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Synthesis and Characterization of Lithium Manganese Phosphate by a Precipitation Method


A142

Journal of The Electrochemical Society, 157 2 A142-A147 2010
0013-4651/2009/157 2 /A142/6/$28.00 The Electrochemical Society

Synthesis and Characterization of

Lithium Manganese Phosphate by a Precipitation Method
Jie Xiao,* Wu Xu,** Daiwon Choi,** and Ji-Guang Zhang**,z
Energy and Environment Directorate, Pacic Northwest National Laboratory, Richland, Washington 99354, USA

LiMnPO4 is synthesized from a MnPO4H2O precursor precipitated via a spontaneous reaction. These MnPO4H2O nanoplates react quickly with the lithium source and form a pure phase of LiMnPO4 that has good electrochemical properties. Thermogravimetric analysis was used to determine the optimum synthesis temperature. The crystallization of LiMnPO4 occurs before 438°C. After full nucleation at 550°C, the samples exhibit a discharge capacity of 115 mAh g1 C/20 rate, 2.5–4.4 V in the rst cycle. The coulomb efciency is maintained at near 100% after the rst few cycles. When the synthesis temperature decreases to 350°C, the particle size of LiMnPO4 is further reduced to 10–50 nm with a reversible capacity of more than 90 mAh g1. For the 550°C synthesized LiMnPO4, 73% of the initial capacity was retained at the 60th cycle. After the high rate 5C discharge, the reversible capacity of LiMnPO4 can be recovered to nearly the original value of 110 mAh g1 at the C/20 rate. This precipitation method is a cost-effective approach for manufacturing high performance LiMnPO4 cathode materials. 2009 The Electrochemical Society. DOI: 10.1149/1.3265440 All rights reserved. Manuscript submitted July 30, 2009; revised manuscript received September 28, 2009. Published December 11, 2009.

Recently, olivine-structured LiFePO4 has attracted much interest as a cathode material for lithium-ion batteries because of its low cost, environmentally benign constituents and high theoretical capacity of 170 mAh g1 with a at operation voltage at 3.4 V when compared with Li/Li+.1,2 The strong covalent bonding between the O2 and P5+ ions in the PO3 groups stabilizes the structure during 4 lithium intercalation and deintercalation while resisting overcharge and thermal degradation.3 Isostructural LiMnPO4 could be another excellent cathode material because its at 4.0 V plateau vs Li/Li+ is comparable with that of commercial 4 V class LiCoO2 cathodes and is close to the maximum voltage limit accessible to most electrolytes. The theoretical specic energy of LiMnPO4 684 Wh kg1 is 20% greater than that of LiFePO4 578 Wh kg1 . Other members of the olivine family e.g., LiCoPO4 4 and LiNiPO4 5 are less attractive because cobalt is expensive and toxic. LiNiPO4 requires a stable electrolyte that works at high voltages i.e., above 5.0 V . One of the primary barriers to the practical application of LiMnPO4 is its very low intrinsic electronic and ionic conductivity 1010 S cm1 , which means it has poor electrochemical properties. The following approaches have been applied in attempts to overcome this deciency: i minimizing the particle size to reduce the lithium diffusion path,6-8 ii coating with carbon to enhance the electronic contact between particles,9,10 iii controlling the orientation of the formed crystalline structure to facilitate lithium transport,11,12 and iv doping with 1 atom % Mg for Mn to improve the rate capability.13 Various synthesis methods based on the approaches described above have been used to prepare electrochemically active LiMnPO4. Delacourt and co-workers reported a method for the direct precipitation of LiMnPO4 from an aqueous solution.7 By strictly controlling pH and temperature, the precipitation of LiMnPO4 occurred in a very narrow pH range i.e., 10.2– 10.7 ; however, the discharge capacity of the directly precipitated LiMnPO4 was low. Bramnik and Ehrenberg used NH4MnPO4H2O precipitated from an aqueous solution as a precursor to prepare LiMnPO4.12 The morphology and size distribution of the precursor NH4MnPO4H2O was preserved after ion exchange, and the electrochemical performance improved after adding 50% of carbon.12 The molecular wiring of LiMnPO4 by ruthenium II –bipyridine complexes14 provided new insight in decreasing the carbon content to make this material a more practical cathode. Although the hydrothermal method used in the preparation of LiFePO4 1 may also be

