that larger corroded-structure-like particles formed which were mixed with GNF. This provided both a large active area and fast transport of lithium ions, which afforded enough active sites for simultaneous intercalation of many lithium ions, leading to improved electrochemical properties of the material. Compared with LiVOPO4, the Li3V2(PO4)3–GNF showed better properties, such as an improved lithium ion diffusion coefficient, improved cyclability, and smaller impedance. Furthermore, the optimized Li3V2(PO4)3–GNF (7%) battery showed the best discharge capacity of 181 mA h g 1 at 0.1 C and lithium ion diffusion coefficient of 6.01 10 9 cm2 s 1. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Li3V2(PO4)3–graphite nanofiber; Solid-state method; Structural conversion; Positive electrode
1. Introduction In recent years, cathode materials based on lithium transition metal oxides have attracted much attention because they are advantageous for applications such as powering electronic devices like cameras, laptops, electrical devices, or hybrid electric vehicles. At present, several materials are under development to overcome certain limitations of these materials related to electron and lithium ion transfer within the lattice, structural stability, low activation energy, low electronic conductivity, and low redox potential versus the Li/Li þ potential. Together, these limitations lead to lowpotential batteries that are not beneficial in terms of energy consumption. Recently, lithium-ion phosphate cathodes with active materials such as LiFePO4 [1–3], LiMnPO4 [4], LiCoPO4 [5], LiNiPO4 [6], and Li3V2(PO4)3 [7] have been extensively utilized n
Corresponding author. Tel.: þ82 62 530 1746; fax: þ 82 62 530 0077. E-mail address: [email protected] (H.-B. Gu).
http://dx.doi.org/10.1016/j.ceramint.2014.12.105 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
for lithium ion batteries. Among them, tremendous attention has been focused on LiFePO4 as an alternative cathode material. In comparison with LiFePO4, LiVOPO4 has the advantages of a higher potential (4.0 V versus Li/Li þ ) with the theoretical capacity of 166 mA h g 1 for charging/discharging and of a triclinic phase [8]. However, the lithium ion diffusion rate in LiVOPO4 decrease with increasing current density because of the separation of VO6 octahedra by PO4 tetrahedra in the monoclinic structure, which leads to a decrease in capacity. Much effort has been devoted to improving the electronic properties of these materials, such as (1) coating them with electronically conductive materials [9], (2) doping them with various elements [10], or (3) modulating the particle properties [11]. With these approaches, LiVOPO4 can be transformed into a better material, namely, Li3V2(PO4)3. Indeed, the phosphate polyanion Li3V2(PO4)3 has been considered a promising material because of its better thermal stability, better ion mobility, higher theoretical capacity (197 mA h g 1), higher operating voltage, and high electronic conductivity of 2.4 10 7 S cm 1 [7].
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Fig. 1. Schematic illustration of LiVOPO4 and Li3V2(PO4)3–GNF preparation.
The structural conversion from LiVOPO4 to Li3V2(PO4)3 can be induced using many methods, such as the hydrothermal method [12–14], the solid-state method [15,16], the sol–gel method [17–19], or spray pyrolysis [20,21]. Despite certain disadvantages such as a long period of calcination, irregular morphology, uncontrollable particle growth, and agglomeration, the solid-state method is still the most commonly used because of its simple synthetic procedure and because it is suitable for mass production [15]. In the solid-state method, solid precursors are ground or ball-milled together and then annealed in a furnace. During annealing, carbon can be directly added or can form by pyrolysis of an organic precursor and can then act as a reduction material to reduce V5þ into V3þ . In this work, graphite nanofiber (GNF) was used as a reduction material. Compared with other carbon sources, GNF has considerably higher surface areas, more inexpensive, a superior performance and lightweight material capable of absorbing and retaining hydrogen at room temperature. Moreover, as in my knowledge, until now there still no report about GNF's properties as carbon source enhancing the electrochemical properties of Li3V2(PO4)3 by transforming the structure from LiVOPO4 with GNF. Therefore, the objective of the present investigation is to examine this method as a way of enhancing the electrochemical properties of Li3V2(PO4)3 by transforming the structure from LiVOPO4 with GNF as a reduction material. Furthermore, the physical, electrical, and electrochemical characteristics of LiVOPO4 and Li3V2(PO4)3–GNF batteries were investigated. 