For the practical use of Lithium-ion battery in electric vehicles, the rate capability should be more improved. Although LiFePO₄ is wildly known as the high power cathode materials, the large mismatch between LiFePO₄ and FePO₄ might still disturb the further improvement of the rate capability. Because the huge volume change during the phase transition require the large activation energy, the suppression of the crystal structure change is one of attracted design principle for cathode materials. Recently, Nishijima et al have reported that Zr-substitution in LiFePO₄ eliminates the volume change between LiFePO₄ and FePO₄ and the cyclability of the doped LiFePO₄ is drastically improved[1]. In this study, we investigate the rate capability of the Zr-dopoed LiFePO₄. Moreover the phase transition behavior is examined by using operando X-ray diffraction measurements.

LiFePO₄, called hereafter undoped LFP, and Li(Fe_0.95Zr_0.05)(P_0.9Si_0.1)O₄, called Zr-doped LFP, were synthesized by the solid solution method in the same manner as reported[1]. The crystal structure of the prepared samples were analyzed by X-ray diffraction (XRD) which were performed at beamline BL02B2, SPring-8 with a wavelength of 0. 699292(4) Å using a Debye-Scherrer camera and an imaging plate detector. The electrodes for the electrochemical tests were prepared by mixing 80% active material, 10% carbon black, and 10% polyvinylidene fluoride (PVDF) with 1-methyl-2-pyrrolidinone solvent. The slurry was then casted onto an Al foil current collector. 1 M LiPF₆ in a 3:7 volume ratio of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) was used as the electrolyte. Li foil was used as a counter electrode. operando time-resolved XRD measurements were performed at SPring-8 with a wavelength of 0.619862(2) Å using a 1D detector, Mythen.

All the peaks of XRD pattern of the both prepared powder samples are indexed to orthorhombic Pnma space group and any impurity is not observed. The calculated lattice volume of Zr-doped LFP is almost similar to that of undoped LFP. SEM measurements indicate that the particle size of both undoped LFP and Zr-doped LFP is approximately 100 nm. The rate capability of Zr-doped LFP is improved compared with undoped LFP. At 10 C rate, 110 mAh/g is still acquired in Zr-doped LFP, while the capacity of undoped LFP is decreased to 90 mAh/g. The polarization of the discharge profile is also suppressed by Zr substitution. Operando time-resolved XRD measurements reveal that the expansion of solid solution reaction of LiFePO₄ and FePO₄end-members in nonequilibrium condition.

REFERENCES

[1]  Nishijima, M.; Ootani, T.; Kamimura, Y.; Sueki, T.; Esaki, S.; Murai, S.; Fujita, K.; Tanaka, K.; Ohira, K.; Koyama, Y.; Tanaka, I., Nat. Commun. 5, 4553 (2014).