applied to the preparation of pure LiMnPO4, the large amount of disordered Mn2+ ions on the Li+ sites may block the Li+ diffusion path when LiMnPO4 is prepared at a low temperature.15 Therefore, this method needs to be improved further to control the particle size and to achieve the desired capacity. Recently, Martha et al.16 compared the surface reactivity and thermal stability of the carboncoated LiMnPO4 with those of conventional transition metal oxide cathodes such as LiNi0.5Mn0.5O2 and LiNi0.8Co0.15Al0.05O2. The lower surface reactivity and excellent thermal stability exhibited by LiMnPO4 encourage more efforts on this promising cathode material. In this paper, we report a precipitation process for the preparation of LiMnPO4 nanopowders. In this process, a rapid spontaneous precipitation process is initially used to prepare MnPO4H2O. After mixing the precursor with a lithium source and carbon black, a nanosized LiMnPO4 /C composite with good electrochemical characteristics is synthesized. This approach provides a cost-effective method as well as the material itself for the mass production of LiMnPO4. Experimental MnPO4H2O was prepared by dissolving 10 g of Mn NO3 24H2O Fluka, 97.0% in 50 mL of anhydrous ethanol Pharmco-AAPER, ACS/USP grade . Then, 10 mL of H3PO4 Sigma-Aldrich, 85 wt %, solution in water, ACS reagent was added to the solution and stirred. Initially, no precipitation was observed, and the mixed solution was purple; however, after 2–3 min, a yellow precipitant MnPO4H2O began to form. The MnPO4H2O precipitant was stirred for 2 h, washed twice in deionized water, and then centrifuged. MnPO4H2O was ballmilled with a stoichiometric amount of LiAc2H2O Sigma-Aldrich, 99.999% and 20 wt % of Ketjenblack EC600JD, Akzo Nobel for 4 h. The mixture was then calcined at temperatures ranging from 350 to 550°C for 3 h in a reducing environment composed of argon with 2.5% hydrogen. A combined differential scanning calorimetry/thermogravimetric analysis TGA instrument Netzsch, STA 449C was used to study the decomposition and reaction of the precursors. The powder sample of MnPO4H2O and the mixture of the starting materials were heated in an argon/2.5% hydrogen reducing environment to 800°C at a ramp rate of 5°C/min. The crystalline structure of the as-prepared LiMnPO4 /C composite was determined by X-ray diffraction XRD using a Scintag XDS2000 0-0 powder diffractometer equipped with a Ge Li solid-state detector and a Cu K sealed tube = 1.54178 . All samples were scanned in a range of 10–60° 2 with a step size of 0.02° and an exposure time of 10 s. A

* Electrochemical Society Student Member. ** Electrochemical Society Active Member.
z

E-mail: jiguang.zhang@pnl.gov

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Journal of The Electrochemical Society, 157 2 A142-A147 2010

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105 100 95
284°C

Weight %

90 85 80 75 70 0 100 200 300 400 500 600 700 800
580°C

Intensity (A.U.)
10

20

30

40

50

60

70

80

Two Theta (Degree)
Figure 2. Color online XRD pattern of the as-prepared MnPO4H2O with its crystalline structure. Solid balls red in the corner: oxygen ions; open balls white in tunnels: hydrogen ions; octahedra purple : MnO6 with Mn2+ residing in the center; tetrahedral magenta : PO4 with P5+ residing in the center.

