Effect of microstructure on low temperature electrochemical properties of LiFePO4/C cathode material

Abstract

The low-temperature electrochemical performance of Li-ion batteries is mainly determined by the choice of cathode material, as evident from a comparison of the low-temperature electrochemical performance of the 18650 batteries with the LiMn2O4, LiNi1/3Co1/3Mn1/3O2, and LiFePO4/C as the cathode, respectively, at −20°C. LiFePO4/C materials with different morphologies and microstructures were prepared by different methods. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), galvanostatic charge–discharge measurements and EIS. The low-temperature performance of the samples and those of the coin cells utilizing the materials as cathodes were measured. The results indicate that the microstructure of LiFePO4/C is a key factor determining the low-temperature performance of LiFePO4/C. A new type of LiFePO4/C with a pomegranate-like spherical structure composed of smaller spherical particles is reported, which shows good process-ability and superior low-temperature performance. The composite has a uniform particle size and carbon network, which delivers a discharge capacity of 89.3mAhg−1 at −20°C at a discharge rate of 0.5 C, with capacity retention rate of 58.7%. The 18650 batteries were prepared with pomegranate-like spherical structure LiFePO4/C composite which delivers a discharge capacity of 1603.7, 1563.8, 1572.28, 1598.0, 1580.1, 1504.2, and 1405.4mAh at 0.5 C, 1 C, 2 C, 5 C, 10 C, 15 C, and 20 C, under 25°C, respectively. Moreover, the batteries also exhibit good low-temperature performance with capacity of 1127.2mAh at −20°C at a discharge rate of 1 C, which is the 72.1% of the same discharge rate at 25°C. Otherwise, the 18650 batteries also exhibit excellent cycling performance and the capacity maintains 83.4% at −20°C after 100 cycles. The superior low-temperature performance of the LiFePO4/C composite material may be attributed to its uniform carbon network and fine primary particles.