Stabilization of Intermediate Phase in LiFePO4 Leading to High-Rate Performance
Authors: Yoshinari Takahiro , Orikasa Yuki , Nishijima Motoaki , Ohira Koji , Esaki Shogo , Sueki Toshitsugu , Ootani Takuya , Kamimura Yuichi , Ohara Koji , Fukuda Katsutoshi , Koyama Yukinori , Ogumi Zempachi , Uchimoto Yoshiharu 
Published: February 2020

INTRODUCTION

LiFePO4 is a commercially successful cathode material due to low costs and high-power energy density. The electrode reaction proceeds divided into two phases, Li-rich phase and Li-poor phase, with a large volume change. We reported the intrinsic high-rate nature that the intermediate phase, which is metastable at room temperature, emerges to moderate the lattice mismatch and gets closer to the single-phase reaction1. Here, we show how the stabilization of the intermediate phase can improve the rate capability using time-resolved in-situ XRD measurements. We also derive general non-equilibrium criteria for two-phase reaction in Li-intercalation compounds using the LiFePO4 system as a specific example.

EXPERIMENTAL

Undoped LiFePO4 and Li(Fe0.95Zr0.05)(P0.9Si0.1)O4, denoted UL and Z2S respectively, were synthesized in the same manner as reported2. The partially delithiated materials which is Li concentration of x = 0.66 in LixFePO4 were prepared by the chemical reduction. The ex situ X-ray diffraction (XRD) patterns were obtained at various Li concentration and elevated temperature. The electrochemical tests were performed using a liquid electrolyte at room temperature and a molten salt electrolyte at elevated temperature. The time-resolved in situ XRD measurements were performed at BL28XU, SPring-8 with a wavelength of 0.619862(2) Å using a 1D detector, MYTHEN 1K. The data were collected in the 2θ range of 10° to 13° with an exposure time of 1 s.

RESULTS AND DISCCUSION

The ex situ XRD patterns of Li0.66FePO4 show that the intermediate phase explicitly emerge at 150°C in Z2S whereas 200°C in UL, which indicate the stabilization of the intermediate phase by the reduced lattice volume change. This result is also supported the electrochemical tests at elevated temperature whether or not the two-step voltage plateau could be observed. The ex situ XRD patterns of various Li concentration and the open-circuit voltage measurements show a typical two-phase characteristics in both cathodes. There is no significant difference in phase transition behavior based on an equilibrium system because the intermediate phase is metastable at room temperature. We performed time-resolved in-situ XRD measurements to access the non-equilibrium phase transition under a large overpotential. At a high rate of 10C, the peak shift mainly occurs in Z2S compared with UL. This suggests the quasi-single phase reaction proceeds via the intermediate phase. The reaction kinetics was then evaluated. At higher rates, the differences of both capacities are lager, which indicates the enhancement of the rate capability in Z2S. The stabilization of the intermediate phase by the reduction of the lattice volume change leads to the ability of high-rate cycling. This insight involves new concepts in non-equilibrium thermodynamics, which may apply other two-phase electrode materials.

REFERENCES

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