Temperature ( C)
Figure 1. TG curve of the MnPO4H2O precursor measured in argon mixed with 2.5% hydrogen.

o

JEOL 5900 scanning electron microscope SEM equipped with a Robinson series 8.6 backscattered electron detector and an EDAX Genesis energy-dispersive spectroscopy EDS system with a Si Li EDS detector were used to investigate the particle morphology. High resolution transmission electron microscopy TEM analysis was carried out on a JEOL JEM 2010 microscope tted with a LaB6 lament and an acceleration voltage of 200 kV. Electrodes were prepared by casting a slurry of the LiMnPO4 /C composite, super P from Timcal , and poly vinylidene uoride PVDF, Kynar HSV900, Arkema Inc. in an N-methyl pyrrolidone Aldrich solvent onto aluminum foil. The weight ratio of LiMnPO4 /C:super P:PVDF was 87.5:2.5:10. The total wt % of carbon in the electrode was 20%. After drying at 65°C, the electrodes were punched into 1.4 cm disks. The active material loading was 2–5 mg cm2. Pure lithium metal was used as an anode in a 2325 coin cell Canadian National Research Council system. The electrolyte consisted of 1 M LiPF6 in a mixture of ethylene methyl carbonate and ethylene carbonate at a 7:3 volume ratio. The coin cells were assembled in an argon-lled MBraun glove box. The electrochemical tests were performed on an Arbin BT-2000 battery tester at room temperature. The cells were cycled between 2.5 and 4.4 V vs Li/Li+ at the C/20 1C = 150 mAh g1 rate unless otherwise mentioned in the rate capability comparison. During the charge process, the cells were charged in a constant current-constant voltage CC-CV mode at a C/20 rate to 4.4 V and then held at 4.4 I/5 CC-CV mode . V until I Results and Discussion The water content in the MnPO4H2O precursor is rst determined by TGA. It is important to know the exact amount of water in MnPO4H2O, as it inuences the nal stoichiometry of the nal LiMnPO4 product. The thermogravimetric TG curve in Fig. 1 shows a gradual weight change of 2.4% between 76 and 284°C, corresponding to the loss of the surface water of around 0.2 mol. A sharp weight loss shown between 284 and 580°C can be assigned to the release of crystalline water in the structure associated with the decomposition of MnPO4H2O into Mn2P2O7, H2O, and O2. The weight loss during the second part is 15.4%, which matches well with the decomposition process of MnPO4H2O. Therefore, the total water content is 1.2 mol including the surface and crystal water in the as-prepared MnPO4H2O. The XRD pattern for the as-prepared MnPO4H2O precursor is shown in Fig. 2. All peaks can be indexed as pure MnPO4H2O in a

monoclinic system with space group C2/c. The lattice parameters were calculated to be a = 6.9341 8 , b = 7.4771 9 , c = 7.378 1 , and = 112.3° by Rietveld renement. The crystalline structure of MnPO4H2O is also shown in Fig. 2. The octahedra and tetrahedra represent the MnO6 and PO4 coordination, respectively. Unlike the structure of LiMnPO4, there is no edge sharing between the MnO6 and PO4 polyhedrons. This precipitate also exhibits a tunnel structure except that hydrogen ions reside in the tunnels instead of lithium ions. The morphology and primary particle size of the as-prepared MnPO4H2O are shown in Fig. 3. The SEM image see Fig. 3a reveals that nanoplates are formed during the precipitation and that they aggregate into yarn-ball-like particles. These secondary particles can easily be separated into small primary particles during the ballmilling process and thus can be mixed homogeneously with lithium salt and carbon. The TEM image Fig. 3b shows that the primary particle size ranges from 100 to 200 nm. By adjusting the pH value and concentrations of the starting materials, the morphology and particle size of MnPO4H2O can be tuned further. Thermogravimetric/differential thermal analysis TG/DTA was used to determine the synthesis temperature of LiMnPO4. Figure 4 shows the TG/DTA curves performed on the precursors of MnPO4H2O, LiAc2H2O, and Ketjenblack in the argon/2.5% hydrogen reducing environment previously described. There is only one main stage in the TG curve from ambient temperature to 438°C. Above 438°C, the weight loss decreases and nally stabilizes at around 73% of the original weight. Two endothermic peaks at 105.3 and 135.1°C are observed in the DTA curve. These peaks correspond to the evaporation of residual water on the MnPO4H2O surface and the release of crystal water. Because MnPO4H2O in the mixtures has been ballmilled with LiAc2H2O for 4 h, it is understandable that the temperatures corresponding to the water loss of MnPO4H2O in the mixtures are decreased when compared with the temperatures corresponding to the water loss of pure MnPO4H2O, as shown in Fig. 1. The decomposition of LiAc2H2O and crystallization of LiMnPO4 occur below 438°C because the weight loss up to this temperature is about 26.4%. Another small exothermic peak related to further nucleation or growth of the LiMnPO4 particles appears at 506.4°C. As mentioned by other groups,6 the optimal synthesis temperature for LiMnPO4 is around 520°C because the suppressive effect provided by carbon occurs up to 600°C. Above that temperature, the particles grow markedly. Thus, 550°C was chosen as the optimal synthesis temperature. There is no organic solvent that needs to be removed in this method; therefore, it would be feasible

