Development of Lithium Iron Phosphate Cathode Materialfor Super Long Life Lithium-Ion Battery

Introduction

LiFePO₄ (LFP) is a well known, low cost and a safe lithium ion cathode material. The charge and discharge process in LFP proceeds viatwo-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.

In the case of LFP, the volume change during charge and discharge is 7% and the difference of the interface plane (mismatch of bc plane) is 1.5%. These values are larger than other cathode materials and this causes 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.

Experiment

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 (Fe_1-xZr_x) (P_1-2xSi_2x) O₄. The calculated volume change ratio of this material was 3.2% at x = 0.125, which shows a very small value compared to 7% of the un-substituted LFP.

Samples were synthesized by the following sol-gel method. First, iron nitrate (Fe (NO₃) 3.6 H₂O) was solved into alcohol, and then lithium acetate (LiCH₃COO · 2H₂O) was add to this solution. After lithium acetate are completely solved, then Zr and Si source was add to this solution. TEOS was used for Si source, and ZrCl₄ was used for Zr source. After that, phosphoric acid (H₃PO₄) was added to this solution. After the adding of propylene oxide (PO), solution changes to solid gel immediately. The obtained gel was allowed to promote gelation by heating for 24 hours at 60 deg in a closed container. The obtained the dried gel was ground in a mortar, and calcined at 550 deg. under a nitrogen atmosphere.

Results and discussion

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%.

From the X-ray diffraction pattern of the sample after full-charging, the lattice constant was calculated. The variation of the volume change ratio and mismatch of bc plane for various substitution was shown in Fig.1. 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 bc plane ratio was also decrease with substitution, and reduced to about 0.4% in the replacement of 12.5%.

Fig. 2 shows the cycle performance of two pouch cells that comprise of 1. A substituted LFP cathode / graphite anode and 2. An un-substituted LFP / graphite anode. For the un-substituted cathode cell, the capacity decreases with cycling. For the substituted LFP cell, the capacity degradation was only 15% after 10,000 cycles. These results show that the substituted cathode that has a low volume change during charge and discharge and this enhances the cell cycle life remarkably.