2. Experimental section LiVOPO4 powders were synthesized using a solid-state method. The lithium carbonate (Li2CO3, Aldrich Co., 499%), vanadium oxide (V2O5, Aldrich Co., 498%), and ammonium dihydrogen phosphate (NH4H2PO4, Aldrich Co., 498%) precursor materials were used in stoichiometric amounts. First, the mixtures were ballmilled by a planetary mono-mill (Pulverisette 6, Fritsch, Germany) with N-methyl-2-pyrrolidone (NMP) solvent at 300 rpm for 12 h. After that, the obtained LiVOPO4 slurry was dried at 90 1C for 12 h in air. In order to obtain the Li3V2(PO4)3–GNF composite, the
LiVOPO4 slurry described above was separately mixed with GNF (segment type, length 30 μm, diameter 300 nm, and Carbon Nano-Material Technology Co.; 3%, 5%, 7%, and 10%) and ballmilled for 10 h. The obtained powders were pelletized and then heat-treated at 900 1C for 12 h in a tube furnace (J-FCA, Jisico, Korea) under nitrogen atmosphere. The synthesis process described above is illustrated in Fig. 1. The crystalline phases of LiVOPO4 and the Li3V2(PO4)3–GNF composites were identified by X-ray diffraction (XRD, Dmax/ 1200, Rigaku, Japan) with scanning steps of 0.021 in the range 10–801. The morphologies of the LiVOPO4 and Li3V2(PO4)3– GNF composites were observed with a field emission-scanning electron microscope (FE-SEM, Hitachi S-4700, Japan), which had an accelerating voltage of 15 kV. The stoichiometric molar composition of LiVOPO4 and Li3V2(PO4)3–GNF was analyzed by inductively coupled plasma mass spectrometer (ICP/MS, Nexion 300X, Perkin-Elmer, Canada). The surface areas of samples were measured by Brunauer, Emmett and Teller (BET, Micrometrics Instruments ASAP 2020, USA). The composite electrodes were made from mixtures of LiVOPO4 and Li3V2(PO4)3–GNF composites with carbon black (SP-270) as a conductive material and polyvinylidene fluoride (PVdF) as a binder in a weight percent ratio of 70:25:5. After ball-milling, the slurry was coated onto aluminum foil and dried at 90 1C for 1 h. The electrodes were roll-pressed (0.6 m min 1 and 20 μm) and dried again at 110 1C for 24 h under vacuum. The CR2032-type coin cells were assembled in an argon-filled glove box using lithium as the anode and 1 M LiPF6/EC-DMC (1:1) (Soulbrain, Korea) as the electrolyte. The separator was a Celgard#2500 membrane. The weights of the LiVOPO4 and Li3V2(PO4)3–GNF samples were 1.5–2 mg cm 2. Cyclic voltammetry (CV) was carried out on automatic charge/ discharge equipment (WBCS3000, WonATech, Korea) with a scanning rate of 0.1 mV s 1. The apparent lithium ion diffusion coefficients of LiVOPO4 and Li3V2(PO4)3–GNF were obtained by varying the scanning rate from 0.01 to 0.2 mV s 1. Electrochemical impedance spectroscopy (EIS) was performed using an impedance measurement system (IM6, Germany) with an AC
V.H. Nguyen et al. / Ceramics International 41 (2015) 5403–5413
voltage of 20 mV in an amplitude over the frequency range from 10 mHz to 2 MHz. The cycling performance was evaluated in the voltage range between 3.0 and 4.8 V at room temperature. The charge–discharge experiment was carried out with WBCS3000 system, the voltage range between 3.0 and 4.8 V at 0.1 C (16.6 mA g 1 for LiVOPO4 and 19.7 mA g 1 for Li3V2(PO4)3– GNF). 3. Results and discussion The XRD patterns of the LiVOPO4 and Li3V2(PO4)3–GNF (7%) samples are depicted in Fig. 2(a). The XRD profile of LiVOPO4, which is identified as that of a triclinic system with a P1 space group (JCPDS card #20537), indicates unit cell