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A144

Journal of The Electrochemical Society, 157 2 A142-A147 2010

(a)
Intensity (A. U.)

550°C 500°C 400°C 350°C

15

25

35

45

55

Two Theta (Degree)
Figure 5. Color online XRD patterns of the LiMnPO4 /C composites formed at different temperatures.

(b)
The XRD patterns for the LiMnPO4 /C synthesized at different temperatures are plotted in Fig. 5. Even at 350°C, a pure phase of LiMnPO4 forms with an olivine structure indexed in Pnma of an orthorhombic system. No characteristic peaks for the carbon are observed, suggesting that carbon in the nal composite is in an amorphous state. The common impurity of Li3PO4 is not found in the product because of the presynthesis of MnPO4H2O. It is difcult for the PO3 ions in MnPO4H2O to react with lithium alone 4 below 550°C because this polyanion has been connected with the MnO6 octahedra by sharing the corner oxygen ion, thereby forming a stable structure, as shown in Fig. 2. In the traditional solid-state method of preparing LiMnPO4, the precursors used always include lithium salt Li2CO3, LiOHH2O, etc. , manganese salt Mn NO3 26H2O, MnAc24H2O, etc. , and NH4H2PO4 or NH4 2HPO4. However, during the calcination process, some Li+ ions may react with PO3+ ions rst to form Li3PO4 impurities in the 4 nal product, especially at low calcination temperatures.6 The formation of 1 mol of Li3PO4 which is inactive in the nal product means the loss of 3 mol of Li+ ions, signicantly affecting the electrochemical properties of LiMnPO4. Therefore, the preformation of MnPO4H2O in the spontaneous precipitation process used in this work can signicantly reduce the impurities produced by the solidstate reaction. The lattice parameters of LiMnPO4 synthesized at different temperatures were calculated from Rietveld renement and are compared in Table I. When the temperature decreases from 550 to 400°C, all the lattice parameters slightly increase suggesting an increase in crystal volume. This conclusion is consistent with the change in the lattice volume in Table I. Similar results were observed in other reported works on LiFePO4.1 However, when the temperature is as low as 350°C, all the lattice parameters decrease instead. This phenomenon is probably related to the incomplete formation of the olivine structure. It has been calculated that the cation disorder between Li+ r = 0.76 and Mn2+ r = 0.80 ions in the 350 and 550°C products are 6.24 and 3.71%, respectively. The increased amount of disordered Mn2+ at lower temperatures also plays a role in the decreased lattice parameters.1,14 However, after annealing, the 350°C calcined LiMnPO4 at 500°C in the reducing environment described earlier, the lattice parameters, and the lattice volume of the reheated LiMnPO4 are similar to those of the 500°C calcinated LiMnPO4. The morphologies of the as-prepared LiMnPO4 /C composites prepared at 550 and 350°C are shown in Fig. 6. At 550°C, the particle sizes range from 100 to 200 nm. At 350°C heating temperature, the particle sizes range from 10 to 50 nm. In both cases, carbon

Figure 3. Morphologies and particle size of the as-prepared MnPO4H2O from a SEM and b TEM.

to continuously decrease the calcining temperature to 350°C to investigate the corresponding electrochemistry of the low temperature product.

