Olivine-Type Cathode Materials for Lithium-Ion Batteries

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Introduction

Lithium-ion batteries (LIBs) have been widely adopted as the most promising portable energy source in electronic devices because of their high working voltage, high energy density, and good cyclic performance. Currently, LIBs are used in electric vehicles and hybrid electric vehicles as well. In these batteries, olivine-type cathode materials such as LiMPO₄ (M=Fe and Mn) have attracted significant interest, especially due to their low cost and high intrinsic safety. However, they show poor electrochemical properties mainly due to their low electrical conductivities. Thus far, a number of attempts have been made to enhance the electrochemical properties of olivine-type cathode materials, including particle-size reduction,¹ cation doping,² carbon coating of LiMnPO₄,³ and use of LiMnPO₄/carbon composite.^2,4 Several synthesis routes have been developed to overcome the weakness of LiMnPO4 3 such as solid-state reaction,^4–6 sol-gel method,^2,3 polyol process,⁷ hydrothermal synthesis⁸ precipitation,⁹ and electrostatic spray deposition.¹⁰ However, the electrochemical performance of LiMPO₄ cathodes prepared using the aforementioned techniques does not yet meet the application needs of commercial cells.

Spray pyrolysis (SP) is a well-known continuous and single-step method for the preparation of fine homogeneous high-purity multicomponent powders. Compared to particles prepared by soft chemistry methods, SP yields a narrow particle size distribution that can be controlled from the micrometer to sub-micrometer scale. Moreover, SP can be used to achieve highly pure powders with easily controllable composition. About a decade ago we first reported that the spherical nanostructured LiMn₂O₄ and its substituted forms could be prepared by spray pyrolysis.^11,12 Recently, we have also reported that LiMPO₄/C (M=Fe, Mn, Co) nanocomposites^13–16 with excellent electrochemical properties can be successfully synthesized using a combination of SP, ball milling, and heat treatment. Some of our recent results on LiMPO₄/C composites are summarized in this article.

Synthesis and Electrochemical Properties of LiFePO₄/C Nanocomposites

LiFePO₄/C nanocomposites were prepared at 500 °C by SP and then wetball milled (WBM) at a rotating speed of 800 rpm. The WBM with ethanol was performed for 3 h in an Ar atmosphere, and the milled mixture was heat-treated at 600 °C for 4 h in a N₂+3% H₂ atmosphere. Transmission electron microscopy (TEM) revealed that the LiFePO₄ nanoparticles, with a geometric mean diameter of 146 nm, were coated with a thin carbon layer of several nanometers (Figure 1).

The first charge-discharge profiles of cells containing carbon-coated LiFePO₄ nanoparticles with increasing charge-discharge rate from 0.1 to 60 C between 2.5 and 4.3 V are presented in Figure 2. At a charge-discharge rate of 0.1 C, the cell has a discharge capacity of 165 mAh g^-1, which corresponds to 96% of the theoretical capacity of LiFePO₄ (170 mAh g^-1) and a much smaller polarization loss and irreversible capacity. A wide flat voltage plateau corresponding to the Fe⁺²/Fe⁺³ redox reaction is observed at 3.4 V. The first discharge capacities at charge-discharge rates of 1, 5, 10, and 20 C are 155, 130, 118, and 105 mAhg^-1, respectively. Even at a rate of 60 C, the electrode containing carbon-coated LiFePO₄ nanoparticles has a discharge capacity of 75 mAh g^-1.

The cyclic performance of the cells containing carbon-coated LiFePO₄ nanoparticles at different charge-discharge rates was investigated over 100 cycles, and the results are given in Figure 3. The cells exhibit an excellent cyclic property. The cells show no capacity fading even after 100 cycles at different charge-discharge rates ranging from 1 to 60 C. These results demonstrate that the structure of the carbon-coated LiFePO₄ nanoparticles is very stable, and the electrochemical lithium-ion insertion/extraction process is quite reversible even at high chargedischarge rates. These results indicate that the preparation approach using the combination of SP and WBE followed by heat treatment enables to achieve a high rate performance for LiFePO₄ composite electrode materials.

