Electrochemical and Crystalstructural Analysis of Intermediate Phase Behavior in Co-Substituted LiFePO4 with Reduced Lattice Mismatch
Authors: Hamaguchi Naoya , 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

Olivine-type LiFePO4 is one of the most promising cathode material for lithium-ion batteries as it exhibits high rate performance. Under high rate cycling, we revealed the metastable phase formation of Li0.6FePO4(LxFP) which acts as a buffer layer between Li-rich phase (LFP) and Li-poor phase(FP) .1 To improve the rate performances of LiFePO4, controlling the lattice strain between two phases was one of the idea. Recently, it is reported that the relative volume change of the LiFePO4 was controlled by the substitution of the cation in the LiFePO4. From the DFT calculation, the relative volume change between two endmembers of Li(Fe1-xZrx)(P1-2xSi2x)O4 (Z2S) was smaller than 3%, and show the better cycle stability.2 However, more detailed reaction mechanisms has not been clarified yet. Recently, the LiFePO4 doped with different cations were also reported. For example, vanadium doped LiFePO4 showed the better rate properties and the temperature of phase transition from two phase to solid-solution was more decreased than undoped LiFePO4. The stability of LxFP was expected one of the key factor for the improvement of the rate properties of the LiFePO4. Here we investigated the relationship between the rate property of Z2S and the appearance of LxFP by using intermediate temperature cell.

EXPERIMENTAL

LiFePO4 (Undoped) and Li(Fe0.95Zr0.05)(P0.9Si0.1)O4 (Z2S) were synthesized in the same manner as reported one.2 The temperature-controlled XRD under Ar atmosphere was performed for the both Li0.66FePO4 powders.(denoted Undoped Li0.66FePO4, and Z2S Li0.66FePO4) The Li0.66FePO4 was prepared by the mixture of the pristine powders and chemically delithiated powders. The temperature was changed from 25°C to 300°C by 5ºC min-1, and XRD measurements was performed after keeping for 10 hours at several temperatures. The 2θrange was 29.0º ~ 31.5º.

 The working electrodes for the electrochemical tests were prepared by mixing 80% active material, 10% carbon black, and 10% polyimmide binder with 1-methyl-2-pyrrolidinone solvent and coating on the aluminum current collector. A binary molten salt electrolyte based on MN(SO2CF3)2(M=Li,Cs) was used as the electrolyte. The temperature range of galvanostatic charge-discharge was performed between 150°C and 170°C, rate performance was measured in 170°C.

RESULTS AND DISCCUSION

 The temperature controlled XRD patterns of Undoped Li0.66FePO4 corresponded with the reported phase diagrams.4 The peaks of LxFP 020 was appeared above 200ºC. On the other hand, Z2S Li0.66FePO4 tends to decrease the solid solution formation temperature. The peaks of LxFP 020 was appeared above 150ºC. Z2S sample stabilized the LxFP phase at lower temperature than undoped LiFePO4. We also performed galvanostatic charge-discharge measurements at 170ºC by using intermediate temperature ionic liquids. The charge curves of Z2S have the two plateau regions despite the curves of undoped LiFePO4 have the single plateau. This phenomena corresponded with the results of the temperature controlled XRD. At a high rate of 30C, the capacity of Z2S was higher than undoped LiFePO4. The difference of the obtained capacity between Z2S and undoped LiFePO4 at high rate was larger than the room temperature conditions. The LxFP phase stabilized by cation substitutions and appearance of LxFP phase leads the high rate performance of the LiFePO4electrodes.

REFERENCES

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