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parameters of a¼ 6.748(0) Å, b ¼ 7.206(0) Å, c¼ 7.922(0) Å, and V ¼ 343.16 Å3. After GNF was added, the LiVOPO4 transformed completely to Li3V2(PO4)3. The XRD pattern of the transformed material is typical for a monoclinic system with a P21/n space group (JCPDS card #69345) with unit cell parameters of a¼ 8.604(0) Å, b ¼ 8.591(0) Å, c¼ 14.723(0) Å, and V ¼ 889.54 Å3. Moreover, no (002) diffraction peak of GNF located around 24.51 was observed [22]. This also proves that GNF has no effect on the Li3V2(PO4)3 crystallites other than acting as a reduction material. The ICP analysis also confirmed the molar stoichiometric composition of the prepared LiVOPO4 and Li3V2(PO4)3–GNF (7%). In the results, the molar ratios of Li:V:P in the LiVOPO4 and Li3V2(PO4)3– GNF (7%) are 0.97:0.98:1 and 2.98:1.97:3. In general, several factors can contribute to the broadening of X-ray diffraction peaks. One factor that directly affects the Li3V2(PO4)3 crystallite is local microstrain [23,24]. The crystallite size L and local microstrain (e2) can be determined by combining Scherrer's equation and Bragg's law for diffraction using following equation: B2 cos 2 θ ¼ 16e2 sin 2 θ þ ðK 2 λ2 Þ=L2
ð1Þ
where B is the full-width at half-maximum, θ is the diffraction angle, and K is a dimensionless shape factor (K=0.9). The plot of the relationship between B2cos2 θ and sin2 θ is reported in Fig. 2(b) and was well fit by straight lines. The values of the microstrain (e2) and crystallite size (L) are provided in Table 1. From a glimpse at the table, we can recognize that the crystallite size increases as the structure conversion from LiVOPO4 to Li3V2(PO4)3, which leads to the increase in the grain size of Li3V2(PO4)3. This suggestion will be confirmed in the morphological image below (Fig. 4). On the other hand, the microstrains of LiVOPO4 and Li3V2(PO4)3– GNF (7%) are the same, which means that the structural conversion does not cause defects in the resulting Li3V2(PO4)3 crystallite. Fig. 3(a) shows the fitting line for the powder X-ray diffraction data of Li3V2(PO4)3–GNF with a space group of P21/n. The good fitting between the observed and calculated patterns of Li3V2 (PO4)3–GNF can be recognized from the figure. As can also be seen from Fig. 3(b), Li3V2(PO4)3 consists of a framework of metal octahedra and phosphate tetrahedra linked together by common apical oxygen atoms. This framework forms a three-dimensional network in which all the Li, V, P, and O atoms occupy Wyckoff position 4e with different coordinates. This unique arrangement forms three-dimensional pathways in the structures of the lithiated and delithiated phases, leading to better electrochemical properties than those obtained with just the one-dimensional pathway in LiVOPO4. This suggestion will be confirmed in the following results. Table 1 Local microstrain and crystallite size of LiVOPO4 and Li3V2(PO4)3–GNF.
Fig. 2. (a) XRD profiles of the LiVOPO4 and Li3V2(PO4)3–GNF (7%) samples and (b) the relationship between B2cos2 θ and sin2 θ.
Samples
Microstrain (e)
Crystallite size (L) (nm)
LiVOPO4 Li3V2(PO4)3–GNF
0.0197 0.0205
8.32 11.59
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Fig. 4 shows FE-SEM images of LiVOPO4 and Li3V2(PO4)3– GNF (7%) before and after heat-treatment. In Fig. 4(a), after ballmilling, the gravel-like particles can be observed. When GNF was added and ball-milled for 10 h, the morphology of the particles changed, as shown in Fig. 4(b). In the presence of GNF, the size of particle becomes smaller and the holes between the particles were decreased. Further, after the heat-treatment, the particle sizes of LiVOPO4 particles become smaller (Fig. 4(c)). Whereas, the Li3V2(PO4)3–GNF consists of larger particles with sizes of about 3–5 mm mixed with GNF. Larger particles with eroded-like structures were formed, as can be observed in Fig. 4(d). This is because after the heat-treatment process LiVOPO4 was completely converted to Li3V2(PO4)3. Despite of the particles size increase, the different structures of the Li3V2(PO4)3–GNF compound are
responsible of the better performances. These structures have the advantages of a large active area and fast transport of lithium ions, which can afford enough active sites for simultaneous intercalation of many lithium ions. Furthermore, the presence of GNF enhances the connections between Li3V2(PO4)3 particles, resulting in the enhanced diffusion of lithium ions between the particles and the enhanced electrochemical properties of Li3V2(PO4)3–GNF batteries. The structure of the synthesized Li3V2(PO4)3–GNF favors the penetration of the electrolyte into the cathode material, decreasing the diffusion lengths of lithium ions and electrons and improving the cyclability of the Li3V2(PO4)3 cathode. Further, the Brunauer–Emmett–Teller (BET) analysis in estimating the surface area of the LiVOPO4 and Li3V2(PO4)3–GNF (7%) sample to be 13.59 