Li3V2(PO4)3-LiMnPO4 Cathode Material for Li-ion Batteries Bull. Korean Chem. Soc. 2013, Vol. 34, No. 2 433 http://dx.doi.org/10.5012/bkcs.2013.34.2.433
Synthesis and Electrochemical Properties of Li
3V
2(PO
4)
3-LiMnPO
4Composite Cathode Material for Lithium-ion Batteries
Jin-Shik Yun, Soo Kim, Byung-Won Cho,* Kwan-Young Lee,† Kyung Yoon Chung, and Wonyoung Chang*
Center For Energy Convergence, Korea Institute of Science and Technology, Seoul 136-791, Korea
*E-mail: [email protected] (B.-W. Cho); [email protected] (W. Chang)
†Department of Material Science and Engineering, Korea University, Seoul 136-701, Korea Received October 2, 2012, Accepted November 14, 2012
Carbon-coated Li3V2(PO4)3-LiMnPO4 composite cathode materials are first reported in this work, prepared by the mechanochemical process with a complex metal oxide as the precursor and sucrose as the carbon source.
X-ray diffraction pattern of the composite material indicates that both olivine LiMnPO4 and monoclinic Li3V2(PO4)3 co-exist. We further investigated the electrochemical properties of our Li3V2(PO4)3-LiMnPO4
composite cathode materials using galvanostatic charging/discharging tests, where our Li3V2(PO4)3-LiMnPO4
composite electrode materials exhibit the charge/discharge efficiency of 91.9%, while Li3V2(PO4)3 and LiMnPO4 exhibit the efficiency of 87.7 and 86.7% in the first cycle. The composites display unique electrochemical performances in terms of overvoltage and cycle stability, displaying a reduced gap of 141.6 mV between charge and discharge voltage and 95.0% capacity efficiency after 15th cycles.
Key Words : Li-ion batteries, Mechanochemical process, Composite, Li3V2(PO4)3, LiMnPO4
Introduction
Lithium cobalt oxide is the most successful commercial cathode material for portable Li-ion battery devices; how- ever, it is limited by the scarcity of cobalt sources and its safety issue during the overcharging process. Recently, alter- nate lithium transition metal phosphates such as LiFePO4, LiMnPO4, and Li3V2(PO4)3 are emerging as promising cathode material candidates for Li-ion batteries.1-3 Compared with the traditional lithium metal oxides, lithium transition metal phosphate compounds show excellent electrochemical and thermal stability because of their rigid phosphate networks.4 However, there is a serious drawback to their electrochemical performances for these materials due to the low electronic conductivity. Li3V2(PO4)3 has the theoretical capacity of 197 mAh g−1, when all of the three lithium ions are utilized, however, it is difficult to extract the third lithium ion, where it also exhibits very low reversibility. LiMnPO4 has a dis- advantage of having low electronic conductivity.1-6
Recently, Zheng et al. reported that utilizing both LiFePO4
and Li3V2(PO4)3 components improve their electrochemical performances.5,6 However, LiFePO4 and Li3V2(PO4)3 operate at the different voltage limits, so that it is disturbed to utilize all three lithium ions in the Li3V2(PO4)3 electrode materials.
In this work, we incorporate LiMnPO4 component with Li3V2(PO4)3, which the operating voltage of LiMnPO4 is at 4.1 V, similar to Li3V2(PO4)3 component. Here, carbon- coated Li3V2(PO4)3-LiMnPO4 composite cathode materials were synthesized by using the complex metal oxide as the precursor via a mechanochemical process, and the electro- chemical performance of our Li3V2(PO4)3-LiMnPO4 com- posite was studied in detail, as well as the electrochemical properties for each sample of Li3V2(PO4)3 and LiMnPO4
were investigated.
Experimental
Mn2V2O7 compounds were first prepared from MnO (Aldrich) and V2O5 (Junsei, Japan) with a mole ratio of 2:1.
Mechanochemical process was performed in a similar manner to our previous works using a planetary mill (FRITSCH Pulverisette 5, 300 rpm, 3 h), followed by the heat treatment in a box furnace at 600 °C for 5 h.7-11 The ball-to-powder weight ratio was optimized at the ratio of 20:1. Mn2V2O7, Li2CO3 (Aldrich), and (NH4)2HPO4 (Aldrich) were mixed with the stoichiometric ratio of 1:2.5:5 to yield the Li3V2(PO4)3- LiMnPO4 composite with a mole ratio of Li3V2(PO4)3: LiMnPO4 = 1:2. We used sucrose as the carbon source, and 6 wt % of the total weight of precursors were added to the mixtures, before we carried out the mechanochemical pro- cess. The precursors were mixed using a mechanochemical process in a wet-milling basis with ethanol (300 rpm, 3 h), subsequently dried at 80 °C oven. The powder was fired at 800 °C for 10 h under a flow of 95% Ar-5% H2 mixture gas to yield the Li3V2(PO4)3-LiMnPO4 composite. We also pre- pared the Li3V2(PO4)3 and LiMnPO4 compounds using V2O5 and MnO as the precursors for further comparisons.
