Electrochemical and Crystalstructural Analysis of Intermediate Phase Behavior in Co-Substituted LiFePO4 with Reduced Lattice Mismatch

INTRODUCTION Olivine-type LiFePO 4 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 Li 0.6 FePO 4 (L x FP) which acts as a buffer layer between Li-rich phase (LFP) and Li-poor phase(FP) . 1 To improve the rate performances of LiFePO 4 , controlling the lattice strain between two phases was one of the idea. Recently, it is reported that the relative volume change of the LiFePO 4 was controlled by the substitution of the cation in the LiFePO 4 . From the DFT calculation, the relative volume change between two endmembers of Li(Fe 1-x Zr x )(P 1-2x Si 2x )O 4 (Z2S) was smaller than 3%, and show the better cycle stability. 2 However, more detailed reaction mechanisms has not been clarified yet. Recently, the LiFePO 4 doped with different cations were also reported. For example, vanadium doped LiFePO 4 showed the better rate properties and the temperature of phase transition from two phase to solid-solution was more decreased than undoped LiFePO 4 . The stability of L x FP was expected one of the key factor for the improvement of the rate properties of the LiFePO 4 . Here we investigated the relationship between the rate property of Z2S and the appearance of L x FP by using intermediate temperature cell. EXPERIMENTAL LiFePO 4 (Undoped) and Li(Fe 0.95 Zr 0.05 )(P 0.9 Si 0.1 )O 4 (Z2S) were synthesized in the same manner as reported one. 2 The temperature-controlled XRD under Ar atmosphere was performed for the both Li 0.66 FePO 4 powders.(denoted Undoped Li 0.66 FePO 4 , and Z2S Li 0.66 FePO 4 ) The Li 0.66 FePO 4 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 2 θ range 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(SO 2 CF 3 ) 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 Li 0.66 FePO 4 corresponded