and 21.40 m2 g 1 respectively (Fig. 5). The CV curves for the LiVOPO4 and Li3V2(PO4)3–GNF batteries obtained at a scanning rate of 0.1 mV s 1 in the voltage range of 3.0–4.8 V are shown in Fig. 6. Reduction and oxidation peaks of LiVOPO4 at around 4.13 and 3.74 V, respectively, are observed in Fig. 6(a). During the charge process, as shown in Fig. 6(b), there are four plateaus around 3.71 (A1), 3.82 (A2), 4.26 (A3), and 4.64 (A4) V, which correspond to a sequence of phase transition processes between the single-phase reaction (solid-state behavior) of LixV2(PO4)3 (x¼ 3.0, 2.5, 2.0, 1.0, and 0). In contrast, in the discharge process, there are three peaks around 3.81 (B1), 3.70 (B2), and 3.48 (B3) V corresponding to a sequence of phase transition processes between the two-phase reaction of LixV2(PO4)3 (x¼ 2, 2.5, and 3) [25]. Compared with LiVOPO4, the cyclic performance of Li3V2(PO4)3–GNF was changed dramatically. This is can be explained in two main reasons: (1) GNF was completely converted LiVOPO4 to Li3V2 (PO4)3 which has better theoretical capacity as well as electrochemical performance, and (2) as mentioned above, Li3V2 (PO4)3–GNF has as structure which allows larger active area and faster lithium ion transport. Moreover, the more GNF is added the better the cyclic voltammograms are. However, when the GNF content increased to more than 7%, the values were dropped. This is because an excessive carbon content will reduce the ratio of the active material and the GNF adding web is intrinsically a physical barrier that hinders the diffusion and transition of lithium ion, both of which leading to capacity loss [26]. The phase transition processes of Li3V2(PO4)3 are given in the equations below, which are consistent with early CV studies on Li3V2(PO4)3. A plausible discharge mechanism can be described as follows [27,28]. In the charge process: Li3 V2 ðPO4 Þ3 – 0:5Li þ –0:5e -Li2:5 V2 ðPO4 Þ3
þÞ V3 þ ; Vð4 ð1=2Þ
ð2Þ Li2:5 V2 ðPO4 Þ3 –0:5Li þ –0:5e -Li2 V2 ðPO4 Þ3
V3 þ ; V4 þ ð3Þ
Li2 V2 ðPO4 Þ3 – Li þ – e -LiV2 ðPO4 Þ3 Fig. 3. (a) Fitting line for the Li3V2(PO4)3–GNF (7%) powder X-ray diffraction data. (b) Crystal structure of Li3V2(PO4)3.
LiV2 ðPO4 Þ3 –Li þ –e -V2 ðPO4 Þ3
V4 þ V4:5 þ
ð4Þ ð5Þ
V.H. Nguyen et al. / Ceramics International 41 (2015) 5403–5413
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GNF
Fig. 4. FE-SEM images of LiVOPO4 and Li3V2(PO4)3–GNF (7%) powders (a) and (b) before heat-treatment, (c) and (d) after heat-treatment.
In the discharge process: V2 ðPO4 Þ3 þ 2Li þ þ 2e -Li2 V2 ðPO4 Þ3
V3 þ ; V4 þ
Li2 V2 ðPO4 Þ3 þ 0:5Li þ þ 0:5e -Li2:5 V2 ðPO4 Þ3
ð6Þ
þÞ V3 þ ; Vð4 ð1=2Þ
ð7Þ Li2:5 V2 ðPO4 Þ3 þ 0:5Li þ þ 0:5e -Li3 V2 ðPO4 Þ3
V3 þ ð8Þ
Fig. 5. BET images for LiVOPO4 and Li3V2(PO4)3–GNF (7%).
To deeply explore the properties of LiVOPO4 and Li3V2(PO4)3– GNF, the apparent lithium ion diffusion coefficient was measured. This diffusion coefficient is affected by the reversibility of the intercalation and de-intercalation reactions, stability of the material, and phase conversions during the reactions. CV tests of LiVOPO4 and Li3V2(PO4)3–GNF have been carried out at various scanning rates from 0.01 to 0.2 mV s 1 (Fig. 7(a–e)). As shown in Fig. 7(a–e), the reduction peaks move to higher values and the oxidation peaks simultaneously move to lower ones with increasing scanning rate. Furthermore, the redox peaks get higher while the redox areas get bigger at high
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processes, resulting in irreversible behavior. In addition, at a high scanning rate, anodic peaks A1 and A2 and cathodic peaks B2 and B3 integrate and disappear gradually, respectively. This phenomenon was also observed in previous studies [28,29]. Therefore, only two oxidation peaks (A3 and A4) and one reduction peak (B1) remain, which can be used to characterize the lithium ion diffusion coefficients in Li3V2(PO4)3. Consequently, the apparent lithium ion diffusion coefficient of LiVOPO4 and Li3V2(PO4)3–GNF was estimated using Eqs. (9 and 10) from all the CV results above and the relationship between the peak current and CV scanning rate [27,29–32]: ip ¼ ð2:69 105 ÞðαnÞ1=2 AΔC Li DLi 1=2 v1=2 =Ep E o = ¼
RT αnF
(
ð9Þ
) 1=2 DLi αnFv 1=2 0:78þ ln þ ln RT k0 ð10Þ
A3 A2 A1
B3
B2
A4
B1
Fig. 6. Cyclic voltammetry profiles of (a) LiVOPO4 and (b) Li3V2(PO4)3– GNF (3%, 5%, 7%, and 10%) obtained at a scanning rate of 0.1 mV s 1.