Powder X-ray diffraction measurements were performed using a Rigaku X-ray Diffractormeter using the monochro- matic Cu-Kα line produced at 40 kV and 100 mA. The diffraction data were collected with a scan speed of 2° min−1 over a 2θ range from 10 to 60°. The morphology of the powders was characterized by a scanning electron micro- scope (SEM, HITACHI S-4200FE-SEM).
The active material was mixed with acetylene black and polyvinyl difluoride (PVDF) in 1-methyl-2-pyrrolidon (NMP)
434 Bull. Korean Chem. Soc. 2013, Vol. 34, No. 2 Jin-Shik Yun et al.
with a weight ratio of 85:10:5 to form slurry. The slurry was mixed in a homogenizer for 1 h then coated onto aluminum foil using a doctor blade and dried 2 h in an oven. These electrodes were assembled in a dry room using Li metal as the counter and reference electrodes with a layer of separator in the 2032 coin-cell configuration (Hoshen Corp., Japan).
Electrolyte with 1 M LiPF6 in EC:DMC:DEC = 1:1:1 by volume was used in this experiment. After assembling, the cells were stored at room temperature for 24 h to ensure complete impregnation of the electrodes and separators with the electrolytic solution. Galvanostatic charge-discharge cycling tests were carried out using a multichannel battery tester (Model 4000, Maccor Inc.) using the constant-current / constant-voltage (CC/CV) method.
Results and Discussion
Figure 1 shows the XRD patterns of transition metal oxides after the ball-milling for the precursor composed of MnO and V2O5 and the sample after the heat treatment at 600 °C to form Mn2V2O7 compound, with a 2θ range from 10 to 60°. After the heat treatment, the raw materials are not observed from the XRD patterns, and the crystal structure is identified to be Mn2V2O7 structure, which is consistent with those previously reported.12
Figure 2(a) shows the SEM images of the Mn2V2O7 sample synthesized at 600 °C. Fine and ball-like particles are observed with the average particle sizes of 500 nm, with some aggre- gations. In Figure 2(b), Li3V2(PO4)3 electrode materials are shown, with the average size of approximately 2 μm. Also, LiMnPO4 electrode materials show various distributions of the particle sizes from 500 nm to 3 μm in Figure 2(c). With the mechanochemical process, it is possible to effectively create alloying between the Li3V2(PO4)3 and LiMnPO4 elec- trode materials, at the same time to prepare particles with uniform size distributions. In addition, carbon coating is known to aid the crystal size control during the heat treat- ment. When we designed our new composite system, we were able to obtain uniform particles as shown in Figure 2(d).
Figure 3 shows the resulting XRD profiles of Li3V2(PO4)3, LiMnPO4, and Li3V2(PO4)3-LiMnPO4 composite synthesiz- ed at 800 °C, with a 2θ range from 10 to 50°. The diffraction patterns of our prepared composite clearly overlay olivine LiMnPO4 and monoclinic Li3V2(PO4)3, which is consistent with those previously reported.13-15 The diffraction patterns were indexed and their lattice parameters are calculated based on Li3V2(PO4)3 structure in Table 1 and LiMnPO4 structure in Table 2. The lattice parameters are clearly
Figure 1. XRD profile of Mn2V2O7.
Figure 2. SEM images of: (a) Mn2V2O7 (b) Li3V2(PO4)3 (c) LiMnPO4 (d) Li3V2(PO4)3-LiMnPO4.
Figure 3. XRD profiles of: (a) Li3V2(PO4)3 (b) LiMnPO4 (c) Li3V2(PO4)3-LiMnPO4.
Table 1. Lattice parameters of Li3V2(PO4)3 in the pristine and composite materials; the structure was carefully calculated using the Li3V2(PO4)3 structure
Sample a (Å) b (Å) c (Å)
Pristine Li3V2(PO4)3 8.6242 8.6103 12.0630
Composite 8.6527 8.6387 12.1029
Li3V2(PO4)3-LiMnPO4 Cathode Material for Li-ion Batteries Bull. Korean Chem. Soc. 2013, Vol. 34, No. 2 435
increased in our composite sample, indicating the change in the lattice distance for the lithium de-/intercalations in the compounds.
Charge and discharge measurements on cells containing our Li3V2(PO4)3-LiMnPO4 composite as the active cathode material were carried out in the voltage range of 2.5-4.8 V at C/50 current rate at room temperature and 55 °C, as dis- played in Figure 4(a). At room temperature, we found the cell containing our composite materials delivers the specific discharge capacity above 100 mAh g−1, and the electro- chemical characteristics of both LiMnPO4 and Li3V2(PO4)3 can be clearly observed. As we increased the operating temperature to 55 °C, Li3V2(PO4)3-LiMnPO4 composite dis- plays an increased discharge capacity of 154.0 mAh g−1.
