1. Introduction

In the recent years, the LiFePO₄ materials with the olivine structure have become a promising cathode material for the lithium ion battery. LiFePO₄ has a lot of advantages, such as high operation voltage, long operational life, low materials cost, environmental friendliness. However, the disadvantages of low electronic conductivity and poor ionic conductivity greatly restrict the commercial applications of LiFePO₄. Metal doping is one of the effective ways to improve materials properties of the LiFePO₄ [1-2]. In this study, the materials properties of LiFePO₄ by doping metal ions were investigated by first-principles calculation and some findings were illustrated.

2. Theoretical Approach

The calculations are carried out to investigate the geometry structures and electronic properties of LiFePO₄ material using Vienna Ab initio Simulation Package (VASP) based on density functional theory (DFT). The generalized gradient approximation (GGA) within Perdew-Wang 91 functional is employed to take account of the electronic exchange-correlation effect. The cutoff energy of 400 eV is used in calculation. LiFePO₄ is an olivine structure. The unit cell with lattice parameters and volume a=10.3324(Å), b=6.0105 (Å), c=4.6922(Å), and V=263.77 (ų), respectively, are constructed. The structure of LiFePO₄ material and the minimum energy path of Li-ion diffusion are as shown in Figs. 1 and 2, respectively. The unit cell of LiFePO₄ and the model to depict four Li ions delithiated away individually from the unit cell of LiFePO₄ are shown in Figs. 3 and 4, respectively.

Figure 1. Structure of LiFePO4 material [3]

Figure 2. Minimum energy path of Li-ion diffusion [3]

Figure 3. Unit cell of LiFePO4

Figure 4. Schematic diagram of Li ion delithiation (a) Li compound phase (b) intermediate phase (c) delithiation phase

3. Results and Discussion

The changes of lattice parameters and volume after discharge of Li ions are shown in Table 1. It is seen that the lattice parameters tend to decrease and the volume change of cell shrinks as large as 8.09% when the Li ions move away from the lattice.

Table 1. Lattice parameters and volume change of LiFePO4 after discharge

During the charging process of Li-ion battery, the extraction of Li ions will induce a significant volume change of cathode material. It is found that the change of bond length of P-O is small, but it is quite large for Fe-O after extraction of Li-ion in LiFePO₄. In other words, the volume change of LiFePO₄ material is mainly due to the change of bond length of Fe-O rather than P-O after discharge. In this work, effects of doping Mg, V and Co metal ions on volume change are evaluated and as shown in Tables 2, and 3, respectively. Compared to the significant change in bonding length of Fe-O, the change in bonding length M-O of doping metals M (M=Mg, V, Co) in doping LiFePO₄ material is much smaller after discharge. These stable structures have a smaller change in volume after discharge.

Table 2. Lattice parameters and volume change of LiFe0.75Mg0.25PO4

Table 3. Lattice parameters and volume change of LiFe0.75V0.25PO4

High performance of Li-ion battery should have high diffusivity in cathode material. However, the diffusivity of LiFePO₄ cathode material is only about 10^-11~10^-13 cm²s^-1, much lower than the 10^-8 cm²s^-1 of LiCoO₂, already popularly used in market. In general, the diffusive behavior of Li-ion is related to the gap of lattice structure, which is the main pathway of Li-ions. Larger gap allows more Li-ion passing through. For instance, LiCoO₂ is characterized by two-dimensional layer structure and the gap between two adjacent layers is large enough and can accommodate a lot of Li-ions to pass through. It, therefore, has a very good diffusivity. On the other hand, LiFePO₄ is featured by P-O tetrahedron and Fe-O octahedral structures, and the pathway of Li-ion is only one-dimensional, which severely restricts the diffusivity of material. In this article, a part of Li atoms in LiFePO₄ are replaced by Na through doping.

The electronic conductivity of LiFePO₄ material is limited due to large energy gap. It was reported in experiment that the V-doped LiFePO₄ cathode exhibits significant enhancement in the electrochemical performance compared with pure olivine LiFePO₄ [4]. In this work, the effects of doping five different elements, i.e., Mg, V, Ti, Mn, and Co, to LiFePO₄ material are studied. It is found in Table 4 that the energy gap can be significantly reduced by doping V, Ti, Mn, and Co. The effect of doping V is the best of all. Compared to Figs. 3 and 4, the decrease of energy gap by doping V is mainly due to the shift of density state toward the Fermi energy in conduction band, as shown in Fig. 5.

Table 4. Energy gap of cathode materials after doping M (M = Co, Mg, Ti, Mn, V)

Figure 2. Molecular structure of LiFePO4

Figure 3. Energy band structure of LiFePO4

Figure 4. Energy band structure of Li0.75Na0.25Fe0.75V0.25PO4

Figure 5. LDOS of metal doping (a) LiFe0.75V0.25PO4 (b) LiFe0.75Mn0.25PO4

4. Conclusions

1. In this work, the effects of doping Mg, V, Ti, Mn, and Co to LiFePO₄ material by replacing Fe atoms to improve the electric conductivity were evaluated. It was found that the energy gap can be reduced by doping V, Ti, Mn, and Co. The effect of doping V is the optimal, and the corresponding energy gap can be significantly reduced from 0.925 eV to 0.207 eV. However, the effect of doping Mn is insignificant. The reason why the energy gap becomes smaller is mainly due to the shift of local electron distributions toward the Fermi energy.

2. During the charge/discharge cycles, the volume variation of LiFePO₄ material becomes significant due to the repeatedly extraction/insertion of Li ions. Higher volume change tends to cause higher possibility of structural damage. It was found that the doping of Mg, V, and Co metal ions can decrease the volume change and improve the life cycles of battery. For these three metals, the effect of Mg is the best. The volume change can be significantly reduced from original 8.1% to 5.4%.

3. The diffusion efficiency of Li ion in LiFePO₄ material can be enhanced by increasing the gap of LiFePO₄. It was found that the gap can be increased by doping Na ion.

ACKNOWLEDGEMENTS

This research was supported by the Ministry of Science and Technology in Taiwan through Grants No. 103-2221-E-006-054.