scanning rates. This is because the peak area divided by the scanning rate yields the capacity of the electrode, which should be a constant [27]. It is noted that the oxidation peaks are slightly higher than the reduction peaks, indicating a slight kinetic difference between the lithium insertion and extraction processes, especially at a scanning rate of 0.2 mV s 1. This is because of disparities in the insertion and extraction capacities the electrode for lithium ions at high scanning rates, which means that the forward and reverse reaction rates have a wide disparity. In particular, the ionic transport across particle boundaries is not the same for the forward and reverse
where Ip is the peak current, n is the number of electrons per lithium ion (n=1), ν is the scanning rate for the CV, ΔCLi is the concentration of lithium ions (3.7 10 3 mol cm 3) [27], A is the electrode's surface area which contacts between electrode material and electrolyte [27,30], /Ep Eo/ is calculated as half of the difference between redox peak potentials, α is the transfer coefficient, and k0 is the standard rate constant. The relationships between /Ep Eo/ and ln ν, between Ip and ν1/2 are illustrated in Fig. 8(a–d). The transfer coefficient (α) is obtained from Fig. 8(a and c) and Eq. (10). The values of DLi for LiVOPO4 and Li3V2(PO4)3–GNF can be obtained from Fig. 8(b and d) with knowledge of the obtained α and Eq. (9). The peak current (Ip) exhibits a linear relation with the square root of the scanning rate (ν1/2), as shown in Fig. 8(b and d). It is found that LiVOPO4 displays an apparent diffusion coefficient lower than that of Li3V2(PO4)3 in the extraction process, which confirms that the extraction of lithium ions from Li3V2(PO4)3–GNF is better than that from LiVOPO4. In particular, when 7% GNF is added to Li3V2(PO4)3, the apparent diffusion coefficient during extraction is 6.01 10 9 cm2 s 1, whereas 8.32 10 11 cm2 s 1 was obtained for LiVOPO4. Detailed results are shown in Table 2. The higher DLi for Li3V2(PO4)3–GNF is due to the charge-transfer interface reaction, which is an important factor in lithium ion diffusion and thus leads to different rate performance. For the case of LiVOPO4 where the most places on a particle surface contact directly with the electrolyte, the reaction can only occur on the selected spots in which the electronic conductive path is built, the effective reaction area is limited. However, after adding carbon sources-GNF, by the appearance of GNF and large active area, lithium ions can move from this particle to the others more easily, thus enhancing the connection availability between particles, leading to the change in the transfer coefficient and the apparent diffusion of Li3V2(PO4)3–GNF, especially with Li3V2(PO4)3–GNF (7%). Further, after adding 10% GNF, as mentioned above an excessive carbon content will reduce the ratio of the active material and the GNF adding web is intrinsically a physical barrier that hinders the diffusion and transition of lithium ion, leading to the decrease in the value of 10% GNF added Li3V2(PO4)3.
V.H. Nguyen et al. / Ceramics International 41 (2015) 5403–5413
Fig. 7. Cyclic voltammetry profiles of (a) LiVOPO4, (b)–(e) Li3V2(PO4)3–GNF (3%, 5%, 7%, 10%) obtained at various scanning rates.
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Fig. 8. Relationships (a) and (c) between /Ep Eo/ and ln ν, (b) and (d) between Ip and ν1/2 for LiVOPO4 and Li3V2(PO4)3–GNF (3%, 5%, 7%, and 10%).
Table 2 Transfer coefficient and apparent diffusion coefficient obtained by CV. Samples LiVOPO4 3% 5% 7% 10%
Peaks
α
DLi (cm2 s 1)
A3 A4 A3 A4 A3 A4 A3 A4
0.35 0.59 0.57 0.56 0.71 0.25 0.30 0.23 0.22
8.32 10 11 2.91 10 10 4.31 10 11 7.07 10 10 7.95 10 11 6.01 10 9 1.20 10 9 4.06 10 9 8.17 10 10
EIS results for the LiVOPO4 and Li3V2(PO4)3–GNF batteries are shown in Fig. 9. The semicircles from the high to medium frequency are mainly related to a complex reaction process at the electrolyte/ cathode interface. The impedance spectra can be interpreted on the basis of an equivalent circuit in which Zw is Warburg impedance, Rct is charge-transfer resistance, Cd is capacitance of a double layer, and Rs is ohmic resistance. Furthermore, the exchange current density is
calculated using following equation: RT ð11Þ nFRct The diameter of the high-frequency combined semicircle of the batteries decreases when GNF is added and is especially small for Li3V2(PO4)3–GNF (7%). The impedance parameters of the batteries prepared from LiVOPO4 and Li3V2(PO4)3–GNF are shown in Table 3. It is found that the charge transfer resistance (Rct) of the Li3V2(PO4)3–GNF battery is significantly smaller than that of the LiVOPO4 battery, and the exchange current density is increased. This phenomena can be explained that after adding GNF, Li3V2(PO4)3–GNF has larger active area which reduces resistance between the active material and the surface film [28], as well as allows solid state diffusion of lithium ion easily, leading to a decrease in the charge transfer resistance. Adding GNF (7%) significantly improved the performance of the Li-battery, leading to the lowest resistance of 40 Ω and the largest exchange current density of 2.14 10 4 mA cm 2. However, when the GNF content increases to 10%,
io ¼
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Fig. 9. EIS results for LiVOPO4 and Li3V2(PO4)3–GNF (3%, 5%, 7%, and 10%).