Figure 4. Color online TG/DTA curves of the LiMnPO4 /C precursor in argon mixed with 2.5% hydrogen.

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Journal of The Electrochemical Society, 157 2 A142-A147 2010
Table I. Lattice parameters of LiMnPO4 prepared at different temperatures. LiMnPO4 °C 550 500 400 350 350 annealed at 500°C a 10.451 10.454 10.4626 10.454 10.452 1 1 7 2 1 b 6.1019 6.1027 6.1127 6.105 6.1029 8 6 4 1 9 c 4.7456 4.7498 4.7501 4.748 4.7477 6 5 3 1 7 V 3 302.748 302.981 303.798 302.928 303.09

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Rp % 11.82 7.17 12.3 7.91 7.89

forms a good contact on the surface of the LiMnPO4, providing good electronic conductivity between the particles. The electrochemical behavior of the 550°C sample tested at the CC-CV mode is shown in Fig. 7. The capacities for the rst charge and discharge are 183 and 115 mAh g1, respectively. The relatively low coulomb efciency of 61% during the rst cycle can be attributed to the decomposition of the electrolyte caused by the

(a)

Carbon

nanosized LiMnPO4 particles. The highly active carbon Ketjenblack used in the ballmilling process of LiMnPO4 exhibits a Brunauer, Emmett, and Teller surface area of 2672 m2 g1. Even after the ballmilling process, Ketjenblack still has a large surface area of 342.4 m2 g1, which may facilitate electrolyte decomposition. The efciency is improved to 85% during the second cycle and almost becomes constant at 100% in the subsequent cycling, as revealed in Fig. 8. The polarization between the charge and discharge curves is only about 0.25 V, suggesting a good energy efciency. Figure 7 shows that the at discharge plateau only contributes about half of the total capacity, while the other half is from the sloped voltage prole. This may be related to the extent of the surface disorder produced during ballmilling.17 Several other parameters during the electrode preparation including carbon content, binder choice, and pressure also play important roles in the extension of the plateau for the LiMnPO4-based cathode materials, so these parameters need to be optimized. After 60 cycles, 73% of the original

5.0 4.5

Voltage (V)

4.0
CC

CV

3.5 3.0 2.5 2.0 0

Specific Capacity (mAh/g)

50

100

150

200

(b)

Figure 7. Charge–discharge proles of LiMnPO4 /C synthesized at 550°C. The rate used is C/20 7.5 mA g1 between 2.5 and 4.4 V. The CC-CV mode is used for the test.

200

Specific Capacity (mAh/g)

160
Charge

120 80 40 0 0 10 20 30 40 50 60
Discharge

Carbon
Figure 6. TEM images of the LiMnPO4 /C composites prepared at a 550 and b 350°C.

Cycle
Figure 8. Color online Cycling performance of LiMnPO4 /C synthesized at 550°C. The rate is C/20 7.5 mA g1 between 2.5 and 4.4 V.

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A146

Journal of The Electrochemical Society, 157 2 A142-A147 2010

140

5.0
C/20 C/20 C/5 C/2

Specific Capacity (mAh/g)

120 100 80 60 40 20 0 0

4.5

Voltage (V)

C/10

4.0 3.5 3.0 2.5 2.0
(a) 350°C

1C

2C

5C

1.5 0 20

10

20

30

40

50
5.0 4.5

Specific Capacity (mAh/g)

40

60

80

100

120

Cycle
Voltage (V)
Figure 9. Color online The reversible capacities at various discharge rates as labeled. The charge rate used is C/20 7.5 mA g1 with 1C equals to 150 mAh g1 for all the measurements.

