Development of Lithium Iron Phosphate Cathode Materials for Super Long Life Lithium Ion Battery

摘要

LiFePO4 (LFP) is a well known, low cost and a safe lithium ion cathode material. The charge and discharge process in LFP proceeds via two-phase reactions. This process produces particles in which two phases coexist with different lattice parameters. For phases which exhibit a large difference in lattice parameters, large stresses will occur in the interface plane between the phases and cause micro-cracks inside the particles during the repeated charge and discharge process. In order to solve this problem, we have used first principles calculations to investigate the effect of various substitute elements into different atomic positions within LFP. These calculations indicate that certain compositions reduce the volume change and the mismatch of interface. Several compositions with a variety of substitutions onto the Li site, Fe site, the P site are calculated. The composition in which P was substituted by Si and Fe was substituted by Zr was the most promising optimal composition, Li (Fel-xZrx) (Pl-2xSi2x) 04. Samples were synthesized by sol-gel method Structural analysis from x-ray diffraction data and subsequent Rietveld refinements show that the substituted materials were single phase with no impurities. The maximum substitution of Zr in Fe was found to be 12.5%. The volume change ratio was 6.5 % for the un-substituted LFP. The volume change ratio decrease with substitution, the volume change ratio of 3.7 % was achieved in 12.5% substituted samples. In addition, the mismatch of be plane was also decrease with substitution, and reduced to about 0.4% in the replacement of 12.5%. Pouch cells were evaluated to check the cycle capacity retention. For the substituted LFP cell, the capacity degradation was only 11% after 10,000 cycles. This result shows that the substituted cathode that has a low volume change during charge and discharge and this enhances the cell cycle life remarkably.