Table 3 Impedance parameters of the cells prepared from LiVOPO4 and Li3V2(PO4)3– GNF (3%, 5%, 7%, and 10%). Samples
Rct (Ω)
i0 (mA cm 2)
LiVOPO4 3% 5% 7% 10%
421 252 206 40 165
6.10 10 5 3.39 10 5 4.16 10 5 2.14 10 4 5.19 10 5
the charge transfer resistance dropped. That is because the higher GNF carbon content is over thick for easily penetration and intercalation of lithium ions. Actually, only appropriate amount of GNF residue would effectively enhance the electrochemical performance as well as cyclic property. This is in accordance with the CV results. Fig. 10 shows the charge–discharge curves and the cycling performance of the LiVOPO4 and Li3V2(PO4)3–GNF batteries charged–discharged at 0.1 C between 3.0 and 4.8 V. The discharge capacities of LiVOPO4 at the 1st and 16th cycles are 113 and 89 mA h g 1, respectively. Thus, only 79% of the initial capacity was retained after 16 cycles. This demonstrates poor stability of the structure. The Li3V2(PO4)3–GNF (7%) retained 94% of the initial capacity even after 16 cycles, since the capacity was reduced from 177 to 164 mA h g 1. Further, the coulombic efficiencies of LiVOPO4 and Li3V2(PO4)3–GNF (7%) are 93% and 98%, respectively. In here, the coulombic efficiencies do not reach to 100% after 10 cycles due to the dendrite phenomenon formed on electrode surfaces. The dendrites may grow large enough to penetrate the separating barrier and touch the cathode. In the charging– discharging process, after some cycles solid electrolyte interfaces (SEI) layer will form once Li metal contact liquid electrolyte. Lithium ions diffuse through SEI layer and deposit on Li surface difficulty. Formation and stability of SEI layer are the main factors
Fig. 10. (a) Charge–discharge performance at first cycle and (b) cycling performance of LiVOPO4 and Li3V2(PO4)3–GNF (3%, 5%, 7%, and 10%) at 0.1 C.
affecting the coulombic efficiency of Li deposition/stripping processes. Taking into account the XRD, SEM, CV, and apparent lithium ion diffusion results, we can conclude that the intercalation/deintercalation of lithium ions into the active material in the Li3V2(PO4)3–GNF (7%) electrode takes place more easily during the charging/discharging process. Furthermore, Li3V2(PO4)3–GNF (7%) delivers the highest discharge capacity of 181 mA h g 1 at the second cycle. Thus, the electrochemical performance was improved by the structure conversion from LiVOPO4 to Li3V2(PO4)3– GNF due to the addition of GNF. Fig. 11 delivers the LiVOPO4 and Li3V2(PO4)3–GNF batteries charged–discharged at different current densities of 0.1 C, 0.2 C, 0.5 C, 1 C and 5 C. The discharge capacity of each sample declines with an increase in current density. When the same current density is applied, Li3V2(PO4)3–GNF (7%) exhibits larger
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of the batteries. Further, by the presence of GNF, lithium ions can move from this particle to the others more easily, thus enhancing the connection availability between particles, leading to the change in the charge transfer resistance and the apparent lithium ion diffusion of Li3V2(PO4)3–GNF. Because of the structural conversion, the Li3V2(PO4)3–GNF (7%) batteries showed the best properties, with apparent lithium ion diffusion coefficient of 6.01 10 9 cm2 s 1, an impedance of 40 Ω, an exchange current density of 2.14 10 4 mA cm 2, and a discharge capacity of 181 mA h g 1 at the 2nd cycle. Acknowledgments This research was received support from Brain Korea 21 Plus. References
Fig. 11. C-rate performance of LiVOPO4 and Li3V2(PO4)3–GNF .
