Consecutive Phase Transition of Lattice Strain Controlled LiFePO₄ for High-Power Li-Ion Batteries

Abstract

INTRODUCTION Lithium iron phosphate, LiFePO4, becomes conspicuous as a commercially important cathode material due to low cost, high safety, and non-toxic nature. Lithium-ion intercalation proceeds through a two-phase reaction between two compositions close to the endmembers LiFePO4 (LFP-phase) and FePO4 (FP-phase). The first-order phase transformation accompanied with a large volume change of 6.8% hinders moving phase boundaries faster than ever. Co-substituted LiFePO4, represented in Li(Fe1-xZrx)(P1-2xSi2x)O4 or Z2S, which decreases the lattice volume change between two phases, shows six times longer cycle life than undoped LiFePO4 (ref. 1). Here we focus on the Z2S, consisting of moderate two-phase interfaces, and investigate kinetics and mechanisms for the two-phase reaction with reduced lattice mismatch using time-resolved X-ray diffraction. EXPERIMENTAL Undoped LiFePO4 and Li(Fe0.95Zr0.05)(P0.9Si0.1)O4, just called hereafter Z2S, were synthesized in the same manner as reported1. The ex situ X-ray diffraction (XRD) measurements were performed at BL02B2, SPring-8 with a wavelength of 0.699292(4) Å using a Debye-Scherrer camera and an imaging plate detector. The cathode materials 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 composite electrodes were placed in original laminate-type cells in an Ar-filled glovebox with lithium metal as the counter and reference electrodes. LiPF6 (1 M) in a 3:7 volume ratio of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) was used as the electrolyte. The 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 every 60 s(1C) and 15 s(10C). RESULTS AND DISCCUSION Both Undoped LiFePO4 and Z2S are the same particle size as about 150 nm, indicating independent of nano-sized effects2 and allowing the comparison of the difference of the lattice volume change. Galvanostatic charge/discharge tests were performed. At a low rate of 1C, both of them show the same capacity as 120 mAh/g and the similar shape of charge/discharge curves. At a high rate of 10C, the capacity of Z2S keeps still 90 mAh/g although that of undoped LiFePO4 falls to 75 mAh/g. The difference of both capacities are lager with increasing rates, which indicates the enhancement of the rate capability in Z2S. In order to get information on the static state, the ex-situ XRD and the open-circuit voltage (OCV) measurements were performed. The ex situ XRD patterns show a typical two-phase characteristics in both cathodes as emerged FP-phase peaks from 0.1Li during a charge process. The OCV curves show the flat voltage plateau in a wide range, which also indicates the coexistence of two phases in the reaction. From these results based on an equilibrium system, there is no significant difference in phase transition behavior between undoped LiFePO4 and Z2S. To try to understand what is going on in lithium intercalation reaction indeed and to achieve high power lithium-ion batteries, time-resolved in-situ XRD measurements were performed. At a low rate of 1C, undoped LiFePO4 proceeds via two phase reaction similarly in the results of the ex situ experiments. In Z2S, the peaks belonging to each LFP-phase and FP-phase simply increase or decrease although accompanying with peak broadening. At a high rate of 10C, on the other hand, the peak shift mainly occurs in Z2S compared with undoped LiFePO4. This suggests the quasi-single phase reaction proceeds in Z2S and explains the enhancement of rate capability in Z2S, which is the small lattice volume change. REFERENCES 1. M. Nishijima, T. Ootani, Y. Kamimura, T. Sueki, S. Esaki, S. Murai, K. Fujita, K. Tanaka, K. Ohira, Y. Koyama and I. Tanaka, Nat. Commun., 5 (2014). 2. N. Meethong, H. Y. S. Huang, W. C. Carter and Y. M. Chiang, Electrochemical and Solid State Letters, 10, A134 (2007).