Consecutive Phase Transition of Lattice Strain Controlled LiFePO 4 for High-Power Li-Ion Batteries

INTRODUCTION Lithium iron phosphate, LiFePO 4 , 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 LiFePO 4 (LFP-phase) and FePO 4 (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 LiFePO 4 , represented in Li(Fe 1-x Zr x )(P 1-2x Si 2x )O 4 or Z2S, which decreases the lattice volume change between two phases, shows six times longer cycle life than undoped LiFePO 4 (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 LiFePO 4 and Li(Fe 0.95 Zr 0.05 )(P 0.9 Si 0.1 )O 4 , just called hereafter Z2S, were synthesized in the same manner as reported 1 . 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. LiPF 6 (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 LiFePO 4 and Z2S are the same particle size as about 150 nm, indicating independent of nano-sized effects 2 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 LiFePO 4 falls to 75 mAh/g. The difference of both capacities are lager with increasing rates,