capacity than LiVOPO4. The discharge capacity of Li3V2(PO4)3– GNF (7%) varies in the range 177–97 mA h g 1 at the rate of 0.1–5 C (113–36 mA h g 1 for LiVOPO4). The improvement may be associated with the decreased charge transfer resistance, increased conductivity and enhanced electrochemical kinetic induced by GNF adding. Li3V2(PO4)3–GNF (7%), with lesser resistance to charge-transfer which will increase the lithium ion diffusion between particles, obviously has the higher stability at the high rate performance. The superior performance and the good stability should be attributed mainly to the suitable content of carbon, which formed not only a homogeneous conductive coating on the surface of each particles but also a uniform web connected the whole active material, and a strong adhesion to maintain the composite film stable during longtime cycling processes. The film derived from higher GNF content (10%) displayed good cyclic stability but frustrating storage properties. The film exhibited only 168 and 85 mA h g 1 at discharge rates of 0.1 C and 5 C, respectively. Since 10% GNF adding was over thick and extensively inhibited the diffusion of lithium ions, resulting in lower intercalation ability, especially at higher discharge current density. 4. Conclusions In summary, LiVOPO4 and Li3V2(PO4)3–GNF were synthesized successfully using the solid-state method. The structural conversion of LiVOPO4 to Li3V2(PO4)3 occurred easily with the help of the GNF reducing agent. No impurity peak was observed in the XRD patterns of LiVOPO4 and Li3V2(PO4)3, and the structural conversion does not cause defects in the resulting Li3V2(PO4)3 crystallite. Larger particles with corroded-like structures formed and were mixed with GNF particles, which provided the advantages of a large active area and fast transport of lithium ions, leading to an improvement in the electrochemical properties
[1] S.Y. Chung, J.T. Bloking, Y.M. Chiang, Electronically conductive phospho-olivines as lithium storage electrodes, Nat. Mater. 1 (2002) 123–128. [2] Y.G. Wang, Y.R. Wang, E.J. Hosono, K.X. Wang, H.S. Zhou, The design of a LiFePO4/carbon nanocomposite with a core–shell structure and its synthesis by an in situ polymerization restriction method, Angew. Chem. Int. Ed. 47 (2008) 7461–7465. [3] V.H. Nguyen, E.M. Jin, H.B. Gu, Synthesis and electrochemical properties of LiFePO4–graphite nanofiber composites as cathode materials for lithium ion batteries, J. Power Sources 244 (2013) 586–591. [4] M. Pivko, M. Bele, E. Tchernychova, N.Z. Logar, R. Dominko, M. Gaberscek, Synthesis of nanometric LiMnPO4 via a two-step technique, Chem. Mater. 24 (2012) 1041–1047. [5] N.N. Bramnik, K. Nikolowski, C. Baehtz, K.G. Bramnik, H. Ehrenberg, Phase transitions occurring upon lithium insertion extraction of LiCoPO4, Chem. Mater. 19 (2007) 908–915. [6] J. Wolfenstine, J. Allen, Ni3 þ /Ni2 þ redox potential in LiNiPO4, J. Power Sources 142 (2005) 389–390. [7] M.Y. Saidi, J. Barker, H. Huang, J.L. Swoyer, G. Adamson, Electrochemical properties of lithium vanadium phosphate as a cathode material for lithium-ion batteries, Electrochem. Solid-State Lett. 5 (2002) A149–A151. [8] B.M. Azmi, T. Ishihara, H. Nishiguchi, Y. Takita, Cathodic performance of VOPO4 with various crystal phases for Li ion rechargeable battery, Electrochim. Acta 48 (2002) 165–170. [9] P. Fu, Y. Zhao, Y. Dong, X. An, G. Shen, Low temperature solid-state synthesis routine and mechanism for Li3V2(PO4)3 using LiF as lithium precursor, Electrachim. Acta 52 (2006) 1003–1008. [10] Y.H. Chen, Y.M. Zhao, X.N. An, J.M. Liu, Y.Z. Dong, L. Chen, Preparation and electrochemical performance studies on Cr-doped Li3V2(PO4)3 as cathode materials for lithium-ion batteries, Electrochim. Acta 54 (2009) 5844–5850. [11] H. Huang, S.C Yin, T. Kerr, N. Taylor, L.F. Nazar, Nanostructured composites: a high capacity, fast rate Li3V2(PO4)3/carbon cathode for rechargeable lithium batteries, Adv. Mater. 14 (2002) 1525–2528. [12] M.R. Gao, Y.F. Xu, J. Jiang, S.H. Yu, Nanostructured metal chalcogenides: synthesis, modification, and applications in energy conversion and storage devices, Chem. Soc. Rev. 42 (2013) 2986–3017. [13] J.J. Chen, M.J. Vacchio, S.J. Wang, N. Chernova, P.Y. Zavalij, M.S. Whittingham, The hydrothermal synthesis and characterization of olivines and related compounds for electrochemical applications, Solid State Ion. 178 (2008) 1676–1693. [14] H. Yu, X.H. Rui, H.T. Tan, J. Chen, X. Huang, C. Xu, W.L. Liu, D.Y.W. Yu, H.H. Hng, H.E. Hoster, Q.Y. Yan, Cu doped V2O5 flowers as cathode material for high-performance lithium ion batteries, Nanoscale 5 (2013) 4937–4943.