In Figure 4(b), olivine LiMnPO4 shows a discharge capa- city of 121.9 mAh g−1 with MnPO4↔ LiMnPO4 phase
transition. Li3V2(PO4)3 shows three plateaus at 3.6, 4.1, and 4.4 V during charging, indicating three lithium ions are being de-intercalated, respectively, with a discharge capacity of 173.2 mAh g−1. It can be clearly observed that Li3V2(PO4)3- LiMnPO4 composite electrode materials display both charac- teristics of the LiMnPO4 and Li3V2(PO4)3. Also, the re- versibility is improved for the composite sample compared with the single compound of LiMnPO4 and Li3V2(PO4)3. Li3V2(PO4)3-LiMnPO4 composite electrode materials exhibit the charge/discharge efficiency of 91.9%, while Li3V2(PO4)3 and LiMnPO4 exhibit the efficiency of 87.7 and 86.7% in the first cycle, respectively. Because the operating voltage of LiMnPO4 is at 4.1 V, similar to the Li3V2(PO4)3 electrode, Li3V2(PO4)3-LiMnPO4 displays a fairly smooth curve, com- pared with the previous research works that incorporating the LiFePO4 electrode materials, which operates approxi- mately at 3.5 V.5,6 Our composite materials have the advant- age in term of specific capacity compared with the pristine LiMnPO4. Additionally, our Li3V2(PO4)3-LiMnPO4 shows smoother phase transitions compared to Li3V2(PO4)3 that exhibiting three different plateaus at 3.6, 4.1, and 4.4 V as lithium ions are being de-intercalated from the compound.
Differential capacity plots are provided in Figure 4(c) to detect voltage of plateaus for all prepared samples. Reduced Table 2. Lattice parameters of LiMnPO4 in the pristine and
composite materials; the structure was carefully calculated using the LiMnPO4 structure
Sample a (Å) b (Å) c (Å)
Pristine LiMnPO4 10.4287 6.0963 4.7460
Composite 10.4613 6.1144 4.7451
Figure 4. First cycle charge-discharge of: (a) Li3V2(PO4)3-LiMnPO4 (RT and 55 °C) (b) LiMnPO4, Li3V2(PO4)3, Li3V2(PO4)3-LiMnPO4 (55
°C) (c) differential capacity plots of LiMnPO4, Li3V2(PO4)3, Li3V2(PO4)3-LiMnPO4 (d) cycle performances of LiMnPO4, Li3V2(PO4)3, Li3V2(PO4)3-LiMnPO4.
436 Bull. Korean Chem. Soc. 2013, Vol. 34, No. 2 Jin-Shik Yun et al.
overvoltage in the differential capacity plot indicates a polari- zation decrease, which can be interpreted as the decrease in the activated energy. As seen in Figure 4(c), LiMnPO4 dis- plays a gap of 198.2 mV between charge and discharge voltage, while the Li3V2(PO4)3-LiMnPO4 composite shows a gap of 141.6 mV. Also, the composite electrode starts to extract the third lithium starting at 4.35 V, compared with Li3V2(PO4)3 electrode at 4.45 V. We believe that a reduced overvoltage occurs because of the improvements in the over- all electronic conductivity with the carbon source additions and also by a close electrochemical interaction between the two components, which our composite materials are closely integrated by the mechanochemical process.
Figure 4(d) shows cycle performances of the all prepared materials for the fifteen consecutive cycles (C/50, 55 °C).
Li3V2(PO4)3-LiMnPO4 and Li3V2(PO4)3 display increased discharge capacities compared with the LiMnPO4. The capa- city efficiency (1st cycle/15th cycle) of Li3V2(PO4)3-LiMnPO4 composite materials is 95.0%, compared with Li3V2(PO4)3 and LiMnPO4 to be 90.9% and 88.0%, respectively. Our composite also displays very smooth phase transition, unlike Li3V2(PO4)3 electrode material, as we discussed in Figure 4(b). Although Li3V2(PO4)3 shows slightly higher discharge capacities, our prepared composite material is more stable and the capacity retention is much higher, which can be used as an alternate cathode material in the future applications.
Conclusion
The composite of Li3V2(PO4)3:LiMnPO4 = 1:2 was syn- thesized by the mechanochemical process. The XRD pattern of the composite material confirms that both olivine LiMnPO4 and monoclinic Li3V2(PO4)3 co-exist and shows that two different crystal structures are well distributed. Our composite material shows improved electrochemical properties such as the overvoltage decrease with the higher reversibility, com- pared with the single compound of Li3V2(PO4)3 and LiMnPO4. We believe that the reduced overvoltage and the cycle stability are caused by the improvements in the electronic conductivity with the carbon source additions and close integrations between the Li3V2(PO4)3 and LiMnPO4 electrode materials. Future works to improve the high rate capabilities
of our composite electrode materials are currently on-going.
Acknowledgments. This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2011-C1AAA001- 0030538) and by the Green City Technology Flagship Pro- gram funded by the Korea Institute of Science and Techno- logy (KIST-2013-2E23997). This work was partially sup- ported by the Global Research Laboratory Program through the National Research Foundation of Koea (NRF), which is funded by the Ministry of Education, Science and Techno- logy (MEST) (grant number: 2011-00115).
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