4.0 3.5 3.0 2.5 2.0 0 50
b) After 500°C annealing

discharge capacity is retained, which indicates a good structural stability. The rate capability was also investigated, and the results are shown in Fig. 9. The C/20 7.5 mA g1 rate was used for all the charge processes, while the discharge rate was varied as labeled in the gure. The reversible capacity at C/10 is about 100 mAh g1 and then decreases to 60 mAh g1 at 1C. However, the discharge capacity recovers to 110 mAh g1 using C/20 after a high rate discharge. This nding indicates that the high rate discharge did not induce any irreversible changes in the structure of LiMnPO4. As discussed previously, even at 350°C a pure phase of LiMnPO4 forms. The electrochemical behavior of the low temperature LiMnPO4 product was evaluated, and the results are shown in Fig. 10. The low temperature product delivers a reversible capacity of 80 mAh g1 at the C/20 rate with the discharge cutoff voltage being extended from 2.5 to 2.0 V. This is because most of the capacity comes from the sloped voltage region as the discharge plateau is signicantly shortened with increased polarization 0.4 V . This result is consistent with our previous conclusion from TGA that the crystallization of LiMnPO4 starts below 438°C. However, the LiMnPO4 synthesized at 350°C is not yet completely crystallized. This condition leads to decreased capacity in both the charge and discharge processes. The increased amount of disordered Mn2+ on the Li+ sites in the low temperature samples may block the onedimensional Li+ diffusion channels in LiMnPO4 prepared at a low temperature and may lead to a decreased capacity, as observed in Fig. 10. The coulombic efciency in the rst cycle increased from 61% for the 550°C sample to 81% for the 350°C product see Fig. 7 . To improve the crystallization of the 350°C calcinated LiMnPO4 samples, they were reheated at 500°C in an argon atmosphere for 1 h. After this annealing step, the rst discharge capacity improves to 90 mAh g1, suggesting a more complete crystallization of LiMnPO4. The interfacial resistance between LiMnPO4 and carbon may also be reduced during annealing.13 However, the 500°C reheated sample shows a larger capacity than the 550°C prepared sample in the rst charge, as shown in Fig. 10b, and thus leads to a lower coulomb efciency. This phenomenon may be related to the further decreased particle size of LiMnPO4 prepared at a low temperature, as shown in Fig. 6b. The increased active surface areas of the LiMnPO4 particles after annealing may also facilitate the electrolyte decomposition. An optimized electrolyte or stabilized particles are required to further increase the discharge capacity and coulombic efciency in the rst cycle.

Specific Capacity (mAh/g)

100

150

200

250

Figure 10. Electrochemical behavior of LiMnPO4 prepared at a 350°C and b after further annealing at 500°C. To observe the maximum reversible capacity of the 350°C calcinated samples, the discharge voltage is extended to 2.0 V in a .

Conclusions A precipitation method has been successfully applied to prepare an electrochemically active LiMnPO4 as a cathode material for lithium-ion batteries. MnPO4H2O nanoplates are rst precipitated into yarn-ball-like particles. After mixing with a lithium source and carbon, nanosized LiMnPO4 particles form, which in turn reects the homogeneous morphology of the precursors. TGA results indicate that the optimal synthesis temperature is around 550°C. XRD results show that decreasing the annealing temperature leads to an increase of the lattice parameters and an expanded lattice volume down to 400°C. For the 550°C annealed samples, the rst discharge capacity is 115 mAh g1 at the C/20 rate. After 60 cycles, 73% of the initial capacity is retained. After a high rate discharge up to the 5C rate of the LiMnPO4 sample, its original capacity was recovered by reducing the discharge rate to C/20. Even for the LiMnPO4 prepared at 350°C, a reversible capacity of 90 mAh g1 was obtained. Thus, this precipitation method provides a cost-effective, solid-state approach to synthesizing LiMnPO4, which has excellent electrochemical properties. Acknowledgment The authors thank Professor M. Stanley Whittingham and Zheng Li at the State University of New York at Binghamton for the XRD measurements and Dr. Chongmin Wang and Dr. Xiaodong Zhou of the Pacic Northwest National Laboratory PNNL for the TEM characterization and TGA test, respectively. The Laboratory Directed Research and Development LDRD Program at PNNL and the U.S. Department of Energy, Ofce of Vehicle Technologies provided funding for this work.
Pacic Northwest National Laboratory assisted in meeting the publication costs of this article.

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Journal of The Electrochemical Society, 157 2 A142-A147 2010 References
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