V.H. Nguyen et al. / Ceramics International 41 (2015) 5403–5413 [15] Y. Zhang, Q.Y. Huo, P.P. Du, L.Z. Wang, A.Q. Zhang, Y.H. Song, Y. Lv, G.Y. Li, Advances in new cathode material LiFePO4 for lithiumion batteries, Synth. Metals 162 (2012) 1315–1326. [16] P. Fu, Y.M. Zhao, Y.Z. Dong, X.N. An, G.P. Shen, Synthesis of Li3V2(PO4)3 with high performance by optimized solid-state synthesis routine, J. Power Sources 162 (2006) 651–657. [17] D. Choi, P.N. Kumta, Surfactant based sol–gel approach to nanostructured LiFePO4 for high rate Li-ion batteries, J. Power Sources 163 (2007) 1064–1069. [18] H. Liu, Y.P. Wu, E. Rahm, R. Holze, H.Q. Wu, Cathode materials for lithium ion batteries prepared by sol–gel methods, J. Solid State Electrochem. 8 (2004) 450–466. [19] W. Yuan, J. Yan, Z.Y. Tang, O. Sha, J.M. Wang, W.F. Mao, L. Ma, Synthesis of Li3V2(PO4)3 cathode material via a fast sol–gel method based on spontaneous chemical reactions, J. Power Sources 201 (2012) 301–306. [20] Y.N. Ko, J.H. Kim, S.H. Choi, Y.C. Kang, Electrochemical properties of spherically shaped dense V2O5 cathode powders prepared directly by spray pyrolysis, J. Power Sources 211 (2012) 84–91. [21] Y.N. Ko, H.Y. Koo, J.H. Kim, J.H. Yi, Y.C. Kang, J.H. Lee, Characteristics of Li3V2(PO4)3/C powders prepared by ultrasonic spray pyrolysis, J. Power Sources 196 (2011) 6682–6687. [22] C. Xu, X. Wang, J.W. Zhu, X.J. Yang, L. Lu, Deposition of Co3O4 nanoparticles onto exfoliated graphite oxide sheets, J. Mater. Chem. 18 (2008) 5625–5629. [23] M. Kope¢, A. Yamada, G. Kobayashi, S. Nishimura, R. Kanno, A. Mauger, F. Gendron, C.M. Julien, Structural and magnetic properties of Lix(MnyFe1 y)PO4 electrode materials for Li-ion batteries, J. Power Sources 189 (2009) 1154–1163.
5413
[24] N. Amdouni, F. Gendron, A. Mauger, H. Zarrouk, C.M. Julien, LiMn2 yCoyO4 (0ry r1) intercalation compounds synthesized from wet-chemical route, Mater. Sci. Eng. B 129 (2006) 64–75. [25] Y. Yang, H.S. Fang, J. Zheng, L. Li, G.S. Li, G.F. Yan, Towards the understanding of poor electrochemical activity of triclinic LiVOPO4: experimental characterization and theoretical investigations, Solid State Sci. 10 (2008) 1292–1298. [26] N. Zhou, E. Uchaker, Y.Y. Liu, S.Q. Liu, Y.N. Liu, G.Z. Cao, Effect of carbon content on electrochemical performance of LiFePO4/C thin film cathodes, Int. J. Electrochem. Sci. 7 (2012) 12633–12645. [27] X.H. Rui, N. Ding, J. Liu, C. Li, C.H. Chen, Analysis of the chemical diffusion coefficient of lithium ions in Li3V2(PO4)3 cathode material, Electrochim. Acta 55 (2010) 2384–2390. [28] X.H. Rui, Q.Y. Yan, M. Skyllas-Kazacos, T.M. Lim, Li3V2(PO4)3 cathode materials for lithium-ion batteries: a review, J. Power Sources 258 (2014) 19–38. [29] S. Franger, C. Bourbon, F. Le Cras, Optimized lithium iron phosphate for high-rate electrochemical applications, J. Electrochem. Soc. 151 (2004) A1024–A1027. [30] L. Cheng, X.L. Li, H.J. Liu, H.M. Xiong, P.W. Zhang, Y.Y Xia, Carboncoated Li4Ti5O12 as a high rate electrode material for Li-ion intercalation, J. Electrochem. Soc. 154 (2007) A692–A697. [31] V.H. Nguyen, W.L. Wang, E.M. Jin, H.B. Gu, Impacts of different polymer binders on electrochemical properties of LiFePO4 cathode, Appl. Surf. Sci. 282 (2013) 444–449. [32] K. Wang, R. Cai, T. Yuan, X. Yu, R. Ran, Z.P. Shao, Process investigation, electrochemical characterization and optimization of LiFePO4/C composite from mechanical activation using sucrose as carbon source, Electrochim. Acta 54 (2009) 2861–2868.