Synthesis and Electrochemical Properties of LiMnPO₄/C Nanocomposites

LiMnPO₄/C nanocomposites were prepared using a combination of SP, WBM and heat treatment. The SP was performed at 300 °C, whereas the heat treatment was performed at 500 °C for 4 h in a N₂+3% H₂ atmosphere. X-ray diffraction (XRD) was used to confirm the ordered LiMnPO₄ olivine structure without any impurity phases. The scanning electron microscopy (SEM) and TEM also confirmed that the final sample was the LiMnPO₄/C nanocomposite with a primary particle size of 100 nm. The TEM of the final sample is shown in Figure 4. Both phases of carbon and LiMnPO₄ could be seen in this image.

Figure 5 shows the initial charge-discharge profiles of the cells evaluated at constant current (CC)-constant voltage (CV) charge mode up to the cutoff voltage of 4.4 V and then CC discharge at various rates. The flat discharge plateau could still be recognized even at discharge rate of 2 C. This is attributed to the deep Li extraction level under very low currents in CV charge stage.

Figure 6 shows the rate capabilities determined for three charging modes: (1) CC charge up to 4.4 V, (2) CC charge up to 4.8 V, and (3) CC charge up to 4.4 V followed by CV charge reaching to theoretical capacity for LiMnPO₄/C nanocomposite containing cell. At the discharge rate of 0.05 C, the cell exhibits the first discharge capacity of 123, 153, and 147 mAh g^-1 in CC-4.4 V, CC-4.8 V and CC-CV modes, respectively. At a discharge rate of 1 C, the initial discharge capacity of 51, 107, and 123 mAh g^-1 were observed for CC-4.4 V, CC-4.8 V and CC-CV modes, respectively. Even with a discharge rate of 10 C, the cell could deliver capacity of 65 mAh g^-1 in CC-CV mode. In spite of the intrinsic low electronic and ionic conductivities of LiMnPO₄, the cells containing LiMnPO₄/C nanocomposites exhibit very good rate capability, which could be attributed to the large specific surface area, the small primary particle size, and a uniform carbon distribution.

The cycle performance comparison between three charge modes is shown in Figure 7. The cells containing LiMnPO₄/C nanocomposites show good cycle performance for all the three different charging modes.

Synthesis and Electrochemical Properties of LiCoPO₄/C Nanocomposites

LiCoPO4/C nanocomposite also was successfully prepared by using a combination of SP and WBM followed by heat treatment. The SP was performed at 300 °C, whereas the heat treatment was performed at 500 °C for 4 h in a N₂+3% H₂ atmosphere. The XRD analysis confirms that the LiCoPO₄/C nanocomposite is well crystallized in an orthorhombic structure with Pmna space group. SEM and TEM equipped with energy dispersive spectroscopy technique reveals that the LiCoPO₄/C nanocomposites are agglomerates of LiCoPO₄ primary particles with a geometric mean diameter of 87 nm, and the carbon is well distributed on the surface of the agglomerates.

Figure 8 presents the first discharge profile of the cells containing LiCoPO₄/C nanocomposites as active cathode materials at 0.1 C. For comparison, the discharge profile of cells containing LiCoPO₄ is also shown in the same figure. The LiCoPO₄/C nanocomposite cathode exhibits a wide and flat voltage plateau around 4.75 V. It can be noted that in comparison to LiCoPO₄, the LiCoPO₄/C nanocomposite cathode delivers a larger discharge capacity (94 mAh g^-1 for LiCoPO₄ vs. 141 mAh g^-1 LiCoPO₄/C nanocomposite). Figure 9 shows the rate capabilities of the cells containing LiCoPO₄ and LiCoPO₄/C nanocomposites. The cells containing LiCoPO4 exhibit first discharge capacities of 99, 94, 78, 53, and 11 mAh g^-1 at 0.05, 0.1, 1, 5, and 20 C, respectively; the cell containing LiCoPO₄/C nanocomposites shows first discharge capacities of 142, 141, 137, 128, and 109 mAh g^-1 at 0.05. 0.1, 1, 5, and 20 C, respectively.

Conclusion

To summarize, the LiMPO₄/C (M=Fe, Mn, Co) nanocomposites were successfully synthesized using a combination of spray pyrolysis, ball milling, and heat treatment. The LiMPO₄/C nanocomposites synthesized using this approach show excellent electrochemical properties with the specific capacities close to that of theoretical values. The results indicate that the combination of spray pyrolysis and ball milling aerosol has the potential to be an effective synthesis route for improving the electrochemical properties of olivine-type cathode materials for lithiumion batteries.